Geopolymer Concrete

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CHAPTER 1:- INTRODUCTION Portland cement production is under critical review due to high amount of carbon dioxide gas released to the atmosphere. In recent years, attempts to increase the utilisation of fly ash to partially replace the use of Portland cement in concrete are gathering momentum. Most of this by-product material is currently dumped in landfills, thus creating a threat to the environment. Geopolymer concrete is a ‘new’ material that does not need the presence of Portland cement as a binder. Instead, the source of materials such as fly ash, that are rich in Silicon (Si) and Aluminium (Al), are activated by alkaline liquids to produce the binder. Hence, concrete with no cement. The rate of production of carbon dioxide released to the atmosphere during the production of Portland cement and fly ash, a by-product from power stations worldwide is increasing with the increasing demand on infrastructure development, and hence needs proper attention and action to minimize the impact on the sustainability of our living environment. De-carbonation of limestone in the kiln during manufacturing of cement is responsible for the liberation of one ton of carbon dioxide to the atmosphere for each ton of Portland cement, as can be seen from the following reaction equation:

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5 CaCO3 + 2 SiO2  3 CaO.SiO2 + 2 CaO.SiO2 + 5 CO2 …

(1)

The production of Portland cement worldwide is increasing 3% annually. The current contribution of green house gas emission from the Portland cement production is about 1.35 billion tons annually or about 7% of the total greenhouse gas emissions to the earth’s atmosphere [1]. Furthermore, Portland cement is also among the most energy-intensive construction materials, after aluminum and steel. Fly ash, “the finely divided residue that results from the combustion of ground or powdered coal and that is transported by flue gases from the combustion zone to the particle removal system” [2], is available abundantly worldwide. In 2001, the fly ash production in the USA was in the order of 68 million tons, but only 32 percent was used in various applications, such as in concrete, structural fills, waste stabilisation/solidification, etc.

[3].

Worldwide, the estimated production of coal ash in 1998 was more than 390 million tons. The main contributors for this amount were China and India. Only about 14 percent of this fly ash was utilized, while the rest was just disposed in landfills [4]. By the year 2010, the amount of fly ash produced worldwide is estimated to be about 780 million tons annually [5]. There are environmental benefits in reducing the use of Portland cement in concrete, and using a by-product material, such as fly ash, as a substitute. With silicon and aluminium as the main constituents, fly ash has great potential as a cement replacement material in concrete. The search of any aspects in this regard is worthy to pursue. To totally replace the use of Portland cement as concrete binder, fly ash needs to be activated, usually using alkaline solutions. Palomo et al [6]

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described two different models of activation of fly ash or other similar materials. In the first model, the material containing essentially silicon and calcium is activated by low to mild concentration of alkaline solution. The main product of the reaction is calcium silicate hydrate (C-S-H). On the contrary, the material used in the second model contains mostly silicon and aluminium, and is activated using highly alkaline solution. The chemical process in this case is polymerisation. Investigations on the first model have a long history in the former Soviet Union, Scandinavian and Eastern Europe countries [7]. A very well known example is the activation of blast furnace slag. On the other hand, only limited research has been carried out on the second model [6]. Therefore many aspects of the material characteristics, reaction mechanisms and so on for the second model are still not clear. For the binders produced using the second model, also known as inorganic alumino-silicate polymers, Davidovits [8] coined the term Geopolymer in 1978 to describe the alkali activated material from geological origin or by-product materials such as fly ash and rice husk ash. Davidovits [9] also stated that often information on geopolymer material is tied to patent-oriented literature. Only from the end of 1990s, more scientific information are becoming available and published; however most of the data deal with small size specimens made of geopolymer paste or mortar.

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CHAPTER 2:-CONCRETE MAKING MATERIALS Concrete is the most widely used manmade construction material. It is obtained by mixing cement, water, fine aggregates and coarse aggregate (and some times admixtures) in required proportions. The mixture when placed in forms and allowed to cure becomes hard like stone. The hardening is caused by chemical action between water & the cement and it continuous for a long time and consequently the concrete grows stronger with age. The hardened concrete may also be considered as an artificial stone in which the voids of larger particles (coarse aggregate) are filled by the smaller particles (fine aggregate) and the voids of fine aggregates are filled with cement. The strength, durability and other characteristics of concrete depends upon the properties of its ingredient, on the proportions of mix, the method of compaction and other controls during placing, compaction and curing.

 PORTLAND CEMENT: The cement commonly used is Portland cement. It is obtained by burning together in a definite proportion, a mixture of naturally occurring argillaceous (containing alumina) and calcareous (containing calcium carbonate or lime) materials to a partial fusion at high temperature (about 1450 degree Celsius). The product obtained on burning called clinker, is cooled and ground to the required fineness to produce a material, known as cement. Its inventor, Joseph Aspdin, called it Portland cement because when it hardened it produced a material resembling stone from the quarries near Portland in England.

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It is important to note that the strength properties of cement are considerably influenced by the cooling rate of clinker.

Main compounds of Portland cement:In addition to the four major compounds, there are many minor compounds formed in the kiln. The influence of these minor compounds on the properties of cement is not significant. Two of the minor compounds namely K2O and NA2O referred to as alkalies in cement are of some importance. Tricalcium silicate and dicalcium silicate are the most important compounds responsible for the strength. Together they constitute 70 to 80% of cement.

Hydration of cement:The extent of hydration of cement and the resultant microstructure of hydrated cement influences the physical properties of concrete. When the cement comes in contact with water, the hydration of products gets deposited. On the outer periphery and nucleus of the unhydrated cement inside gets gradually diminished in volume. The reaction of cement with water is exothermic. The reaction liberates considerable quantity of heat a considerable quantity of heat. This liberation of heat is called heat of hydration. The study and control of the heat of hydration becomes important in the construction.

 AGGREGATES:Crushed or uncrushed material derived from the natural sources such as rocks, gravels boulders and sand for production of concrete are called as aggregates.

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The aggregates provide about 75% of the body of the concrete and hence its influence is extremely important.

Classification of Aggregates:The aggregates are classified as: a) Fine aggregates b) Coarse aggregates c) All-in-aggregate d) Single-size-aggregate a) Fine aggregate:it is the aggregate most of which passes 4.75 mm IS sieve. Sand is generally considered as fine aggregate. b) Coarse aggregate:The aggregates most of which are retained on the 4.75 mm IS sieve and contain only so much of fine material as is permitted by the specifications are termed coarse aggregates. The graded coarse aggregate is described by its nominal size, i.e. 40 mm, 20mm, 16mm and 12.5 mm, etc. c) All-in-aggregate:Sometimes combined aggregates are available in nature comprising different fractions of fine and coarse aggregates, which are known as all-inaggregates. The all-in- aggregates are not generally used for making high quality concrete. d) Single-Size- Aggregate: Aggregates comprising particles falling essentially within a narrow limit of size fractions are called single-size-aggregates. For example, a 20mm single-size-aggregate means an aggregate most of which passes 6

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through a 20 mm IS sieve and the major portion of which is retained in a 10mm IS sieve.

Fineness Modulus:The fineness is a numerical index of fineness, giving some idea of the mean size of the particles present in the entire body of the aggregate. The sieves that are to be used for the sieve analysis of the aggregate for concrete as per IS: 2386 (Part 1)- 1963, are80mm,40mm,20mm,10mm,4.75mm,2.37mm,1.18mm,600 micron,300 micron, and 150 micron. The purpose of the fineness modulus is to know the character of the aggregate to be used, as to whether it is fine, medium or coarse.

 WATER:Water is the most important and least expensive ingredient of concrete. Generally, cement requires about 3/10 of its weight of water for hydration. Hence, the minimum water-cement ratio required is 0.35. According to IS: 456-1978, the presence of tannic acid or iron compounds in the curing water is objectionable.

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CHAPTER 3:-CONCRETE The good quality of concrete is a homogenous mixture of water, cement, aggregate and other admixtures. Concrete making is not just a matter of mixing ingradients to produce a plastic mass, but good concrete has to satisfy performance requirements in the plastic of green state and also the hardened state. In the plastic state the concrete should be workable and free from segregation and bleeding. Segregation is the separation of coarse aggregate and bleeding is the separation of cement paste from the main mass. The segregation and bleeding results in a poor quality concrete. In its hardened state concrete should be strong, durable and impermeable and it should have minimum dimensional changes. Among the various properties of concrete, its compressive strength is considered to be the most important and is taken as, an index of its overall quality. Many other properties of concrete appear to be generally related to its compressive strength.

 FACTORS AFFECTING STRENGTH OF CONCRETE: The following are different factors affecting the strength of concrete:

1) Water Cement ratio: Strength of the concrete depends on the strength of the cement paste. The strength increase with increase in cement particles. According to

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Abraham (1919) the strength of concrete is only dependent upon water/cement ratio providing the mix is workable.

2) Cement: The effect of cement on concrete strength depends of the chemical composition and fineness of the cement. With increaser in fineness leads to higher strength and also gain strength more rapidly.

3) Aggregate: The aggregate parameters that are shape, texture and maximum size of aggregate are most important. The larger maximum size aggregate gives lower surface for development of gel bonds which is responsible for the lower strength of the concrete. Secondly bigger aggregate size causes more heterogeneity in the concrete which will prevent the uniform distribution of load when stressed. Generally, high strength concrete or rich concrete is adversely affected by the use of the larger size aggregates. But in lean mixes or weaker concrete, the influence of size of the aggregate gets reduced. It is interesting to note that in lean mixes larger aggregates gives highest strength while in rich mixes it the smaller aggregate yields higher strength.

4) Compaction: Compaction can be done by both pressure and vibration. Compaction is necessary to reduce or eliminate the air present in the concrete. By this density of concrete increases. Lack of compaction will result in air voids whose damaging effect on strength and durability is equally or more predominant than the presence of capillary cavities. 9

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To enable the concrete to be fully compacted with given efforts, normally a higher water/ cement ratio than that calculated by theoretically considerations may be required. That is to say the function of water is also to lubricate the concrete. The following methods are adopted for compaction of concrete: a) Hand compaction b) Rodding c) Ramming d) Tamping e) Compaction by Vibration f) Internal vibrator ( Needle vibrator) g) Formwork vibrator (External vibrator) h) Table vibrator i) Platform vibrator j) Surface vibrator ( Screed vibrator) k) Compaction by pressure and jolting l) Compaction by Spinning

5) Curing: Concrete while hydrating releases high heat of hydration. This heat is harmful from the point of view of volume stability. If the heat generated is removed by some means, the adverse effect due to the generation of heat can be reduced. This can be done by a thorough water curing. The present concrete items are normally immersed in curing tanks for certain duration. Pavement slabs, roof slabs etc. are covered under water by making small ponds. Vertical retaining wall or plastered surfaces or concrete 10

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columns etc. are cured by spraying water. In some cases, wet coverings such as wet gunny bags, Hessian clot, jute matting, straw etc. are wrapped to vertical surface for keeping the concrete wet. For horizontal surfaces saw dust, earth or sand are used as wet covering to keep the concrete in wet condition for a longer time so that the concrete is not unduly dried to prevent hydration.

 WORKABILITY: The diverse requirements of mixability, stability, transportability, placeability, mobility, compactibility and finishability of fresh concrete are referred to as workability. The workability of fresh concrete is thus a composite property. It is difficult to define precisely all the aspects of the workability in a single definition. IS: 6461 (Part V11) – 1973 defines workability as that property of freshly mixed concrete of mortar which determines the ease and homogeneity with which it can be mixed, placed, compacted and finished. Sometimes the terms consistency and plasticity are used to denote the workability of a concrete mix and a wetter mix need not have all the above desired properties. On the other hand an extremely wet mix may cause seggration and may be difficult to place in moulds. Plasticity is the cohesiveness of the mix to hold the individual grains together by the cement matrix.

Factors Affecting Workability: 11

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a) Water content b) Mix proportion c) Size of aggregate d) Shape of aggregates e) Surface textures of aggregates f) Grading of aggregates g) Use of admixtures

a) Water Content: Water content in a given volume of concrete will have significant influences on the workability. The higher the water content per cubic meter of concrete, the higher will be the fluidity of concrete which is one of the important factors affecting the workability. b) Mix proportions: Aggregate/Cement

ratio

is

an

important

factor

influencing

workability. The higher the aggregate/cement ratio, the leaner is the concrete. In leaner concrete, less quantity of paste is available for providing lubrication per unit surface area of aggregate and hence the mobility of aggregate is restrained. On the other hand in case of rich concrete with lower aggregate/ cement ratio, more paste is available to make the mix cohesive and fatty to give better workability. c) Size of Aggregate: The bigger the size of the aggregate the less is the surface area and hence less amount of water is required for wetting the surface and less 12

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matrix or paste is required for lubricating the surface to reduce internal friction. For a given quantity of water and paste a bigger size of aggregate will give higher workability. d) Shape of Aggregate: The shape of the aggregate considerably influences workability. Angular, elongated, or flaky aggregate makes the concrete very harsh when compared to rounded aggregate or cubical shaped aggregate. Contribution to better workability of rounded aggregate will come the fact that for the given volume or weight it will have less surface area and also less voids than angular or flaky aggregate. e) Surface Texture: The influence of surface texture on workability is again due to the fact that the total surface area of rough textured aggregate is more than the surface area of smooth rounded aggregate is of same volume. A reduction of inter particle frictional resistance offered by smooth aggregates also contributes to the workability. f) Grading of aggregate: This is one of the factors which will have maximum influence on workability. A well graded aggregate is the one which has fewer amounts of voids in a given volume. Other factors being constant, when the total voids are less, excess paste is available to give better lubricating effect. With excess amount of paste the mixture becomes cohesive and fatty which prevents segregation of particles. Aggregate particles will slide paste each

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other with the least amount of compacting efforts. The better the grading the less is the void content and higher the workability.

g) Use of admixtures: Certain admixtures greatly influence the workability. Use of air entraining agent and super plasticizing admixtures greatly increase the workability when other conditions are constant. It can be viewed that the air bubbles act as a sort of ball bearing between the particles to slide past each other and give easy mobility to the particles. 

MEASUREMENT OF WORKABILITY: The term workability has been generally used for concrete and can

mean the following: Compactibility or the ease with which concrete can flow into the formwork, around steel and remolded. Stability or ability of the concrete to remain a stable, cohesive and homogenous mass while handling and vibrating without the constituents segregating. The following tests are commonly employed to measure workability. a) Slump test b) Compaction factor test c) Flow test d) Vee Bee Consistometer test

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a) Slump test: Slump test is the most commonly used method of measuring consistency of concrete which can be employed either in laboratory or at site of work. It is not a suitable method for a very wet or very dry concrete. If one half of the cone slides down, it is called shear slump. In case of a shear slump the slump value is measured as the difference in height between the height of the mould and the average value of the subsidence.

b) Compaction Factor test: The compaction factor test is designed primarily for use in the laboratory but it can also be used in the field. It is more precise and sensitive than the slump test and particularly useful for concrete is to be compacted by vibration. The compaction factor=Weight of partially compacted concrete Weight of fully compacted concrete

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CHAPTER 4:-ENVIRONMENTAL PROBLEMS WITH CONCRETE The environmental problems with concrete are given below.

 CO2 emissions The most serious problem with our cement industry is that it is a major CO2 emitter causing global warming. With every ton of cement produced, almost a ton of CO2 is emitted. About 0.5 tons comes from the decomposition of the limestone and the balance is generated by the power plant supplying the electricity to turn the kiln and ball mills to grind the cement plus the fuel burned to fire the kiln. All other generation such as operating ready mix trucks adds only a minor amount to the CO2 emissions. In terms of conventional concrete mixtures (i.e. not using fly ash, slag or silica fume), about 480 kg of CO2 is emitted per cubic meter of concrete or 20 kg of CO2 per 100 kg of concrete produced. All of this amounts to about 7% of the total CO2 generated worldwide. Enhanced efficiency is not likely to change this but replacement of some of the cement by a supplementary cementing material not associated with CO2 emission can substantially reduce these emissions.

 Nitrous oxide emissions

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Nitrous oxide emissions come from burning gasoline, coal or other fossil fuels. Ozone is formed when nitrogen oxides and volatile organic compounds mix in sunlight. The volatile organic compounds come from sources ranging from industrial solvents to volatile resins in trees. Ozone near the ground can cause a number of health problems such as asthma attack, sore throat, coughing and other health difficulties. In addition, nitrous oxide, carbon dioxide and methane are the most important greenhouse gases. The Nitrous oxide emissions from Canadian cement kilns range from 1.5 to 9.5 kg/tonne of clinker produced with a proposed limit of 2.3 kg of Nitrous oxide per tonne. Using 2.3 kg of Nitrous oxide per tonne, the world release of Nitrous oxide by the 1625 million tons of cement produced in the year 2000 would be 3.7 million tons of Nitrous oxide. This is a fifth of the Nitrous oxide released in all of continental Asia in a year Reduction in nitrous oxides is normally achieved by reducing the burning temperature or by injecting ammonia compounds into the high temperature exhaust stream (4). This seems like a good idea but when these actions are taken to reduce the Nitrous oxide in coal fired electric power generating stations, it adversely affects the quality of the fly ash produced. The fly ash then needs to be treated to remove the unburnt coal and ammonia gas before it can be used in concrete mixtures and several plants doing this are in operation. Several research programs are currently underway to find out how best to beneficiate fly ash to correct this problem.

 Adverse health effects Currently the health of employees is being adversely affected by the increased chromium content of the cement. The increased chromium content in the cement is mostly derived from the burning of waste products.

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The only solution seems to be to prevent contact of fresh concrete with human flesh and most containers of cement carry such a warning.

 Water pollution On average each ready mix truck returns about one half cubic meter of cement per day. After this concrete is discharged there is still about 300 kg of solids (cement, sand and stone) that is washed out with about 1000 liters of water. In the past the returned concrete and the solids were dumped in a pit at the job site or at the plant. Considering that this represents 2 to 4% of the total concrete produced, it is now considered too valuable to waste and can be recycled or reclaimed as sand and gravel. To reclaim the sand and gravel a “reclaimer” is used. It involves adding water to the returned concrete and then agitating it followed by wet screening to obtain the sand and gravel. Also, the cement-water slurry from the reclaimer, the wash out water, water to clean the outside of the truck, plus any storm water in the past usually was directed into somewhat inefficient settling basins and then into a local water course. Recent developments have enabled the concrete retained and any concrete clinging to the inside of the truck drum plus any wash out water to be stabilized overnight or over the weekend by the addition of a hydration stabilizing admixture.

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CHAPTER 5:-GEOPOLYMER CONCRETE Geopolymer is used as the binder, instead of cement paste, to produce concrete. The geopolymer paste binds the loose coarse aggregates, fine aggregates and other un-reacted materials together to form the geopolymer concrete. The manufacture of geopolymer concrete is carried out using the usual concrete technology methods. As in the Portland cement concrete, the aggregates occupy the largest volume, i.e. about 75-80 % by mass, in geopolymer concrete. The silicon and the aluminium in the fly ash are activated by a combination of sodium hydroxide and sodium silicate solutions to form the geopolymer paste that binds the aggregates and other un-reacted materials.

 PAST RESEARCH ON GEOPOLYMER MATERIAL:In geopolymers, the polymerisation process involves a chemical reaction under highly alkaline conditions on Al-Si minerals, yielding polymeric Si-O-Al-O bonds, as described by [8]: 19

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M n [ – ( Si – O2 ) z – Al – O ] n . wH2O

(2)

where M is the alkaline element, the symbol – indicates the presence of a bond, z is 1,2, or 3, and n is the degree of polymerisation. The chemical composition of geopolymers is similar to zeolites, but shows an amorphous microstructure [10]. The structural model of geopolymer material is still under investigation; hence the exact mechanism by which geopolymer setting and hardening occur is not yet clear [8, 11]. The mechanism of geopolymerisation may consist of dissolution, transportation or orientation, and polycondensation [10], and takes place through an exothermic process [6, 8]. The strength of geopolymer depends on the nature of source materials. Geopolymers made from calcined source materials, such as metakaolin (calcined kaolin), fly ash, slag etc., yield higher compressive strength when compared to those synthesised from non-calcined materials, such as kaolin clay. The source material used for geopolymerisation can be a single material or a combination of several types of materials [12]. A combination of sodium or potassium silicate and sodium or potassium hydroxide has been widely used as the alkaline activator [6, 10, 11, 13], with the activator liquid-to-source material ratio by mass in the range of 0.25-0.30 [6, 13]. Because heat is a reaction accelerator, curing of fresh geopolymer is carried out mostly at an elevated temperature [6]. When curing at elevated temperatures, care must be taken to minimize the loss of water. However, curing at room temperature has successfully been carried out by using

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calcined source material of pure geological origin, such as metakaolin [8, 14]. The geopolymer material can be used in various applications, such as fire and heat resistant fibre composites, sealants, concretes, ceramics, etc., depending on the chemical composition of the source materials and the activators. Davidovits [8] suggested that the atomic ratio of Si-to-Al of about 2 for making cement and concrete. Geopolymer can also be used as waste encapsulation to immobilise toxic metals [15].

 WHAT IS FLY ASH? Fly ash is the finer part of the Coal Combustion Products (CCP) captured by the electro-static precipitators in coal fired thermal power plants (Figure 1). Fly ash closely resembles volcanic ashes used in production of the earliest known hydraulic cements about 2,300 years ago. Some of these cements were made near the small Italian town of Pozzuoli - which later gave its name to the term "pozzolan." About 600 million tones of flyash was produced in year 2000 according to Manz (1989). This poses a major challenge in terms of their safe disposal preferably via bulk utilisation in a commercially profitable manner. At present only about 20% of all fly ash gets productively used while the rest get dumped in landfills one way or the other. Fly ash is divided mainly into two categories based mainly on its calcium content (Table 1). ASTM Class F fly ash is low in calcium and mainly pozzolanic (i.e., they react with calcium hydroxide in moist condition to produce cementitious compounds), whereas class C fly ash has higher fraction of calcium oxides, which give them cementitious

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characteristics. Class C fly ash is often marketed directly in many countries and finds easy application in concrete as a part replacement of cement.

Class F fly ash (most Indian fly ash fall in this category) is primarily used as a cement replacement under certain conditions of quality in terms of chemical composition, particle size distribution and loss on ignition (which is an indicator of carbon content). Other applications for class F fly ash besides High Volume Fly ash Concrete (HVFC) studied in detail by Malhotra (1999), are in the area of alkali activated concrete, blended cements, ternary blends including fly ash and slag or silica fume or lime. A lot of research was carried out in the ex-soviet block countries, particularly at the Kiev Institute in Ukraine. Professors Krivienko and Gloukhovsky led the way in the research in to alkali-activated cements based on fly ash, slag and other industrial by-products (Krivienko, (1994), Gloukhovsky, (1994)).

 ACTIVATION OF FLY ASH:Mechanism of activation of fly ash: The mechanisms of activation of fly ash can be divided in two main stages: Dissolution: Initially the vitreous component of the fly ash (aluminosilicate glass) in contact with the alkali solution is dissolved, forming a series of complex ionic species; Polymerization:

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These small molecules present in the dissolution can join each other and form large molecules that precipitate in the form of gel, in which some degree of short range structural order can be identified (10). The composition and structure of this alkali aluminosicilate gel depends essentially on the size, structure and concentration of the ionic species present in the medium, as well as on the synthesis temperature on the curing time, and the pH of the mixture. The hypothetical evolution of this gel would be to form some sort of zeolite crystal. However, due to the very low “alkaline solution / fly ash” ratio used in the synthesis of this type of materials (or prevailing under experimental conditions), this evolution is extremely slow. The studies done up to today concerning the manufacturing of mortars and/or concretes of activated fly ash following the alkaline process (without Portland cement) turned out into very promising results. The above mentioned studies have demonstrated for example that the properties of the concrete of alkali activated fly ash are influenced, like those of the conventional concrete, by a set of factors related to the dosing of the mixture and to the conditions of the curing process. Nevertheless, these new concrete can manage to develop very high mechanical strength within a few hours. These strengths continue to grow more slowly with time. Also, the mentioned studies have manifested the high potential of these materials that could be used in the near future in the construction industry, especially in the precast industry. This is the reason why the goal of this work is focused on the establishment of some engineering properties for this type of materials (mechanical strength development, matrix-steel adherence and shrinkage). Finally, there is a specific case of an industrial application of these new concrete for the manufacturing of pre-stressed mono-block sleeper son railroads. 23

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 GEOPOLYMER CONCRETE: Geopolymer

is

an

inorganic

alumino-silicate

polymer

synthesized from predominantly silicon and aluminium materials of geological origin or by-product materials such as fly ash. The term geopolymer was introduced by Davidovits represent the mineral polymers resulting from geochemistry. The polymerisation process involves a chemical reaction under highly alkaline conditions on Al-Si minerals, yielding polymeric Si-O-Al-O bonds The polymerisation process may be assisted by applied heat, followed by drying. The chemical reaction period is fast and the required curing period may be within 24 to 48 hours. Geopolymer is used as the binder, instead of cement paste, to produce concrete. The geopolymer paste binds the loose coarse aggregates, fine aggregates and other un-reacted materials together to form the geopolymer concrete. The manufacture of geopolymer concrete is carried out using the usual concrete technology methods. As in the Portland cement concrete, the aggregates occupy the largest volume, i.e. about 75-80 % by mass, in geopolymer concrete. The silicon and the aluminium in the fly ash are activated by a combination of sodium hydroxide and sodium silicate solutions to form the geopolymer paste that binds the aggregates and other un-reacted materials. Geopolymer-based materials are environmentally friendly, and need only moderate energy to produce. They can be made using industrial by-products, such as fly ash, as the source material. In geopolymer concrete, the geopolymer paste serves to bind the coarse and fine aggregates, and any un-reacted material. Geopolymer concrete can be utilised to manufacture precast concrete structural and 24

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non-structural elements, to make concrete pavements, to immobilize toxic wastes, and to produce concrete products that are resistant to heat and aggressive environments. Davidovits proposed that the ranges of the oxide molar ratios suitable to produce geopolymeric materials may be as follows: 1)

0.2