Understanding Cement An introduction to cement production, cement hydration and deleterious processes in concrete
Nicholas B Winter WHD Microanalysis Consultants Ltd
Published by WHD Microanalysis Consultants Ltd Iken House, 8 Acer Road, Rendlesham, Woodbridge, Suffolk IP17 1PL United Kingdom Copyright © 2009 N B Winter. All rights reserved. This work is registered with the UK Copyright Service Registration No: 307063 NOTICE: This e-book is sold for individual use only and may be stored on a single computer. It MAY NOT be stored on any computer network or other retrieval system that allows access by more than one person. You MAY print a single copy for your own personal use. You DO NOT have the right to reprint or resell this e-book. You also MAY NOT give away, sell or share the content herein. Please contact us for details of CORPORATE or EDUCATIONAL multiple user licensing If you obtained this e-book from any source other than through http://www.understanding-cement.com, you have a pirated copy. Please help stop Internet crime by reporting this to: mailto:
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Contents
Introduction
vi
1
A brief history of cement
1
2
Cement basics
5
2.1 2.2
5 6
3
Portland cement composition and microstructure 3.1 3.2 3.2.1 3.2.2 3.3 3.4 3.5 3.5.1 3.6 3.7
4
Some definitions Some non-Portland cements
9
Portland cement minerals Portland cement composition and cement notation Note on oxide analysis Cement chemistry notation Microstructure of clinker and cement Hydraulic properties of the main clinker minerals Proportions of the main clinker phases The Bogue calculation Common parameters used in cement manufacturing Other Portland cement types
10 12 12 13 15 18 19 20 26 28
Portland cement manufacturing - the main components of a cement plant
32
4.1 4.2 4.3 4.4
32 33 39 40
The The The The
quarry kiln cooler cement mill
5
6
7
Portland cement manufacturing – from raw materials to cement
43
5.1 5.2 5.3 5.4 5.5 5.5.1 5.5.2 5.5.3 5.5.4 5.6 5.7 5.8 5.9
43 43 45 49 50 50 51 52 53 54 57 59 61
Raw material blending Minor constituents Combinability temperatures Raw material proportioning Reactions in the kiln Reactions before the burning zone Reactions in the burning zone Reducing conditions Cooling of Clinker Clinker sulfate phases Clinker grinding and gypsum addition Other additions Calculation of clinker minerals in cement
Hydration of cement – chemical and physical properties of cementitious materials 6.1 Hydration of cement: heat evolution 6.2 Hydration of cement: main types of hydration product 6.3 Hydration of cement: further considerations 6.3.1 AFm and AFt phases 6.3.2 Flash set and false set 6.3.3 Hydration of cement: other hydration products 6.3.4 Description of cement hydration 6.4 Hydration of cement: paste microstructure and water/cement ratio 6.5 Hydration of cement: pore structure of cement paste and the Powers-Brownyard model 6.6 Some physical properties of cementitious materials 6.6.1 Concrete workability 6.6.2 Concrete strength 6.6.3 Concrete permeability 6.6.4 Other factors
63
Composite Cements
84
7.1 7.1.1 7.1.2
84 84
7.1.3 7.1.4 7.1.5 7.2 7.2.1 7.2.2 7.2.3 7.3 7.3.1
Introduction Background Difference between pozzolanic and latently hydraulic mineral additions Effects of mineral additions on hydration products and paste microstructure Summary of benefits of using mineral additions Potential problems of using mineral additions Blastfurnace slag Blastfurnace slag composition Blastfurnace slag as a cementitious material Hydration products in mixes containing slag Low-lime fly ash Fly ash composition
64 65 68 69 71 71 71 73 77 79 79 81 82 82
85 85 85 87 88 88 88 91 93 93 93
7.3.2 7.3.3 7.4 7.5 8
9
10
11
Fly ash as a cementitious material Hydration products in mixes containing fly ash Microsilica (silica fume) Limestone
94 96 98 99
Cement variability
102
8.1 8.2
102 104
In defence of the cement producer Causes of cement variability
Deleterious processes in concrete
108
9.1 9.1.1 9.1.2 9.2 9.2.1 9.2.2 9.2.3 9.2.4 9.2.5 9.3 9.4 9.5 9.6 9.7
108 108 115 116 118 119 122 124 124 125 131 132 133 134
Reactions between cement paste and aggregate Alkali-silica reaction Alkali-carbonate reaction Sulfate attack in concrete and mortar External sulfate attack Internal sulfate attack Thaumasite form of sulfate attack (TSA) Sulfate attack in mortar Identification of sulfate attack Carbonation Steel corrosion Leaching Frost damage (freeze-thaw action) Efflorescence on masonry
Standards for Portland Cement
137
10.1 10.2 10.2.1 10.2.2 10.3 10.4
138 140 140 142 145 145
ASTM C 150-07 European Standard EN-197 European Standard EN-197, composition European Standard EN-197, strength classes Other specified cement properties Don’t ever mix the standards!
Cement concepts
147
11.1 11.1.1
147
11.1.2 11.2 11.3 11.4 11.4.1 11.4.2 11.5
Altering the properties of cement Altering the properties of cement: modify the Portland cement Altering the properties of cement: combine Portland cement with other materials A “mind’s eye image” of cement Cement clinker Hydrated cement Some useful principles Hydration of different cements Towards quantifying cement hydration products
148 150 152 154 160 160 162 174
12
Making cement greener
176
12.1 12.2
176 177
Cutting back on burning fossil fuels Reducing CO2
Appendix 1 - further reading
180
Introduction This e-book is an informal introduction to cement. Perhaps you are just starting a cement or concrete-related career; maybe you are a student, or perhaps you already know quite a bit about cement but want to refresh your mind on some cement chemistry. Whatever your interest in cement, if you are looking for a basic guide to cement science, I believe that you will find this e-book helpful. No one fully understands cement; the title of this e-book should really be something like “Understanding Cement a Bit Better.” Cement is like any other fascinating subject; the more you find out, the more you realise just how huge the gap really is between what you know and what there is to be known. I’ve worked with cement for nearly thirty years, first at the research division of a major cement producer, then as an independent consultant and I’m only too happy to talk about cement with anyone who is interested, so thank you for reading this e-book. Much of my work has been to do with cement and concrete microscopy and as a result, this e-book is very “visual”. They say that a picture is worth a thousand words, and in the following pages there are a lot of images taken using optical and scanning electron microscopes; pictures really do help in gaining an understanding of how cement is made and how it works. Until recently, “cement” generally meant Portland cement, the normal grey powder, and Portland cement forms the main subject of this e-book as it is still the principal cement used. However, increasingly, Portland cement is used in conjunction with other cementitious materials such as fly ash and slag. The extent to which these “mineral additions” are used varies in different parts of the world, but looks set to increase everywhere. The reasons are varied, but include enhanced cement performance, conservation of virgin raw materials and a reduction in carbon dioxide emissions. We will look first at Portland cement production, then at Portland cement hydration. With this firmly established, we then widen the scope of the term “cement” to include composite (ie: blended) cements, looking at why they are used and how they alter the cement hydration products. After a quick look at some physical properties of hydrated cement, we move swiftly on to consider why variations in Portland cement might occur between one day and the next before considering the basics of the principal deleterious processes that can affect concrete such as sulfate attack and alkali-silica reaction. The penultimate chapter draws everything together in the form of “thought experiments” that consolidate the main subjects from the earlier chapters. Finally, we look briefly at how cement is becoming “greener”. The material presented here is either mainstream cement science that has undergone peer review before publication in journals or textbooks, or is industry data regarded as non-controversial. Of course, if you should find something you disagree with, please contact me via the Understanding Cement web site and I would be delighted to discuss it.
“Understanding Cement” has its origins in a one-day seminar on cement that Arthur Harrisson and I first gave back in 1991. Since then, the seminar content and cement science have both evolved, and the technology for presenting it has progressed hugely, from handwritten overhead projector slides to the internet. This e-book is intended to be a fairly gentle introduction into the slightly arcane world of cement science and I very much hope that the basic grounding it provides will enable you to go on and read with confidence some of the many other textbooks available on cement and concrete. Excellent though they mostly are, they can be a little daunting at first, so “Understanding Cement” is here to get you started. References are provided at the end of each chapter; many are references to more detailed treatments in some of the standard works on cement, particularly Taylor’s “Cement Chemistry” and Lea’s “The Chemistry of Cement”. There are also some references to individual papers where these are especially relevant. However, I have limited the range of external references to some extent because not all readers will have easy access to a technical library where these often slightly obscure references may be obtained. It is the purpose of this e-book to be helpful and encouraging, not to handicap anyone who does not have a university library next door. In Appendix 1 are my suggestions for the nucleus of a small “cement library” that will be of value for many years to anyone interested in cement science. Together, these books contain more references than anyone is likely to want to read in a lifetime of cement study. I am very grateful to the many people who have helped me with this e-book, especially the two groups of reviewers who so kindly read the draft version. The “expert reviewers” were people who have spent their working lives in cement manufacturing, concrete production, masonry, education or related fields; they contributed greatly to the technical content. The “readability reviewers” had a technical background but no specific knowledge of cement; they identified many areas where I had not explained the subject matter sufficiently clearly. I am also very grateful to those who have helped in other ways too numerous and diverse to describe in detail. I particularly thank Don Ashcroft, Geoff Bowler, Tom Burnham, Mike Burton, Josette Camilleri, Mike Connell, Ian Ferguson, Ron Green, Arthur Harrisson, Paul Livesey, Robert Matthews, Kelly Park, Lindon Sear, Anthony Tidder and Anna Wright.
NOTES Spelling The spelling used is standard “British English,” except for “sulphur” and related words where I have adopted the International Union of Pure and Applied Chemistry (IUPAC) standard spelling of “sulfur”. How to use this book Some people can happily read a book from a computer screen. If you can, that has the advantage that you can use the “search” function of Acrobat Reader. Many people, however, including me, prefer to read from the printed page and if that is you, then print this e-book out and bind it with a comb binder or similar, or (even better) get someone to do it for you. The margins are set so that the document should print on both A4 and US letter paper without adjustment. Printing on both sides of the paper will make the book more manageable. Microscope images This e-book contains many images taken using either a scanning electron microscope (SEM) or petrographic microscope. SEM images are black-and-white images as electrons don’t have colour; petrographic microscope images are in colour. Microscope images have scale bars to indicate the size of features. These are shown in microns, or micrometres (µm). A micron is 10-6 metre, so there are 1,000 microns in a millimetre and 1,000,000 microns in a metre. The microscope images do not show the magnification (eg: x1000) since this will vary depending how the image is displayed. An image that is x100 displayed fullscreen on a 15-inch computer monitor will be x160 on a 24-inch monitor, so indicating a fixed magnification is meaningless unless you know the size of the original image. It is the scale bar that is important, not the magnification. If you want to work out the magnification of an image, measure the scale bar as displayed on your monitor, or on the page if you have printed it, and work it out. For example, if a 1000 µm (=1 mm) scale bar measures 10 mm on your monitor, the magnification is x10. If a 100 µm scale bar measures 10 mm (=10,000 µm), the magnification is 10,000/100 = x100. If a 50 µm scale bar measures 20 mm, the magnification is 20,000/50 = x400. Glossary You may find it helpful to download the cement glossary, available if you sign up to the free Newsletter, from www.understanding-cement.com.
Contact details
Feedback Comments, suggestions, bouquets, brickbats – all feedback will be gratefully received. Please do tell me what you think of “Understanding Cement”. Your comments will be very useful in improving future editions of this e-book. The best way to send feedback is to use the feedback form at: www.understanding-cement.com/ucebookfeedback.html
E-book technical problems If you have any technical problems with the e-book (eg: printing it out), contact us using the Contact Form on the Understanding Cement web site and we will try to help. http://www.understanding-cement.com/contact.html
Consultancy If you have any queries about the consultancy services offered by WHD Microanalysis Consultants Ltd., I can be reached at:
[email protected]
Nick Winter December 2009
Cover picture courtesy Castle Cement
IMPORTANT! I hope you know this already, but the first thing anyone should know about cement, is that it is HIGHLY ALKALINE. If you get wet cement, or dry cement powder, on your skin you will get ALKALI BURNS. These can be severe and in extreme cases can result in the amputation of limbs, or even death. You should therefore avoid contact with cement by taking appropriate precautions, including wearing suitable protective clothing and equipment. If you do get cement on your skin, or in your eyes, wash it off immediately and seek medical advice if necessary.
On that happy note, read on!
1 A brief history of cement
Cementing materials were used widely in the ancient world. The Egyptians used calcined gypsum as a cement. The Greeks and Romans used lime made by heating limestone, and then added sand to make mortar, with coarser gravel for concrete. This process of heating limestone so that it decomposes to lime (calcium oxide) and carbon dioxide gas is called “calcination”. Before the lime is used in building, it is normally mixed with water (“slaking”) to convert the calcium oxide to calcium hydroxide. The Romans found that a cement could be made which set under water and this was used for the construction of harbours. The cement was made by adding crushed volcanic ash to lime, and was later called a ‘pozzolanic’ cement named after the village of Pozzuoli near Vesuvius. Where volcanic ash was scarce, crushed brick or tile was used instead. The Romans were therefore probably the first to manipulate the properties of cementitious materials for specific applications and situations. Marcus Vitruvius Pollio, a Roman architect and engineer in the 1st century BCE wrote in his ‘Ten books of Architecture,’ a revealing insight into ancient technology (1): “There is also a kind of powder from which natural causes produces astonishing results… This substance, when mixed with lime and rubble, not only lends strength to buildings of other kinds, but even when piers are constructed of it in the sea, they set hard under water.” The Romans used pozzolanic material more widely than just in marine construction. Vitruvious says: “First I shall begin with the concrete flooring, which is the most important of the polished finishings, observing that great pains and the utmost precaution must be taken to ensure its durability….”
A brief history of cement “…On this, lay the nucleus, consisting of pounded tile mixed with lime in the proportions of three parts to one, and forming a layer not less than six digits thick.” Vitruvious’ ‘Ten books of architecture’ is a real historical gem bringing together history and technology, although anyone wishing to follow his instructions might first need to find a thousand or so slaves to dig, saw, pound and polish... After the end of the Roman Empire, much of their building technology was lost certainly in Europe with regard to cement. Mortars hardened mainly by carbonation of lime, a slow process, and it wasn’t until the late Middle Ages that the benefits of pozzolanic material mixed with lime were rediscovered. The great European mediaeval cathedrals were clearly built by highly skilled masons but it would probably be fair to say they did not have the technology to manipulate the properties of cementitious materials in the way the Romans had done a thousand years earlier. The Renaissance and Age of Enlightenment brought new ways of thinking and compelling new reasons to develop technology, including cement technology. For example, maritime nations needed to build lighthouses on exposed rocks to reduce shipping losses. John Smeaton, the “father of civil engineering” in England, found that a mix of hydraulic lime, produced by burning an impure limestone and then mixing the lime with a natural pozzolan, produced a mortar that hardened under water (2). Smeaton used his mortar with interlocking stone blocks in the construction of the third Eddystone lighthouse (1759) off the coast of Cornwall in Southwestern England. This mixture for a mortar of lime and pozzolan was specified for government contracts until 1867, 43 years after Aspdin’s patent for Portland cement.
Figure 1.1 Ardnamurchan lighthouse, Scotland, built by Alan Stevenson and completed in 1849. The Stevensons were a Scottish family of engineer lighthouse builders who used similar construction technology to Smeaton’s. Alan Stevenson was the uncle of author Robert Louis Stevenson, who wrote “Treasure Island” and many other books.
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A brief history of cement Joseph Aspdin took out a patent in 1824 for “Portland Cement,” a material he produced by firing a mixture of finely-ground clay and limestone. What Aspdin did that was different was to calcine the limestone, then mix the lime with clay and fire it again. He called the product “Portland Cement” because the concrete made from it looked like Portland stone, a widely-used building stone in England. While Aspdin is usually regarded as the inventor of Portland cement, Aspdin’s cement was not produced at a high-enough temperature to be the real forerunner of modern Portland Cement. His son, William, found that a higher temperature (around 1400 C) was beneficial. At this increased temperature, a small proportion of the material melts but the bulk of it remains solid. This process is known as “sintering” and it produces a material called clinker. A ship carrying barrels of William’s cement went aground off the Isle of Sheppey in Kent in 1848 and the barrels of set cement, minus the wooden staves, were later incorporated into the wall of an inn in Sheerness. They are still there now and the landlord occasionally receives requests from cement enthusiasts for small pieces. In 1845 William’s main competitor, Isaac Johnson, had also made an improved cement, by firing a mixture of chalk and clay at higher temperatures (1400 C– 1500 C), similar to those used today. Because both William Aspdin and Johnson burnt at higher temperatures than had been used previously, minerals were produced which were very reactive and more strongly cementitious. While they both used similar materials and temperatures to make Portland cement as we use now, three more important developments in the manufacturing process lead to modern Portland cement:
Development of rotary kilns (Ransome, 1885; Hurry and Seaman, 1895)
Addition of gypsum to control setting
Use of ball mills to grind clinker and raw materials
A rotary kiln is a long, rotating, tube slightly tilted from the horizontal and will described more fully in Chapter 3. Ransome’s kiln did not work properly in trials in 1887. Later, Hurry and Seaman in the USA resolved the problems and produced the first modern rotary kiln. These gradually replaced the original vertical shaft kilns from the early 1900s. Rotary kilns heat the clinker mainly by radiative heat transfer and this is more efficient at higher temperatures, enabling higher burning temperatures to be achieved. Also, rotary kilns give a more consistent product because the clinker is constantly moving inside the kiln; this gives a more uniform temperature in the burning zone, the hottest part of the kiln. The two other principal technical developments, gypsum addition to control setting and the use of ball mills to grind the clinker, were also introduced at around the end of the 19th century.
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A brief history of cement This brief and selective whirlwind tour of cement development sets Portland cement in its historical context. If you’re particularly interested in cement history, the Blezard paper and his chapter in Lea referred to below, contain more information with numerous references.
References, Chapter 1 1. Vitruvius, “The Ten Books of Architecture,” Dover Publications, 1960. 2. “Reflections on the history of the chemistry of cement,” R G Blezard, Society of Chemical Industry, 1998. (www.soci.org/SCI/publications/2001/pdf/pb72.pdf)
Further reading Lea, Chapter 1, “The History of Calcareous Cements,” R G Blezard. This first chapter in Lea contains a lot of information on cement history, plus 90 references.
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2 Cement basics
2.1
Some definitions
There are many different types of cement used in the construction industry. By far the most important in terms of volume is Portland cement and cement that is based on Portland cement. Cement may be “pure” Portland cement, or, alternatively, it may be made from Portland cement mixed with other materials that also have cementitious properties, such as blastfurnace slag from iron smelting or fly ash from coal-fired electricity power stations. Cements composed of mixtures of Portland cement with these other materials can enhance concrete properties; these mixtures are widely used and are called “composite cements” or “blended cements”; the exact terminology varies according to local custom. We’ll focus initially on Portland cement and return to composite cements later. “Pure” Portland cement was the most widely used cement in most parts of the world until recently. In some parts of the world, it still is but technical developments as well as environmental concerns have led to the increased use of other cements, particularly composite cements. The meanings of the words “cement” and “concrete” are rather blurred in general use, so let’s start by defining these words more carefully, along with a few others.
Cement basics
Portland cement
Cement
Aggregate Concrete Mortar Grout
Material made by heating a mixture of limestone and clay in a kiln at about 1450 C, then grinding the resulting clinker to a fine powder with a small addition of gypsum. Usually taken to mean either Portland cement or, more recently, a cement with Portland cement as one of the main constituents but also containing other materials. It could also mean any other type of cement, depending on the context. Cobbles, pebbles, crushed rock, gravel, sand and silt – the ‘rock’ component of all particle sizes in concrete. Synthetic rock made using cement mixed with water and aggregate. Mixture of cement and fine aggregate, mainly sand. Used typically to bond bricks, blocks and building stone. Mixture of cement (possibly of various types) and other fine material such as fine sand. Used in a wide range of applications from filling the gaps between bathroom tiles to oil wells.
Portland cement itself can be divided into a number of different types, each cement having different characteristics. “Normal,” grey, cement for general-purpose use will be referred to in this e-book as “ordinary” Portland cement. Other types of Portland cement include white Portland cement and sulfate-resisting Portland cement, of which more later.
2.2
Some non-Portland cements
As a brief aside, there are many other types of cement that are not based on Portland cement. However, the quantities of these other cements used are small compared with Portland cement and composite cements based on Portland cement; here, we will briefly mention three and then get back to the main subject of Portland cement:
Calcium aluminate cements
Lime concrete/mortar
Expansive cements
Calcium aluminate cements (CACs) These cements used to be called ‘high alumina cements.’ They are made from lime or limestone mixed with bauxite (aluminium ore) or other high-alumina material heated in a furnace until the raw materials have completely melted, then cooled and the solid material is then ground to produce cement.
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Cement basics
Concrete made with CAC typically develops strength quickly and is resistant to chemical attack. CACs have a wide range of compositions, mainly with different ratios of lime to alumina, and they are generally brown, grey or black in colour. One common type of CAC, ‘Ciment Fondu’, is grey but CACs can also be white if made from pure alumina. As well as being used in concrete, CACs are also used in grouts and other specialised applications, often mixed with Portland cement and other materials such as gypsum. Lime mortar and concrete Lime mortar and concrete was used for thousands of years until eclipsed by other cements particularly Portland cement. Its use declined to a very low level towards the end of the 20th century, being used mainly in the rebuilding or repair of historic or ancient buildings. More recently, the use of lime mortar and concrete in the construction of new buildings has increased in some countries, the UK being an example. Although it is not as strong, there are benefits in using lime mortar instead of a mortar based on Portland cement, in particular:
Cracks that develop in lime mortar tend to heal themselves, unlike conventional mortar made with Portland Cement.
Lime mortar is usually weaker than mortars made with Portland cement and so can be removed from the brick or stone at the end of the useful life of the building. Particularly in the case of bricks, this means that they can be recycled, saving energy otherwise needed to make new bricks. If mortar made with Portland cement is used, bricks generally can’t be reused as it is almost impossible to remove the mortar.
Lime is produced at a lower temperature than Portland cement, so other things being equal, it takes less energy to produce a lime mortar compared with a mortar made with Portland cement.
Lime mortar and concrete gain strength largely by carbonation, the process of re-absorbing carbon dioxide from the atmosphere. This converts calcium hydroxide to calcium carbonate, removing an equivalent amount of carbon dioxide from the atmosphere as was released when the limestone was calcined. (This, of course, neglects the CO2 emissions from the fuel used to heat the lime but the CO2 reabsorbed still represents a substantial part of the total CO2 emitted during manufacture).
Lime mortars and plasters allow a building to “breathe” more than if gypsum plaster and mortar based on Portland cement is used. This results in fewer problems with condensation.
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Cement basics
Expansive cements These are special cements designed to exert an expansive force on their surroundings after the cement has set. (With most cements, manufacturers go to a lot of trouble to make sure the cement is not expansive). Expansive cements are used mainly in demolition and also in mining.
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3 Portland cement composition and microstructure
Portland Cement is made by heating suitable raw materials, typically ground limestone and clay, at a temperature of about 1450 C to produce a dark grey nodular material called clinker. At this temperature, much of the clinker remains solid but perhaps 20%-30% of the clinker is liquid, thinly-dispersed within the nodules. When cool, the clinker is ground up to a fine powder and a small amount of gypsum is added to control the setting properties of the cement. Here are a few familiar, and perhaps less familiar, views of cement.
Figure 3.1 Bulk cement road tankers on a Figure 3.2 Cement bagging plant. quay. (Photo courtesy Rugby Cement.) Bagged cement is sold mainly to small builders and the do-it-yourself market. (Photo courtesy Rugby Cement.)
Portland cement composition and microstructure
Figure 3.3 The familiar grey powder. The coin is 23mm in diameter.
Figure 3.4 Cement particles viewed in a scanning electron microscope.
A more complete description of the cement production process follows.
3.1
Portland cement minerals
A more precise definition of Portland Cement is that it is the ground product of Portland cement clinker, usually interground with a small amount of calcium sulfate to bring the total sulfate content of the cement to about 2.5%-4% by weight. In turn, Portland cement clinker is produced from a mixture of finely-ground calcareous and siliceous components, together with a proportion of alumina and iron, as well as some impurities, fired in a kiln at a temperature of about 1450 C. Clinker is composed of rounded, dark grey or grey-green nodules, ranging in size from less than 1 mm to 30 mm or more. The clinker in Figure 3.5 is a typical example of Portland cement clinker. It isn’t always as clean or as coarse as this, the nodules can be smaller with a lot of fine sub-millimetre dust.
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Portland cement composition and microstructure
Figure 3.5 Typical Portland cement clinker nodules: these nodules are 10mm – 15mm across.
Portland cement clinker contains four principal minerals, or ‘phases’:
Alite Belite A calcium aluminate phase A calcium alumino-ferrite phase
A “phase” in cement chemistry or petrology may be defined as: “A part or parts of a system occupying a specific volume and having uniform physical and chemical characteristics which distinguish it from all other parts of the system.” (1) Note that “system” includes temperature. Cooled clinker contains four main phases – the four principal clinker minerals – but in the burning zone of the kiln, it contains mainly alite, belite, free lime and the liquid phase. (“Free lime” is lime that has not yet combined to form alite or belite or other minerals). The compositions of the four main clinker minerals in any particular cement clinker vary a little, depending on the compositions of the raw materials. Their compositions are often simplified by approximating them to the following pure compounds:
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Portland cement composition and microstructure
Table 3.1 Mineral names and approximate compositions of the four principal clinker minerals. Mineral name Approximate composition Formula Alite: tricalcium silicate (Ca3SiO5) Belite: dicalcium silicate (Ca2SiO4) Calcium aluminate phase: tricalcium aluminate: (Ca3Al2O6) Calcium alumino-ferrite phase: tetracalcium aluminoferrite: (Ca4Al2Fe2O10) or (Ca2AlFeO5) These simplifications are useful but need to be used carefully.
3.2
Portland cement composition and cement notation
A certain amount of chemical notation is unavoidable from now on. However, it won’t be anything difficult – nothing beyond second or third year at High School. A typical analysis of a Portland cement is given below (Table 3.2). Table 3.2 Typical analysis of a Portland cement, expressed as oxides in weight %. SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O SO3 LOI IR Total 20.7 5.7 2.5 64.0 1.0 0.6 0.2 2.7 1.5 0.5 99.5 Balance is due to minor oxides, typically P2O5, Mn2O3, TiO2, Cr2O3. LOI=loss on ignition (due to water from traces of hydrates, carbon dioxide). IR=insoluble residue (ie: acid-insoluble residue, mainly comprising unreacted silica or feldspar).
3.2.1
Note on oxide analyses
Cement analyses are usually shown as oxides, eg: CaO or SiO2. This is a standard form of analysis in physical chemistry and geology. For anyone unfamiliar with it, the following example may help: Consider the mineral quartz, composed of silicon dioxide (SiO2). We could represent the analysis of quartz as in Table 3.3.
Table 3.3 Quartz composition shown as individual elements. Percentage of element Si 46.67 O 53.33
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Table 3.4 Quartz composition shown as an oxide. Percentage of oxide SiO2 100.00
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However, the proportion of silicon to oxygen in quartz has a fixed ratio of 2:1, determined by the valency of silicon and oxygen. Silicon has a valency of +4 and oxygen -2; this means that a silicon atom can share four electrons with neighbouring atoms and oxygen can share two. Since quartz is not electrically charged, it follows that each silicon atom will bond with two oxygen atoms: +4 + (2 x -2) = 0 This means that it isn’t necessary to show the oxygen separately; the data in Table 3.4 gives the same information as that in Table 3.3, but in a more convenient form. For more information, see Reference 2 at the end of this chapter, or try a basic chemistry textbook.
3.2.2
Cement chemistry notation
Chemical compositions of cement clinker phases are expressed as oxides and this is convenient in many ways but can lead to rather long-winded formulae. To simplify them, cement chemists have adopted a form of notation that seems strange to chemists not familiar with cement. Using cement chemistry notation, the formulae are abbreviated. Remember that these compositions are approximate because the minerals contain impurities. Alite: Ca3SiO5 in terms of its oxides is 3CaO.SiO2. The CaO term is shortened to C and the SiO2 to S. In cement chemistry notation, the compound becomes C3S. Belite: Similarly, Ca2SiO4 is 2CaO.SiO2, which is shortened to C2S. Tricalcium aluminate: Ca3Al2O6 is 3CaO.Al2O3. The Al2O3 term is shortened to A and the compound becomes C3A. Tetracalcium aluminoferrite: Ca2AlFeO5 can be written as (Ca4Al2Fe2O10) or 4CaO.Al2O3.Fe2O3. Fe2O3 is shortened to F and the compound becomes C4AF. (With names like “tetracalcium aluminoferrite”, the need for snappier names is all-too clear.) In other words, for each of the clinker main minerals, we now have at least three possible descriptions, as below:
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Portland cement composition and microstructure
Alite or tricalcium silicate or C3S
Belite, or dicalcium silicate or C2S
Tricalcium aluminate (or the ‘aluminate phase’) or C3A
Calcium alumino-ferrite (or the ‘ferrite’ phase) or tetracalcium aluminoferrite or C4AF
There are also the full chemical formulae for the pure compounds, eg: C3S or Ca3SiO5. Although strictly, these do not mean the same thing, they are frequently used indiscriminately. This lax use means that names like ‘C3A’ should not usually be taken to signify a definite composition in the sense of a pure compound, unless this is indicated by the context. To take another example, strictly speaking tricalcium silicate is a pure compound, while alite is a mineral composed largely of tricalcium silicate but also with a significant quantity of impurities, mainly magnesium, iron and aluminium. All the oxides commonly found in cement are abbreviated as follows: Table 3.5 Cement chemistry notation for the principal oxides in Portland cement clinker. C=CaO T=TiO2 S=SiO2 H=H2O A=Al2O3 F=Fe2O3 and K=K2O _ N=Na2O S=SO3 M=MgO _ P=P2O5 C=CO2
_ _ S and C are spoken as ‘S-bar’ and ‘C-bar’ respectively, but are less used now than they once were. (Probably, they are a victim of technology - fine when notes were hand-written or typed on a typewriter, but just too complicated to do on a word-processor. Have a try and see why it isn’t used so much these days!) People unfamiliar with cement notation sometimes have trouble adjusting to the notion of S representing SiO2, for example, but do stick with it – it is usually a great time saver and it does work.
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Portland cement composition and microstructure
3.3
Microstructure of clinker and cement
Consider an individual nodule of cement clinker (Figure 3.6). If we take that nodule, or pieces of it, we can make a polished section (Figure 3.7) and view the individual minerals in the nodule using a microscope.
Figure 3.6 Single clinker nodule and a coin, 23mm in diameter, for scale.
Figure 3.7 Polished section containing pieces of clinker nodules. Section diameter is 30 mm.
The polished section could be examined using either an optical microscope or a scanning electron microscope (SEM); the techniques for preparing polished sections are broadly the same for either. Clinker nodules are porous and it is desirable to fill these pores with epoxy resin; nodules are embedded in the resin using vacuum impregnation to force the resin into as many small pores as possible. In Figure 3.7 the nodules have been separated into different size fractions by sieving and the fragments held in place during specimen preparation by card divisions, which remain visible. When the epoxy resin had set, the hardened resin block was sawn to reveal the nodules in section, then polished using diamond polishing compound in successively finer grades. For examination using an optical microscope, various etches – liquids that react with the surfaces of the crystals - may be used to highlight particular minerals. Etches are not normally used for SEM examination. Clinker appears quite different when examined using a petrographic microscope compared with when using a scanning electron microscope. The obvious main difference is that the petrographic microscope image is a colour image while the SEM image is black-and-white. Both have their advantages and disadvantages. Examples of a petrographic microscope image of clinker and an SEM image are shown below.
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Portland cement composition and microstructure
Figure 3.8 Polished section of Portland cement clinker, examined in reflected light using a petrographic microscope, showing crystals of individual minerals. Brown crystals are alite and blue crystals are belite. The bright material between the alite and belite is a mixture of aluminate and ferrite. Grey areas are pores filled with epoxy resin. The colours are the product of etching; the section was etched with hydrofluoric acid vapour in order to distinguish between the different minerals. Belite is not actually blue and alite is not brown.
Figure 3.9 Polished section of Portland cement clinker, backscattered SEM image. Alite (‘a’) and belite (‘b’) can be clearly distinguished, alite being light grey and belite darker grey –examples arrowed. Black areas are epoxy resin. Small bright, nearly white, crystals are ferrite and tiny darker crystals close to the ferrite are mainly aluminate phase, but these are hard to distinguish at this magnification. SEM images are always black and white, unless they have had false colours added. This is a different clinker to that in Figure 3.8 but the magnification is similar.
Alite crystals are elongated, typically 20 µm-60 µm in length and hexagonal in shape, as can be seen in Figures 3.8 and 3.9. Belite crystals are generally rounded in shape and 10 µm-30 µm across. In both images, since the crystals are randomly orientated, many of them will not appear in polished sections showing their full length. Some nodules are highly porous and others much denser. The small nodule in Figure 3.10 is of fairly typical density. In this low-magnification view of most of a nodule, alite is the mid-grey mineral comprising the bulk of the nodule. The darker ‘patches’ are belite clusters.
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Portland cement composition and microstructure
Figure 3.10 SEM image of polished section of a whole small nodule, approximately 2mm across. The pores in the nodule structure are the black areas which are filled with epoxy resin. In this clinker, most of the belite occurs in large clusters (arrowed, dark grey); ideally, the belite would be distributed more uniformly. The uneven distribution suggests that some of the siliceous particles in the raw feed were too coarse. Despite relict structures of coarse raw feed particles, this nodule is well-combined with no free lime visible.
Ferrite and aluminate phase vary in appearance from one clinker to another and within the same clinker. Sometimes ferrite and aluminate crystals are small and intergrown but they can also be coarse separate ‘blocky’ crystals (Figure 3.11).
Figure 3.11 SEM image of polished section, showing coarse blocky aluminate (dark grey, examples shown by white arrows) and ferrite (bright, examples shown by black arrows). Most of the material in this image is alite, with a small elongated belite cluster towards the centre. Again, black areas are pores in the clinker filled with epoxy resin used in specimen preparation.
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Portland cement composition and microstructure
When the clinker is ground up to produce cement, it is obvious that much of the microstructure of the nodules is lost (Figure 3.12).
Figure 3.12 SEM image of polished section of cement particles set in epoxy resin. The four main clinker minerals are visible (aluminate only just visible). Much information that would be visible in a clinker nodule is lost on grinding, such as alite crystal sizes, belite cluster sizes and nodule porosity.
3.4
Hydraulic properties of the main clinker minerals
Alite (C3S) Alite is very reactive in the presence of water. It is the main constituent of Portland cement, typically between 45% and 70% of the clinker by weight. It is the main strength-giving component of cement and is thus strongly hydraulic. Belite (C2S) Belite is reactive and hydraulic, but less so than alite. The proportion of belite in clinker typically varies between 5% and 30%. Aluminate (C3A) The aluminate phase is the most reactive constituent of clinker. However, it is not a major contributor to strength in Portland cement, in other words it is only very weakly hydraulic. Although only weakly hydraulic, because it is very reactive it has a strong influence on early setting properties of cement and it produces a lot of heat. In order to control this reactivity, gypsum is added to cement to control the aluminate phase hydration.
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Portland cement composition and microstructure
Ferrite (C4AF) Ferrite phase is moderately reactive, but only weakly hydraulic. It reacts quickly initially when mixed with water but the rate of reaction slows and unhydrated ferrite can be found in concrete over a hundred years old. Ferrite is black and gives cement its characteristic grey colour. Aluminate and ferrite are often referred to as “flux phases”. Why are aluminate and ferrite present in cement? If alite and belite are the main hydraulic minerals in cement, the minerals that give strength to concrete, what do the aluminate and ferrite phases do? Why not just make cement containing alite and belite? The short answer is that it would be very difficult to make cement that did not contain at least some aluminate or ferrite. We’ll be looking at the reactions in the cement kiln later, but broadly cement-making is a clinkering (sintering) process. This means not all the material in the kiln melts – at the hottest part of the kiln, the burning zone, about three-quarters of the clinker remains solid and only a quarter is liquid. Remember that what we are trying to do in the kiln is make calcium silicates, particularly alite. The solid material entering the burning zone of the kiln is, roughly speaking, belite and free lime that we want to combine to make alite. The liquid is mainly composed of oxides of calcium, iron and aluminium; when cooled, this liquid crystallises into the aluminate and ferrite phases. The rôle of the liquid is to accelerate the reactions in the clinker. Ions are transferred through the liquid much more easily than through a solid. All other things being equal, the higher the proportion of liquid, the easier it is to combine the belite and free lime to make alite. In cement-making terminology, the liquid is also called the flux. The liquid from which aluminate and ferrite crystallise is therefore critical to the process of making cement. Without the liquid, ion transfer would be much slower; belite and free lime would not combine adequately. The liquid is essential and the aluminate and ferrite form from the liquid is it cools, even though they do not contribute greatly to the strength of concrete.
3.5
Proportions of the main clinker phases
As we’ll see, the proportions of each of the main minerals (that is, the quantity of alite, belite, etc.) are of major importance in determining the properties of the cement produced from the clinker. The term “phase composition” is often used – it means the proportions of the clinker minerals in a cement or clinker.
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Portland cement composition and microstructure
The “actual” phase composition is dependent on:
The quantities of each of the main oxides (CaO, SiO2, Al2O3 and Fe2O3) in the raw materials.
The extent to which they have combined to form the main clinker phases.
The compositions of the phases (including impurities).
The actual phase composition is not easy to calculate without additional information, in particular, the true compositions of the minerals. (These can be determined by SEM and X-ray microanalysis.) The “potential” phase composition simplifies things by ignoring the extent to which the oxides have actually combined. It means: “these are the mineral proportions you would expect if everything were to combine and all the chemical reactions reached equilibrium as the clinker cooled”.
3.5.1
The Bogue calculation
The Bogue calculation is a very useful method of calculating the approximate quantities of the four main clinker minerals in a clinker or cement. In essence, it is a calculation of the potential phase composition based on some simplifying assumptions. If we know the composition of the clinker from its oxide analysis, and if we also know the compositions of the four main clinker minerals, we can calculate how much of each mineral is present. The Bogue calculation can be used in two slightly different ways:
It can be used prescriptively – a good example is the ASTM C-150 standard for Portland cement, in which the Bogue calculation is used to specify limits for the proportions of the different clinker minerals in different types of cement.
It can be used for troubleshooting; as an example, suppose you had two cements that were nominally similar but one gave better strengths than the other and you wanted to know why. A good starting point would be to use the Bogue calculation to calculate the proportions of clinker minerals in either the cement or the clinker from which the cement was made.
The calculation used will vary slightly, depending whether you are applying it to cement or to clinker, and on what you are trying to achieve:
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Portland cement composition and microstructure
To apply the calculation to cement, you need to take into account any material that was added to the clinker, such as gypsum and fine limestone.
To use the calculation to compare the phase proportions of two particular cements or clinkers, you would want to correct for uncombined lime (free lime).
The following version of the Bogue calculation is applied to clinker and corrects for free lime; it does not allow for added gypsum or fine limestone and so should not be used for cement in this form. We’ll look at applying the calculation to cement later (Chapter 5.9). The calculation assumes that the four main clinker minerals have the actual compositions given by:
Alite, C3S, or tricalcium silicate
Belite, C2S, or dicalcium silicate
Aluminate phase, C3A, or tricalcium aluminate
Ferrite phase, C4AF, or tetracalcium aluminoferrite
(At the risk of becoming repetitive, these are only approximations of the true compositions of the minerals.) To make clinker, we are combining lime and silica and also lime with alumina and iron. If some of the lime remains uncombined, we need to subtract this from the total lime content before we do the calculation, or we will overestimate the actual alite content. For this reason, a clinker analysis normally gives a figure for free lime. The calculation is simple in principle:
Firstly, according to the assumed mineral compositions, ferrite phase (C4AF) is the only mineral to contain iron. The iron content of the clinker therefore fixes the ferrite content.
Secondly, the aluminate content is fixed by the total alumina content of the clinker, minus the alumina in the ferrite phase. We can now calculate this, since we have a figure for the amount of ferrite.
Thirdly, we assume all the silica is present as belite and calculate how much lime is needed to form belite from the total silica content of the clinker. There will be a surplus of lime.
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Portland cement composition and microstructure
Fourthly, we allocate the lime surplus to the belite, converting some of it to alite.
In practice, the above process of allocating the oxides can be reduced to the following equations: C3S
=
4.0710CaO-7.6024SiO2-1.4297Fe2O3-6.7187Al2O3
C2S
=
8.6024SiO2+1.0785Fe2O3+5.0683Al2O3-3.0710CaO
C3A
=
2.6504Al2O3-1.6920Fe2O3
C4AF
=
3.0432Fe2O3
Here’s a worked example. First, we need a clinker analysis (Table 3.6). Table 3.6 An example of a typical clinker analysis (note that the analysis in Table 3.2 was of a cement, not a clinker). SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O SO3 LOI IR Total 21.5 5.2 2.8 66.6 1.0 0.6 0.2 1.0 1.5 0.5 98.9 Free lime = 1.0% CaO
Example of a Bogue calculation using the clinker data in Table 3.6: Combined CaO = (Total CaO – Free CaO) = (66.6% - 1.0%) = 65.6% This is the figure we use for CaO in the calculation. So, for the four oxides we have: CaO=65.6%; SiO2=21.5%; Al2O3=5.2% and Fe2O3=2.8% The Bogue calculation is: C3S C2S C3A C4AF
= = = =
4.0710CaO-7.6024SiO2-1.4297Fe2O3-6.7187Al2O3 8.6024SiO2+1.1Fe2O3+5.0683Al2O3-3.0710CaO 2.6504Al2O3-1.6920Fe2O3 3.0432Fe2O3
Therefore: C3S C2S C3A
= = =
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(4.0710 x 65.6)-(7.6024 x 21.5)-(1.4297 x 2.8)-(6.718 x 5.2) (8.6024 x 21.5)+(1.0785 x 2.8)+(5.0683 x 5.2)-(3.0710 x 65.6) (2.6504 x 5.2)-(1.6920 x 2.8)
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=
3.0432 x 2.8
= = = = =
64.7% 12.9% 9.0% 8.5% 95.1%
So: C3S C2S C3A C4AF Total
To the total, we can add the 1% free lime we deducted at the start to give 96.1%; note that the total still does not add up to 100%. This is because the calculation neglects impurities in the four clinker minerals, and because of other minor constituents in the clinker not accounted for, such as clinker sulfate and, possibly, periclase (MgO). It should be stressed that the Bogue calculation does not give the exact amounts of the four main clinker phases present, although this is sometimes forgotten. These differ from the ‘true’ amounts mainly because the actual mineral compositions differ a little (occasionally more than a little) from those assumed in the calculation, as shown in Table 3.7.
Table 3.7 Comparison of clinker mineral compositions calculation, with typical compositions. SiO2 Al2O3 Fe2O3 CaO MgO Bogue C3S 26.3 0.0 0.0 73.7 0.0 Bogue C2S 34.9 0.0 0.0 65.1 0.0 Bogue C3A 0.0 37.7 0.0 62.3 0.0 Bogue C4AF 0.0 21.0 32.9 46.1 0.0
assumed by the standard Bogue K2O 0.0 0.0 0.0 0.0
Typical 25.2 1.0 0.7 71.6 1.1 0.1 Alite Typical 31.5 2.1 0.9 63.5 0.5 0.9 Belite Typical 3.7 31.3 5.1 56.6 1.4 0.7 ‘Aluminate’ Typical 3.6 21.9 21.4 47.5 3.0 0.2 Ferrite *Balance is mainly P2O5, Mn2O3, TiO2, Cr2O3 Typical mineral compositions are from Reference 3 at the end of
Na2O 0.0 0.0 0.0 0.0
SO3 0.0 0.0 0.0 0.0
Total 100 100 100 100
0.1
0.1
99.9*
0.1
0.2
99.7*
1.0
0.0
99.8*
0.1
0.0
97.7*
this chapter.
It is clear from Table 3.7 that there are appreciable differences between the compositions assumed by the Bogue calculation and those typical of minerals in cement clinker. For example, alite and belite both contain aluminium and iron, and the aluminate phase (C3A) is not strictly tricalcium aluminate as it contains some iron and silicon. That said, the Bogue calculation gives a useful approximation and is widely used within the cement industry. It provides a standardised format for estimating the clinker mineral proportions and the approximations made are not normally
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Portland cement composition and microstructure important. Of much greater interest are relative changes over time in the indicated mineral content rather than the absolute values. If concrete early strengths suddenly drop and if alite content as calculated using the Bogue calculation also shows a drop, the lower strength may well be due to the lower alite content. The calculation can be expressed in other ways, of which perhaps the most useful is as a set of four simultaneous equations. In the following, (A)= alite; (B)=belite; (Al)=aluminate; (F)=ferrite and (bulk) refers to the oxide composition of the clinker. S(A) + S(B) + S(Al) + S(F) A(A) + A(B) + A(Al) + A(F) F(A) + F(B) + F(Al) + F(F) C(A) + C(B) + C(Al) + C(F)
= = = =
S(bulk) A(bulk) F(bulk) C(bulk)
For convenience, these equations can be solved using a spreadsheet by the method of inversion of matrices. If the true mineral compositions, as distinct from the assumed ideal compositions, are known (eg: from SEM/X-ray microanalysis) the actual phase proportions can be calculated more accurately. Note that C(bulk) should have the free lime subtracted before the calculation for the best estimate of the alite content. Similar calculations can be performed for cement, provided that gypsum and any other added material, such as fine limestone, is taken into account (see Chapter 5.9).
Free lime: to subtract or not to subtract In the above calculation, we subtracted the free lime from the total CaO content in order to obtain the “best estimate” of the alite content. However, as mentioned above, if used prescriptively in a standard specification the form of the calculation specified may not require free lime to be deducted. In the ASTM standard for Portland cement, ASTM C150-07, the free lime content is not deducted from the total lime in calculating the phase composition; it isn’t even mentioned. This is because the objective of the calculation is different. The specification defines limits to the phase composition of certain cement types to ensure that the cements are suitable for specific purposes. Differences in free lime content in individual cements are not an issue and would be an unnecessary complication. In summary, the form of calculation to use depends on the context. If the calculation relates to a standard specification, you would obviously use the calculation given in that specification. If you are a cement producer trying to work out why one clinker is making better cement than another, you would want the best estimate of the alite content; that would mean deducting the free lime
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Portland cement composition and microstructure from the total CaO first. Free lime is lost alite To see how alite content and free lime content are related, look at the effect of free lime on the mineral content of a typical clinker, as shown by the Bogue calculation (Table 3.8). Table 3.8 Effect of uncombined lime on alite and belite contents. SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O SO3 LOI IR Total 21.5 5.2 2.8 66.6 1.0 0.6 0.2 1.0 1.5 0.5 98.9 LSF=97.6; SR=2.69; AR=1.86 (see Chapter 3.6). LOI=Loss on ignition (weight loss when heated to approx. 1000 C), due to small amounts of carbon dioxide or water) IR=Insoluble residue - insoluble material which has not combined, usually silica or feldspar. Clinker A Clinker B Clinker C 0% Free CaO 1% Free CaO 2% Free CaO Alite 68.7 Alite 64.7 Alite 60.6 Belite 9.8 Belite 12.9 Belite 15.9 Aluminate 9.0 Aluminate 9.0 Aluminate 9.0 Ferrite 8.5 Ferrite 8.5 Ferrite 8.5 Total 96.1 Total 95.1 Total 94.1
Applying the Bogue calculation gives an alite content of about 69% if no free lime is assumed to be present (Clinker A) but only about 61% if 2% free lime is assumed (Clinker C). This illustrates why a full clinker analysis should also show the free lime content if the purpose of the analysis is to indicate the actual phase proportions of the clinker. Cements made from Clinkers A, B and C in Table 3.8 would probably perform differently. If early strengths were dependent solely on alite content, Clinker A with the highest alite content should give the best early strengths (from about 1 day to 7 days). With 2% free lime (Clinker C) the lower alite content should result in the lowest early strengths. At later ages, the higher belite content of Clinker C would start to partly close the gap with Clinker A, although this may take several weeks or months. However, the mineral proportions are not the only factors controlling the performance of a cement, although they are very important. For example, as mentioned previously a hard-burned clinker may contain calcium silicates which are less reactive than those in a clinker burned with a lighter touch. In other words, Clinker A with little or no free lime, was likely to have been very hard-burned in order to get all the lime to combine. Clinker C would be much lighter-burned (all other things being equal) and Clinker B somewhere inbetween. Clinker B might well have the best compromise between alite content and silicate reactivity and cement made from it may out-perform cements made from Clinkers A and C in terms of both early and late strengths. Comparing Figure 3.10, a normally-burned clinker nodule, with the underburned nodule in Figure 3.13, the underburned nodule is clearly more porous and of
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Portland cement composition and microstructure lower density. Harder burning would cause the nodule to coalesce. Nodule density is therefore, very approximately, a measure of how hard the clinker was burned. Indeed, the ‘litre weight’ (weight of a known volume, ie: density - more strictly the bulk density) of a clinker is often used as a rough measure of burning. Usually, the clinker sample for calculating the litre weight has been sieved, so that the nodule sizes are within defined upper and lower limits.
Figure 3.13a This clinker nodule is underburned; it was not at burning temperature for a sufficient time for adequate combination to take place. This image shows much belite (dark grey) in large, arcuate, clusters. Some alite is present (light grey) and much free lime (“FL” - white). This nodule has an open, porous, structure and contains more belite than alite. Compare with the wellcombined nodule in Figures 3.10-3.11 shown at a similar magnification.
3.6
Figure 3.13b Detail of Figure 3.13a, showing free lime (“FL”), alite (“a”) and belite “b”). Harder burning would have resulted in a densification of the microstructure and enabled more lime to combine with belite to produce more alite.
Common parameters used in cement manufacturing
Cement clinker is frequently characterised in terms of three compositional parameters, based on the bulk oxide analysis of the clinker. The three parameters are:
Lime Saturation Factor (LSF)
Silica Ratio (SR, or S/R)
Alumina Ratio (AR)
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Portland cement composition and microstructure Lime Saturation Factor The LSF is a ratio of CaO to the other three main oxides. Applied to clinker, it is calculated as: LSF=CaO/(2.8SiO2 + 1.2Al2O3 + 0.65Fe2O3) Often, the LSF is referred to as a percentage, and therefore multiplied by 100. The LSF controls the ratio of alite to belite in the clinker. A clinker with a higher LSF will have a higher proportion of alite to belite than will a clinker with a low LSF. LSF values in clinkers typically range between 92%-99%. Values above 1.0 (100%) indicate that free lime is likely to be present in the clinker. This is because, in principle, at LSF=1.0 all the free lime should have combined with belite to form alite. If the LSF is higher than 1.0, there will be excess lime that has nothing with which to combine. In practice, there are always regions within the clinker where the LSF is locally a little above, or a little below, the average for the clinker as a whole. This is partly due to imperfect mixing and partly to particle size; a large particle will cause the composition locally to deviate from the average composition. This means that there is almost always some residual free lime, even where the LSF is considerably below 1.0. It also means that to convert all the belite to alite, an LSF slightly above 1.0 is needed; there will inevitably be free lime remaining. Silica Ratio (SR) The silica ratio is defined as: SR = SiO2/(Al2O3 + Fe2O3) A high silica ratio means that more calcium silicates are present in the clinker and less aluminate and ferrite. SR is typically between 2.0 and 3.0. The silica ratio is also called the ‘silica modulus.’ Alumina Ratio (AR) The alumina ratio is defined as: AR=(Al2O3/(Fe2O3) This determines the potential relative proportions of aluminate and ferrite phase in the clinker. An increase in clinker AR (also sometimes written as A/F) means there will be proportionally more aluminate and less ferrite in the clinker. In ordinary Portland cement clinker, the AR is usually between 1 and 3.
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Portland cement composition and microstructure
What’s the best clinker composition? A simple question, you may think, but there isn’t a simple answer. It depends on the requirements of the concrete to be made from the cement. For example, if high early strengths were required, we would need: A high SR – this means a high proportion of the clinker will be in the form of calcium silicates. A high LSF – this means the bulk of the calcium silicates will be in the form of alite. But, there is a “down side” to this… Reactions in the kiln are harder to achieve with high LSF and SR mixes - they are more difficult to “burn,” or to combine. This results in higher costs, and also in increased CO2 emissions due to the higher CaO content of the cement and an increased fuel requirement. To take another example, if we wanted a Portland cement with a low heat of hydration (perhaps for use in large concrete pours) we should limit the alite and also the aluminate, as these minerals give the strongest exothermic reactions on hydration. We would be looking for a cement containing a lot of belite and not too much aluminate; this would be given by a clinker of low LSF and low-medium AR. (In practice, in many parts of the world, a concrete mix containing Portland cement with a high proportion of slag or fly ash would probably be used rather than a mix with Portland cement only. We will be discussing slag and fly ash in Chapter 7.)
3.7
Other Portland cement types
We have just considered very brief examples of how changing the clinker composition can change the characteristics of the cement made from that clinker. The majority of Portland cement produced is “general purpose” cement, but other types of Portland cement are manufactured in some countries for specific applications. Broadly, these are:
Sulfate-resisting Portland cement
White Portland cement
Rapid-hardening Portland cement
Low heat of hydration Portland cement
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Portland cement composition and microstructure
These different Portland cement variations are treated differently by the various national standards; ASTM C 150 lists five different types of Portland cement, plus three air-entrained sub-types, not all of which have equivalents in other standards. Chapter 10 has more on standard specifications. Sulfate-resisting Portland cement Sulfate-resisting cement (Type V cement in the ASTM C 150 specification) is used where concrete may be exposed to high levels of sulfate in solution. An example might be use in foundations where the groundwater is unusually high in sulfates. Reactions between hydration products of the aluminate phase and sufficient sulfate cause expansive reactions within the concrete – a condition known as ‘sulfate attack.’ Concrete made with sulfate-resisting cement remains sounder for longer in more extreme conditions of exposure to sulfate solution compared with ordinary Portland cement. It can do this because it contains a minimal amount of aluminate phase. There are several different forms of sulfate attack and these will be considered later (Chapter 9.2). So, how can we make cement that contains less aluminate phase?
In theory, we could have less flux overall by increasing the silica ratio.
We could make ferrite phase as a high a proportion of the flux as possible, by lowering the alumina ratio.
In practice, if we increase the silica ratio too much, there will be insufficient liquid at the burning temperature and good combination will be more difficult to achieve, so this isn’t a good option. If we can’t have much (or any) aluminate phase in the clinker, we will need to lower the alumina ratio. This may mean that harder burning is necessary to achieve combination (we’ll see why in Chapter 5.3) but we can offset this by having more liquid. In other words, we can make sulfate-resisting cement by adjusting the silica ratio downwards, as well as the alumina ratio. This means more liquid is formed overall, but almost all of it will crystallise as ferrite. We can’t take alumina out of the raw materials, but we can add iron. The ferrite that crystallises from the liquid in sulfate-resisting cement is more iron-rich than the ferrite in ordinary Portland cement. Because of this, the normal assumption made by the Bogue calculation that ferrite has the composition C4AF is even more wrong than it is for normal clinker. ASTM C150 provides an alternative calculation for use with Type V (sulfate-resisting) cement. Composite cements, containing ordinary Portland cement and granulated
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Portland cement composition and microstructure blastfurnace slag (gbs), or ordinary Portland cement and fly ash also provide good resistance to sulfate solutions. These composite cements are increasingly used to make sulfate-resisting concrete. In Europe, composite cements have largely taken over from sulfate-resisting cement where sulfate resistance is required; in the UK, for example, the demise of sulfate-resisting Portland cement is almost complete, as it is no longer produced. White Portland Cement White Portland Cement is used for architectural purposes where a white concrete finish is wanted. It is not treated as a separate type of cement in national standards but is produced to meet the same criteria as normal grey cement. Normal cement contains ferrite phase. Ferrite is dark grey or black, giving cement its characteristic grey colour. So, to make white cement, we need to make cement that contains little or no ferrite phase. The principal element that gives the dark colour to ferrite is iron, at least in terms of quantity. Other elements, particularly chromium and manganese are also strong colorants. Titanium, copper and vanadium are also potential sources of colour. In other words, we want as little of these elements as possible in the clinker, achieved by careful selection of raw materials. Magnesium may also affect colour. Typical raw materials used for making white cement are pure limestone, such as chalk, china clay and silica sand. Of course, there is still likely to be a little iron, and other undesirable elements, present in the raw materials. To prevent this from forming ferrite, white clinker is usually burnt under slightly reducing conditions, converting Fe(III) to Fe(II). Fe(II) substitutes for calcium and then resides in the clinker minerals, thus avoiding ferrite formation and discolouration. The Bogue calculation will overestimate the true ferrite content of a white clinker, as it assumes that all iron is present in ferrite. White cement clinker is commonly water-quenched; the rapid temperature decrease helps to prevent Fe(II) oxidising to Fe(III). Rapid-hardening Portland cement Rapid-hardening Portland cement (Type III cement in the ASTM C 150 specification) is basically ordinary Portland cement that has been ground more finely so that it reacts more quickly with water. It is used where fast strength growth is required (eg: precast concrete products) or special applications such as sprayed concrete. As well as being more finely ground, the cement also may have a higher alite content than typical ordinary Portland cement. Typically, rapid-hardening cement at 3 days has a similar strength to ordinary Portland cement at 7 days. Since it reacts more quickly than ordinary Portland cement, it is unsuitable for use in large pours as the heat evolution may be too rapid.
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Portland cement composition and microstructure
References, Chapter 3 1. “Chemical Fundamentals of Geology,” Robin Gill, pub. Chapman and Hall, 1996. 2. http://www.understanding-cement.com/basic-chemistry.html 3. “Cement Chemistry,” H F W Taylor, Thomas Telford, 2nd ed. 1997, Table 1.2.
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4 Portland cement manufacturing - the main components of a cement plant
4.1
The quarry
We’ll follow a logical path looking at the main pieces of equipment at a cement plant as the raw materials go from the quarry to the kiln and then to the cement mill. The main basic components of a cement works are: Crushers and Silos for raw Kiln Clinker Cement Quarry (raw materials) Mills for raw material store mill material blending
If you happen to be a geologist, the quarry is probably the most interesting part of a cement works (although the kiln might be if you view the clinkering process as igneous rocks in the making).
Figure 4.1 Limestone quarry. (Picture courtesy Figure 4.2 Limestone blocks being Castle Cement.) removed for crushing after blasting. (Picture courtesy Castle Cement.)
Typical rock types used in cement production are:
Portland cement manufacturing - the main components of a cement plant
Limestone (supplies the bulk of the lime)
Clay, marl or shale (supplies the bulk of the silica, alumina and ferric oxide)
Other supplementary materials such as sand, fly ash or ironstone to achieve the desired bulk composition
Quarry management is a complex process; most quarries will have ‘good material’ from which cement can easily be made, but they will also have lessgood material, possibly of harder material, or of less convenient composition. If the ’good stuff’ is all used up first, it may be difficult to make cement out of what is left. Detailed forward planning is needed to make the best use of all the materials available. Raw materials are extracted from the quarry, then crushed and milled as necessary to provide a fine material for blending in the required proportions. They may also need to be dried. The raw materials may be milled together or separately, depending on how hard they are and whether additional mills are available. The fineness of the material is usually expressed in terms of the percentage retained on a 90µm sieve, typically between 5% and 15%. The blended raw material, the raw meal, is stored in a silo before being fed to the kiln. The silo provides a stock of raw meal, enabling production to be maintained for a day or two in the event of failure of equipment or in the supply of materials.
4.2
The cement kiln
Rotary kilns were first developed in the 1890s, and became widespread in the early part of the 20th century. They were a great improvement on the earlier shaft kilns, giving continuous production and a more uniform product in larger quantities.
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Portland cement manufacturing - the main components of a cement plant
Figure 4.3 A dry-process kiln, showing the raw meal silo (A), the pre-heater tower (B) and the kiln (C ). (Picture courtesy Castle Cement.)
In a modern works, the blended raw material passes from the silo (A in Figure 4.3) to the preheater tower (B) and then to the kiln (C). In the preheater tower, hot gases from the kiln, and often the cooled clinker at the far end of the kiln, are used to heat the raw meal. As a result, the raw meal is already hot before it enters the kiln. We’ll look first at the general principle of the rotary kiln (Figure 4.4). Ignore the whimsical wheelbarrow – not standard equipment in a modern kiln. A rotary kiln is basically a long cylinder rotating about its axis about 1-3 times a minute. At one end of the kiln is a flame. The kiln is inclined at a slight angle, the end with the flame being lower. The rotation of the kiln causes the raw meal to gradually pass along the kiln from where it enters at the cool end, to the hot end where it eventually drops out as clinker and cools.
Figure 4.4 General principle of a rotary kiln.
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Portland cement manufacturing - the main components of a cement plant
Wet process kilns The original rotary cement kilns were called ‘wet process’ kilns and in their basic form, they were relatively simple compared with new modern kilns. The raw meal was fed into the kiln at ambient temperature in the form of a slurry. A wet process kiln may be up to 200 m long and 6 m in diameter. It has to be long because a lot of water has to be evaporated and the process of heat transfer in a wet process kiln is not very efficient. Clearly, slurry contains water that has to be evaporated, perhaps 35% or more of the total feed. This takes a lot of energy and various developments of the ‘wet process’ were aimed at reducing the water content of the raw meal. An example of this is the ‘filter press.’ Imagine a musical accordion 10-20 metres long and several metres across and you’ll get the concept. Such adaptions were described as ‘semi-wet.’
Figure 4.5 Basic principle of a wet-process kiln. The raw meal is fed into the kiln unheated (eg: at 20 C) as slurry. All the drying and heating takes place in the kiln.
The wet process has survived for over a century because many raw materials are suited for blending as slurry. Also, for many years, it was technically difficult to get dry powders to blend adequately. Quite a few wet process kilns are still in operation, usually now with higher-tech bits bolted on. However, most new cement kilns are of the ‘dry process’ type and we will focus on these.
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Portland cement manufacturing - the main components of a cement plant Dry process kilns A dry process kiln is much more thermally efficient than a wet process kiln. Firstly, and most obviously, this is because the meal is a dry (or nearly dry) powder and so there is no water that has to be evaporated. Secondly, and less obviously, the process of transferring heat is much more efficient in a dry process kiln.
Figure 4.6 Basic principle of a dry process kiln with precalciner. This type of kiln has two burners. The raw meal is a fine powder and is more-or-less dry. It is heated by hot gases from the first burner, or precalciner. Also (not shown in the diagram) the meal is heated by hot gases from both the kiln and the clinker cooler before passing into the kiln at a temperature of 900 C – 1000 C.
The basic dry process system consists of the kiln and a “suspension preheater”. The suspension preheater is a heat exchanger housed in a tall tower, consisting of a series of cyclones in which fast-moving hot gases from the kiln keep the meal powder suspended in air. All the time, the meal gets hotter and the gas gets cooler until the meal is at almost the same temperature as the gas. The high open structure made of steel girders (B) in Figure 4.3 is a preheater tower containing the cyclones. The raw meal is fed in at the top of the preheater tower and passes through the series of cyclones in the tower before entering the kiln. Hot gas from the kiln and, often, hot air from the clinker cooler are blown through the cyclones. Heat is transferred efficiently from the hot gases to the raw meal. The heating process is efficient because the meal particles have a very high surface area in relation to their size and because of the large difference in temperature between the hot gas and the cooler meal. Typically, 30%-40% of the meal is decarbonated before entering the kiln.
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Portland cement manufacturing - the main components of a cement plant
A development of this process is the ‘precalciner’ kiln (Figure 4.7). Most new cement kilns are of this type. The principle is similar to that of the dry process preheater kiln but with the addition of another burner, called the precalciner. This is placed in the preheater tower, either as a separate unit or within the ‘riser duct,’ conveying kiln gas from the kiln to the last preheater cyclone. The meal, which is already hot, reaches about 900 C in a few seconds and about 85%-95% of the meal is decarbonated before it enters the kiln.
Figure 4.7 Basic diagram of a dry process kiln with a suspension preheater showing three cyclones and a precalciner (P).
In practice, a preheater tower is likely to have four to six stages; only three are shown in Figure 4.7 for simplicity. Some preheater designs are much more complex but all share the principle of maintaining the feed particles in suspension in a flow of hot gases, while transferring heat from the gases to the feed. Optimising the efficiency of this process will entail some elegant mathematics and applied physics to determine airflows and the best shape for the ducting. In a preheater tower, just small parts of the total system are visible at each floor level; only in drawings can they be seen in their entirety and their graceful form appreciated. Since meal enters the kiln at about 900 C, rather than 20 C in a wet process kiln, a dry process kiln can be shorter and of smaller diameter for the same output. This reduces the capital costs of a new cement plant. A dry process kiln might be only 70 m long and 6 m wide but produce a similar quantity of clinker (usually measured in tonnes per day) as a wet process kiln of the same diameter but 200m in length.
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Portland cement manufacturing - the main components of a cement plant At ‘A’ in Figure 4.7, most of the meal is decarbonated and the feed enters the kiln at about 950 °C. At ‘B’ the feed has reached about 1100 °C – 1200 °C and consists largely of belite, free lime and intermediate minerals. At ‘C’, the burning zone, the feed temperature reaches 1400 °C – 1500 °C; clinker nodules form and most of the remaining free lime reacts with belite to form alite. The next chapter will go into this in more detail. The kiln is made of a steel casing lined with refractory bricks. There are many different types of refractory brick and they have to withstand not only the high temperatures in the kiln but reactions with the meal and gases in the kiln, abrasion and mechanical stresses induced by deformation of the kiln shell as it rotates.
Figure 4.8 View inside a kiln, showing the burner pipe and flame. (Picture courtesy Rugby Cement.)
Bricks in the burning zone are in a more aggressive environment compared with those at the cooler end of the kiln (the ‘back end’), so different parts of the kiln are lined with different types of brick. Periodically, the brick lining, or part of it, has to be replaced. Refractory life is reduced by severe changes in temperature, such as occur if the kiln has to be stopped suddenly. As the cost of refractories is a major expense in operating a cement plant, kiln stoppages are avoided as far as possible. As the meal passes through the burning zone, it reaches clinkering temperatures of about 1400 C – 1500 C. Clinker nodules form as the burning zone is approached; when the clinker has passed the burning zone, it starts to cool, slowly at first, then much more quickly as it passes over the ‘nose ring’ at the end of the kiln and drops out into the cooler.
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Portland cement manufacturing - the main components of a cement plant
Fuels The most common fuels used in a cement kiln are probably coal and petroleum coke. Gas is also used. Recently, particularly in the last ten years or so, many cement producers have reduced their consumption of fossil fuels by supplementing them with other “greener” types of fuel (Chapter 12). Examples of these substitute fuels include bonemeal, car tyres (chopped into small pieces, or “chipped”) and waste paper. Hazardous waste materials such as solvents are also burned.
4.3
The clinker cooler
There are various types of cooler – we will consider only one, the ‘grate cooler.’ The purpose of a cooler is, obviously, to cool the clinker. This is important for several reasons:
From an engineering viewpoint, cooling is necessary to prevent damage to clinker handling equipment such as conveyors.
From both a process and chemical viewpoint, it is beneficial to minimise clinker temperature as it enters the clinker mill. The clinker gets hot in the mill and excessive mill temperatures are undesirable, as we will see later. If the clinker is cool as it enters the mill, this is clearly helpful.
From an environmental and a cost viewpoint, the cooler reduces energy consumption by extracting heat from the clinker, enabling it to be used to heat the raw materials.
From a cement performance viewpoint, faster cooling of the clinker enhances silicate reactivity; slow-cooling is undesirable as it allows the clinker minerals to convert to more stable, less reactive, crystal types. However, the most critical part of the cooling from the viewpoint of clinker reactivity takes place in the kiln after the clinker has passed the burning zone, before it reaches the cooler.
The cooler also heats the air entering the kiln around the main burner; hot air (800-1000 ºC) gives a better flame.
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Portland cement manufacturing - the main components of a cement plant
Figure 4.9 Clinker cooler: hot clinker from the kiln is carried on a moving grate towards the front of the picture. Cool air is blown from underneath. (Picture courtesy Rugby Cement.)
The cooled clinker is then conveyed either to the clinker store or directly to the clinker mill. The clinker store holds several weeks’ supply of clinker (or more), so that deliveries to customers can be maintained when the kiln is not operating.
4.4
The cement mill
Cement clinker is usually milled using a ball mill. A ball mill is essentially a large rotating drum containing steel balls, or grinding media. As the drum rotates, the motion of the steel balls crushes the clinker. The mill rotates approximately once every couple of seconds. A mill will usually have two or three chambers, with different size grinding media. As the clinker particles are ground down, smaller media are more efficient at further reducing particle size. Mills are either ‘open circuit’ or ‘closed circuit’ mills:
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Portland cement manufacturing - the main components of a cement plant
In an open circuit mill, the clinker passes through the mill and the feed rate of clinker into the mill is adjusted to achieve the desired fineness of the product.
In a closed circuit system, particle size is controlled more closely; coarse particles are separated from the finer product and returned to the mill for further grinding.
Cement from an open circuit mill has a wider particle size range than cement from a closed circuit mill; in an open circuit mill, some particles don't get ground sufficiently while others are ground extremely finely. Closed circuit milling is more efficient because it concentrates the milling effort more effectively on those particles that require further grinding. Clinker is interground with gypsum in the mill. Gypsum is added in order to control the setting properties of the cement, of which more later. The milling process uses a lot of energy and cement in the mill becomes hot. High milling temperatures result in the gypsum becoming dehydrated, with potentially undesirable results in concrete - more on this later as well.
Figure 4.10 Inside a stationary ball mill, showing steel grinding media and part-ground cement. (Picture courtesy Castle Cement.)
Recently, an alternative type of mill, the vertical roller mill (VRM) has also become popular because of its lower power consumption.
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Portland cement manufacturing - the main components of a cement plant
Further reading, Chapter 4 “Portland Cement: Composition, Production and Properties,” G C Bye, pub. Thomas Telford Ltd., 2nd edition, 1999. ISBN-13: 978-0727727664. Chapters 2 and 3.
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5 Portland cement manufacturing – from raw materials to cement
In Chapter 4 we looked at the different parts of a cement plant. In this chapter, we will consider what happens as the raw materials are transformed into cement.
5.1
Raw material blending
Suppose we have a source of limestone and a source of clay and that we have had them both analysed. Ideally, we would like to control all three clinker compositional parameters: LSF, SR and AR. We can blend the limestone and clay in the correct proportions to give whatever value for LSF we like, say 97%. However, the SR and AR will then be fixed by whatever the composition of the raw materials determines them to be. Since SiO2, Al2O3 and Fe2O3 are mainly contributed by the clay, it is the clay composition that will largely determine these parameters. In general terms, two types of raw material, such as limestone and clay, can be proportioned to fix any one parameter only, say the LSF. To fix x ratios, x+1 materials of suitable composition are needed, so to control all three parameters, LSF, SR and AR, a cement plant needs to blend four different materials of suitable composition. On a coal-fired plant, the composition of the coal ash also needs to be allowed for, since the ash also ends up in the clinker and combines with the other materials. In practice, a cement plant may have five or six different raw materials in order to control composition.
5.2
Minor constituents
Although the four main oxides, lime, silica, ferric oxide and alumina, comprise typically 94% - 97% of a Portland cement clinker, the other minor constituents have an important effect both on reactions in the cement kiln and on cement hydration.
Portland cement manufacturing – from raw materials to cement
You can’t use just any limestone or clay for cement-making - minor components in the raw materials also need to be considered. For example, standard specifications prescribe limits on the magnesium oxide that can be present in the clinker, so a dolomitic limestone – a limestone containing magnesium carbonate as well as calcium carbonate – may be unsuitable. The most common minor constituents normally present in clinker and therefore in the raw materials, are:
Alkali (Na2O, K2O) – perhaps 0.5% - 0.8%
Sulfate (SO3) – typically about 1%, usually between 0.5% and 1.5%
Magnesia (MgO) – typically about 1% - 2%
TiO2 – typically about 0.3%
P2O5 – typically about 0.1% – 0.2%
We’ll look at the first three of these, alkalis, sulfates and magnesia in more detail. Alkalis and sulfates In a cement-making context, ‘alkalis’ usually means potassium and sodium. Alkalis are generally present combined with sulfates. Potassium sulfate, as the mineral arcanite, is a common type of alkali sulfate present in clinker. Sulfate can be introduced from fuel as well as being present in the raw materials. While alkalis and sulfate will combine where possible, it is unlikely that the proportions of both will be present in just the right amounts to make, for example, potassium sulfate. It is more likely that there will be an excess of either alkali or sulfate in the clinker. Normally, a small amount of alkali enters into the alite, belite and ferrite phases. A slightly larger proportion of alkali is normally contained in the aluminate phase, perhaps up to about 1%. Separately, some sulfate also enters into the structures of the four main clinker minerals, particularly belite, which may have up to about 0.5% SO3 within the belite but not present as alkali sulfate. Belite sulfate contents generally increase as the total clinker sulfate increases. It is normally beneficial for the clinkering process, and for subsequent cement hydration, that there is an excess of sulfate compared with alkali. The surplus sulfate normally combines with lime to form a double-sulfate called calcium langbeinite (2CaSO4.K2SO4). This is usually present intimately mixed with arcanite.
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Portland cement manufacturing – from raw materials to cement
On the other hand, if there is an excess of alkali compared with sulfate, the excess alkali enters the belite and aluminate phases. This is detrimental and to be avoided if possible. Alkalis and sulfates will be considered further when we look at the reactions in the kiln. Magnesia (MgO) Small amounts of magnesium can replace calcium in the four main clinker minerals. If the main clinker minerals can’t accommodate all the magnesium within their crystal structures, the excess forms crystals of periclase (magnesium oxide). Small amounts of periclase are not a problem but large amounts are undesirable, as the periclase may hydrate within hardened concrete; the conversion of magnesium oxide to magnesium hydroxide is an expansive reaction and can cause damage. For this reason, cement standard specifications impose a maximum limit on the MgO content of cement, typically 4%-6%. In addition to limiting the potential for expansion by controlling composition, standard specifications generally also impose a test for expansion under specified experimental conditions to identify expansive cements. Other minor constituents Some heavy metals retard cement hydration (eg: lead, zinc); the presence of these would obviously be undesirable from the viewpoint of the cement, as would emission of toxic heavy metals during manufacture. Fluoride can retard the setting of cement but it can also act as a mineraliser together with sulfate, enabling alite formation in cement clinker at a lower temperature, with consequent savings in fuel. Mineralised cements characteristically have good early strengths.
5.3
Combinability temperatures
Combinability is about getting the raw materials to react, and how easily they do it. If we were looking at potential raw materials for cement production, in addition to just looking at the analyses of the materials, we would need to know how well they combined with each other. There are different approaches to this, but the basic idea is to do a series of trial mixes and assess how well they combine in a laboratory furnace. First, we establish a fixed burning regime, eg: 30 minutes at the required temperature, intended to represent the time in the burning zone of the kiln.
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Portland cement manufacturing – from raw materials to cement For example, we might mix limestone and shale at an LSF of 0.94 and burn ten samples of this mix at different temperatures for 30 minutes – we’ll call this Mix A. When cool, we measure the free lime in each and draw a combinability curve for Mix A by plotting free lime against burning temperature. We then repeat this for other mixes, eg: the same limestone and shale mixed to an LSF of 0.98; we’ll call this Mix B. For two mixes, A and B, we now have data relating burning temperatures and free lime (Figure 5.1). We would expect hotter burning to give lower free lime contents.
Figure 5.1 Examples of typical combinability curves for two mixes.
We then decide on a free lime level that we consider represents acceptable combination (eg: 1%, but it could be, say, 1.5% or 2%). We’ll use 1%. The combinability temperature for each mix can be read from the plots at the intersection of the curve with the line CaO=1%. In Figure 5.1, the combinability temperatures, T(A) and T(B) for Mixes A and B are about 1430 ˚C and 1560 ˚C respectively. Combinability depends on composition, specifically on the LSF, SR and AR. Combinability also depends on particle fineness, so we would have to include a measure of fineness, such as the 90µm raw feed residue in the tests as well. The composition of the coarse residue is also important; a particle of silica 100µm across is more difficult to combine than a particle of limestone of the same size. Combinability depends additionally on how well the raw materials react together; a marl might be composed of small particles of calcium carbonate, silica and clay all together in one rock type in approximately the desired proportions to make cement. The small particles, already in close proximity with each other, will probably react more easily than a mix of the same overall composition containing a blend of three rock types say, chalk, silica and clay.
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Portland cement manufacturing – from raw materials to cement In order to find out how each of these parameters affects how particular raw materials will combine, we need to do a large number of test burns. Assuming we have a suitable range of raw materials to control all three parameters (at least four materials) individual parameters can be changed to show how each parameter affects the combinability temperature for that mix. First, we produce a series of mixes with varying LSF but the same SR and AR and fineness, we then burn each of these mixes at different temperatures to find the combinability temperature for that mix (ie: the temperature that gives us 1% free lime). We then repeat, but varying the SR and then the AR. Finally, we repeat the whole exercise, this time using material of a different fineness. (Fineness can mean the blended raw materials collectively, or we could test variations in the fineness of each component). It will be clear that combinability experiments may involve doing tens or even hundreds of individual burns of test mixes and that this is not a trivial exercise. Many hours of fun later, we would be able to produce standard plots of combinability temperatures against the other variables. We would typically expect to find that: Combinability temperatures increased with increasing LSF (Figure 5.2). Combinability temperatures increased with increasing SR (Figure 5.3). Combinability temperatures showed a minimum at about AR=1.4 (Figure 5.4). The coarser the material, the more difficult it is to combine (Figure 5.5). We could expect that the combinability temperature would increase with LSF; to produce a clinker in which the calcium silicates are all in the form of alite will require the constituents to be evenly mixed. Any heterogeneities will result in areas of excess lime or silica and these will be difficult to combine. With decreasing LSF mix heterogeneities become less important. We would also expect the combinability temperature to increase with increasing SR because, as the SR increases, there will be less liquid to facilitate combination. The reason for a minimum in the combinability temperature at around AR=1.4 is less obvious. Alumina Ratios for ordinary Portland cement - normal grey cement are in the range 1 to 4. However, at the lowest temperature at which liquid will form (1338 ˚C), the amount of liquid will be at a maximum at AR=1.38. Since it is the liquid flux that mainly facilitates combination, mixes of AR ratio of 1.38 will be easiest to burn. Strictly, this applies to the CaO-SiO2-Al2O3-Fe2O3 system. Minor constituents can alter the optimum AR; magnesium oxide, for
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Portland cement manufacturing – from raw materials to cement example, increases it. Combination is also affected by the viscosity of the liquid phase; a less viscous liquid will be more mobile and this will aid combination. Liquid viscosity increases with increasing AR and decreases with increasing temperature. Finally, the coarseness of the reactants affect the rate of any chemical reaction, so it is no surprise that coarser material is harder to combine than fine material.
Figure 5.2 Example of a typical curve showing combinability temperature variation with LSF. With increasing LSF, it becomes progressively harder to achieve combination. Above 100% LSF, there will always be some uncombined free lime.
Figure 5.3 Example of a typical curve showing combinability temperature variation with SR. With increasing silica ratio, mixes become harder to burn, as there is less liquid present for ion transport, without which lime cannot combine with belite to form alite.
Figure 5.4 Example of a typical curve showing combinability temperature variation with AR at fixed LSF and SR. Combinability temperatures are at a minimum at about AR=1.3-1.5.
Figure 5.5 Example of a typical curve showing combinability temperature variation for a fixed composition with 90µm sieve residue. Coarser material is harder to combine.
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Portland cement manufacturing – from raw materials to cement So, returning to the question of what the best clinker composition might be, from the previous section, suppose we want a high content of calcium silicates, with a high alite content. This means both a high LSF and a high SR. However, the curves in Figures 5.2 and 5.3 show that either will make the raw materials harder to combine, so in a mix with both a high LSF and a high SR, good combination might be difficult to achieve. In practice, the main factor influencing the clinker composition will be the compositions of the main raw materials to be used. For example, if a fairly pure limestone and a clay are to be used, the clay composition will largely determine the silica ratio and the alumina ratio, because the limestone won’t contain much silica and alumina. Much more control would be possible for the lime saturation factor, by adjusting the ratio in which the limestone and clay are mixed. Of course, as we have seen, technically a cement works can have almost complete control over clinker composition by blending raw materials of different composition to produce the desired result. Compositions are often a compromise between what is ideally wanted and what can be achieved in reality, given the composition of the available raw materials. Other important factors will include fuel and raw material grinding costs, and practical issues such as how finely the raw material mills can grind material at the required throughput.
5.4
Raw material proportioning
Proportioning calculations show how much of each raw material is needed to achieve the desired clinker composition. Before computers became widely available these calculations, done with pencil and paper, were an entertaining part of cement technology courses. Where coal is the fuel for the kiln, the calculation also needs to take into account the effect of coal ash. Most of the ash will become incorporated into the clinker and the quantity of ash is enough to have a significant effect on clinker composition; ash may represent perhaps 2%-8% of the clinker. To calculate the effect of coal ash, we would need to know: how much coal is needed to produce a tonne of clinker; the proportion of ash retained in the clinker (probably most of it); the proportion of ash in the coal and the composition of the ash. The principle of the proportioning calculation is simple but the actual calculation is tedious when done manually. Computers can do it instantly by solving simultaneous equations, so we’ll save a few hours and move swiftly on.
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5.5
Reactions in the kiln
We’ve blended our milled raw materials in the correct proportions (trusting our computer) and the raw feed now goes to the kiln. We’ll consider the reactions in the kiln under three broad headings:
Reactions before the burning zone, below about 1350 C
Reactions in the burning zone (1350 C - 1500C)
Cooling of the clinker.
5.5.1
Reactions before the burning zone
In a dry-process kiln, some reactions will take place in the preheater tower before the feed enters the kiln. Any moisture will evaporate, some or most of the limestone will be calcined and the surfaces of siliceous particles in the feed will start to combine with adjacent lime particles. In a wet-process kiln, all the reactions obviously take place in the kiln. Water evaporation and calcining In isolation, decarbonation of calcium carbonate at one atmosphere takes place at 894 C. This temperature is reduced to 500 C - 600 C if the reaction takes place in contact with quartz or the decomposition products of clay minerals. In a wet-process kiln, water evaporation and calcination takes place in the kiln, within the moving mass of feed. This situation is not ideal because heat transfer has to take place through a large mass of material and water vapour and CO2 have to escape outwards, taking heat with them, when the objective is to transfer heat inwards. Calcining takes place after the water has been driven off, about a third of the way down the kiln. In a dry process preheater kiln, any moisture in the feed particles is evaporated and up to about half of the limestone is decarbonated before the feed enters the kiln. Surface reactions on siliceous particles will occur to a limited extent. In a dry process kiln with a precalciner, most of the limestone is calcined before the feed enters the kiln. Calcination takes place in a few seconds, rather than the half an hour or so inside a wet kiln at the same temperature. The surfaces of siliceous particles will react with lime to a greater extent than in a kiln with a preheater only, due to the higher temperatures. In a preheater kiln, especially one with a precalciner, low temperature sulfate melts condensing from the kiln gases onto the feed can aid combination of lime and siliceous particles.
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Formation of early and intermediate compounds As lime is calcined, it starts to react with other components of the raw feed. The main initial silicate product is belite. Some calcium aluminate (CA and C12A7) and ferrite phases also start to form. Some minerals are formed in the kiln at relatively low temperatures compared with the temperature in the burning zone. Most of these minerals dissociate in the burning zone and are not therefore present in the clinker; these minerals are termed “intermediate compounds”. Some intermediate compounds assist in forming the final clinker minerals. Before decarbonation is completed, a calcium silicate carbonate known as spurrite frequently forms. Its composition can be written as 2C2S.CaCO3, suggesting it is a step on the way to forming belite. It can cause problems in the presence of solid condensation products at the kiln back end (cool end); spurrite can reinforce these deposits by sticking feed particles together, producing rings or blockages that are difficult to remove. Other intermediate products that may form include calcium sulfosilicate (2C2S.CaSO4) and Klein’s compound (C3A.CaSO4). The extent of these is dependent on the overall level of sulfate in the clinker. A sulfate melt phase may form, consisting mainly of alkali sulfates (Chapter 5.6); this liquid phase can aid combination of lime and silica and is not miscible with the main liquid phase. Alkali chlorides may be also present.
5.5.2
Reactions in the burning zone
In the burning zone, above about 1350 C, reactions take place quickly. The clinker is in the burning zone for perhaps 15-30 minutes but in this time a lot happens:
The proportion of clinker liquid increases and nodules form.
Intermediate compounds dissociate to form liquid and belite.
Belite reacts with free lime to form alite.
Some volatile phases evaporate.
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Portland cement manufacturing – from raw materials to cement Clinker liquid and nodule formation Above about 1350 C, the proportion of liquid starts to increase. By 1450 C, perhaps 20-30% of the mix is liquid. The liquid forms from melting ferrite and aluminate phases and some belite, in addition to any sulfate melt phase already present. The liquid content is more than the sum of the aluminate and ferrite phases in the cooled clinker mainly because of dissolved lime and silica. The additional liquid causes feed particles to stick together, leading to the formation of clinker nodules.
Dissociation of intermediate compounds As the temperature rises, the intermediate phases that formed at lower temperatures (CA and C12A7, spurrite and calcium sulfosilicate) dissociate to form mainly aluminate phase and belite.
Alite formation Alite forms mainly by the reaction of lime with belite. Some alite also forms directly from free lime and silica. These reactions occur rapidly once the clinker temperature is above about 1400 C.
Evaporation of volatiles Alkalis, sulfates and chlorides are present in the burning zone chiefly as sulfate melts and are liable to volatilise. The volatilised material passes down the kiln, where it condenses on the cooler, part-reacted, feed. It then travels back towards the burning zone, where it may volatilise again; it may follow this cycle many times. These recycling volatiles are known as the recirculating volatile load. A high recirculating volatile load may cause adverse clinker characteristics. It can also result in kiln rings when condensed volatiles adhere to the kiln wall causing rings to form, or blockages in the riser duct or cyclones. These can interfere with production if the kiln has to be shut down to remove the blockage. Clinker sulfate phases will be looked at in Chapter 5.6.
5.5.3
Reducing conditions
A kiln is a metal box containing a flame. A controlled supply of air (secondary air) is admitted to allow combustion but if oxygen loss in the burning zone is extreme, due for example to very high burning temperatures, poor flame direction or poor mixing of secondary air or insufficient secondary air, reducing
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Portland cement manufacturing – from raw materials to cement conditions can result. This adversely affects cement quality. Reducing conditions cause Iron (III) to be reduced to Iron (II). Iron (II) can substitute for calcium and so can be present in all the clinker phases. The consequences of this are numerous, including:
Less liquid in the clinker, because there is less Iron (III). Effectively, the SR is increased, so alite formation is inhibited, necessitating harder burning.
As the clinker cools, some or most of the iron (II) reoxidises to iron (III). As it does so, some alite dissociates into belite and free lime because iron that was substituting for calcium in the alite is no longer present; the lower alite content results in some loss of cement hydraulicity.
Iron may reduce further to form metallic iron. Sulfides also form, altering the balance between alkali and sulfate in the clinker (Chapter 5.6).
The effect on cement made from clinker produced under reducing conditions is generally to reduce cement performance. Concrete strengths may be lower and setting times erratic. Overall, reducing conditions in the kiln are undesirable. The only exception to this is that of white cement. White cement clinker is burned deliberately under slightly reducing conditions to reduce iron (III) to iron (II) – that way the cement is whiter. As there is little iron present anyway, the negative aspects of reduction are minimal.
5.5.4
Cooling of Clinker
As soon as the clinker passes the burning zone, it begins to cool, even though it is still inside the kiln. The cooling rate from the burning temperature down to perhaps 1000 ºC is especially important; much of this cooling occurs before the clinker reaches the cooler. Fast cooling of clinker is advantageous; it makes for more hydraulically-reactive silicates and lots of small, intergrown, aluminate and ferrite crystals. Slow cooling gives less hydraulically-reactive silicates, allows alite to decompose to belite and free lime, and produces coarse crystals of aluminate and ferrite. Over-large aluminate crystals can lead to erratic setting characteristics. Rapid cooling probably makes the clinker easier to grind by inducing thermal cracking.
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5.6
Clinker sulfate phases
Most clinkers contain sulfate, expressed as SO3, in amounts ranging from about 0.5% to 2%. Around 1% SO3 is fairly typical. Despite this relatively low proportion of the total clinker, sulfates and related alkalis play an important part in both clinker mineral formation and in cement hydration. In the kiln, sulfate is beneficial as it forms a sulfate melt phase which promotes reactions by acting as a flux, helping ion transfer in the same way as the main liquid phase. Although clinker leaving the kiln may typically contain perhaps 1% SO3, the proportion of sulfate in the partly-combined material in the kiln will be higher, possibly much higher; this is because of the recirculating volatiles, of which alkali sulfates are likely to be the principal component. Within the kiln, the proportion of sulfate melt phase could be 5% or more of the charge. When clinker is ground to make cement, gypsum is added to control the setting of the cement. However, the clinker alkali sulfate phases can also contribute to controlling the set. Since sulfates are largely combined with alkalis, sodium and potassium in the cement-making context, alkalis and sulfates have to be considered together. Clinker alkali sulfate forms mainly in pores in the clinker (Figure 5.6).
Figure 5.6 Clinker alkali sulfate in a pore in a clinker nodule (arrowed).
It is unlikely that the amount of available sulfate will exactly equate with the amount of available alkali to make alkali sulfates; there will normally be an excess of one over the other. An excess of available sulfate over available alkali is the more desirable of the two alternatives.
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Portland cement manufacturing – from raw materials to cement If there is an excess of sulfate over alkali, calcium combines with the excess sulfate to form another double sulfate known as calcium langbeinite (K2O.2CaO.3SO3). Calcium langbeinite is an effective set retarder. Clinker alkalis can enhance early strength development in concrete as they accelerate the rate of alite hydration by increasing the alkalinity of the pore fluid,. The types of clinker sulfate present are determined by the ratio of clinker alkali to sulfate and by the types of alkali. The main forms of clinker alkali sulfate are:
Arcanite - mainly K2SO4 often with minor replacement of potassium by sodium
Calcium langbeinite (K2SO4 . 2CaSO4)
Clinkers of higher Na/K ratio may contain aphthitalite (mainly 3K2O.Na2O.4SO3)
Clinkers with high Na and low K may contain thenardite (Na2SO4)
High sulfate/low alkali clinkers may contain anhydrite (CaSO4)
While most alkali and sulfate in clinker is present as alkali sulfate inclusions in pores in the clinker, some alkali and some sulfate are incorporated into the four main clinker phases. Alkali and sulfate, uncombined with each other, are generally present in belite; typical levels are 0.2% SO3 and 0.9% K2O although this varies considerably. Belite often also contains small inclusions of alkali sulfate. Normal aluminate generally contains little sulfate but perhaps typically a total of about 1% Na2O and K2O. Normal aluminate has a cubic crystal structure, but another form of aluminate with a prismatic crystal structure also occurs in some clinkers. Prismatic aluminate (or alkali aluminate) has a higher alkali content, up to 4%-5% K2O and Na2O combined. It occurs where there is an excess of alkali over sulfate in the clinker; it is detrimental in itself, leading to erratic setting characteristics of cement, and is symptomatic of undesirable clinkering conditions.
Clinker alkalis and sulfates – it is all a matter of balance Suppose we have a clinker which has an excess of available alkali over sulfate. For simplicity, suppose all the alkali is K2O. ‘Available’ means neglecting alkali and sulfate bound in the solid phases, alite and belite, in the burning zone and not available to form alkali sulfates. (Free lime does not normally contain much alkali or sulfate). ‘Excess of alkali over sulfate’ means that there is alkali left over after all the available sulfate has combined with alkali. Potassium sulfate (K2SO4) consists of about 54.1% K2O and 45.9% SO3.
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For a clinker with 0.50% available sulfate, alkali and sulfate are in balance if the clinker also contains about 0.59% available K2O. A small excess of available alkali over sulfate is not usually important, unless it is abnormal for clinker from that particular cement plant. If so, cement characteristics may be altered and customers may notice. A large excess of available alkali over sulfate is to be avoided if possible because:
In a clinker with excess alkali over sulfate, the excess alkali will be present as an elevated alkali content in both the aluminate and belite phases.
Excess alkali in belite will inhibit combination with free lime to make alite in the burning zone.
Excess alkali in the aluminate may cause setting problems.
Water-soluble alkali aids early strength development; if alkali is tied up in the main clinker phases it is not available and early strengths may decline.
Suppose now that the clinker sulfate content is gradually increased (perhaps by burning a higher-sulfur fuel), while the alkali content is maintained. There will be less alkali in the aluminate and belite and more alkali sulfate as arcanite. At some point, the available alkali and sulfate will be balanced, with all alkali sulfate present as arcanite. Combinability and cement performance should improve. If the sulfate level is increased further, the excess sulfate combines with calcium to form the double sulfate calcium langbeinite (K2SO4.2CaSO4). The clinker will then contain a mixture of calcium langbeinite and arcanite. With yet more sulfate addition, at some point all of the alkali sulfate will be present as calcium langbeinite. With still more sulfate, anhydrite will start to form, although this is unusual. As the clinker sulfate increases, recirculating sulfate loads and deposit build-ups may cause kiln operating problems.
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Summary of role of sulfate in clinker
The clinker sulfate melt phase contributes to ion transfer in the same way as the main liquid phase - if there is less sulfate melt phase, its fluxing contribution will be less and combination of the clinker minerals will be more difficult to achieve.
Clinker sulfate combines with available alkali, preventing it entering belite and aluminate. If there is an excess of available alkali compared with available sulfate, there will be an undesirable excess of alkali over sulfate.
If the recirculating volatile load is too high, production problems are likely with sulfate-rich build-ups causing blockages.
Clinker sulfate contributes to set control during hydration.
Alkalis from clinker sulfate aid early strength development in concrete.
5.7
Clinker grinding and gypsum addition
Cement mills (Figure 4.10) are usually tube mills with two or more separate chambers containing different sizes of steel balls, or grinding media. A large mill might be about 15 metres long and 5 metres in diameter. Diaphragms perforated with holes or slots permit the passage of cement from one chamber to the next, allowing cement particles through, but not the grinding media. The grinding of clinker requires a lot of energy. How easy a particular clinker is to grind - ‘grindability’ - is difficult to predict, but rapid cooling of the clinker is thought to improve grindability due to the presence of microcracks in alite and to the finer crystal size of the flux phases. The fineness of cement is one of the main factors controlling its behaviour when mixed with water. Cement ground to a finer particle size will clearly react more quickly than the same cement milled more coarsely. Fineness is measured s the specific surface area in square metres per kilogram (m2 kg-1). This is typically determined by an air permeability test, eg: the Lea and Nurse method or the Blaine method. These methods give similar results, typically 350-450 m2 kg-1; rapid-hardening (high early strength) cements will be at the higher end of this range. Other test methods may give different values. As part of the grinding process, calcium sulfate is added as a set regulator, usually in the form of gypsum (CaSO4.2H2O). Natural anhydrite may also be added to fine-tune the rate at which sulfate dissolves when the cement is mixed with water. It also may reduce the tendency of the cement to agglomerate during storage, known as air-setting.
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Portland cement manufacturing – from raw materials to cement Air-setting is due to small crystals of syngenite (calcium potassium sulfate hydrate, CaSO4.K2SO4.H2O) sticking cement particles together; the water for the formation of syngenite can come from moist air, or from the water of hydration in gypsum, or from residual internal cooling water if the mill temperature falls. Since the clinker gets hot in the mill due to the heat generated by grinding, gypsum can be partly dehydrated. It then forms hemihydrate, or plaster of Paris CaSO4.1/2H2O. On further heating, hemihydrate dehydrates further to a form of calcium sulfate known as soluble anhydrite (CaSO4 with a trace of water); this has a similar solubility in water to hemihydrate, which in turn has a higher solubility than either gypsum or natural anhydrite. Cement mills need to be cooled to limit the temperature rise of the cement. This is done by air-cooling, or air-cooling and water-cooling. The relative proportions and different solubilities of these various types of calcium sulfate are of importance in controlling the rate of aluminate hydration and consequently of cement set retardation. Changes in soluble sulfate availability can also affect the performance of some concrete admixtures. Problems associated with setting and strength characteristics of concrete can often be traced to one or more of:
changes in the relative proportions of gypsum and hemihydrate due to hot milling.
changes in the relative proportions of clinker sulfate compared with added gypsum.
variations in cooling rate of the clinker in the kiln and subsequent changes in the proportions or size of the C3A crystals.
Solubilities of the different sulfate types in decreasing order are approximately: arcanite > calcium langbeinite > hemihydrate ≈ soluble anhydrite > gypsum > natural anhydrite. In controlling the rate of aluminate hydration to regulate the setting of cement, the most important feature of the aluminate is not necessarily the absolute amount present, but the amount of surface available to water for reaction. This will be governed by many factors, such as the surface area of the cement, the grinding characteristics of the different phases and also the size of the aluminate crystals. Over-large crystals can lead to erratic setting characteristics. To summarise this complex issue of clinker sulfate and added gypsum and other sulfate:
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In a cement from any given source (ie: same kiln), the clinker sulfates may vary in type and quantity over time.
At the clinker mill, sulfate will be added to a target figure, typically about 3% SO3, but because total clinker sulfate may vary, the proportion of clinker sulfate to added sulfate may also vary.
As different forms of sulfate have different solubilities, these variations may have a detectable effect on cement hydration and concrete properties.
5.8
Other additions
Materials other than gypsum may be incorporated into cement at the cement mill, but different standard specifications differ over what are permissible additions. The European standard specification EN 197-1 permits the use of 5% by mass of “minor additional constituents” including limestone, fly ash or kiln dust.
Grinding aids Grinding aids are small quantities (typically up to 0.2% by mass of clinker) of usually organic liquids such as triethanolamine or solid organic amines that are added because they improve grinding efficiency; in other words, less energy is required to achieve the same cement fineness. They operate by inhibiting the formation of coatings of powder on the grinding media, and by facilitating crack propagation. Some grinding aids are also designed to affect cement hydration. Limestone Small amounts of limestone, typically up to 5% limestone, may be interground with clinker where permitted by national standard specifications. This is a relatively recent development in many countries. Limestone is softer than clinker and so generally grinds preferentially, resulting in very fine limestone particles. Cynics may say that cement companies like to add limestone because limestone is cheaper than cement. While this cost comparison is undeniable, there is a bit more to it than that. In the past, cement mills were open circuit mills and the cement produced by them had a wide range of particle sizes, from very fine to over-coarse. More recently, cement mills have tended to be closed circuit mills; cement from a closed circuit mill tends to have a narrower particle size range compared with cement from an open-circuit mill and this increases the water demand of the cement.
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Portland cement manufacturing – from raw materials to cement Concrete made with cement that has a higher water demand will tend to be of lower strength because the water/cement ratio is higher. By intergrinding limestone, the very fine limestone particles act to reduce the water demand of the cement because they occupy space between the cement particles that would otherwise be occupied by water. The presence of a small amount of interground limestone therefore can increase concrete strengths by reducing the water/cement ratio required when mixing the concrete. For related reasons, fine limestone has the positive effects of reducing bleeding and/or allowing the cement to be ground slightly coarser for the same strength. Fine limestone can also be beneficial by providing nucleation sites on which hydration products can form; this “fine filler effect” increases the rate at which material is precipitated from solution. In turn, the rate at which dissolution can occur at the surfaces of the cement particles is also increased, resulting in a faster rate of cement hydration and improved early strengths. The fine filler effect may also improve later strengths by producing a more uniform hydrate microstructure. In summary, the benefits of intergrinding small amounts of limestone (up to about 5%) are:
Reduces the water demand of the cement
Reduces bleeding
Allows cement to be ground slightly coarser, saving energy
Reduces the quantity of clinker in the cement, saving energy and raw materials
Acts as nucleation sites, aiding early cement hydration and improving the hydrate microstructure
Small amounts of fine limestone in cement can therefore reduce the energy required to produce the same quantity of cement, and have beneficial effects on concrete properties. The added limestone acts as a filler, but a proportion of it will react with the cement and contribute to the composition of the cement hydration products (Chapters 6.3 and 11). Reducing agents to limit Chromium VI Some countries impose limits on the Chromium VI content of cement. This is because chromium causes skin sensitisation, resulting in allergy problems. Not all chromium in cement is chromium VI, the majority is generally Chromium III; however, Chromium VI compounds are more water-soluble and so are more likely to cause problems.
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Portland cement manufacturing – from raw materials to cement Reducing agents are added to control the Chromium VI content of the cement. Typical reducing agents are ferrous sulfate, either the monohydrate (FeSO4.H2O) or heptahydrate (FeSO4.7H2O) forms, or stannous sulfate (SnSO4). By mass of cement, approximately 0.5% ferrous sulfate is typically added, and approximately 0.05% stannous sulfate. At such low levels of addition, no significant adverse effects on cement performance would normally be expected. In the European Union, the Chromium VI Directive came into effect in 2005 and prohibits the use or supply of cements containing more than 2 ppm water-soluble hexavalent chromium by mass of cement.
5.9
Calculation of clinker minerals in cement
The Bogue calculation (Chapter 3.5.1) can be applied to cement in modified form. This is more complex than for a clinker as we need to consider what other materials may have been added to the cement at the grinding stage, or any other time. Firstly, gypsum will be present, so we have to allow for the proportion of CaO assumed to be present as calcium sulfate. One approach is to subtract 0.7 x (SO3) from the CaO in the bulk composition of the cement, in the same way that free CaO is subtracted. For many years, this was a standard procedure. Of course, this introduces a slight error as it neglects any clinker sulfate, but if no other material has been added, this is still the simplest approach. After making the deduction for sulfate (and for free lime if required), the calculation can be done in the usual way as outlined in Chapter 3.5.1. However, in addition to gypsum, many cements now also contain up to 5% interground limestone. (Some standard specifications permit the addition of other materials also, such as fly ash, but that is a further complication we won’t consider here). To allow for limestone in the calculation, the CO2 content of the cement should be determined and the limestone content estimated. If the purity of the limestone is known, a more accurate determination can be made; for example, pure limestone (100% calcium carbonate) contains 44% CO2 but if the limestone is 80% pure, it will contain only 35% CO2. The American standard specification for Portland cement ASTM C 150-07 contains a modified version of the Bogue calculation that allows for both added gypsum and limestone:
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MODIFIED BOGUE CALCULATION TO ALLOW FOR INTERGROUND GYPSUM AND LIMESTONE C3S C2S C3A C4AF
= = = =
4.071CaO-7.600SiO2-1.430Fe2O3-6.718Al2O3-2.852SO3-5.188CO2 2.867SiO2-0.7544C3S 2.650Al2O3-1.692Fe2O3 3.043Fe2O3
As given above, this is a calculation of the potential phase composition, since it does not allow for any free CaO. For any calculations related to that standard specification, the calculation stated in the standard should obviously be followed. As discussed previously (Chapter 3.5.1), in some other circumstances, it may be useful to first subtract the free lime content of the cement (if known) before performing the calculation, in order to obtain an improved estimate of the actual alite content.
Further reading, Chapter 5 “Portland Cement: Composition, Production and Properties,” G C Bye, Thomas Telford Ltd., 2nd edition, 1999. ISBN-13: 978-0727727664. Chapters 2-5.
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6 Hydration of cement – chemical and physical properties of cementitious materials
Cement hydration is the process by which cement and water react together. When cement and water are mixed in suitable proportions, the result of the reaction is a solid mass, composed of gel and crystalline material, which binds together the constituents of a concrete mix. In a Portland cement composed of clinker and interground gypsum, the reactants in the process are:
From the clinker: alite, belite, aluminate and ferrite, plus alkali sulfates and free lime
Gypsum interground with the clinker
Water
The main products of the reactions are:
Calcium silicate hydrate (C-S-H)
Ettringite (up to 1-2 days)
Monosulfate (after about 1-3 days)
Calcium hydroxide
A mix of cement and water forms a hard cement paste. Concrete is a mix of cement, water and aggregate (ie: sand and gravel or crushed stone) in which the aggregate is bonded in a matrix of cement paste. Because it is the cement paste that binds all the constituents together as a single, solid, mass it follows that the properties of the concrete (strength and durability for example) will be critically influenced by the properties of the cement paste.
Hydration of cement – chemical and physical properties of cementitious materials
6.1
Hydration of cement: heat evolution
Portland cement is composed largely of the four main clinker minerals, plus some clinker alkali sulfate and gypsum. When cement is mixed with water, virtually all the alkali sulfate from the clinker dissolves rapidly. Calcium sulfate dissolves until the solution becomes saturated. Of the four main clinker phases, the aluminate phase is the most reactive, followed by alite, then belite. Ferrite is initially very reactive but the hydration subsequently slows appreciably. When cement and water are mixed, exothermic reactions occur. The rate at which heat is evolved varies for the first three days or so and then gradually declines. The general shape of this plot is useful in understanding the reactions that occur as the cement hydrates (Figure 6.1). The shape of the plot shows a rapid exothermic reaction at first (I in Figure 6.1); the “*” in Figure 6.1 symbolises a fast increase in heat evolution when water is added, then a fast drop, too near the y-axis to be shown.
Figure 6.1 General form of heat evolution curve of hydrating Portland cement. ‘H’ represents the rate of heat evolution, measured in watts per kilogram of cement. Stages I to V are described in the text.
The initial reaction lasts only a few minutes, or less, and is followed by a dormant, or induction, period (between I and II in Figure 6.1) of several hours during which there is little further heat evolution. Subsequently, the rate of hydration picks up again; during this second period of heat evolution, the
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Hydration of cement – chemical and physical properties of cementitious materials concrete sets and gains strength. The rate of heat evolution in this second period reaches a peak (III in Figure 6.1) after 10-15 hours then, apart from possible small increases at around 20 hours (IV in Figure 6.1) and 30-40 hours (V in Figure 6.1), gradually tails off over the following days and weeks. The main characteristics of the plot in Figure 6.1 can be interpreted as:
The initially high rate of heat evolution at I in Figure 6.1 is due mainly to the hydration of the aluminate and alite phases and the hydration of calcium sulfate hemihydrate to the dihydrate form (gypsum).
The rate of hydration then slows to a low level after a few minutes (II) due to the deposition of hydration products on the surfaces of the cement grains. During this dormant, or induction, period before the paste sets, the concrete is plastic or flows freely.
Gradually, the rate of aluminate and alite hydration increases again and belite also begins to hydrate (III).
Small subsequent increases in heat evolution (IV and V) are probably due to ettringite formation and ferrite hydration respectively.
After a few days, the rate of heat evolution gradually tails off as the unhydrated cement particles become scarcer, and as what cement remains is surrounded by dense hydration product that slows any further reaction.
6.2
Hydration of cement: main types of hydration product
We should first distinguish between “setting” and “hardening”. “Setting” is the transition of the concrete, or cement paste, from a fluid to a rigid state. “Initial set” and “final set” are terms commonly used to refer to setting characteristics and are arbitrary definitions of early and later set. There are laboratory procedures for determining these using weighted needles penetrating into the cement paste. “Hardening” refers to the increase in strength of the concrete. The rate of hardening, the rate of increase in strength of the concrete, is independent of the rate of setting. Rapid-hardening cement may have similar setting times to normal ordinary Portland cement. The cement paste sets when solid particles in the mix become tenuously connected by the products of the hydration reactions, mainly ettringite and calcium silicate hydrate. As time passes, the links between the solid particles the unhydrated cement particles, the hydration products and the aggregate become thicker and the concrete gains strength, or hardens, as hydration
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Hydration of cement – chemical and physical properties of cementitious materials products continue to form. These physical changes can be explained more fully by first considering the hydration products that form as hydration proceeds. The four main clinker minerals produce hydration products broadly as follows:
Alite: alite hydration produces calcium silicate hydrate gel plus a lot of calcium hydroxide.
Belite: belite hydration produces calcium silicate hydrate gel plus, perhaps, a little calcium hydroxide.
Aluminate: aluminate (“C3A”) hydration, in the presence of sufficient sulfate, produces ettringite (more on this later), and other phases.
Ferrite: ferrite hydration also produces ettringite, together with other phases.
Let’s look at these cement hydration products in a little more detail. Calcium silicate hydrate: calcium silicate hydrate is a gel, in other words an amorphous or poorly-crystalline form of calcium silicate hydrate, that forms as a result of the hydration of alite and of belite during normal cement hydration. It may be conveniently shortened to C-S-H in cement chemistry notation; the dashes indicate that no particular ratios of C, S and H are intended - writing it as CSH would incorrectly indicate a 1:1:1 ratio of C, S and H respectively. In Portland cement, the ratio of calcium to silicon in C-S-H is somewhat variable, but typically approximately 2:1. Belite (C2S in the pure form) also has a Ca:Si ratio of about 2:1 and it follows that C-S-H will be the principal product of belite hydration, with little excess calcium or silicon available to produce other hydration products. Usually, a small amount of calcium hydroxide is likely to form. In contrast, alite (C3S in the pure form) has a Ca:Si ratio of approximately 3:1. Alite hydration will therefore result in excess calcium if the C-S-H that forms has a Ca:Si ratio of 2:1. The excess calcium oxide produces calcium hydroxide by reacting with water. Calcium silicate hydrate is the main strength-giving component of hydrated Portland cement and it is therefore usually advantageous to maximise the proportion of C-S-H produced. Calcium hydroxide: calcium hydroxide, or CH in cement chemistry notation, is produced primarily from the hydration of alite, as described above. CH will also form from the hydration of free lime in the cement. It usually forms as hexagonal plates, 1µm-20µm across, where it can grow freely in water. Dense growths of CH can also infill pores and the hexagonal crystal shape is not then apparent. Ettringite: when cement and water are first mixed, the aluminate phase reacts with dissolved calcium sulfate to form ettringite. Ettringite, in cement chemistry notation, has the formula:
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Hydration of cement – chemical and physical properties of cementitious materials _ C6AS3H32 or C3A.3CaSO4.32H2O (mixing cement chemistry and normal notation) Ettringite crystals are generally thought to form a coating over the surfaces of hydrating crystals of aluminate phase, acting as a temporary barrier to further hydration. However, more recently, this classical interpretation of the process by which aluminate hydration is retarded has been questioned and the mechanism may be more complex. Nevertheless, the concept of retardation due to a coating of ettringite or other hydration products over the surface of the cement grains is simple and more than adequate for most purposes. Ettringite forms initially as acicular crystals – thin rods a micron or so in length. Subsequently, these crystals may grow to perhaps 5µm-10µm if sufficient waterfilled space is available. These sparse, long and thin crystals of ettringite play an important role in the setting of cement but are not sufficiently strong to contribute greatly to compressive strength in normal Portland cement hydration.
Figure 6.2 SEM image of a fracture surface of cement paste after two days’ hydration. The important features in this image are the long, thin, crystals of ettringite (e); the calcium silicate hydrate (C-S-H) and the hexagonal plates of calcium hydroxide (CH). Towards the left of this image, the paste is porous, with numerous capillary pores (see Chapter 6.4) up to about 5µm across, partly bridged by hydration product growing from hydrating cement particles (C). Towards the right of the image, the C-S-H is much denser and most of the pores are filled; evidently, the paste at the right will be stronger than the paste at the left. This picture illustrates a transitional stage where some regions of the paste remain porous and weak, while others are becoming denser and stronger. After another day or so, the paste at the left will resemble the paste at the right, as the microstructure of the paste as a whole becomes more uniformly dense.
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Hydration of cement – chemical and physical properties of cementitious materials Another important phase is: Monosulfate phase: calcium monosulfate aluminate hydrate, abbreviated to monosulfate, tends to form at later ages in concrete mixes, typically after 1-3 days. Monosulfate resembles ettringite in being a hydrated compound containing calcium, aluminium and sulfur, but the proportions are different. Monosulfate can be expressed as: _ C4ASH12 or C3A.CaSO4.12H2O The ratio of C3A:CaSO4 in monosulfate is therefore 1:1, whereas in ettringite it is 1:3. In the early stages of cement hydration, most of the clinker sulfate and the gypsum is available for reaction, while much of the aluminate phase is contained within the cement particles where it is protected from reaction with water. Initially, therefore, the ratio of available sulfate to aluminate is high. Subsequently, as more cement hydrates and the supply of gypsum and clinker sulfate becomes exhausted, the ratio of available sulfate to aluminate decreases. Monosulfate then forms instead of ettringite, and ettringite that has already formed converts to monosulfate. Monosulfate typically forms as thin platelets a few microns across.
6.3
Hydration of cement: further considerations
The foregoing describes the hydration products of a Portland cement composed of clinker and interground gypsum only. For many years, such a cement was normal and was specified in national standards. More recently, the addition of small amounts of other materials to Portland cement has been permitted in national standards, particularly fine limestone interground with the clinker. Fine limestone affects cement hydration in two principal ways:
Limestone is often regarded as being inert in a cementitious system but this is incorrect; finely-ground limestone reacts with the cement and contributes to the formation of the hydration products (1).
Fine limestone also has a physical effect on cement hydration through the “fine filler effect” (Chapter 5.8).
In discussing cement hydration products, we've looked at ettringite and monosulfate phase; to get a more complete view of cement hydration products, we need to expand this a little.
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6.3.1
AFm and AFt phases
Ettringite is one of a group of minerals termed AFt phases. Monosulfate is one of another group termed AFm phases. In both of these groups, iron (the “F” in AFm/AFt) can partially replace aluminium (the “A”). The general composition of an AFt phase is: C3(A,F).3CX.nH2O where X represents a doubly-charged anion (eg: SO32-, CO22-). Singly-charged anions may be accommodated to a limited extent. “n” is variable; n=32 for ettringite.
The general composition of an AFm phase can be written as: C3(A,F).CX2.nH2O (again, mixing cement and ordinary notation) X represents an anion with a single negative charge (eg: Cl- or OH-) or ‘half’ an anion with a double negative charge (eg: SO32-, CO22-); n indicates that the amount of water is variable, eg: n=12 (usually) for monosulfate. For example, neglecting any iron substitution for aluminium: if X were chloride, the formula for that form of AFm would be: C3A.CaCl2.10H2O if X were sulfate, the formula for that form of AFm would be: C3A.CaSO4.12H2O The ‘m’ - or ‘mono’ - in AFm refers to the single unit of CX2 in the formula for AFm phases. The ‘t’ - or ‘tri’ - in AFt refers to the triple unit of 3CX in the formula for AFt phases. For example, the most significant AFt phase in cement hydration is ettringite, where X is sulfate. Neglecting any iron substitution for aluminium, the formula for ettringite can be written as: C3A.3CaSO4.32H2O There are other AFt phases, but ettringite is overwhelmingly the most important. Unless you are doing a PhD in cement hydrate composition, just note that there might be other AFt phases present in small amounts, but otherwise forget about them.
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Hydration of cement – chemical and physical properties of cementitious materials AFm phases are numerous; in the “good old days” when cement was just clinker and gypsum we could assume the main AFm phase was monosulfate. Where interground limestone is present in the cement, as it commonly now is, that assumption is no longer true. The most significant AFm phases in the context of cement hydration are:
_ Monosulfoaluminate, or monosulfate: C3A.CaSO4.12H2O or C4ASH12 _ Monocarboaluminate, or monocarbonate: C3A.CaCO3.11H2O or C4ACH11 _ Hemicarboaluminate, or hemicarbonate: C4C0.5H12
Hydroxy-AFm C4AH13
Just to summarise where we have got to so far, the main products of cement hydration are:
Calcium silicate hydrate (C-S-H)
Calcium hydroxide (CH)
AFt phase
AFm phase(s)
Ettringite will be the main AFt phase, but which AFm phases are present in any particular hydrated cement will depend on the balance between available alumina and sulfate, and the availability of other ions, particularly carbonate. Temperature may also have an effect.
For a typical Portland cement with no interground limestone or fine limestone from aggregates, the main hydration products once the cement has mostly hydrated (after, say, a month) will be calcium silicate hydrate and calcium hydroxide. There will also be other minor phases, principally AFm and AFt phases. Ettringite will be the AFt phase and monosulfate the main AFm phase; how much of each there is will depend on the balance of available sulfate and alumina.
For a typical Portland cement containing fine limestone, the main hydration products once the cement has mostly hydrated will be calcium silicate hydrate and calcium hydroxide. There will also be other minor phases, principally AFm and AFt phases. Ettringite will be the AFt phase. AFm phases will be monosulfate and hemicarbonate, or hemicarbonate and monocarbonate, or monocarbonate and residual fine limestone; how much of each there is will depend on the balance of available sulfate, carbonate and alumina. For more information, (much more information!) see (2).
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Hydration of cement – chemical and physical properties of cementitious materials
6.3.2
Flash set and false set
Soon after mixing, if insufficient sulfate is available from the added gypsum and the clinker sulfates to slow the hydration of C3A, the resulting rapid hydration of the C3A produces AFm phase in the form of hydroxy-AFm. These crystals link up the particles (cement, aggregate, hydrates) in the mix and quickly cause the mix to become unworkable. This is termed a “flash set”. The rapid hydration of the C3A results in the evolution of much heat. Concrete strengths where flash setting has occurred may be lower than normal. If an excess of sulfate is present in the form of hemihydrate, the hemihydrate dissolves and then reprecipitates as gypsum. Again, this causes a rapid set due to the linking of the particles in the mix. However, unlike flash set, there is no rapid evolution of heat because C3A hydration is inhibited by the sulfate. Another difference between false set and flash set is that a false set may be reversed if continued mixing can break up the gypsum crystals bonding the solids. The resulting gypsum fragments then slowly dissolve and the concrete should set normally and show fairly normal strength growth.
6.3.3
Hydration of cement: other hydration products
As described above, aluminate hydration in the absence of sufficient available sulfate in the cement pore fluid rapidly produces AFm phase in the form of hydroxy-AFm (C4AH13); this results in a flash set.
Ferrite hydration may result in the formation of iron hydroxide or a phase called hydrogarnet. Alternatively, the iron from ferrite hydration may enter AFm phase; there is not complete agreement in the literature on this.
Minerals similar to hydrotalcite (a hydrated and carbonated aluminium magnesium hydroxide) are likely to form in mixes containing slag, or, in very small quantities, in Portland cements of high MgO content.
6.3.4
Description of cement hydration
Suppose we have a cement that contains just Portland cement clinker interground with gypsum. We add water at time t=0: t=0: at the instant of mixing, the mix contains water and unhydrated particles of cement.
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Hydration of cement – chemical and physical properties of cementitious materials t=a few minutes-a few hours: when mixed with water, there is a high initial rate of reaction for a short period as aluminate and alite hydrate. Sulfate from the gypsum and clinker sulfate starts to dissolve and an amorphous gel containing alumina, lime, sulfate and silica forms. Small crystals of ettringite also form. A period of lower activity follows (the dormant period). The initial set typically occurs at about 2 hours and hydration restarts in earnest after 3-6 hours. t=6-12 hours: Alite hydration produces C-S-H and large crystals of calcium hydroxide (CH) up to 10µm or more across. Continued aluminate hydration produces larger rod-like ettringite crystals. C-S-H forms dense shells around cement grains and a delicate sheet structure bridging the water-filled gaps between the cement grains. By this time, the concrete has set but it has little strength. A small fluid-filled gap less than a micron across appears between the surface of the unhydrated cement grain and the surrounding dense shell of C-S-H. The cement reacts in two ways. Firstly, it reacts by a process of dissolution at the surface of the reacting cement grain, with dissolved material passing through the coating and reprecipitating in the large water-filled gaps between the cement grains. Secondly, cement hydrates in-situ; this means that hydration product (mainly C-S-H) forms within the volume occupied by the cement particle. Often, the crystal shapes of former and alite and belite crystals can be seen in microscopic examination of polished sections where the alite and belite have been replaced entirely by C-S-H. t=1-3 days: after a day or so, most of the available sulfate has been used, but aluminate phase continues to hydrate. This causes a gradual change in the ratio of available sulfate to aluminate, causing a change from ettringite formation to monosulfate formation. In ettringite, the aluminium: sulfur ratio is 2:3 whereas in monosulfate it is 2:1. With this decrease in sulfate availability, platey crystals of monosulfate start to form instead of ettringite. After a few days, the ettringite already present begins to decompose and is replaced by monosulfate. C-S-H and CH continue to form in the large water-filled gaps between the cement grains; the concrete gains strength as the C-S-H in the large water-filled gaps becomes denser. t=3 days onwards: the small gap between the anhydrous cement and the dense C-S-H coating starts to fill. Smaller cement grains have fully reacted. Once the small gap between the anhydrous cement grain and surrounding dense shell has filled, reaction between the solid cement grain and the solid shell of hydration product continues at a slower rate. Consequently, C-S-H and CH formation in the large water-filled gaps between the cement grains continues also at a slower rate and strength development continues at a slower rate reflecting the slower C-S-H formation. As alite becomes depleted, belite hydration begins to contribute more to strength gain. Belite hydration produces C-S-H but little CH. The ratio of Si/Ca in C-S-H is approximately 0.50-0.60. By about 4 weeks, hydration and further strength gain continues but at an ever-slowing rate. The hydration products at four weeks are
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Hydration of cement – chemical and physical properties of cementitious materials not quite the “final products” but are approaching them. After a year, any further change will be very slow; most of the available sulfate is present in monosulfate phase but a small excess of sulfate allows a little ettringite to persist.
6.4
Hydration of cement: paste microstructure and water/cement ratio
The water/cement ratio is a crucial parameter in cement hydration because it directly affects porosity and permeability. Consequently, the water/cement ratio influences concrete strength and resistance to water ingress, frost attack, leaching and other detrimental chemical and physical processes. The watercement ratio (often abbreviated to w/c) is defined as: (Mass of water)/(mass of cement) Most concrete has a w/c ratio between 0.3 and 0.7. With increasing water/cement ratios, the paste fraction of the concrete becomes increasingly porous. This has two direct consequences:
The paste becomes weaker due to the increased porosity.
The paste becomes more permeable because the pores are interconnected.
These effects are apparent in the sequence of four images that follow, showing cement pastes made using different water/cement ratios. However, in order to understand these, it is useful to be able to identify the different features visible in SEM images of polished sections of cement paste. Look first at Figure 6.3; this image shows one of the images in the sequence that follows in Figures 6.4-6.7, but with the different features indicated.
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Figure 6.3 Polished section of cement paste, age 2 years, made using ordinary Portland cement, w/c=0.40. Key: c - unhydrated cement; C-S-H - calcium silicate hydrate; CH – calcium hydroxide; p – pore; Circle ‘A’ contains C-S-H, CH and is assumed also to contain AFm and AFt phases. CH is just visible, AFm and AFt are not visible as they are too small.
The cement paste shown in Figure 6.3 contains the following features:
Partly-reacted cement particles (c); the remaining unhydrated material appears very bright in the SEM image. These particles have reacted to a considerable extent, as shown by the dense rims, composed mainly of CS-H, that surround them.
The outer edges of these rims indicate the approximate size of the original cement particle and the hydration products composing the rims have therefore formed within the volume occupied by the original cement particle. The hydrated material in these rims is therefore called “inner product” or “in-situ” hydration product. Although it looks solid and dense in the image, calcium silicate hydrate is a gel and contains huge numbers of tiny pores, called gel pores, approximately 1 nm across (1 nm=10-9 m). At this magnification, the gel pores are far too small to be distinguished.
Elsewhere in Figure 6.3, there are smaller areas of dense C-S-H, corresponding to smaller cement particles that have fully hydrated.
Also visible are areas of calcium hydroxide (CH) – these appear slightly brighter than the C-S-H but darker than the unhydrated cement.
“Circle A” contains CH (just visible) and also C-S-H. AFt and AFm phases (probably ettringite and monosulfate) are also likely to be present but are too small to be seen at this magnification. The hydration products in Circle A have formed in a region that was occupied by water when the cement and water were mixed; these hydration products formed from the precipitation of dissolved material are known as “outer product” or “undifferentiated hydration product”.
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Of particular importance in Figure 6.3 are the small black features; these are capillary pores. Capillary pores generally range in size from a few tens of microns down to less than 1 µm; these are the remnants of the volume originally occupied by water when the cement and water were first mixed, most of this original water-filled volume having been filled by cement hydration products. The pores became filled with epoxy resin when the polished section was prepared and so the black features are actually epoxy resin, but the extent of the resin shows the extent of the original pore system.
The capillary pores in Figure 6.3 are generally small, up to about 5 µm across, and not very numerous. They appear not to be connected, but it is important to remember when looking at polished sections that the third dimension is not shown and that pores may be connected out of the plane of the section. In this case, the pores clearly are interconnected, or they could not have become filled with epoxy resin. Now that you know what you are seeing in these images of cement paste, try looking at the four images in Figures 6.4-6.7. Changing the water/cement ratio has clearly altered the microstructure of the pastes. Pastes made with different water/cement ratios contain similar features to those shown in Figure 6.3, but the proportions are different. Firstly, look at the image in Figure 6.5; this is familiar as the same image as in Figure 6.3, but without all the labels getting in the way of parts of the picture. Now look at the image in Figure 6.4, the paste with the lowest water/cement ratio (0.33). As in Figure 6.3, the brightest features are unhydrated cement; these particles contain all four main clinker minerals. The capillary pores (black, as in Figure 6.3) are not numerous and are generally small ( ~1 in the mineral addition, there should be a net gain of C-S-H. In slag, the Si/Al atom ratio of the glassy fraction is typically between 2:1 and 3:1. In low-lime fly ash, the bulk Si/Al is typically approximately 1.5:1 but this includes alumina bound in unreactive crystals such as mullite; the Si/Al ratio in the reactive glass will be higher.
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7.1.4
Summary of benefits of using mineral additions
The advantages of using mineral additions may be summarised as:
Improvement in some aspects of concrete performance
Possible reduction in costs by the use of a byproduct or waste material.
Reduction in the volume of material sent to landfill.
Reduction in the rate of depletion of raw materials used to produce Portland cement.
Reduction in the energy required for production of cementitious materials per unit volume of concrete.
Reduction in the total carbon dioxide emissions per unit volume of concrete.
Most of the above are self-evident. Figures for the CO2 emissions for Portland cement production vary widely but are generally gradually declining as the cement producers make efforts to lower them; somewhere between 800-1000 kg tonne-1 probably covers the majority of production. Taking the United Kingdom as an example, figures for the “indicative embodied CO2” (ECO2) for the main cementitious materials in concrete have been agreed by the cement, slag and ash industries (2), based on data from 2007 for the calculations (Table 7.1). Table 7.1 Indicative embodied CO2 for cementitious constituents of concrete (source UK Quality Ash Association). Material Embodied CO2 (kg CO2 tonne-1) Portland cement (CEM I) 930 Ground granulated blastfurnace slag 52 Fly ash (from coal burning power generation) 4 Limestone 32
These illustrative figures demonstrate that significant savings in CO2 emissions can be achieved by the use of mineral additions. The actual savings will obviously depend on the level of replacement of Portland cement. Figures for factoryproduced cements (3) show ECO2 varying from 930 kg CO2 tonne-1 for Portland cement (CEM I) to 420-590 kg CO2 tonne-1 for pozzolanic cement made using siliceous fly ash (CEM IV/B-V cement). Cements containing other combinations of cementitious materials have intermediate ECO2 figures.
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7.1.5
Potential problems of using mineral additions
Although the final strength of mixes containing composite cements can be higher than equivalent Portland cement-only mixes, the rate of strength growth is usually lower, as the rate of reaction of the mineral addition is less than that of Portland cement. Possible exceptions are microsilica and metakaolin, both of which are very reactive. The slower strength gain is important for two reasons:
More care is needed in concrete curing, and the overall curing time will be longer.
Concrete is generally specified for 28-day strength. For an equivalent 28day strength to a concrete made with Portland cement only, if the cementitious material includes mineral additions it may be necessary to use a higher content of cementitious material.
A higher total cementitious content (ie: Portland cement plus slag, flyash etc.) would probably be needed for equivalent 28-day strength if fly ash were used, or if slag replaced more than 30%-50% of the Portland cement. However, many variables can have an effect, including the total cementitious content of the concretes being compared, aggregate type and any admixtures used. Much may also depend on the concrete curing temperature. Test data on cubes, cylinders and prisms obtained at ambient temperatures (usually 20 ºC) may not accurately reflect strengths in large pours of concrete where the heat of hydration has raised the temperature of the concrete. Using mineral additions may make the concrete more susceptible to carbonation (Chapter 9.3) because depletion of calcium hydroxide from the cement paste to produce additional C-S-H lowers the alkalinity of the pore fluid. To some extent, this susceptibility is offset by the lower permeability to CO2 of concretes containing mineral additions.
7.2 7.2.1
Blastfurnace slag Blastfurnace slag composition
Slag used in concrete is a by-product of iron smelting. Limestone is added as a flux during smelting to combine with silica and other impurities in the iron ore. If the resulting liquid is tapped off and cooled rapidly, it produces a latentlyhydraulic glass. If the liquid is cooled slowly, an unreactive crystalline material is produced, known as air-cooled slag, and is used as an aggregate.
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Composite Cements Slag used in concrete is largely composed of a calcium aluminosilicate glass, often with some crystalline phases, particularly gehlenite (C2AS) and merwinite (CMS2). The crystalline fraction is unreactive. When allowed to cool slowly the crystalline content of the slag increases, with a consequent decrease in hydraulicity. Rapid cooling is generally achieved by one of two methods:
Granulated blastfurnace slag is produced by pouring liquid slag into a high volume of water (Figure 7.1); the water may be running through a trough, or it may be a still pond. The resulting cooled slag generally contains 90%-98% glass.
Pelletised slag is produced by pouring the liquid slag onto a rotating drum cooled by water (Figure 7.2). The slag breaks into pellets and is propelled through the air in a chamber full of water spray. Pelletised slag used in concrete generally has a lower glass content than granulated slag, typically 70%-90%.
A small amount (~5%) of crystalline material in slag may be beneficial; increases in the crystalline content of the slag up to a total of about one-third may result in only a slight loss of concrete strength (4). Chemical moduli have been used to evaluate slags. For example, British Standard BS EN 15167-1: 2006 specifies that (CaO+MgO)/(SiO2)>=1 and that CaO+MgO+SiO2 must be at least 2/3 the total mass. An earlier version, BS6699: 1986 used the modulus (CaO+MgO+Al2O3)/(SiO2) >=1.
Figure 7.1 Production of granulated blastfurnace slag: left - a fast-moving stream of water; right - molten slag flows into the water stream and is quenched (courtesy Hanson Cement).
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Figure 7.2 Principle of production of pelletised slag (courtesy Hanson Cement).
Figure 7.3 Granulated blastfurnace slag, before grinding.
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Figure 7.4 SEM image of granulated blastfurnace slag (gbs) after grinding.
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Composite Cements Slag is an impure calcium aluminosilicate glass (Table 7.1). Table 7.1 Chemical composition of a typical blastfurnace slag used in concrete in the UK, (source: Hanson Cement, UK). SiO2 Al2O3 FeO CaO MgO K2O Na2O S2LOI Total 36.0 13.0 0.5 40.0 8.0 0.5 0.3 0.8 0.5 99.6 Balance is due to minor oxides, typically P2O5, MnO, TiO2, and possibly a trace of SO3.
Because of the need for precise control of the iron production process, the slag from a given source tends to have a consistent composition. As used in cement, the material is essentially a crushed glass and so the composition varies little between individual particles. Whether a slag is suitable for use as a cementitious material in a composite cement depends mainly on how reactive the material is. Reactivity will depend particularly on the composition of the glassy fraction, total glass content and the particle size to which the material is ground, but the relationships are not straightforward. For example, a fast-cooled slag with a high glass content will have the same bulk composition as a slag from the same liquid that was cooled more slowly. However, the composition of the glassy fraction will be different because the composition of the liquid will have changed during cooling as crystals formed from the melt. The glassy fractions of the two slags cooled at different rates may therefore have different reactivities.
7.2.2
Blastfurnace slag as a cementitious material
When used as a mineral addition, slag typically comprises 20%-70% of the total cementitious material, although higher levels are used for specific applications. The main technical benefits of using slag in a composite cement are:
Will give higher later strengths, but generally lower early strengths.
Reduced concrete permeability.
Improved durability (sulfate resistance; reduced likelihood of alkali-silica reaction; reduced chloride penetration).
Lower heat of hydration reduces risk of thermally-induced cracking.
Lighter colour of concrete.
In addition, the general benefits of using composite cements discussed in Chapter 7.1 apply to the use of slag, especially the following:
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Using slag reduces the rate of depletion of raw materials used to produce Portland cement.
Using slag reduces the energy required for production of cementitious materials per unit volume of concrete.
Using slag reduces the total carbon dioxide emissions per unit volume of concrete.
Let’s look at these in a little more detail: Strengths Partial replacement of Portland cement by slag generally results in lower early strengths, due to the slower rate of reaction of the slag. Later strengths should be higher in a suitably-designed mix. In mature concrete, more residual unhydrated slag is likely to persist than unhydrated Portland cement in an equivalent mix. After a year, perhaps 50%-75% may have reacted, but the actual figure will be dependent on factors including the w/s ratio, the intrinsic reactivity of the slag, the proportion of slag in the mix and the fineness of the slag. Higher later strengths, in a properly designed and executed mix, are mainly due to enhanced C-S-H formation. Permeability and durability Mature concrete made with cement containing slag is generally of lower permeability to water than concrete made with Portland cement only; this should give improved durability. Chloride and sulfate penetration should be lower, so chloride-induced steel corrosion or sulfate attack are less likely in a well-designed and implemented concrete mix. Alkali silica reaction (ASR) is also less likely, partly because partial replacement of PC by slag reduces the total available alkali but also because slag hydration lowers the calcium hydroxide content and the alkalinity of the cement pore fluid. Heat of hydration Compared with Portland cement, slag typically hydrates more slowly as it is a less-reactive material. The heat of hydration of a composite cement containing slag is therefore less than that of a comparable PC-only mix. Both the rate of heat evolution and the total heat evolved are lower in mixes where Portland cement is partially replaced with slag. Colour The grey colour of Portland cement is due to the ferrite phase. Slag does not normally contain ferrite or other strongly-coloured minerals so partial replacement of Portland cement with slag should result in a lighter colour of concrete. Although slag powder is almost colourless, fresh fracture surfaces of concrete containing slag are usually a blue-green colour. This is thought to be due to sulfide from the slag entering the hydration products, probably the AFm phase. After a few days, the blue-green colour fades as the sulfide oxidises.
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7.2.3
Hydration products in mixes containing slag
The hydration products in a composite cement containing Portland cement and slag are similar to those of Portland cement. There is likely to be more C-S-H and less CH, and the C-S-H will have a higher Si/Ca ratio. Because of the additional available alumina from slag hydration, most sulfate is likely to be contained in AFm phase and there may be little or no ettringite, unless fine limestone is also present (Chapters 6.3 and 12.2). Hydrotalcite (see Chapter 6.3): ([Mg0.75 Al
0.25
OH)2](CO3)0.125(H2O)0.5
is likely to be present in small quantities; it may also occur in small amounts in an ordinary Portland cement mix if the MgO content of the cement is high. Its effect on strength will probably be negligible, but it will slightly alter the balance between available sulfate and alumina and so could affect the relative proportions of AFm phases (eg; monosulfate: hydroxy-AFm: monocarbonate) and ettringite.
7.3
Low-lime fly ash
Fly ash (pulverised fuel ash, or pfa) is produced by power stations burning pulverised coal and is the non-combustible mineral residue separated from the flue gas. The composition of the ash will evidently depend on the minerals present in the coal.
7.3.1
Fly ash composition
Low-lime fly ash used in Europe typically contains high proportions of silica and alumina (Table 7.2) and contains a high proportion of glass. This broadly corresponds to a Class F ash in the USA. Table 7.2 Chemical composition of a low-lime fly ash used in concrete (UK-sourced ash). SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
SO3
Total
53.0
26.6
8.0
2.2
1.2
4.2
1.6
1.0
97.8
Balance is mainly carbon, with minor oxides including P2O5 and Mn2O3.
The glass content of fly ash is typically 70%-90%, with the bulk of the crystalline material consisting of quartz (SiO2) and mullite (an aluminium silicate, Al6Si2O13 or Al6Si3O15) The bulk of most fly ash is composed of spherical particles, ranging in size from under 1µm to about 50µm, although some particles may be up to 100µm in
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Composite Cements diameter. Most particles are solid (with some air bubbles) although some are hollow shells.
Figure 7.5 Coal fired power station, source Figure 7.6 SEM image of fly ash particles, of fly ash used in concrete. (Photo courtesy as used in concrete. UK Quality Ash Association.)
Most fly ash particles are mainly composed of an impure glassy aluminosilicate. However, because they are formed individually from mineral assemblages in coal, the composition of each particle is slightly different, or sometimes very different. While most of the aluminosilicate particles are of broadly similar composition, some particles are iron-rich or silica-rich and some ‘oddities’ are rich in other oxides eg: titanium dioxide. Some fly ash is calcareous; these ashes are typically higher in CaO, SO3 and MgO and contain less SiO2 and Al2O3 compared with siliceous fly ash. These ashes are described as Class C fly ash in the USA if the sum of the SiO2 and Al2O3 contents is less than 70%. The following focuses on siliceous fly ash.
7.3.2
Fly ash as a cementitious material
When used as a mineral addition, fly ash typically comprises up to 30% of the total cementitious material, sometimes more. The main technical benefits of using fly ash in a composite cement are:
Can give higher later strengths in large pours of concrete; ultimate strengths in small pours or mixes cured at ambient temperature will be similar to, or lower than, those of Portland cement mixes for the same total cementitious content.
Because fly ash particles are spherical, mixes containing fly ash require less water for the same workability compared with a mix containing Portland cement only; this reduces permeability.
The intrinsic paste permeability is reduced, probably due to an increase in
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Composite Cements the Si/Ca ratio of the C-S-H, (Chapter 7.1).
Improved durability (sulfate resistance; reduced likelihood of alkali-silica reaction; reduced chloride penetration).
Lower heat of hydration reduces risk of thermally-induced cracking.
Less efflorescence.
Less shrinkage and creep.
In addition, the general benefits of using composite cements outlined in Chapter 7.1.2 apply to the use of fly ash. The main properties determining whether an ash is suitable for use in concrete are its pozzolanic reactivity and the ability of the ash to reduce the concrete water demand. Too much carbon in the ash increases the water demand, affects the colour of the concrete and can affect the action of concrete admixtures. Carbon in ash can usually be accommodated in the mix design, provided the carbon content does not change; a variable carbon content is more difficult to deal with. Strengths Concrete in which fly ash has replaced some of the Portland cement would be expected to have lower early strengths due to the slower rate of reaction of the ash particles compared with Portland cement. However, the basis of any comparisons has to be considered carefully as one of the principle benefits of the use of fly ash is that the w/s ratio can be lowered for the same workability, due to the particle shape of the ash. Other adjustments to the mix design may also be made. For the same total cementitious contents cured at ambient temperature, a concrete containing Portland cement and fly ash would be expected to show both lower early strengths and lower 28-day strengths compared with a Control mix containing just Portland cement. At later ages, the strength growth curve for the Control mix would flatten off but the fly ash mix would continue to gain strength and may overtake after three months to a year. In large pours of concrete where the heat of hydration raises the concrete temperature appreciably, the mix containing fly ash will still be weaker than the Portland cement Control mix at early ages, but will tend to overtake the Control later; the larger the pour and higher the temperature, the sooner the fly ash concrete is likely to become the stronger of the two mixes. (Note that the curing temperatures will not be the same; for equivalent pours, the mix containing fly ash will have a lower heat of hydration, see below). Cement and fly ash finenesses will affect the rates of strength growth in these comparisons.
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Composite Cements Permeability and durability The benefits of using fly ash in a composite cement in relation to permeability and durability are broadly similar to those of using slag (Chapter 7.2.2). Heat Low-lime fly ash reacts more slowly than ordinary Portland cement and consequently it is widely used in large concrete pours, such as dams, where high fly ash contents are often used. The maximum temperature reached by the concrete is much reduced, lessening the chances of thermal fracturing and increasing the final concrete strength. Other benefits Benefits cited by the ash industry, in addition to the above, include a reduction in efflorescence and in shrinkage and creep. Efflorescence is caused by soluble salts reaching the surface (particularly calcium hydroxide); these salts are usually derived from the cement. If the cement content of a concrete is reduced by partial replacement of Portland cement with fly ash, it follows that less soluble material will be in the mix and so the likelihood of efflorescence occurring will be lessened. Additionally, if fly ash reduces the permeability of the paste, less soluble material will reach the surface, again reducing efflorescence. The magnitude of any drying shrinkage and creep in concrete is related to the w/c or w/s ratio of the mix. Since using fly ash as a mineral addition in cement enables the use of a mix of lower w/s ratio for the same workability, shrinkage and creep should be reduced.
7.3.3
Hydration products in mixes containing fly ash
Silica released from fly ash hydration forms C-S-H, the necessary lime being derived from CH from cement hydration and by increasing the Si/Ca ratio of the C-S-H. Alumina is also released; some enters the C-S-H structure, increasing the Al/Ca ratio of the C-S-H. The remaining alumina forms other compounds, including AFm phase, hydrogarnet (mainly C3AH6) and stratlingite (C2ASH8). Typically, between a quarter and a half of the ash may eventually react (this might take months or years), depending on factors including ash glass content, hydration temperature, percentage of cement replacement and the alkali content of the Portland cement. It is primarily the glassy phase that reacts; the crystalline inclusions, mainly quartz and mullite, are generally inert. Mullite and quartz crystals can occasionally be seen in SEM images of fracture surfaces, protruding from part-reacted ash particles where glass has been extracted by reaction with the cement pore fluid. While fly ash contributes to later strength gain through the pozzolanic reaction, the slow rate of reaction means that, initially, fly ash is almost chemically inert in the early stages of Portland cement hydration. Before any significant fly ash reaction has occurred, as with fine limestone particles, the ash particles provide www.whd.co.uk
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Composite Cements nucleation sites on which hydration products can form from solution, increasing the rate of Portland cement hydration at early ages. This partly offsets the slower rate of strength growth due to the lower fly ash reactivity compared with the Portland cement it has replaced. The pozzolanic reaction process by which fly ash contributes to concrete strength, is believed to be that sodium and potassium in the pore fluid reacts with silicate material extracted from the fly ash by the action of hydroxyl ions. An alkalisilicate gel is formed, which is unstable in the presence of calcium and is rapidly converted to C-S-H. The presence of sodium and potassium hydroxide in the cement pore fluid will increase the pore fluid alkalinity and accelerate fly ash reaction. However, lime (calcium hydroxide) does not contain sodium or potassium hydroxide although a lime/fly ash mix produces an hydraulic binder. This shows that sodium and potassium hydroxide are not essential to fly ash reaction; the alkalinity produced by the lime is sufficient, although the reaction rate will be slower.
Figure 7.7 Polished section of concrete made using Portland cement (70%) and fly ash (30%). Key: c-(and other similar bright features) are relicts of partly-hydrated cement particles; s-silica sand; arrowed features are examples of fly ash particles, but there are many more. Note that the fly ash particles are not all the same grey level; this is because each particle has a different composition, usually only slightly. Most are medium-grey and composed of aluminosilicates; a few are bright, iron-rich, particles. (NB: in backscattered electron images, grey level is approximately proportional to mean atomic number: Fe2O3, for example, will appear brighter than SiO2.).
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Figure 7.8 Polished section of concrete made using Portland cement (30%) and slag (70%). Key: circled features are relicts of partly-hydrated cement particles; s-sand particles, mainly silica and feldspar; arrowed, light grey particles are slag particles. A dark reaction rim can just be seen around some of the slag particles. In contrast with the fly ash particles in Figure 7.7, all the slag particles here are of the same grey level, as they are all of similar composition having been quenched from the same liquid.
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7.4
Microsilica (silica fume)
Microsilica is a by-product of silicon production. It is composed largely of structurally-amorphous silicon dioxide spheres, typically 0.1 µm across. It is a highly pozzolanic material, white, grey or almost black in colour and has a high pozzolanic activity, more so than fly ash. Because it has a high surface area, it has a high water demand and this limits the level of cement replacement by microsilica unless water-reducing admixtures are used. As it is composed largely of silica, the main chemical effect of microsilica is to increase the C-S-H content of the paste by reaction with CH from Portland cement hydration. Unlike fly ash or slag, where a significant proportion of the material remains unreacted, microsilica will usually react largely or fully. The increased proportion of C-S-H should reduce the overall porosity of the concrete and increase the strength; also, the increased Si/Ca ratio of the C-S-H, as discussed above (Chapter 7.1) may reduce the interconnectivity of the pores. The main benefits of using microsilica are:
An increase in strength; microsilica is often used in high-strength concrete.
The high surface area of microsilica reduces bleeding at the concrete surface.
A decrease in permeability to water (by a factor of 10-100 compared with a Portland cement-only mix) and hence improved durability and resistance to chloride or sulfate ingress.
A reduction in the likelihood of ASR due to the decreased pH as a result of consumption of CH to form C-S-H; the lower permeability to water should also be beneficial in this regard.
Microsilica is available in several forms: densified, undensified and as a slurry; each type is best suited for particular applications. It is important to disperse microsilica adequately, since undispersed clumps will behave essentially as larger particles of reactive silica, producing alkali-silica gel. For more information on microsilica in concrete, see Lea (5) or Taylor (6).
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Composite Cements
7.5
Limestone
Limestone is a mineral addition widely used in cement. It is neither pozzolanic nor latently hydraulic. Small additions of limestone (up to 5%) have been considered in Chapter 5.8. In the context of mineral additions, the proportions of limestone in the cement are much higher. For example, European Standard EN 197-1 specifies cements containing 6%-20% limestone (CEM II/A-LL)1 and 21%-35% limestone (CEM II/B-LL). Generically, such cements are referred to as Portland-limestone Cement (PLC). PLC has been used in Europe for many years and is now becoming more widely used elsewhere. To take an example of how PLC is used, in the UK it is widely used in both precast and ready-mixed concrete; in Europe as a whole, PLC represents about a quarter of the cement market. The main benefits of PLC are:
Cost
Shrinkage control
Reduction in bleeding
Reduced CO2 emissions
PLC may be produced by intergrinding limestone with clinker, or by blending limestone powder in the mixer. (These alternative methods are likely to produce different clinker and limestone particle sizes with possible effects on cement characteristics). Limestone interground with clinker grinds preferentially, as it is softer than clinker; PLC therefore is usually ground finer than Portland cement to ensure the clinker particles are sufficiently fine. Fine limestone in cement is often regarded just as a filler and indeed it does fill gaps between clinker particles. However, as already discussed (Chapter 6.3), limestone is not inert, as some of it takes part in the hydration process. The dilution effect of replacing some of the clinker with limestone can be offset by grinding the clinker more finely. For the same water/cement ratio, PLC concrete may have a lower 28-day strength than a concrete containing Portland cement only, and a higher cement content may be needed to produce similar strengths. For the same concrete strength, PLC has similar durability characteristics to Portland cement in most respects, although its resistance to sulfate attack is still being evaluated, particularly with regard to thaumasite formation. (Sulfate attack and thaumasite are discussed in Chapter 9). 1
Or CEM II/A-L; CEM II/B-L for lower purity limestone, see Chapter 10.
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Composite Cements
In PLC concrete, the rate of hydration of alite is increased by the presence of fine limestone, giving good early strengths. Hemicarbonate and monocarbonate are formed from the reaction of aluminate phase with some of the limestone; most of the sulfate in the cement is likely to be present as ettringite. Only a small proportion of the limestone will react, although the proportion will increase in concrete containing Portland cement with a higher aluminate content, or if additional alumina is available from slag or fly ash in the mix. Monocarbonate and hemicarbonate formation fill pore space within the concrete, reducing permeability and thereby increasing durability. They may also make some contribution to strength. The increased use of PLC is highly likely if the proportion of Portland cement in concrete is to be reduced for environmental reasons. Limestone is readily available, but other mineral additions (particularly slag and fly ash) are in limited supply. Also, it is undesirable to transport bulk materials further than necessary. The availability of fly ash and slag depends on coal-fired power generation or iron smelting; they are not available everywhere without being transported over long distances but limestone is readily available wherever cement is produced. Herfort describes the hydration and performance of PLCs (7).
References, Chapter 7 1. “Cement Chemistry,” H F W Taylor, Thomas Telford, 2nd ed. 1997, pp291293. 2. Source: http://www.ukqaa.org.uk/Datasheets_PDF/Datasheet_83_Feb_2009.pdf 3. Source: http://www.ukqaa.org.uk/Datasheets_PDF/Datasheet_8-4_Feb 2009.pdf 4. Demoulian et al, 7th International Congress on the Chemistry of Cement, Vol 2, 1980. 5. Lea, particularly Chapter 12; also other references in Lea – check Lea index 6. “Cement Chemistry,” H F W Taylor, Thomas Telford, 2nd ed. 1997, p284. 7. “Portland Limestone Cements”, D Herfort, The First International Conference on New Cements and their Effect on Concrete Performance" HBRC-Helwan University, Cairo, 16-18 December 2008. Currently in press in the HBRC journal. http://www.hbrc.edu.eg/journal/Default.aspx
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Composite Cements Additionally: Some excellent datasheets (downloadable pdf files) on the use of fly ash in concrete are available from the UK Quality Ash Association web site at: http://www.ukqaa.org.uk/Publications.html#TechnicalDatasheets Similarly, equally-excellent datasheets on the use of slag in concrete in addition to Reference 1 above are available from the Hanson UK web site at: http://www.heidelbergcement.com/uk/en/hanson/products/cements/ggbs_and_r elated_products/ggbs_.htm There is more on slag at the UK cementitious slag makers association web site: http://www.ukcsma.co.uk/index.html (Web sites have an irritating habit of changing, so if these pages change and the links do not work, look around the web site as the pdf files will probably still be available somewhere.)
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8 Cement variability
Here, we use ‘cement variability’ to mean the extent to which cement from the same source, and produced to the same specification, varies over time. In other words, if you buy a bag of cement produced from clinker made on Monday and then another bag from clinker made on Friday, will the cement in the two bags behave in exactly the same way when mixed with water? On a larger scale, if you receive a delivery of bulk cement to a silo today, will the cement in this delivery behave identically to that delivered last week or last month? The short answer is: “Yes, probably”. For most practical purposes, they should all behave in such a similar way that it won’t be easy to distinguish cement from the two bags or deliveries. However, you might just possibly notice differences in cement characteristics. These differences may be so slight as to make no practical difference but occasionally they may cause problems. The main differences that may be noticed typically relate to colour, setting times, early strengths, later strengths or how the cement responds to admixtures. If you are manufacturing pre-cast concrete products, early strengths are likely to be important as they will control how quickly you can de-mould the product and re-use the mould. If you are an engineer requiring a minimum 28-day strength, then later strengths will probably be your prime concern.
8.1
In defence of the cement producer
The cement producer may sometimes be forgiven for feeling unloved. He (or she) is blamed for the perceived high price of his product and, when anything goes wrong, everyone immediately blames the cement. Most cement producers normally go to great lengths to ensure a consistent product. However, achieving this is this is not always easy. A typical modern cement plant may produce 2000-3000, or more, tonnes of cement every day. The raw materials from the quarry will inevitably be of variable composition, fuels will vary in their ash content, ash composition and calorific value. These days, in efforts to reduce fossil fuel use, alternative fuels
Cement variability used to make cement may include waste paper, car tyres or solvents in addition to the normal gas, oil or coal. The availability and composition of each of these may vary on a daily basis, requiring constant adjustments to the raw meal composition and fuel mix. Given all this variability, it would be surprising if the behaviour of the product never fluctuated just a little. Despite all of the above problems, in most places modern cement is more consistent and of a higher quality than ever before. Improvements have been made possible through developments in the major components of the cement plant - the kiln and the raw materials and cement mills - but especially in computer-controlled operations and in on-line analysis. Prior to a few decades ago, analyses of everything - raw materials, blended raw materials, clinker and cement - would have been carried out by taking samples, dissolving them in suitable reagents, then quantifying compositions by titration or gravimetrically, followed by calculation using pencil and paper. This might all take several hours, or even up to 24 hours. Today, online sampling and analysis is carried out automatically in minutes using X-ray fluorescence and X-ray diffraction. If, for example, the raw feed composition starts to drift from the target values, the change should be noticed quickly and corrective action taken, again probably automatically. Such early recognition of deviations from target compositions, followed by rapid corrective action, should minimise variability in the composition of the cement. Tests carried out on the cement by the manufacturer will depend on the standard to which the cement is produced, but will typically include tests on composition, tests for expansion on hydration and tests on strength in standard mixes. These tests should ensure that the cement conforms to the appropriate standard. It is simply not possible for a cement producer to test his cement with all of the materials with which it may be used. These could include fly ash, slag, microsilica and any of hundreds of admixtures. Even a few basic permutations of all of these would produce thousands of mixes to be tested - not just once, but every day, or hour or whatever analysis interval was deemed appropriate. This would clearly be unrealistic and anyway would add greatly to the cost of the cement. The price of cement is often a key factor in making purchasing decisions. A major cement user will often contract with a particular supplier for a period of time (eg: a year) and a pound or dollar or two either way in the cost per tonne may make all the difference in deals worth millions. Adding an expensive battery of tests that will be largely unnecessary won’t appeal to either the cement producer or purchaser. The cement maker’s lot is not, then, always a happy one. He has to contend with variable raw materials and variable fuels. Increasingly, he also has to comply with environmental legislation that requires him to reduce CO2 emissions or pay heavy penalties; in turn this may involve the use of alternative fuels that may themselves have the potential to cause more variability in the product. He has to comply with other environmental laws limiting what can be emitted from his site. In more densely-populated areas he has the general public living
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Cement variability nearby, for whom the cement producer is invariably responsible for dust from any source falling onto their house or car. He has customers for whom price often appears to be a more important factor than the supply of a consistent product or good customer service - at least until something goes wrong. On top of all that, he has shareholders looking for a return on the tens or hundreds of millions that they invested to build the cement plant in the first place. If he fails to deliver, the shareholders will sell him to another cement producer who will demolish his kiln and turn his quarry into a shopping mall. It’s enough to make anyone wish they had listened to mum and dad and become a “...lawyer or a doctor or a civil engineer”.
8.2
Causes of cement variability
Let’s restore some objectivity. If it is unrealistic to expect cement from a given source never to vary, what degree of variation might be expected and what might be the cause(s)? Obviously, the purchaser can expect that all cement will conform to the standard to which it was produced. However, the standard is likely to permit a range of physical and compositional characteristics. Usually, a purchaser has the choice of several different sources of cement, all nominally similar and produced to the same standard, and he will probably have a preferred source. This preference may be based on price or on technical considerations. Perhaps one source has a particularly dark or light colour that suits the application better than other cements, or maybe it has a particularly good early strength, or it just works better in his process than other alternatives. Suppose you are a producer of concrete products who has been using cement from a particular source for some months without any problems; suddenly, you notice a change in the strength of the concrete, or that the cement has a different colour. What may have happened? Firstly, it might be a different cement, or cement made from a blend of the usual clinker with other imported clinker. This might happen, for example, when a cement kiln is shut down for periodic maintenance or because of a technical problem. The cement producer may bring clinker in from elsewhere to supplement the lost production. Usually, he will advise his larger customers in advance that this will happen, or he should do. If a change in cement characteristics coincides with a kiln shutdown, this may well be the explanation. With luck, normal cement characteristics will be restored when the kiln restarts. However, suppose you notice changes in behaviour over time in cement from a single source, even though no major alterations in the raw materials or clinker composition were apparently made over this period. Surely something must have changed? Well, yes it might have.
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Cement variability
Colour: changes in the colour of cement may be due to differences in the amount of ferrite in the clinker; since ferrite is dark grey or black, it gives clinker its characteristic grey or black colour. If the proportion of ferrite changes, the colour may also change. A change in the ferrite content may be due to a change in the overall oxide composition, but differences in burning conditions can also affect colour. Strength: changes in strength may be due to a number of causes; obviously, lower strengths tend to be the cause of more concern than increased strengths. A few of the most common causes of lower strengths include:
Cement fineness: the cement particles may be coarser. This could be due to a difference in the overall fineness of the cement, or to differences in particle size distribution. This might be due to an intentional change in cement fineness, or it might be due to a temporary mill problem. Note that this is primarily a physical difference, rather than chemical. It is clearly related to the performance or operation of the cement mill, although other factors may have been the trigger. One example might be that the raw feed to the kiln contains more coarse silica resulting in large clusters of belite, hard and resistant to being ground, getting into the cement. Another could be that the normal cement mill is under repair and the alternative mill produces a different particle size distribution.
Lower alite content: concrete strength is mainly due to calcium silicate hydrate formation from the hydration of alite and belite. Alite is more reactive than belite, so if less alite is present in the cement, early strengths are likely to be lower. If there is less alite, there is likely to be more belite (provided that the silica ratio of the clinker has not altered appreciably); later strengths may be restored to near-normal as belite hydration continues.
Different kiln conditions: for good early concrete strength it is not simply the total amount of alite present that is important, but the reactivity of the individual alite crystals. The reactivity of the alite (and belite) crystals is higher if the clinker is brought to burning temperature rapidly; the temperature maintained for as long as is necessary and the clinker cooled as rapidly as possible. The length and temperature of the flame in the kiln is of prime importance in this. Other factors are also relevant, for example, the size of the clinker nodules will affect the rate at which the clinker cools-the interior of coarser nodules will cool more slowly. Overburning or underburning the clinker can also cause strength loss, as can reducing conditions in the kiln.
Sulfates - fixed w/c ratio v fixed slump: sulfates affect setting properties by controlling C3A hydration. As discussed in Chapter 6.3, if there is insufficient sulfate in solution in the pore fluid, flash setting may occur, with the rapid formation of AFm phase. Conversely, if there is excess sulfate in solution (possibly caused by high mill temperatures), false setting may occur due to the growth of gypsum crystals. Flash or false setting are both extreme cases; it is more likely that a very small
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Cement variability amount of either may occur, insufficient to cause a set but enough to slightly stiffen the mix or alter the performance of an admixture. Gypsum is often considered to be the main form of sulfate in cement. In practice, as we've seen, cement is likely to contain more than one form of sulfate. Some will be clinker sulfate and some will be added at the cement mill as ‘gypsum’. Clinker sulfate may typically contain several different sulfates (eg: calcium langbeinite, arcanite, aphthitalite) and the added ‘gypsum’ may contain gypsum and natural anhydrite. The gypsum may be decomposed during milling to hemihydrate or anhydrite. The cement might then contain up to five or six different forms of sulfate, all with different solubilities. The objective is that sufficient sulfate should be in solution to control C3A hydration, but not enough to cause the precipitation of gypsum. This is a delicate balance. Waste calcium sulfate is increasingly used; usually, this is gypsum in the form of materials such as plasterboard and can sometimes have deleterious effects. Syngenite formation (CaSO4.K2SO4.H2O - calcium potassium sulfate hydrate) can occur in cements with a high K2O content. This may cause false set due to crystal growth. Since syngenite formation removes sulfate from the pore fluid, flash set may then also occur. Sulfate solubilities are affected by temperature. The different forms of calcium sulfate have lower water solubilities as temperatures increase; arcanite (potassium sulfate) behaves more conventionally, becoming more soluble at higher temperatures. Whether incipient flash or false set occurs may therefore depend on the temperature of the mix. Provided that test concrete (or mortar) mixes are prepared at a constant water-cement ratio, minor imbalances in the rate of supply or consumption of sulfate should not make much difference to strengths. Specifications for cement in national standards normally specify testing at particular water-cement ratios. However, the ready-mixed concrete industry, and others, generally prepares concrete to a particular slump, not to a particular water-cement ratio. Suppose in a concrete produced at a ready mix plant, the cement pore fluid becomes slightly super-saturated with respect to gypsum; a small amount of gypsum will precipitate and the mix will stiffen slightly. The operator’s response will probably be to add more water to restore the desired slump. The hardened concrete will then be weaker at all ages, due to the additional water. We now have the interesting situation in which the cement manufacturer correctly says that his tests show that his cement produces mortar or concrete of normal strengths. Conversely, the concrete producer equally correctly says that, using the same cement, his concrete strengths are lower than normal. The problem is that the testing procedures are not the same. This situation could arise, for example, where two cements have a similar oxide composition but one has been milled hotter than the other, decomposing the gypsum to hemihydrate. As hemihydrate is more soluble
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Cement variability than gypsum, a small amount of gypsum may form in an incipient false set, even though the mix is still workable. At the ready mix plant, the reduction in slump is noticed and more water is added to compensate. Anything that affects the rate at which the C3A requires sulfate to control setting, or the rate at which sulfate becomes available from all sources, may have a similar effect. This could be a change in the total cement sulfate content, a change in the proportion of clinker sulfate to total sulfate, a change in cement fineness, a change in the C3A content of the cement, a change in C3A reactivity or a change in the extent to which C3A and ferrite crystals are intergrown in the clinker.
Optimum gypsum: the gypsum content of the cement affects the rate of strength development and the volume stability of the concrete.
Given all the things that could go wrong, of which the above is really just an introduction, you might feel that maybe cement isn’t quite so reliable after all. However, the extent to which that impression is wrong is really a tribute to the evolution of the manufacturing process over many years. Of course, things can occasionally go badly wrong, but, fortunately, that is now unusual. Or, at least, it should be.
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9 Deleterious processes in concrete
9.1
Reactions between cement paste and aggregate
Reactions between hydrated cement and aggregate in concrete, or “alkaliaggregate reaction,” can be divided into two main types: alkali-silica reaction and alkali-carbonate reaction. In alkali-silica reaction, alkalis in the cement paste fraction of the concrete react with with certain types of reactive silica to form an expansive gel. In alkali-carbonate reaction, aggregate containing dolomite reacts with hydroxyl ions in the paste to form calcium carbonate plus magnesium hydroxide and carbonate ions. The products of the reaction occupy a greater volume than the initial reactants and so an expansive force is exerted on the surrounding concrete. We’ll consider these processes in more detail below.
9.1.1
Alkali-silica reaction
Background Alkali-silica reaction (ASR) can cause serious expansion and cracking in concrete, resulting in major structural problems and sometimes necessitating demolition. Although first recognised in the 1940s in the USA, it wasn’t until the 1970s that ASR really became the focus of attention of concrete science worldwide. It also came to the attention of the media, who promptly dubbed it “concrete cancer”. Extensive research was undertaken, which later led to better practise in the production of concrete to minimise the risk of ASR in the future. Of course, it wasn’t purely scientific interest that had led to research funding suddenly being available; rebuilding major structures damaged by ASR was becoming expensive. The reaction ASR is caused by a reaction between the hydroxyl ions in the alkaline cement
Deleterious processes in concrete pore solution in the concrete and reactive forms of silica in the aggregate. These reactive forms of silica are typically chert, quartzite, opal and strained quartz crystals. Other materials, such as glass can also produce ASR. Pyrex glass and calcined flint have been used as test reactive aggregates in experimental concrete mixes. Ordinary glass (as in household jars and bottles) is also reactive. Undispersed agglomerations of microsilica can also cause ASR. Silicate anions are detached from the reactive aggregate by hydroxyl ions in the cement pore fluid; sodium and potassium ions are the ions most readily-available to balance the silicate anions and an alkali-silicate gel is formed. This can take up (“imbibe”) water and is mobile. The gel is unstable in pore fluid containing dissolved calcium, and calcium silicate hydrate (C-S-H) is produced. This releases sodium and potassium from the gel into the pore fluid, increasing the pore fluid hydroxyl ion concentration, with the potential for continued reaction.
Figure 9.1 Alkali-silica reaction in multistorey car park, UK. (Photo courtesy The Concrete Society).
Figure 9.2 Alkali-silica reaction in highways structure (Photo courtesy The Concrete Society).
The alkali-silicate gel increases in volume by taking up water, and so exerts an expansive pressure. If the reactive particle is impermeable to the pore fluid, gel forms at the surface. Some types of silica (eg: chert and opal) are permeable to alkali in pore fluid and gel can form inside the aggregate, often near the centre of the particle. Where gel has formed inside an aggregate particle, the aggregate particle usually cracks, with the crack extending into the surrounding concrete (Figures 9.3-9.5).
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Figure 9.3 ASR in concrete viewed in thin-section; alkali-silica gel (arrowed) almost fills the crack extending from the chert particle at the right beyond the left of the image. Yellow material is resin used in specimen preparation.
Figure 9.4 SEM image of ASR in chert particle (centre); internal microcracking is widespread and some cracks extend into the surrounding concrete.
Figure 9.5 Detail of Figure 9.4 showing gel extruded from chert particle into a crack in the concrete. Ettringite is also present (lower right) in a gap between the cement paste and a sand particle.
In unrestrained concrete (that is, without any reinforcement), ASR typically causes the characteristic 'map cracking' associated with ASR, with a repeating pattern of three radiating cracks 120 degrees apart. Usually, the best method to confirm that ASR has occurred is to examine the concrete in either thin section using a petrographic microscope, or in polished section using a scanning electron microscope (SEM). Gel may be seen in cracks and in aggregate particles. The process of alkali-silica reaction is believed to be broadly similar to the pozzolanic reaction, as occurs normally in concrete containing fly ash, for example. However, there are important differences.
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Deleterious processes in concrete In the pozzolanic reaction where a pozzolan is used as a partial cement replacement, the particles are small. As there is much calcium available in young concrete, the alkali-silicate gel forms in a thin layer around the pozzolanic particle and quickly converts to C-S-H and no expansion results. In the case of alkali-silica reaction, the reaction usually occurs much later, possibly years after the concrete was placed. Large aggregate particles (large, that is, compared with cement-sized pozzolan) generate a significant volume of gel, which then takes up water and expands within the hardened, mature, concrete. Because the concrete is mature, calcium availability is limited, as most of the calcium is bound up in stable solid phases. The rate of supply of calcium is therefore insufficient to convert the gel quickly to C-S-H, especially if the gel has formed within an aggregate particle such as opal or chert, where calcium is scarce.
Figure 9.6 This is the same image as in Figure 9.5, with X-ray spectra superimposed showing how alkali-silica gel composition changes with time to become more like that of the surrounding calcium silicate hydrates. At A the gel spectrum shows large peaks due to silicon and potassium and only a very weak peak due to calcium. At B the calcium peak has become much stronger and the potassium peak much weaker. At C the potassium peak has disappeared entirely and the gel has approximately the same composition as the normal calcium silicate hydrate comprising the bulk of the cement paste. Clearly, the gel is older with increasing distance from the aggregate particle in which it originated - the 'oldest' gel has had more time in which to take up calcium from the surrounding paste, and has now become calcium silicate hydrate.
Expansion of the gel as water is taken up, is likely to result in damage to the surrounding concrete. Over time, the gel will slowly take up calcium and
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Deleterious processes in concrete eventually the composition of the gel may become similar to that of the calcium silicate hydrate in the cement paste (see Figure 9.6). By then, though, the concrete may already be severely damaged.
Conditions necessary for ASR and how to limit expansion Three conditions are necessary for ASR to occur in concrete:
A sufficiently high alkali content of the cement pore fluid.
A reactive aggregate, such as chert.
Water - needed for the gel to expand; expansion due to ASR will not occur if there is no available water in the concrete.
Given these three conditions, ways in which we can prevent expansion in concrete due to ASR may appear obvious; we avoid using reactive aggregates and we limit the available alkalis. However, restrictions that are too severe will in themselves cause other problems.
Restricting the alkali in the cement The cement is usually the main source of alkali in the pore fluid in concrete, derived from alkali sulfate and from alkalis in the main clinker minerals, mainly in belite and aluminate. So if we restrict the alkali content of the cement, won’t that limit the alkali in the pore fluid? Yes, it will, but this does not allow for different cement contents in concrete, so simply imposing a limit on cement alkalis is not a complete answer to the problem. Also, we need to remember that higher alkali cements tend to give better early concrete strengths; alite and belite hydrate faster as pore fluid alkalinity increases. Imposing limits on cement alkali content that were “too low” (however defined) may therefore affect cement performance. It would also increase the cost of cement production, as bypass dust and precipitator dust high in alkalis could not be returned to the kiln and would have to go to landfill. Additionally, some raw materials high in alkali could not be used and would have to be replaced by possibly more expensive alternatives. So, although restricting the cement alkali content makes an important contribution to controlling ASR in concrete where potentially reactive aggregate is to be used, it is not necessarily a complete answer to the problem. Alkali limits are usually expressed as “sodium equivalent”. This conveniently combines the potassium and sodium oxides, and is useful for other purposes as
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Deleterious processes in concrete well. “Sodium oxide equivalent” is defined as: Na2Oeq=(Na2O+0.658K2O)1. Upper limits for sodium oxide equivalent on low alkali cement are typically of the order of 0.60%-0.75%; check the standards or codes of practice applicable to your location.
Restricting the alkali in the concrete An alternative approach is to limit the alkali content of the concrete. This would then take into account that different concretes have different cement contents. Limiting the alkali content can be achieved by using lower alkali Portland cement, by using a lower cement content in the concrete or by partially replacing the Portland cement with slag, fly ash or microsilica. The use of slag or a pozzolan in the concrete mix as a partial cement replacement can reduce the likelihood of ASR occurring as these reduce the alkalinity of the pore fluid. Slag, fly ash, microsilica and metakaolin have all been found in various studies to reduce or prevent expansion due to ASR. However, this is a complex area because some of these cement replacement materials also contribute alkali when they react. For example, using some typical Figures from Taylor, fly ash typically contains 1.5% Na2O and 4.2% K2O (1) and slag contains approximately 0.4% Na2O and 0.7% K2O (2). Compared with the alkali levels in Portland cement, slag is therefore broadly similar and fly ash is considerably higher. However, both are less reactive than Portland cement and even in mature concrete some slag or fly ash will remain unreacted. Only the reacted fraction of the slag or fly ash will have released alkali into the pore fluid. As this proportion is not known precisely, and anyway will vary between different concrete mixes, it is difficult to calculate just how much alkali from the slag or fly ash has become available for reaction. National Rules will determine what substitution levels are effective and what allowance to make for slag or PFA alkalis.
Restricting the water Can we restrict water availability? In most cases, not really, unless the concrete is entirely under cover and will never get wet. While expansion due to ASR is unlikely in concrete that remains dry inside buildings, even building interiors can become saturated in flood conditions. In any case, most concrete is used in foundations, roads, bridges or other open structures where it will be exposed to water.
1
The constant 0.658 is derived from the ratios of atomic weights of sodium and potassium oxides: Na 22.99; K 39.10; O 15.99; therefore Na2O (22.99 x 2)+15.99=61.97; K2O (39.10 x 2)+15.99=94.19; 61.97/94.19 = 0.658. www.whd.co.uk
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Tests for aggregate reactivity If concrete does not contain reactive aggregate, it follows that there will be no expansion due to ASR. So, can we prevent expansion by not using reactive aggregate in concrete? Yes, we can, but we need to decide what we mean by “reactive” and we also need some method by which we can determine whether or not a particular aggregate is reactive. With some aggregates, expansion due to ASR increases broadly in proportion with the amount of reactive aggregate in the concrete. The majority of aggregates (eg: chert) show what is called a “pessimum” effect; if the proportion of reactive aggregate in test mixes is varied while other factors are kept constant, maximum concrete expansion occurs at a particular aggregate content. Higher or lower proportions of reactive aggregate will give a lower expansion. In some areas, chert represents the bulk of aggregate used in concrete, with no resulting expansion due to ASR because the proportion of chert in the concrete is well above the pessimum. If we excluded all potentially reactive aggregate, we would not be able to use chert, but such a restriction is unnecessary since it can clearly be used safely. We just need to avoid aggregate that contains chert near the pessimum proportion. There are broadly three types of test for alkali reactivity of aggregate:
Examination in thin-section using optical microscopy (ie: petrography)
Chemical tests (eg: immersion in an alkaline solution such as sodium hydroxide)
Expansion tests using mortar bars or concrete prisms
Variations on all three of these approaches have been described in different standards. Standard microscopy procedures include: ASTM C295-08 Standard Guide for Petrographic Examination of Aggregates for Concrete and British Standard BS 812: Part 104: 1994 Testing aggregates - method for qualitative and quantitative petrographic examination of aggregates. Chemical tests include ASTM C289 - 07 Standard Test Method for Potential AlkaliSilica Reactivity of Aggregates (Chemical Method). Expansion tests include ASTM C227-03 Standard Test Method for Potential Alkali Reactivity of Cement-Aggregate Combinations (Mortar-Bar Method) and British Standard BS 812: Part 123: 1999 Testing aggregates - method for determination of alkali-silica reactivity – concrete prism method”.
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Deleterious processes in concrete Petrographic examination of aggregate is widely used and is an effective diagnostic test, assuming of course that the microscopist is able to recognise potentially reactive forms of silica and also that the sample examined is representative of the aggregate as a whole. Chemical tests can also be effective in identifying potentially reactive aggregate but may not be a good predictor of expansion where the aggregate shows a pessimum effect. Conventional expansion tests with mortar bars or concrete prisms are the closest simulation to ‘real concrete’ but may require two years or more to show results; this may not always be convenient. Accelerated expansion tests can shorten this timescale but do not simulate real conditions as closely. In summary, all these tests for aggregate reactivity can be very useful – indeed essential - but all have potential problems and the perfect test for ASR susceptibility has yet to be developed.
So, is it practicable to limit expansion due to ASR? Yes, definitely. Despite the limitations we’ve looked at above, the incidence of expansion due to ASR has declined markedly over the last 10-20 years where these different approaches to limiting expansion have been applied. For example, in the UK, there have been no reported cases since the current rules were introduced in the 1980s. Generally, codes of practice and national standards define how these approaches are used in practice in different parts of the world. Usually, several strategies to limit expansion are applied at the same time. For example, aggregate producers will routinely have their aggregate examined petrographically to determine the rock types and their potential reactivity. These aggregates then go into concrete that contains cement that has limits on alkali content specifically to minimise the risk of ASR; the concrete may well also contain fly ash or slag or other mineral addition to limit the available alkali. Of course, these relatively recent ways of dealing with the problem only control the problem in concrete produced using these methods. Instances of ASR still occur in some older concrete because there were fewer controls over what was used to produce it. Even where good controls are apparently in place, there is always the potential for things to go badly wrong.
9.1.2
Alkali-carbonate reaction
Alkali-carbonate reaction (ACR) is an expansive reaction between the alkaline pore fluid in concrete and some aggregates in which dolomite is present. Many aggregates containing dolomite do not give rise to ACR. Dolomite is a mineral composed of calcium magnesium carbonate (CaMg(CO3)2). Dolomite is often present in limestone; while limestone is usually composed mainly of calcium carbonate (calcite, CaCO3), some limestones also contain dolomite and these are www.whd.co.uk
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Deleterious processes in concrete known as dolomitic limestones. Other aggregate types may also contain dolomite. The reaction is also known as de-dolomitization and occurs between dolomite in aggregate and hydroxyl ions in the pore fluid. This forms calcium carbonate and magnesium hydroxide, and releases carbonate into the pore fluid: CaMg(CO3)2 + 2OH- CaCO3 + Mg(OH)2 +CO32The dissolved carbonate can then react with calcium hydroxide to form calcium carbonate, releasing hydroxyl ions into the pore fluid, which can then react with more dolomite: CO3 + Ca(OH)2 CaCO3 + 2OHACR was first identified in Canada and has also been confirmed in China and the Middle East (and probably other places too) but it is rare overall in comparison with ASR. There is some difference in emphasis in the literature regarding expansion due to ACR, including whether the dolomite has to be argillaceous dolomite for the reaction to occur and also just how expansive the de-dolomitization process actually is. Also, some people have attributed the expansion to the swelling of the argillaceous component of the aggregate rather than to de-dolomitization. A further complication arises if the aggregate also contains alkali-reactive silica as this may produce alkali-silica reaction; both ACR and ASR may occur in the same aggregate.
Identification of ASR and ACR In unrestrained concrete (where no reinforcement is present), expansion from a point source characteristically causes the typical “map-cracking” often observed in association with ASR. However, if reinforcement is present, the crack pattern will be altered. In any case, the most reliable method of identifying ASR or ACR is microscopic examination of the concrete. Typically, cores are taken from the concrete for petrographic examination using either a petrographic microscope to examine thin sections of the concrete, or by SEM examination of polished sections of pieces of the concrete.
9.2
Sulfate attack in concrete and mortar
Concrete and mortar made with Portland cement can be damaged when exposed to solutions containing sulfate. Groundwater often contains sulfate, and so the foundations of buildings and other structures are frequently where damage due to sulfate attack is found. Most of what follows refers particularly to concrete; sulfate attack in mortar is discussed in Chapter 9.2.4.
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Deleterious processes in concrete The majority of instances of sulfate attack have two main effects on the cement paste:
Sulfate reacts with hydrated aluminate phases, particularly monosulfate phase, and any unreacted aluminate (C3A), to produce ettringite. Ettringite formation within the paste exerts an expansive force, causing cracking.
Calcium hydroxide (CH) in the paste supplies calcium to produce the ettringite; when CH becomes scarce, the required calcium for continued ettringite formation is supplied by the C-S-H, which becomes partly decalcified.
The need for additional calcium (and water) to form ettringite from monosulfate is shown by the following equation: C3A.CaSO4.12H2O + 2Ca2+ + 2SO42+ + 20H2O C3A.3CaSO4.32H2O (monosulfate) + (2 calcium ions) + (2 sulfate ions) + (20 water molecules) (ettringite)
To recap: the cement paste is weakened because calcium to make ettringite is provided by the CH and decalcified C-S-H. The sulfate is provided by the source of sulfate that is causing the sulfate attack, eg: ingressing groundwater containing dissolved sulfate. We therefore typically have two processes occurring at the same time. Cracking is generally due to the expansive pressure caused by the growth of ettringite within the cement paste. Weakening of the paste by decalcification of the calcium silicate hydrate (C-S-H) that gives concrete its strength, causes further damage. Eventually, the concrete disintegrates. Usually, sulfate attack involves a deterioration of the cement paste in the concrete with the aggregate taking no part in the reaction. However, occasionally the aggregate can be the cause of the problem if it contains soluble forms of sulfate (eg: gypsum). Sulfate attack causes expansion and cracking, and a general loss of concrete strength, with ettringite often widespread within the cement paste in affected areas. However, the quantity of ettringite that can form is limited by the available alumina from the cement. When this is exhausted, gypsum may form if there is a continued supply of sulfate; gypsum formation also requires additional calcium, obtained from CH or C-S-H decalcification, and the deterioration of the cement paste continues. Sulfate attack can be 'external' or 'internal'. External sulfate attack is due to penetration of sulfates in solution into the concrete from outside. Internal sulfate attack occurs when a source of soluble sulfate is incorporated into the concrete at the time of mixing, an example might be the presence of
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Deleterious processes in concrete gypsum particles in the aggregate.
9.2.1
External sulfate attack
This is the more common type and typically occurs where water containing dissolved sulfate penetrates the concrete. A fairly well-defined reaction front can often be seen in polished sections; ahead of the reaction front the concrete is normal, or near normal. Behind the reaction front, the composition and microstructure of the concrete will have changed. These changes may vary in type or severity but commonly include:
Extensive cracking
Expansion
Loss of bond between the cement paste and aggregate
Alteration of paste composition, with monosulfate phase converting to ettringite and, in later stages, gypsum formation.
The effects described so far are typical of attack by solutions of sodium sulfate or potassium sulfate. Solutions containing magnesium sulfate are generally more aggressive, for the same concentration. This is because the magnesium also takes part in the reactions, replacing calcium in the solid phases with the formation of brucite (magnesium hydroxide) and magnesium silicate hydrates. The displaced calcium precipitates mainly as gypsum. Sources of sulfate, other than groundwater already mentioned, which can cause sulfate attack include:
Seawater - typically contains about 0.27% sulfate as SO4.
Oxidation of sulfide minerals in clay adjacent to the concrete: this can produce sulfuric acid, which then reacts with the concrete.
Bacterial action in sewers: anaerobic bacteria produce sulfur dioxide, which dissolves in water and then oxidises to form sulfuric acid.
Pollution from industrial waste.
In masonry, sulfates are present in some bricks and can be gradually released over a long period of time, causing sulfate attack of the mortar, especially where sulfates are concentrated due to moisture movement. For this reason, the soluble salt content of bricks in many countries is limited by national standards.
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Figure 9.7 SEM image of sulfate attack in concrete; ettringite has replaced C-S-H within the marked areas.
9.2.2
Figure 9.8 SEM image of sulfate attack in concrete; here, ettringite (examples - e) has largely replaced C-S-H throughout this field of view.
Internal sulfate attack
This can occur when a source of sulfate in sufficient quantity is incorporated into the concrete when mixed. Examples include the use of sulfate-rich aggregate, excess of added gypsum in the cement, oxidised sulfide minerals in the aggregate, or contamination. Proper screening and testing procedures should generally avoid internal sulfate attack.
Delayed ettringite formation Delayed ettringite formation (DEF) is a special case of internal sulfate attack. A key point in understanding DEF is that ettringite is destroyed by heating above about 70 °C. DEF occurs if ettringite that formed normally during hydration is decomposed through heating, but then subsequently re-forms in the hardened concrete. Delayed ettringite formation has been a significant problem in many countries. It typically occurs in concrete that was cured at elevated temperatures, for example, where steam curing has been used - it was originally identified in steam-cured concrete railway sleepers (railroad ties). It can also occur in large concrete pours where the heat of hydration has resulted in high temperatures within the concrete. DEF may also occur in older concrete subjected to heating (eg: due to fire). DEF causes expansion of the concrete due to expansive ettringite formation within the paste and it can cause serious damage to concrete structures. DEF
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Deleterious processes in concrete normally arises from the sulfate in the cement and may occur some time – months or years – after the concrete was placed. In normal concrete where no sulfate ingress has occurred, the total amount of ettringite that forms as the result of DEF is evidently limited by the sulfate contributed by the cement initially. It follows that the quantity of ettringite forming due to DEF is relatively small. Ettringite crystals form widely-dispersed throughout the paste. If expansion due to DEF causes cracking, this is particularly likely around aggregate particles; ettringite may subsequently form in the cracks but this does not necessarily mean the ettringite in the cracks was the cause of the initial cracking. DEF causes a characteristic form of damage to the concrete. While the paste expands because ettringite crystals are forming within it, the aggregate does not expand. Consequently, gaps form around these non-expanding 'islands' of aggregate within the paste (Figures 9.9-9.11). Often, these peripheral gaps become filled with ettringite (Figure 9.11).
Figure 9.9 Two-dimensional representation of DEF expansion; at A the blue cement paste is in contact with the red aggregate. At B, the blue paste has expanded uniformly, but the red aggregate has not. Consequently, a gap has formed between the aggregate and the paste.
Once gaps have formed at the periphery of aggregate particles, any further ettringite formation is likely to occur within these gaps, where it can form freely, rather than within the paste where the crystals would have to exert an expansive force in order to create space in which to crystallise.
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Figure 9.10 DEF expansion: SEM image showing a limestone particle with a gap between the limestone and the cement paste (arrowed).
Figure 9.11 DEF expansion: SEM image showing ettringite (e) filling a gap between a limestone aggregate particle (L) and the cement paste. Small silica sand grains (s) remain bonded to the cement paste and do not show peripheral gaps.
The images in Figures 9.10 and 9.11 are characteristic of damage to concrete due to DEF. Where peripheral gaps form around aggregate particles, the aggregate is no longer contributing to concrete strength, since it is effectively detached from the cement paste. Often, these gaps become filled with ettringite; the ettringite does not form a strong bond between the paste and aggregate and so the concrete is still weakened. Conditions necessary for DEF to occur are:
High temperature (>70 C approx.), usually during curing but not necessarily.
Water - intermittent or permanent saturation after curing. An ettringite molecule contains 32 molecules of water so ettringite formation evidently requires wet conditions.
The effect of cement composition on DEF is not well understood. Some factors correlate strongly but the causes are not clear. In laboratory tests, DEF expansion has been shown to correlate positively with cement-related factors, including:
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high sulfate
high alkali
high MgO
cement fineness
high C3A
high C3S
A pessimum cement sulfate content of 4%, at which maximum expansion occurs, has been suggested (3); this sulfate level is higher than permitted in some national standard specifications for cement. Composite cements tend to show good resistance to DEF. In laboratory tests, limestone coarse aggregate has been found to reduce expansion, although, as shown in Figures 9.8-9.9, the use of limestone aggregate does not confer immunity. In general, careful control of curing temperature has proved to be a reliable precaution against DEF. Expansion due to DEF and ASR have been linked; in one study, initial expansion due to ASR was enhanced by subsequent expansion due to DEF (4). From the above, you will probably have guessed that delayed ettringite formation is not fully understood.
9.2.3
Thaumasite form of sulfate attack (TSA)
The thaumasite form of sulfate attack (often abbreviated to TSA) requires sources of sulfate and carbonate. Thaumasite occurs as a natural mineral as an alteration product of limestone, although it is rare. Thaumasite can form in both concrete and mortar. The normal cement hydration products, mainly calcium silicate hydrate and calcium hydroxide, are decomposed as a result of both sulfate attack and of carbonation. Thaumasite itself is a weak and friable mineral, so when it replaces the calcium silicate hydrate, the concrete is severely weakened. Thaumasite has the chemical formula: [Ca3Si(OH)6.12H2O] (SO4)(CO3) or CaSiO3.CaCO3.CaSO4.15H2O
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It tends to form at low temperatures, typically about 4 ºC - 10 ºC. As it forms, the concrete or mortar converts to a friable material, often described as a “mush”. Concrete severely affected by thaumasite formation can easily be broken with the fingers and the coarse aggregate lifted out. A source of additional water is also required for thaumasite formation. Damp cementitious render over brickwork, especially where the render is cracked, and concrete and masonry in cool, damp cellars are typical examples of where thaumasite may occur. Normal sulfate attack usually results in the formation of ettringite. This uses aluminium provided by the cement and clearly this is limited in quantity in normal concrete. However, thaumasite formation does not involve aluminium; given an adequate supply of sulfate and carbonate, thaumasite can continue to form until the calcium silicate hydrate is completely decomposed. Consequently, while the use of sulfate-resisting Portland cement provides some defence against normal sulfate attack, it does not give any particular protection against thaumasite formation. Sulfate can be supplied from a range of sources; groundwater or bricks are common examples, or oxidation of sulfide aggregate. Carbonate can be supplied from atmospheric CO2 or from limestone present in the concrete or mortar. Serious damage in concrete due to thaumasite formation is not common, even in cool, damp climates, but it can occur.
Figure 9.12 Polished section showing degraded concrete containing white crystalline thaumasite formed around coarse limestone aggregate (large dark particles) and in cracks. Examples of thaumasite are arrowed. This polished section was 40mm in diameter but thaumasite formation is so extensive that it requires little magnification to be clearly visible.
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9.2.4
Sulfate attack in mortar
Sulfate attack in mortar can be due to similar causes as sulfate attack in concrete. Sources of sulfate are typically the cement in the mortar, sulfate in bricks or stone and sulfate in groundwater, or a combination of these sources. Damage in mortar, or render, can be due to expansive ettringite formation, causing loss of bonding between cement paste and sand, and between the cement paste and the brick or stonework. The mortar becomes friable, with cracking and scaling readily visible. Often, however, ettringite is present in voids in masonry, where it causes little or no damage; air-entrained mortar contains numerous voids in which ettringite can form without exerting an expansive force. Efflorescence (Chapter 9.7) may be associated with sulfate attack, with white crystalline growth on the surface of the damaged masonry or render. Damage to mortar, and masonry generally, can be due to the formation of crystals of sodium, potassium or magnesium sulfate within the mortar, brick, stone or render; these can exert an expansive force on crystallisation and damage may be severe. Magnesium sulfate can be especially damaging. Damage due to thaumasite formation in masonry can occur, especially in cold, damp conditions where there is also a source of carbonate. The carbonate may be present in cement, stone or groundwater. TSA in masonry typically occurs in cold, wet conditions such as cellars in cool climates, or foundations. DEF would not normally be expected to occur in mortar unless the mortar had been heated by an external source; the heat of hydration of the cement would normally be dissipated by the bricks or stone.
9.2.5
Identification of sulfate attack
Severe damage to concrete and mortar can be readily identified from visual inspection. A range of different analytical techniques can be used to confirm that sulfate attack has occurred. A calculation of the cement content by chemical analysis, together with a determination of the sulfate content, can indicate that it may have occurred if the sulfate content is appreciably higher than would be expected from sulfate normally present in the cement. This is perhaps the simplest test, although it is not conclusive - for example it would not normally indicate that DEF had occurred, since sulfate will probably not be present in excess. X-ray diffraction may indicate the presence of gypsum and ettringite, or thaumasite; if present in appreciable amounts, sulfate attack is likely to have occurred.
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Generally, the best technique for identifying sulfate attack is microscopic examination of concrete cores. This could be a petrographic examination of the concrete in thin-sections, or by scanning electron microscopy (SEM) and X-ray microanalysis using polished sections.
9.3
Carbonation
Although included in this chapter on deleterious processes, not all of the effects of carbonation on concrete are bad. It increases concrete density and so reduces permeability. Carbonation is associated with the corrosion of steel reinforcement and with shrinkage but it also increases both the compressive and tensile strength of concrete. Carbonation occurs when carbon dioxide dissolves in the pore fluid of the concrete to produce carbonate ions. These react with calcium to produce calcium carbonate, usually in the form of calcite, although aragonite can form in hot conditions. Because the process involves dissolved carbon dioxide, a film of pore fluid on the hydration products in the concrete is necessary for carbonation to occur. As carbonation proceeds, calcium is supplied by the calcium hydroxide in the cement paste, and also by the calcium silicate hydrate (C-S-H). Calcium hydroxide is depleted and the ratio of calcium to silicon in the C-S-H decreases. In fully carbonated cement paste, most of the calcium is present as calcium carbonate. The C-S-H has lost its calcium and is now an impure silica gel, also containing aluminium from other cement hydration products and sodium and potassium from the pore fluid (Figure 9.13). Saturated concrete carbonates slowly, because the capillary pores in the concrete are full of water and this inhibits further ingress of water or gas. In permanently dry conditions, carbonation proceeds slowly or not at all, as there is not enough water in contact with the hydration products. Normal wetting and drying of concrete produces conditions conducive to carbonation, especially by rainwater containing dissolved carbon dioxide, but carbonation proceeds most rapidly under humid but non-saturated conditions, with a relative humidity of 50%-70%. The surface of fresh concrete carbonates very quickly, within a few hours or days, but the depth to which the concrete is carbonated is, at first, very limited perhaps only a few microns. Over time, the ‘carbonation front’ gradually moves further into the concrete. After a year, the distance from the surface that the carbonation front has reached may be 1 mm to perhaps 5 mm, depending on the permeability of the concrete. Dense, impermeable, concrete made with a low water/cement ratio will carbonate much more slowly than porous concrete made using a high water/cement ratio. Carbonate ions have to travel a greater distance from the surface to the carbonation front as the carbonation depth increases. Consequently, the rate of progress into the concrete of the carbonation front gradually slows.
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Figure 9.13 SEM image of polished section of concrete roof tile, approximately one year old. The carbonation depth can be seen from the grey-level change of the cement paste at a depth of about 50µm from the surface (arrowed).
The depth of the carbonated zone in concrete - often abbreviated to the ‘carbonation depth’ - can be measured from the image above, but making a polished section and using a microscope is a complicated process just to make a rapid test. A much easier way is to use an indicator sensitive to a suitable pH value. Phenolphthalein indicator is commonly used to measure carbonation depth and is available from chemical suppliers. Phenolphthalein is a white or pale yellow crystalline material. For use as an indicator it is dissolved in a suitable solvent such as isopropyl alcohol (isopropanol) in a 1% solution. The indicator solution is a clear liquid but turns purple above a pH of about 8.5. If you apply phenolphthalein indicator to concrete and it quickly turns purple, the concrete pore fluid is strongly alkaline. In Figure 9.14, the carbonation depth shown by this rapid test is clearly about 5mm-7mm. The only tools needed are the indicator liquid and a hammer and perhaps a cold chisel to fracture the concrete. The phenolphthalein indicator solution is applied to a fresh fracture surface of concrete. If the indicator turns purple, the pH of the concrete pore fluid is above 8.5. Where the solution remains colourless, the pH of the concrete is below 8.5, suggesting carbonation. A fully-carbonated paste has a pH of about 8.4. In practice, a paste with a pH of 8.5 may only give a faintly discernible slightly pink colour. A strong, immediate, colour change to purple suggests a pH that is rather higher, perhaps pH 9 or 10.
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Figure 9.14 Fresh fracture surface through a concrete slab, with phenolphthalein indicator solution applied. The indicator has not changed colour near the top and bottom surfaces, suggesting that these near-surface regions are carbonated to a depth of at least 5 mm from the top surface and 7 mm from the lower surface. Where the indicator has turned purple - the centre of the slab - the pH of the concrete pore fluid remains high (above 8.5, probably nearer 10). Whether the cement paste here is completely uncarbonated is unclear, despite the strong purple indicator colour; a more complete assessment would require microscopic examination.
Warning: both phenolphthalein itself and isopropyl alcohol are harmful and, since it contains alcohol, the indicator solution is flammable. Ingestion, or contact with skin or eyes should be avoided, as should breathing the vapour. Possible effects on the human body include cancer and damage to kidneys and the central nervous system. For more information, do an internet search for something like: phenolphthalein indicator hazard. For example, see: http://fscimage.fishersci.com/msds/45376.htm.
Obviously, the phenolphthalein test is rather crude, but its simplicity makes it very useful. It is important to make sure the concrete surface being tested is a freshly-fractured surface; testing an old surface won’t tell you anything as it will have started to carbonate. Ideally, you would break the concrete (or take minicores or use drilling dust) then test it immediately.
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The carbonation depth is proportional to the square root of time d=k√t where d is the carbonation depth, t is time and k is a constant of proportionality. Occasionally, this can be quite useful. Suppose you find that a piece of concrete that you know to be nine years old has a carbonation depth of 12 mm; for that piece of concrete in that particular environment, k is therefore 4. You can then work out the likely carbonation depth at any other time; after three months it was only 2 mm and after 100 years it will be 40 mm. Don’t expect this relationship between time and carbonation depth to work perfectly. “All things being equal,” it should work well. However, in cement and concrete all things are never equal. Measurement of carbonation depth is imprecise because of distortions in the carbonation front due to factors such as aggregate, microcracks and local variations in concrete porosity. Also, the carbonation depth may be affected locally by wind direction, nearby sources of carbon dioxide (eg: road traffic) and probably a myriad of other factors. Also, while the use of phenolphthalein indicator is straightforward and a very useful test, things are actually a bit more complicated when examined in more detail. Normal concrete pore solution is saturated with calcium hydroxide and also contains sodium and potassium hydroxide; the pH is typically pH 13-14. Concrete with a pore solution of pH 10-12 is less alkaline than sound concrete, but would still produce a strong colour change with phenolphthalein indicator. It therefore follows that the indicator test is likely to underestimate the depth to which carbonation has occurred, as will be seen below. Optical microscopy or scanning electron microscopy often show indications of early-stage carbonation even where the phenolphthalein indicator changes colour to purple. Microscopy shows a gradual transition between concrete near the surface that is fully carbonated, and concrete at depth that is entirely normal and uncarbonated. As an example, Figure 9.15 shows a montage of SEM images of a polished section from the same concrete slab as the piece shown in Figure 9.14. The montage extends to a depth of about 7 mm from the surface. The concrete appears intensely carbonated to a depth of about 4 mm, where there is a distinct change in the appearance of the paste. Less than 4 mm from the surface, very little unhydrated cement remains and the paste has converted almost entirely to calcium carbonate and impure silica gel (see Inset A, 250 µm depth). Below about 4 mm depth, unhydrated cement particles remain and the paste microstructure has not obviously altered greatly from its normal, high-alkaline, state (Inset B, 7 mm depth). However, even here, the paste appears slightly altered, becoming a little denser and with a loss of calcium hydroxide (compare with the image in Figure 6.5).
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The concrete mid-way through the slab is shown in Figure 9.16. Even here, there are signs of slight carbonation. X-ray microanalysis of the C-S-H would show a slight increase in the Si/Ca ratio. This particular example of concrete paving is clearly porous and permeable. Microscopy shows indications of carbonation throughout the thickness of the slab, even though phenolphthalein indicator showed the central region to be uncarbonated (Figure 9.14). The pH at the slab centre is probably above pH10 (as shown by a rapid colour change on application of the indicator solution) but below pH13-14 as is normally present in uncarbonated concrete. Denser concrete would have much smaller carbonation depths and the interior would remain unaffected by carbonation. The concrete roof tile in Figure 9.13 is an extreme example in the other direction; after about one year, the dense paste has limited carbonation to only a few tens of microns. Nevertheless, despite its limitations, the indicator test is very useful as a means of making a quick and easy initial assessment and it is very widely used. Use it, but be aware of its limitations.
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A: Depth 250µm – intensely carbonated
Highly carbonated
~4mm depth
Less carbonated
B: Depth 7mm: less altered by carbonation than near the surface but some alteration has still occurred. See also Figure 9.16. Figure 9.15 Polished section of concrete from the same slab as in Figure 9.14.
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Figure 9.16 View of slab at a depth of about 15mm – approximately at the centre, and about twice the depth shown in Figure 9.15. Even here, there are signs of slight carbonation, with little CH visible and a densification of the microstructure.
9.4
Steel corrosion
The corrosion of steel reinforcement is probably the single most common form of concrete deterioration. Its importance is demonstrated by the mountain of literature on the subject and also by the many enterprises specialising in sophisticated methods of repairing concrete damaged through steel corrosion. The following is the briefest of summaries describing the problem. Steel that has sufficient cover of dense, impermeable, concrete (the “cover zone”) should last almost indefinitely, as the alkaline conditions in the concrete protect it from corrosion. However, the importance of quality control to ensure adequate cover cannot be overemphasised; porous and permeable concrete, made with a high water/cement ratio and containing steel near the surface, is particularly vulnerable to damage. Normally, a thin layer of iron oxide called the ‘passivation layer’ around the steel prevents it from oxidising further. Provided the pH of the pore fluid remains high (above approximately 11.5), the steel should remain in good condition. However, if the alkalinity of the surrounding concrete pore fluid falls, as occurs when concrete carbonates, the passivation layer is lost and the steel corrodes. The corrosion process is expansive due to the formation of rust. This causes pressure to be exerted on the surrounding concrete, resulting in cracking and spalling. Also, the bond between the steel and the surrounding concrete is weakened or broken and the steel is weakened through becoming thinner. In addition to damage to the concrete, the benefits of the presence of the steel reinforcement are therefore also lost.
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Deleterious processes in concrete The passivation layer is impaired in the presence of chloride, even if the pH remains high. This effect is particularly important in concrete used in highway structures where de-icing salt is used, and also in marine environments such as harbours or sea defences. The corrosion process is electrochemical, with electrical cells forming due to variations within the steel or concrete. These variations could be due to smallscale damage to the steel oxide layer, or to physical or chemical variability within the concrete. For corrosion to occur, the steel must be in contact with the pore solution, which acts as the electrolyte. Oxygen must be also present. This need for oxygen means that corrosion may not occur in carbonated concrete provided it is sufficiently dense to restrict the supply of oxygen. For corrosion to be induced by chloride, the chloride ions have first to penetrate the concrete. The permeability of the concrete will be important in controlling how quickly chloride can diffuse through the microstructure and reach the steel. An additional factor affecting the rate of corrosion is the extent to which the cement hydration products can bind chloride ions; AFm phase can bind chloride to form ‘Friedel's salt’ (C3A.CaCl2.10H2O). Friedel’s salt is frequently seen when using scanning electron microscopy and X-ray microanalysis to examine concrete in polished section that has been exposed to road de-icing salt.
9.5
Leaching
Leaching in concrete is the process by which soluble material is extracted from the concrete by flowing water. The water may flow over the concrete surface, or percolate through cracks. Water readily extracts sodium and potassium hydroxide from the concrete pore fluid, lowering the pH; it also progressively dissolves calcium hydroxide and decomposes calcium silicate hydrate (C-S-H), ettringite and AFm phase. Since it is C-S-H that gives concrete its strength, C-S-H decomposition gradually reduces the strength of the affected concrete until it disintegrates. Calcium is leached from the C-S-H, leaving an impure hydrous silica gel. As leaching progresses, the concrete becomes more porous and permeable (Figure 9.17) and may also be susceptible to damage by other mechanisms such as frost attack.
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Figure 9.17 SEM image of a polished section showing concrete that was once dense but is now highly porous due to the loss of hydration product due to leaching. Even where the paste remains, it is clearly porous and weak and extensively carbonated. Black areas are pores filled with epoxy resin when the polished section was prepared.
Dense concrete, made using a low water/cement ratio mix, is obviously likely to be less affected by leaching than porous concrete made using a high water/cement ratio mix. Other factors affecting how quickly leaching can damage concrete include whether the water is hard or soft, water temperature and what is already dissolved in the water. Concrete exposed to acidic water, such as from peaty moorland, is particularly at risk.
9.6
Frost damage (freeze-thaw action)
Freezing conditions can damage concrete, particularly where there is a repeating pattern of freezing followed by thawing. In laboratory tests, the number of freeze-thaw cycles a particular concrete can undergo before showing signs of damage is a measure of the concrete’s ability to withstand frost damage. The affected concrete surface typically has a rough appearance, with flakes of concrete gradually becoming detached. Porous and permeable concrete, made with a high water/cement ratio, is particularly susceptible to damage. Conversely, dense, impermeable concrete made with a low water/cement ratio is resistant to damage. Air entrainment of concrete can greatly improve the resistance of concrete and mortar to frost damage. A surfactant admixture is used to lower the surface tension of the pore fluid, resulting in the entrapped air in the concrete being distributed between myriads of tiny bubbles. These small voids are typically 10µm-200µm in diameter.
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Deleterious processes in concrete The voids prevent or limit damage by providing space into which the pore fluid can expand on freezing. To be effective, the voids need to be closely spaced. The “void spacing factor” is the average value of the maximum distance from any point in the paste to the nearest void. Typical values range from 100µm -200µm but the void spacing factor necessary to give good frost protection varies from one concrete to another.
Figure 9.18 Bridge parapet damaged by frost, Scotland. Note scaling of concrete surface and exposed aggregate.
9.7
Efflorescence on masonry
Efflorescence often occurs on masonry, especially in damp climates. It does not usually indicate that sulfate attack has occurred in the sense that physical damage has been caused. Most efflorescence is due to soluble salts such as sodium or potassium sulfate on the surface of new masonry. It normally disappears over the course of a year or so after construction and any damage is usually aesthetic rather than physical. The sulfate salts are typically derived mainly from the cement, although brick or stone may also contribute to efflorescence if they also contain soluble sulfate salts. The problem is more likely to occur where bricks have been left unprotected and are saturated before construction.
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Deleterious processes in concrete
Figure 9.19 Efflorescence on recently-built wall, south-east England. Photo courtesy Peter-William Eaves, MRICS.
Occasionally, efflorescence can be of a more persistent form, usually composed of gypsum or syngenite; once it has appeared on the surface of masonry it is probably permanent unless removed. It can be associated with the use of nonstandard air-entraining agents in the mortar, such as washing-up liquid (5), although other causes are possible.
References, Chapter 9 1. 2. 3. 4.
“Cement Chemistry,” H F W Taylor, Thomas Telford, 2nd ed. 1997 p273 “Cement Chemistry,” H F W Taylor, Thomas Telford, 2nd ed. 1997 p263 “Cement Chemistry,” H F W Taylor, Thomas Telford, 2nd ed. 1997 p378 S Diamond and S Ong, Cement Technology (Ceramic Transactions, Vol 40, p79). Eds. Gartner and Uchikawa, American Ceramic Society, 1994. 5. 'Investigation into causes of persistent efflorescence on masonry.' G K Bowler and N B Winter, Masonry International 11, 1, 1997.
Further reading, Chapter 9 Alkali-silica reaction: Concrete Society Report TR 30: “Alkali-Silica Reaction - Minimizing the risk of damage to concrete”, published by the Concrete Society in the United Kingdom. Price: £90.00. BRE Digest 330, 2004: Alkali-silica reaction in concrete: 2004 edition - four part set.
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Alkali carbonate reaction: “Cement Chemistry,” H F W Taylor, Thomas Telford, 2nd ed. 1997, p383. “Lea’s Chemistry of Cement and Concrete”, 4th edition, ed. Peter Hewlett, Elsevier, 1998, p964. Delayed ettringite formation: Lawrence C D “Laboratory Studies of Concrete Expansion Arising from Delayed Ettringite Formation,” (1993) published by the British Cement Association. Lawrence C D (1995) in Cement and Concrete Research, Vol 25, p903. Kelham S (1996) in Cement and Concrete Composites, Vol. 18, p171. Steel Corrosion “Cement Chemistry,” H F W Taylor, Thomas Telford, 2nd ed. 1997, p360. Or, for (much) more detail, see: “Corrosion of Steel in Concrete: Understanding, Investigation and Repair,” John P Broomfield, 2006, Published by Taylor & Francis. ISBN-13: 978-0415334044. Leaching “Cement Chemistry,” H F W Taylor, Thomas Telford, 2nd ed. 1997, p380. Air entrainment “Cement Chemistry,” H F W Taylor, Thomas Telford, 2nd ed. 1997 p328. ASTM C457-09 Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete. ASTM International, West Conshohocken, PA, USA. Available for download from: www.astm.org. Price $43.00.
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10 Standard Specifications for Portland Cement
Cement is one of the materials fundamental to the infrastructure of the modern world. In many countries, cement was one of the first materials for which a standard specification was devised. For example, one of the first specifications drawn up by the British Standards Institute after it was formed in 1901 was a standard specification for Portland cement (BS 12, 1904). Other early specifications were for electric power cables (BS 7), steel for use in railway lines or railroad tracks (BS 11), tramway and dock rails and fishplates (BS 2) and steel for steam boilers (BS 14). Such materials were fundamental to the brave new world of industrial society and so these were the priorities for quality control. Cement was fundamental then and it still is, of course. This e-book is not really about standards but about the principles of how cement works but we’ll just have a quick look at two important cement standards. While Portland cement is basically limestone and clay (or other suitable materials) heated at around 1450 C, standards for Portland cement differ in detail. The compositional requirements differ to some extent and the methods of testing cements for compliance with the standards are also different. The two principal worldwide standards for Portland cement are: EN 197-1 (Europe) and ASTM C 150 (USA). Additionally, although no longer used in the United Kingdom, the British Standard BS12: 1996 is still often specified in international cement trading. EN 197-1 covers not only Portland cement but also cements containing Portland cement and other materials. ASTM C150 applies to Portland cement only.
Standard Specifications for Portland Cement
10.1 ASTM C 150-07 The ASTM Portland cement standard current at the time of writing is: C 150-07 Standard Specification for Portland Cement Five types of Portland cement are described (Table 10.1), with three more airentrained sub-types. Table 10.1 Portland cement types described in ASTM C150-07.
Type I Type IA Type II Type IIA Type III Type IIIA Type IV Type V
For use when the special properties specified for any other types are not required. Air-entraining cement for the same uses as Type I, where airentrainment is desired. For general use, more especially when moderate sulfate resistance or moderate heat of hydration is desired. Air-entraining cement for the same uses as Type II, where airentrainment is desired. For use when high early strength is desired. Air-entraining cement for the same uses as Type III, where airentrainment is desired. For use when a low heat of hydration is desired For use when high sulfate resistance is desired.
Specified constituents of the cement are limited to:
Portland cement clinker.
Water or calcium sulfate or both: includes gypsum and other forms of calcium sulfate and associated bound water; also any other bound water such as may occur in hydration products if the cement should have hydrated slightly.
Limestone, up to 5% by mass.
Processing additions (small amounts of material to aid in the production or handling of the cement, eg: grinding aids, kiln dust).
Air-entraining addition (for air-entraining cements).
Compositional and physical requirements are given for each of the cement types, with references to appropriate ASTM test methods. Annex A1 of the standard shows how to calculate the potential cement phase composition. This calculation allows for the presence of calcium sulfate and interground limestone by using a modified form of the traditional Bogue
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Standard Specifications for Portland Cement calculation. The quantity of limestone in the cement is calculated on the basis of the measured carbon dioxide content of the cement and of the limestone used. A sample “Mill Test Report” is shown, indicating the chemical and physical requirements of the specification.
Some other cement-related ASTM standards Cements containing other constituents such as fly ash and slag are specified in: ASTM C595 / C595M-09 Standard Specification for Blended Hydraulic Cements and ASTM C1157 / C1157M-09 Standard Performance Specification for Hydraulic Cement The USA currently uses a lower proportion of composite (blended) cements compared with Europe or Asia. The performance and environmental benefits of using composite cements may result in their increased use over time. Methods of cement analysis are specified in: ASTM C114-09 Standard Test Methods for Chemical Analysis of Hydraulic Cement. Other relevant standards include: ASTM C109 / C109M - 08 Standard Test Method for Compressive Strength of Hydraulic Cement Mortars using 2 in. or 50 mm cube specimens ASTM C115 - 96a (2003) Standard Test Method for Fineness of Portland Cement by the Turbidimeter ASTM C183 - 08 Standard Practice for Sampling and the Amount of Testing of Hydraulic Cement ASTM C186 - 05 Standard Test Method for Heat of Hydration of Hydraulic Cement ASTM C191 - 08 Standard Test Methods for Time of Setting of Hydraulic Cement by Vicat Needle ASTM C266 - 08 Standard Test Method for Time of Setting of HydraulicCement Paste by Gillmore Needles
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Standard Specifications for Portland Cement
10.2 European Standard EN-197 The principal European standard for cements current at the time of writing is: EN 197-1:2000 Cement Composition, specifications and conformity criteria for common cements. This standard covers twenty-seven common cement types available in Europe under five general headings, including Portland cement. Cements are specified by both their constituents and by their strength class. Initially, this looks a bit complicated but all will become clear.
10.2.1
European Standard EN-197, composition
First, we need to look at the cement constituents; these dictate the basic cement type (Table 10.2). Table 10.2 Types of cement described in EN 197-1. Designation Description CEM I Portland cement, with up to 5% of minor additional constituents (macs) CEM II Portland-composite cements: Portland cement with up to 35% of other single constituents, including: Portland-fly ash cement (CEM II/A-V, CEM II/B-V) Portland-slag cement (CEM II/A-S, CEM II/B-S) Portland-limestone cement (CEM II/A-L (LL), CEM II/B-L (LL) Portland-silica fume cement (CEM II/A-D) CEM III Blastfurnace cements: Portland cement with more than 35% blastfurnace slag: (CEM III/A, CEM III/B, CEM III/C) CEM IV Pozzolanic cements Portland cement with more than 35% pozzolana (CEM IV/A, CEM IV/B) CEM V Composite cements: Portland cement with more than 35% blastfurnace slag and pozzolana or fly ash
To be designated as “CEM” cement of any type containing Portland cement and additions, the cement has to be factory-produced by either blending or intergrinding.
A CEM I cement contains the maximum amount of cement clinker (up to 100%, less gypsum).
A CEM I cement is therefore a basic Portland cement.
CEM II, CEM III, CEM IV and CEM V cements all contain Portland cement mixed with other materials.
There is also provision for low-heat cements.
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Standard Specifications for Portland Cement
Macs All types of cement may contain up to 5% “macs”. Macs are “minor additional constituents.” Typical examples of macs are fine limestone, kiln dust or fly ash. A cement containing a mac is considered for specification purposes to be the same as if the mac were not present and the cement has to meet the same performance criteria. For example, a CEM I cement is a Portland cement. A CEM I cement with 5% limestone as a mac is also a Portland cement and should have the same characteristics for setting time, strength, soundness and composition as if the mac were not present.
Codes Other than for CEM I, the following codes are necessary in order to interpret the cement types:
The suffix “A” means a low level of addition of the mineral addition and the suffix “B” means a high level of mineral addition (Table 10.3). For CEM III blastfurnace cements, suffixes A, B and C indicate increasing proportions of slag.
For the CEM II cement type, codes for the second main constituent are specified as follows: S – blastfurnace slag D – silica fume P – natural pozzolana Q – natural calcined pozzolana V – siliceous fly ash W – calcareous fly ash L or LL – limestone (LL means high-purity limestone) T – burnt shale M – two or more of the above
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Standard Specifications for Portland Cement
Table 10.3 EN 197-1 CEM II cements: these are some of the more common cement types. Designation CEM II/A-S CEM II/B-S CEM II/A-D CEM II/A-V CEM II/B-V CEM II/A-L (or -LL) CEM II/B-L (or – LL)
Name
Second main constituent
% Second main constituent 6-20
% Clinker
Portland-slag cement Portland-slag cement Portland-silica fume cement Portland flyash cement Portland flyash cement Portlandlimestone cement Portlandlimestone cement
Blastfurnace slag Blastfurnace slag Silica fume
21-35
65-79
6-10
90-94
Fly ash
6-20
80-94
Fly ash
21-35
65-79
Limestone
6-20
80-94
Limestone
21-35
65-79
80-94
As will be inferred from the list of codes for the second CEM II constituent, the list of cement types in Table 10.3 is not complete; the cements shown are the more common types but there are others specified in the standard, containing for example, calcareous fly ash or burnt shale.
10.2.2
European Standard EN-197, strength classes
Cements are also specified according to their strength class. Three standard strength classes are defined, based on the minimum 28-day mortar prism strength: 32,5 42,5 52,5 (NB: The use of the comma is part of the specification; units are MPa.) Additionally, early strength development is indicated by a suffix: L: Low early strength N: Normal early strength R: High early strength The minimum and maximum strengths of the three strength classes are shown below (Table 10.4).
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Standard Specifications for Portland Cement
Table 10.4 EN 197-1 cement strength classes for all CEM cements. Strength Minimum Minimum Minimum class 2-day strength 7-day 28-day strength strength 32,5 N 16 32,5 32,5 R 10 32,5 42,5 N 10 42,5 42,5 R 20 42,5 52,5 N 20 52,5 52,5 R 30 52,5
Maximum 28-day strength 52,5 52,5 62,5 62,5 -
For example, a bag of cement labelled: CEM I 52,5 R is a high early strength Portland cement producing at least 52.5 MPa at 28 days and at least 30 MPa at 2 days, based on mortar prism strength tests. Two more examples: CEM II/A-LL 42,5N: this is a Portland composite cement with a low proportion of the second constituent (the “A”), the second constituent being high-purity limestone (the “LL”). The proportions would be 80%-94% Portland cement clinker and 6%-20% high-purity limestone. The cement is classed as producing normal strength development (the “N”): at least 42.5 MPa at 28 days and at least 10 MPa at 2 days, based on mortar prism strength tests. (CEM II/B-V 32,5R): this is a Portland composite cement with a high proportion of the second constituent (the “B”), the second constituent being siliceous fly ash. The proportions would be 65%-79% clinker and 21%-35% fly ash. The cement is classed as producing rapid strength development, at least 10 MPa at 2 days, and at least 32.5 MPa at 28 days, based on mortar prism strength tests. There are additional low early strength classes for CEM III cements covered by EN 197-4:2004. Although the system may appear complex, the combination of the cement designation and the strength class conveys a lot of information very succinctly. It tells you what the cement contains, and in roughly what proportions; it also indicates the early and the 28-day strengths based on the standard mortar prism test.
Sulfate-resisting Portland cement You might have noticed a significant omission in CEM I – there is no sulfateresisting Portland cement. Progress on a European standard for sulfate-resisting Portland cement has been slow and, in the UK, for example, SRPC is still covered by residual British Standard BS 4027 although this is largely academic since SRPC is no longer manufactured in the UK.
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Standard Specifications for Portland Cement
Where sulfate resistance is required, in most concrete used in Europe now it is achieved by using mixes containing Portland cement and mineral additions such as slag or fly ash to achieve satisfactory sulfate resistance.
Some other cement-related European standards Cements with a very low heat of hydration are specified as “Special” and are included in EN 14216; these have a characteristic heat of hydration of not more than 220 J/g. BS EN 14216:2004 Cement: composition, specifications and conformity criteria for very low heat special cements Also: BS EN 197-4:2004 Composition, specifications and conformity criteria for low early strength blastfurnace cements The standard for the testing of cement is EN 196-series: methods of testing cement. There are ten of these but one is missing…: EN 196-1: 2005 Methods of testing cement. Determination of strength EN 196-2: 2005 Methods of testing cement. Chemical analysis of cement EN 196-3: 2005 Methods of testing cement. Determination of setting time and soundness There is no Number Four…!1 EN 196-5: 2005 Methods of testing cement. Pozzolanicity test for pozzolanic cements EN 196-6: 1992 Methods of testing cement. Determination of fineness EN 196-7: 2007 Methods of testing cement. Methods of taking and preparing samples of cement EN 196-8: 2003 Methods of testing cement. Heat of hydration. Solution method EN 196-9: 2003 Methods of testing cement. Heat of hydration. Semiadiabatic method EN 196-10: 2006 Methods of testing cement. Determination of the water soluble chromium (VI) content of cement
EN 196-4 is absent because of practical difficulties in distinguishing some constituents chemically in cement, such as microsilica. A draft text for Part 4 is in Technical Report CEN/TR 196-4: 2007 Methods of testing cement - quantitative determination of constituents. This gives a useful guide to techniques. 1
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Standard Specifications for Portland Cement
10.3 Other specified cement properties Both the ASTM C150 and EN 197-1 standards specify other physical and chemical properties, including: composition, loss on ignition, insoluble residue, setting times, limits on minor oxides (EN 197-1: SO3, clinker MgO, Cl); (C 150: SO3, MgO) and limits on expansion using prescribed tests.
10.4 Don’t ever mix the standards! The distinctions between different Portland cement types made by ASTM C150-7 have no parallel in EN 197-1, where all Portland cement is CEM I. The majority of CEM I cement will be, roughly speaking, similar to ASTM Type I or II cements, but that this is not to say that they are necessarily equivalent. While the compositional criteria for Portland cements under EN-197 and C 150 Portland cements may be broadly similar, they are not identical and the testing procedures are quite different. For example:
The upper limit for loss on ignition for CEM I cements under EN-197 is 5%; under ASTM C 150-07 it is 3.0% for all types except Type IV, where the limit is 2.5%.
The upper limit for Insoluble Residue under EN-197 for CEM I is 5%; under ASTM C 150-07 it is 0.75% for all cement types.
EN-197 uses mortar prisms to evaluate strengths while C 150 specifies a mortar cube test; the mortar compositions are different and the strength data from these different tests are not interchangeable.
If you were just laying a base for your garden shed, you might not worry too much about the details of the specification process. However, suppose you were an engineer on a major construction project who authorised the use of a cement other than that specified. Faced with newspaper headlines like: “Hundreds crushed as new hotel collapses-wrong cement used” you would not be starting from a strong position in trying to argue that the cement had not contributed to the problem, whether or not it actually had. So, don’t mix the standards.
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Standard Specifications for Portland Cement
Further reading, Chapter 10 The most useful further reading would clearly be to obtain copies of the standard(s) most relevant to your location. Note that all standards are copyright. C 150-07 ASTM Standard C 150-07, 2007, "Standard Specification for Portland Cement," ASTM International, West Conshohocken, PA. Available for download from: www.astm.org Price $37.00. EN 197-1 BS EN 197-1:2000 Cement. “Composition, specifications and conformity criteria for common cements.” Available for download from the British Standards Institute shop at: www.bsigroup.com/en/ Price £168.00 (UK Pounds). (No, that isn’t a typo – EN 197-1 will cost you £168, or very roughly $275 US at the exchange rate at the time of writing. However, you do get 52 pages, compared with 8 pages for ASTM C 150-07, and EN 197-1 covers a wide range of cements not just Portland cement.)
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11 Cement concepts
So far we have looked at many individual components of cement and concrete. Each one of these is necessary; now we can bring them all together to form a more complete picture. What follows is partly revision, but we will look at cement from a different perspective and apply what we covered in the previous chapters. In the first part of this chapter, we are going to look at the two principal ways that cements for different applications can be produced:
Modify the Portland cement clinker
Combine Portland cement with other materials, eg: slag, fly ash
In the second part of this chapter, we dig deeper. The aim is to develop an intuitive understanding, a “mind’s eye image” in order to visualise both clinker and hydrating systems.
11.1 Altering the properties of cement First, let’s look at two different approaches to producing cements that have different characteristics, then in the next section we can practise visualising them using our mind’s eye image. The requirements of cement differ depending how concrete (or mortar) made from it is to be used. For example, pre-cast products usually need a cement that gives high early strength so that the concrete can be de-moulded and the moulds re-used. Large pours of concrete need a cement with low heat of hydration to minimise thermal cracking. Where sulfate is present in groundwater, the concrete needs to be resistant to deterioration through sulfate attack. These different demands need different cements with which to make the concrete. The characteristics of Portland cement can be altered in two principal ways. The
Cement concepts first is to modify the cement clinker to produce the desired characteristics and the second is to mix Portland cement with other materials such as slag or fly ash. The first approach is exemplified in the American ASTM C 150 standard specification, which describes five different types of Portland cements, neglecting a further three air-entrained sub-types. ASTM C 150, particularly Table 1, is an elegant and concise summary of how to control the properties of Portland cement by varying its composition. The second approach can be seen in the European EN-197 standard specification, which describes only one Portland cement, CEM I, and a long list of other cements containing mixtures of Portland cement and other materials. Of course, blended cements are available in the USA, and some concrete in Europe is still made with CEM I only, although the proportion has been decreasing for some years. In the UK, CEM I is still available in bulk, but in bagged form it is scarce, except for white cement. Typically, a bag of cement at a builder’s merchant is CEM II/A-LL, a Portland-limestone cement, or CEM II/B-V, a Portland-fly ash cement.
11.1.1
Altering the properties of cement: modify the Portland cement
High early strength: Suppose you wanted to make a high early strength Portland cement. You would want to produce a clinker that had a high alite content and a low belite content, since alite is more reactive than belite. You might also want to increase the alkali content, as this will increase the alkalinity of the cement pore fluid and accelerate hydration; of course, this might lead to problems with alkali-silica reaction if reactive aggregates are present in the concrete, but here we are just talking about principles. You would also grind the cement finer as smaller particles will react more quickly. You might also produce a mineralised cement; alite in mineralised cement is more reactive than alite in normal Portland cement. Of the above approaches, the two most commonly used are finer grinding of the cement and increasing the alite content. In ASTM C150-7, there are no particular limits on C3S and C2S contents for Type III cement but the limits for fineness applicable to other cements are removed, and minimum one-day compressive strengths are specified together with higher minimum compressive strengths at 3 days than for other cements. In EN 197, the specification for high early strength CEM I is similarly empirical – the cement has to meet the required strengths at 2 days and 28 days for that strength class.
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Low heat of hydration: If you wanted to make a cement that had a low heat of hydration, you would want to limit the proportions of the most reactive clinker minerals, the alite and aluminate. The cement would then have a substantial belite content. To illustrate this, the ASTM C 150-7 standard specification prescribes limits for the clinker phases in cements of low or moderate heat of hydration:
For a Type II cement (moderate sulfate resistance or moderate heat of hydration) the specified maximum C3A content is 8% and the sum (C3S+4.75C3A) must be 100 or less. (Note the weighting applied to the C3A; this is because it is highly reactive, generating heat quickly, and because limiting the C3A content reduces the potential for sulfate attack).
For a Type IV cement (low heat of hydration), the maximum C3S content is limited to 35% and there should be at least 40% C2S. Also, the maximum permitted C3A content is 7%.
(NB: phase proportions here are calculated using the prescribed Bogue-type formula; these limits do not apply in C 150-07 if optional heat of hydration limits are to be applied).
Sulfate-resisting: To make a sulfate-resisting Portland cement, the main consideration would be to minimise the C3A content by making a clinker with a low Alumina Ratio; normally, this would be achieved by adding an iron-rich component to the raw feed. Instead of producing C3A, most of the aluminium will be then present in the ferrite phase, which will have a higher ratio of iron to aluminium compared with the ferrite in normal Portland cements. The objective in minimising the aluminate content is obviously to limit the potential for the formation of ettringite in sulfaterich environments. In ASTM C 150-7, the cement C3A content is limited to a maximum of 5%; Annex A1 provides a modified calculation for the ferrite phase content if the cement AR is less than 0.64, on the basis that such cements will not contain any C3A.
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11.1.2
Altering the properties of cement: combine Portland cement with other materials
We’ve just seen that one way of varying cement properties is to adjust the composition of the Portland cement clinker. Another way is to blend Portland cement with other cementitious materials, just as the Romans did when they blended lime mixed with volcanic glass or ground brick and tile to produce pozzolanic cements. In the terminology of the European cement standard EN-197, mixtures of Portland cement with other cementitous mineral additions are called “Portland composite cements” if they are produced at the cement plant by intergrinding or blending1. If mixed by blending at the concrete batching plant, they are “combination cements”. In American usage, mixtures of Portland cement with other cementitious materials are widely known as “blended cements” whether blended or interground. Here, we will use “composite cements” to mean blended cements with at least two cementitious components, regardless of how or where they were blended. Taking the three headings as before for variants of Portland cements:
High early strength: To make a high early strength cement, the simplest approach to achieve the highest early strengths would be not to blend anything with Portland cement; most materials you might add, such as slag or fly ash, react more slowly than Portland cement. However, microsilica and metakaolin are highly reactive and may improve early strengths. Sprayed concrete mixes may contain metakaolin or microsilica. Mixes of Portland cement and CAC can give early strength due to flash setting. Composite cements may be designated an “R” suffix under EN-197, indicating high early strength, but in this context, “high early strength” is a relative concept. Composite cements with the “R” suffix are almost always in the 32,5 or 42,5 strength classes and only have to achieve 10 MPa or 20 MPa respectively at 2 days. For the highest strength class, 52,5, a cement has to achieve 30 MPa at 2 days; 52,5 R cements are almost all CEM I (ie: pure Portland cements) ground to increased fineness and often with raised alite contents.
“Intergrinding” means that the additional material (eg: slag or fly ash) is added to the clinker in the cement mill. “Blending” means that the materials are mixed in some other way, usually using specialised dry-blending equipment. 1
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Low heat of hydration: Combining other cementitious materials with Portland cement is an ideal way to produce a cement with low heat of hydration. Composite cements containing Portland cement with additions such as fly ash or slag will have a lower heat of hydration because of the slower rate of reaction of slag and fly ash. In EN-197 terminology, an example of a low heat cement would be a cement containing a high proportion of blastfurnace slag: a cement labelled “CEM III B 32.5N-LH” would contain 66%-80% slag and 20%-34% clinker. The “LH” suffix indicates a low heat cement. In EN-197, “low heat” is defined as a cement with a characteristic heat of hydration of not more than 270 J/g.
Sulfate-resisting: Composite cements made with Portland cement and either fly ash or slag can give at least equivalent sulfate resistance characteristics to that obtained when using sulfate-resisting Portland cement. Suitably-designed mixes containing a Portland fly ash cement with, for example, 70% Portland cement and 30% fly ash should have good sulfate-resisting qualities. Alternatively, a Portland blastfurnace cement could be used with a high slag content, for example 30% Portland cement and 70% slag. To give an example of a sulfate-resisting composite cement in terms of the EN 197-1 standard is not possible, since EN 197-1 does not currently cover sulfateresisting cements. However, a cement labelled: “CEM II/B-V 32,5R” would have a fly ash content of 26% – 35% and such cements are sold as having equivalent sulfate resistance to that of sulfate-resisting Portland cement, if the cement has been shown to meet additional criteria relating specifically to sulfate resistance. These additional criteria are not specified in any European standard but in individual national standards and the cement is labelled with an additional suffix, eg: “+SR”. However, these bureaucratic niceties are not what we are focussing on here; the main point is that composite cements containing slag or fly ash can offer equivalent, or even better, performance for sulfate-resistance compared with sulfate-resisting Portland cement.
Other benefits revisited: The headings above don’t really do justice to the positive characteristics of composite cements, so, by way of revision, here are some more (see Chapter 7 for more detail): Depending on composition, in addition to reduced heat of hydration and better sulfate resistance, benefits of composite cements also include: possibly increased
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Cement concepts later strengths; lower permeability and better durability; reduced efflorescence; reduced shrinkage and creep; reduction in risk of ASR; reduced CO2 emission per unit volume of cement; reduced consumption of natural resources. In the interests of balance, we should also say that composite cements are more prone to carbonation and might want more care with curing.
11.2 A “mind’s eye image” of cement Call it a “mind’s eye image”, a “conceptual model”, or whatever you want; anything we do is a little easier if we have some idea of the processes involved and how they work. If you drive a car, it helps if you know what the gearbox and clutch do in order to drive it well. You don’t have to be able to strip down the engine, but you do need to have some “mind’s eye image” of each of the main parts of the car informing you what they do and, roughly, how they work in relation to the car controls. Have you tried to teach an aged relative how to use a computer? They might be an absolute whizz, of course, but quite often older people have difficulties with computers because they lack an understanding of them at a conceptual level. You can’t use Microsoft Word by trying to memorise a series of instructions; rote learning for computers, or other complex systems, just doesn’t work. Similarly, rote learning isn’t much good with cement. Most people interested in cement are interested because they make it or they use it or they are researching it; whatever your interest or level of understanding, it helps to have a conceptual model of cement appropriate to the situation. The model can be simple or complex, depending on what is most useful. A simple model might be: “Cement is grey powder you use to make concrete that gives you nasty burns if you get it on your skin.” This is a very useful and practical model that even the most distinguished professor researching cement should have uppermost in his mind when concreting his driveway. (If he does concrete his driveway, that is.) However, having come as far as this, we might perhaps try to come up with something a bit more illuminating. You should have the essential components of a better model now, from the previous chapters. All you need to do is to practise using your “mind’s eye image”. I’ve been using scanning electron microscopy and optical microscopy to examine cement and concrete since 1981, during which time I must have looked at thousands of samples of clinker, cement and concrete. My experience of cementitious materials is therefore primarily visual and so I’ve developed very simple “visual models” of clinker, cement and concrete, based on standard images. I use these models automatically, not just when examining material
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Cement concepts microscopically, but whenever I’m thinking about cement, or someone is describing to me a cement-related problem. I call them models - you could just as well call them techniques or methods. The point is that they are structured ways of conceptualising cementitious material, be they clinker or concrete or anything else. They help to understand how cement made from a particular clinker might behave, and they help to understand the hydration products that are likely to form from a Portland cement, or from a composite cement. By adopting a structured approach to thinking about cement, you can break down complex problems that appear hopelessly confused at first sight into manageable pieces you can make sense of more easily. So, while these models are simple, they are powerful, and therefore very useful. As with all models, they represent only an approximation to reality; they are each a “work in progress” that contains simplifying assumptions. If you wish, you can improve or otherwise alter them and make them as complicated as you like. There is one model for clinker and one for hydrating cement. Depending whether you are a producer of cement or a user, one may be of more general use for you than the other, but either way, have a go at using both.
Clinker composition: get a “mind’s eye” view of Portland cement clinker and how differences in composition affect cement properties. Cement hydration: get a “mind’s eye” view of the hydration process. For each of these, there are two basic components:
A concept of a reference material, either clinker or hydrated cement; this is your “archetype”.
A structured, logical, approach to working out how the reference material would change if you changed the inputs.
An archetype is “an original model on which something is patterned,” (definition from Nisus Thesaurus). “Inputs” here means clinker or cement composition or cement type, or time or temperature or some other factor. In other words, you use your “mind’s eye image” to view an archetype that you understand, and then you alter it by gradually adding in new conditions.
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By changing the inputs, you can adjust the archetype in your mind using “thought experiments” to allow for changes in compositions or other circumstances so that your model is flexible and applicable to a wide range of materials and situations. “Thought experiments” are experiments you don’t actually do, of course; instead you explore ideas. Some thought experiments, like “Schrodinger’s Cat”, can’t be done in practice at all; those described here are far less obscure and could be done in practice, but thought experiments do save a lot of time. If you can use your “mind’s eye image” to work through each of the thought experiments in the following two sections, you will have working conceptual models that you can apply to cement clinker and to hydrating systems. You might develop the models further over the years by refining them or adding new pieces, and that would be very valuable, but the basic models, as described here, should serve most people well for a long time. Some of these thought experiments are easier than others. If you find any of them difficult, don’t worry. Come back to them again later and they should all make sense eventually.
11.3 Cement clinker If you are involved in cement manufacturing, this section is important because it focuses on ideas that are fundamental to the chemistry of Portland cement production. If you don’t make cement but you use it or have some other interest in it, this section is still important to you because it illustrates some of the basic reasons why cement properties may change. If the cement you had delivered on Thursday seems to be behaving differently from the cement you received on Monday, the key parameters discussed here would be a good place to start looking for the cause. The starting point of these thought experiments is an image of the minerals in a Portland cement clinker as seen in polished section, see Figures 11.1 and 11.2 below. Get this image fixed in your mind; this is your clinker archetype, the starting point for your “mind’s eye view”. Focus on it, remember it, visualise it and get to know it so you can recognise the clinker minerals. Once it is firmly fixed in your mind, you can start to play with it and adjust it using thought experiments based on the clinker compositional parameters for Lime Saturation Factor, Silica Ratio and Alumina Ratio.
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Figure 11.1 Clinker archetype, SEM image of polished section. Key: a-alite; b-belite; c-aluminate (“C3A”); f-ferrite. Black areas are pores filled with epoxy resin.
Figure 11.2 Clinker archetype, same image as in Figure 11.1 but without the distraction of the labels.
“What’s special about this clinker?” you may ask. Well, the point about this clinker is that there is nothing special about it. It is just a normal, well-produced clinker – although perhaps a little slow-cooled. It is a typical clinker and we'll assume it could equally well be used to produce cement compliant with either ASTM C 150 Type I, or EN-197 CEM I. Look at the individual crystals of clinker minerals. Identify the alite crystals. Now look at the belite, the aluminate and the ferrite. Figures 11.1 and 11.2 both show our standard image; Figure 11.2 is the same image without the labels showing examples of the four main clinker minerals. You don’t want your standard clinker image to be cluttered with labels - just use the image to get to know what each of the clinker minerals looks like, then focus on the image without the labels. To help with adjusting this archetypal image in your mind’s eye in the thought experiments that follow, the clinker compositional parameters for Lime Saturation Factor, Silica Ratio and Alumina Ratio, are shown below for convenience. These formulae govern the proportions of the clinker minerals and are a big help in visualising the clinker as its composition changes. Lime Saturation Factor: LSF=CaO/(2.8SiO2+1.2Al2O3+0.65Fe2O3) Controls the proportion of calcium silicates present as alite - a higher LSF means more alite, assuming the clinker is well-burnt with no additional free lime. (There are other variations of this formula - use one of those if you are more familiar with it.) Silica Ratio: SR=SiO2/(Al2O3+Fe2O3) Controls the proportion of silicate minerals in the clinker, ie: proportion of (alite+belite). Alumina Ratio: AR=Al2O3/Fe2O3 Controls the ratio of aluminate to ferrite phases.
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Clinker: thought experiment 1, changing the LSF Suppose that this image is typical of the clinker produced by a cement plant on a particular day and that the Lime Saturation Factor (LSF) is 94%, the Silica Ratio (SR) is 2.5 and the Alumina Ratio (AR) is 1.6. The exact numbers don’t matter too much, we are just going to look at what happens as we change them. Imagine you alter the raw materials going into the kiln by increasing the proportion of limestone (assume it is pure calcium carbonate). The clinker CaO content will increase, and so the LSF will also clearly increase; the other oxides will decrease. The SR and AR will remain the same. What effect does this have on your archetypal clinker image? As you mentally add CaO, the proportion of alite in your standard clinker image should increase and the proportion of belite should decrease. When you reach an LSF of 100% there is very little belite, just a mass of alite, and the ferrite and aluminate phases. This clinker will be harder to burn than the clinker in the standard clinker image, but cement made from it might have slightly higher early strengths, so long as the clinker wasn’t overburned and all other things were equal (particularly cement fineness, particle size). The heat of hydration will be higher because of the increased proportion of alite. Now imagine increase the LSF to something a little over 100% and visualise free lime crystals forming - see Figure 3.13 for a picture of free lime in clinker. (Of course, remember that in reality a little free lime will always occur below an LSF of 100% due to clinker heterogeneities, especially coarse particles of limestone or silica in the feed, or if the clinker is underburned, but don’t worry about that now). Now picture the LSF slowly decreasing from over 100% to about 90%; first, imagine the free lime disappearing and then the proportion of alite diminishing and the proportion of belite increasing. This clinker will be much easier to burn, but cement made from it will have lower early strengths than cement made from the standard clinker (again, all other things being equal). Later strengths should largely catch up, though it might take a year or so. The cement will have a lower heat of hydration.
Clinker: thought experiment 2, changing the SR Now imagine you increase the Silica Ratio from 2.5 to 2.7 by increasing the proportion of silica in the raw feed. As the SR increases, the LSF decreases and the AR stays the same. What effect does this have on your archetypal clinker image? The SR governs the proportion of total silicates - alite plus belite, so visualise the alite and belite increasing and the aluminate plus ferrite decreasing. The AR remains the same, so the relative proportions of aluminate and ferrite stay the
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Cement concepts same, however, the LSF is reduced, so picture a litle more belite and less alite. The increased SR means that there will be less liquid, making the clinker harder to burn; at the same time, though, the LSF is lower, making it easier to burn so these effects will tend to cancel each other. Overall the clinker burnability may be broadly similar to our standard clinker. All other things being equal, cement made from this clinker would have lower early strengths than cement made from the standard clinker because it contains less alite. However, the later strengths should be at least as good and possibly better as the total silicate content is slightly higher. Now think of reducing the silica in the raw feed and the SR decreasing from 2.7 back through 2.5 down to 2.4. The LSF increases to about 100% and the AR stays the same. Alite increases markedly and belite becomes scarce, as there is now sufficient lime to form alite from all the available silica. There is a little more ferrite and aluminate, in the same ratio as previously, and fractionally less calcium silicates in total. All other things being equal, cement made from this clinker would have higher early strengths than cement made from the standard clinker because it contains more alite. The later strengths may be fractionally lower as the total calcium silicate content is lower, although the difference will be small. Clinker: thought experiment 3, changing the AR Now imagine increasing the Alumina Ratio to 2.0. You could do this by increasing the Al2O3 content or by decreasing the Fe2O3. For this thought experiment, just add some alumina and decrease the SiO2, CaO and Fe2O3 proportionally, don't think about what raw materials you change to do this – it might get complicated. The LSF and SR will both decrease. What effect does this have on your archetypal clinker image? A lower LSF means less alite and a lower SR means less silicates in total and more flux phases; a higher AR means the proportion of aluminate to ferrite will increase. Visualise the alite proportion decreasing and the belite proportion increasing; imagine also the total of aluminate and ferrite increasing. Because there is more alumina and slightly less iron, there will be more aluminate phase and less ferrite, so picture an increasing ratio of aluminate to ferrite. The increased AR will make this clinker harder to burn (maximum liquid at the lowest temperature is at an approximate AR of 1.38) but this will be offset by the lower LSF and SR, both of which will make it easier to burn. Which effect would win out is unclear, so let’s suppose burnability will be similar to the standard clinker. Cement made from this clinker will have a lower alite content and lower silicates in total, so early and later strengths would probably be lower compared with the cement made from the reference clinker (again, all other things being equal). Although there will be less alite, there will be more aluminate and, since aluminate is the most reactive of the four main clinker minerals, the heat of hydration is likely to be higher compared with cement made from the reference clinker.
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Starting with the standard clinker, try thinking of decreasing the AR to 1.4 but this time by adding iron oxide. LSF and SR will decrease, so the total silicates will decrease and there will be proportionally less alite and more belite. Compared with the standard clinker, this clinker will be easier to burn as there will be maximum liquid at minimum temperature and the SR will be lower, so there will be more liquid overall. Also, the LSF will be lower, making combination easier. There will be less alite and less aluminate, so the heat of hydration will be lower. Early strengths of cement made from this clinker will be a little lower; later strengths may also be a bit lower as the total proportion of silicates will be less and so there will be less hydraulic material present in the cement. Note that if you lowered the AR by reducing the Al2O3 content, the SR and LSF would increase.
Clinker: thought experiment 4, changing the free lime Our archetypal image in Figure 11.1 doesn’t show any free lime, although there was some elsewhere in the nodule, say about 1.5% in the clinker overall. Suppose we alter the burning conditions, while keeping the kiln feed composition constant. Suppose we initially had 56% alite and 18% belite (using the Bogue calculation), and then we burned the clinker harder. The free lime content will decrease and we will have more alite and less belite. If that gave us 0.5% free lime instead of the original 1.5%, we might expect to have around 60% alite and 15% belite. More alite is good, so we would have better cement strengths, wouldn’t we? Well, no, probably not. Yes, in principle, we want to make alite. However, there comes a point where trying to get every bit of alite that you can becomes counter-productive. As we said in Chapter 3, overburning produces less-reactive alite. Suppose we now burn less hard and the free lime rises from 0.5% to maybe 2.5% or 3%. The alite should become more reactive and give better cement strengths but there will be less of it; at some point, the optimum trade-off between alite content and alite reactivity will be passed and cement strengths will start to decrease again. If we allowed the free lime to rise further, we would eventually start to have problems with expansion due to the hydration of free lime within hardened concrete or mortar. Where that optimum free lime content lies is likely to depend on a range of factors, including the clinker LSF, feed combinability and the related issue of raw material fineness (especially fineness of silica particles).
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Based on UK experience, a free lime of much less than 1% is likely to indicate an overburned clinker. At least one manufacturer is happy to have 3% free lime in a clinker of 100% LSF. The optimum may also depend on how the cement is to be used. In a composite cement, the presence of free lime, when hydrated, provides a ready source of immediately-available CH to produce additional C-S-H or other hydration products as the slag or fly ash, or other mineral addition, reacts. Of course, the bulk of the CH will still be provided by alite hydration and the clinker LSF will have a stronger effect in determining the available CH. The practical effects of clinker LSF and free lime variability on paste microstructure in composite cements are unclear and could make an interesting research project. Softer burning has other benefits as well, saving fuel and prolonging the life of the kiln lining.
A few general points… In all of the above, the mantra should be:
“all other things being equal.”
A second mantra should be:
“with cement, all other things are never equal,”
but, as these are thought experiments, it is OK to pretend. If the visualised images get a bit hazy and you aren’t quite sure what should be happening, go back and look at Chapter 5 on clinker, especially the plots of combinability curves (Figures 5.1-5.5). All should become clear. One main reason why all things might not be equal in the real world is that raw materials are not pure oxides; it is difficult to add just one oxide, as most raw materials contain more than one, and this would cause unintended secondary effects. For example, if you added clay or fly ash to raise the Alumina Ratio, you would also be adding more silica.
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Your freedom to compensate for this would depend what other raw materials you were using; if you were also using milled silica sand, you could reduce the proportion of sand to compensate for the additional silica in the fly ash. Even then, other minor oxides in the fly ash may also have an effect; if the fly ash contained magnesium, the temperature at which the melt formed on heating might change. However, practical difficulties like this can be ignored in thought experiments if appropriate - that is one of their great advantages. If you later wanted to do one of the experiments for real, you would first identify these secondary effects and apply the conceptual model to them also to predict the results. In other words, think what you want to achieve first, then work out how close you can get to it in reality. Thought experiments can help with both. Finally, if you have a cement made on Thursday that seems different from cement made on Monday, but seems similar in compositional terms, the chapter on cement variability looks at other reasons why this could happen.
11.4 Hydrated cement 11.4.1 Some useful principles Everything to do with cement leads eventually to what happens when we add water, whether in concrete, mortar, grout or some other cementitious material; having an understanding of what happens as cement hydrates is therefore our ultimate objective. With clinker, the formulae for LSF, SR and AR help to visualise how clinker will change as its composition changes. Unfortunately, as yet we have no such helpful formulae for hydrating systems, so we adopt as our archetype the process of hydration of the simplest hydrated Portland cement system, containing clinker and interground gypsum only. From this, using thought experiments, we can consider how changes in water/cement ratio, or additions of fine limestone, slag, fly ash or microsilica, or anything else we might care to add, are likely to affect the hydration products that form. Factor in the paste porosity and permeability as well and we can get an idea of how strong concrete made from that cement might be and how resistant to deleterious processes.
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Seven useful principles: 1. Alite hydration produces calcium silicate hydrate (C-S-H) plus calcium hydroxide (CH); belite hydration produces C-S-H, and maybe a little CH. 2. Aluminate and ferrite hydration release alumina (also lime and a little iron); the alumina is of prime importance as it is required to form ettringite and AFm phases. 3. The ratio of available alumina to sulfate in the cement paste controls the relative proportions of monosulfate (AFm) phase and ettringite, but see also Principle 4. 4. Principle 3 is modified in the presence of other anions that can substitute for sulfate in AFm phase. In particular, carbonate from fine limestone can substitute for sulfate in AFm to form hemicarbonate and then monocarbonate; the displaced sulfate forming ettringite. 5. Fine filler effect: finely-divided inert material in cement acts as nucleation sites at which dissolved material can precipitate to form hydration products. An increase in the number of nucleation sites results in faster dissolution and re-precipitation, allowing the cement to hydrate more quickly. This results in more rapid strength growth at early ages. It also seems to improve later strengths, probably by helping to produce a more uniform microstructure. 6. The presence of additional available silica, due to the reaction of pozzolanic or latently hydraulic additions to the cement, will enable calcium hydroxide to react with the silica to form more C-S-H than would be produced by Portland cement alone. C-S-H is the principal strengthgiving hydration product, so more C-S-H should mean higher strengths. 7. The ratio of calcium to silicon in C-S-H varies a little with calcium availability in the hydration products. If more calcium is available, the proportion of calcium hydroxide will increase. So, roughly speaking, if the proportion of CH increases, the Ca/Si ratio in the C-S-H will increase. Conversely, if there is less CH, the Ca/Si ratio in the C-S-H will decrease. This liberates additional calcium from which more C-S-H can be produced. Summary: if there is more CH in the paste, the C-S-H will have a higher Ca/Si ratio and vice versa.
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It also helps to keep in mind the compositions of the reacting materials and of the principal hydration products: Alite: C3S (approximately) Belite: C2S (approximately) Aluminate: C3A (approximately) Ferrite: C4AF (very approximately) Free lime: C Fly ash, fly ash , (low lime): mainly aluminosilicate glass Slag: mainly calcium aluminosilicate glass (with more SiO2 and Al2O3 and less CaO compared with Portland cement) Calcium silicate hydrate: C-S-H, Si/Ca ratio approximately 0.5-0.6 in Portland cement Calcium hydroxide: CH Ettringite: C3A.3CaSO4.32H2O Monosulfate: C3A.CaSO4.12H2O Monocarbonate: C3A.CaCO3.11H2O Hydroxy-AFm: C4AH13 and _ Hemicarbonate: C4AC0.5H12 + Other minor phases
11.4.2
Hydration of different cements
To start with, think of a Portland cement that is composed only of ground clinker and gypsum added at the cement mill. It has no interground limestone, or grinding aid that affects cement hydration. It contains no carbonate and no carbonate is allowed to penetrate from outside. (In Europe, this almost has to be a thought experiment because most CEM I contains interground limestone, unless you use white cement.) The image in Figure 11.3 below, reproduced from Figure 6.3 for convenience, is a good archetype and we'll use it for the thought experiments that follow.
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Figure 11.3 Archetypal image: polished section of cement paste, age 2 years, made using ordinary Portland cement, w/c=0.40. Key: c - unhydrated cement; C-S-H - calcium silicate hydrate; CH – calcium hydroxide; p – pore; Circle ‘A’ contains C-S-H, CH and is assumed also to contain AFm and AFt phases. CH is just visible, AFm and AFt are not visible as they are too small.
In most of what follows, we will be thinking of the cement paste fraction of the concrete (or mortar etc.). Concrete is not just cement paste with stones in it, because the presence of aggregate introduces other important physical factors such as the water demand of the aggregate and the interface between the aggregate and the paste. However, we want to concentrate on the hydration process and this is most simply done by thinking of pastes - just cement and water. You may first want to review the cement hydration process outlined in Chapter 6.3.4.
Thought experiment 5: effect of water/cement ratio The next thought experiment is to visualise some cement pastes with different water/cement ratios. It should be really easy, because there are some pictures showing exactly this in Chapter 6; for convenience, here they are again (Figures 11.4-11.7).
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Figure 11.4 Polished section of cement paste, Figure 11.5 Archetypal image: polished section of cement paste, age 2 years, made age 2 years, made using ordinary Portland using ordinary Portland cement, w/c=0.40. cement, w/c=0.33.
Figure 11.6 Polished section of cement paste, Figure 11.7 Polished section of cement paste, age 2 years, made using ordinary Portland age 2 years, made using ordinary Portland cement, w/c=0.60. cement, w/c=0.50.
In this particular experiment, don’t think about individual hydration products, just concentrate on three components: the cement hydration products as a whole, the pores and the remaining cement particles that have not hydrated. Implicitly, there is also a fourth component, water, filling the pores. Think firstly of mixing 1 kg cement with 400 g water to give a water/cement ratio of 0.4, then curing the paste under water for a year or two, until it is mature and little further hydration will occur. Picture the hydrating cement: as time passes, the proportion of hydration products increases, the proportion of unhydrated cement decreases and the water-filled gaps are gradually replaced by hydration product. In principle, according to the Powers-Brownyard model, at w/c=0.4, most or all of all the cement should hydrate because we supplied additional curing water. In practice, it won’t, because of the formation of thick coatings of hydration product around the unhydrated relicts of larger cement grains that restrict further access to water. By 28 days, perhaps about 80%-90% of the alite has hydrated, together with www.whd.co.uk
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Cement concepts about half of the belite, almost all of the aluminate and perhaps half the ferrite. Some of what remains unhydrated will probably never hydrate, although hydration will continue at a slow, and ever slower, rate almost indefinitely. After a year or two, hydration will have mostly ceased. The archetypal image in Figure 11.3 (and in Figure 11.5) is our archetype; it shows paste made with a water/cement ratio of 0.4 after 2 years. It is composed largely of cement hydration products, with a little unhydrated calcium silicate (top left) and some ferrite (bright, in larger relict cement grains). There are some pores visible; these are capillary pores and would have been water-filled. In the polished section, water in capillary pores has been replaced by epoxy resin. To summarise, a mature Portland cement paste (w/c=0.4) contains hydration product, some pores, some unhydrated cement, a little water or pore fluid and maybe some entrapped air. Next, visualise how a mature paste made with w/c=0.5 might appear. (Don't cheat and look at the images, just think about it!) There will be more water and less cement in the paste, so we would expect a higher porosity and fewer remaining unhydrated cement grains. Now have a look at Figure 11.6, which shows exactly this; the paste microstructure is considerably more porous than the paste made with w/c=0.4 in Figure 11.5. There is still some residual ferrite; some of this may have hydrated, we can't really tell from the images, but what is there now will probably persist almost indefinitely. While thinking again of the paste made with w/c=0.4, now picture a mature paste made with w/c=0.6. There will be much less cement and much more fluidfilled pore space; the hydration products will be more porous, with larger capillary pores; again, this is just what Figure 11.7 shows. There is a clear trend in the characteristics of the images in terms of porosity and residual unhydrated cement as we go from w/c=0.4 through w/c=0.5 to wc=0.6. Finally, consider a paste made with w/c approximately 0.3. The trend continues in the opposite direction; we would expect a lot of unhydrated cement and little porosity. According to the Powers-Brownyard model, not all the cement can hydrate because there is not enough volume to accommodate all of the hydration product. That is apparent from Figure 11.4; very little porosity is visible but unhydrated cement grains are numerous. By extension, since there is little capillary porosity visible, the available water is also very limited. Assuming hydration product to occupy 2.2 times the volume of the unhydrated cement, it is clear that most of the remaining cement cannot hydrate; there is not enough space for the hydration product, or sufficient available water.
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Thought experiment 6: changing the sulfate/alumina ratio This expands Principle 3 above. Although they represent only a small proportion of the total paste, ettringite and AFm phases are important; they affect concrete setting properties and can have an effect on long-term durability. The balance between available alumina and sulfate in the cement paste controls the relative proportions of AFm phases and ettringite. In this thought experiment, think of three cements, all made from the same clinker as used in our cement archetype above, but containing different amounts of gypsum added when the cement was milled. We’ll call these “low sulfate,” “medium sulfate” and “high sulfate” mixes; the actual numbers aren’t important here but think of them as, say, 1.5%, 3.0% and 4.5% SO3 if you like. The cements are used to make pastes, all at the same water/cement ratio and cured until they are mature, say for a year or more. Assume there is sufficient sulfate to prevent a flash set at the lower sulfate contents and that any false-set that might occur is mixed through at the higher sulfate contents. Don’t worry about the exact numbers shown, they are only intended to be illustrative, and just follow the arguments for each paste.
In a mature paste, what hydration products are present? Medium sulfate mix: suppose the cement in the archetype mix above contained a “medium” level of sulfate. In our mature paste, hydration has virtually completed and the principal hydration product is C-S-H. Some CH is present, and there is sufficient available sulfate to combine with all the available alumina to form monosulfate phase. A small surplus of available sulfate produces a little ettringite from some of the monosulfate. This is our reference paste to compare with the following pastes as we change the amount of sulfate. This paste contains: C-S-H; CH; AFm as monosulfate and a small amount of ettringite.
Suppose there is less sulfate: Low sulfate mix: in this paste, the available alumina is about the same as in the medium sulfate mix; there is less available sulfate, all of which is present in AFm phase as monosulfate, but there is still an excess of available alumina. Consequently, there is no ettringite at all, because this requires a higher ratio of sulfate to alumina. The small deficit of sulfate is accommodated by the formation of hydroxy-AFm phase, in which sulfate is partly replaced by hydroxide. This paste contains: C-S-H; CH; AFm as monosulfate and hydroxy-AFm; no ettringite.
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Cement concepts Suppose there is more sulfate: High sulfate mix: compared with our “medium sulfate” reference paste, this paste with a higher sulfate content has more available sulfate and there is now sufficient to convert almost all the available alumina to ettringite, with a little residual monosulfate. This paste contains: C-S-H; CH; ettringite; a little AFm as monosulfate. At higher still levels of sulfate, all the available alumina will have formed ettringite; there will be some residual gypsum as no other sulfate-containing phases can form.
This experiment has considered what might be present in mature pastes because it is easier to do this when the reactions have largely stopped. At early ages, the reactions are more complicated and there will be much local variability. Cement particle size and crystal microstructure, to name just two factors, affect the rate at which reactants become available. Also, not all the sulfate in the cement is ultimately present in ettringite or monosulfate; some is taken up by the C-S-H, as is some alumina. The use of the term “available” is used to allow for these uncertainties.
Thought experiment 7: add interground limestone In recent years, an increasing proportion of Portland cement worldwide has contained a small amount of interground limestone, typically up to 5%. The next change to our archetype of a basic Portland cement should therefore be to add some interground limestone. With closed-circuit cement milling, the particle size range is less than with an open-circuit mill. There are fewer coarse particles and a much smaller proportion of very fine particles; limestone is softer than clinker, so intergrinding it with clinker in a closed-circuit mill will produce very fine limestone particles (
