01 Cement Chemistry

Cement Manufacturing, Chemistry and Characterization 231M103

Learning Objectives At the end of this presentation you will be able to:

Suggested video watch: http://www.youtube.com/watch?v=n-Pr1KTVSXo

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•Define Portland Cement •Explain the 4 stages of the cement manufacture process and their impact on cement quality •Describe the 4 main components of the Portland Cement and their role during cement hydration •Explain the 5 periods of the hydration process and how each of them determines the thickening time and compressive strength of a cement •Describe the 4 physical and 4 chemical properties of cement •Define the 8 main classes in which cement is classified •Name the 4 methods for cement characterization

Portland Cement Definition:

Portland Cement is a hydraulic cement that reacts with water to develop compressive strength API – ISO standards define oil well cements

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Portland Cement is made by heating a mixture of limestone and clay or similar materials to a temperature of 1500°C and grinding it with gypsum to form cement

4 Stages of the cement manufacture Raw Material Preparation 

Obtain the raw materials from mining the limestone, clay or shale core Schlumberger Private

Limestone Quarry

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Raw Materials Calcareous - 2 Parts

Argillaceous - 1 Part Clays

Calcite

Marl

Cement rocks

Shales

Chalk

Slate and Mudstone

Marine shells and coral

Blast furnace slag

Alkali wastes

Ashes (fly ash)

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Limestone (CaCO3)

4 Stages of the cement manufacture Raw Material Preparation Obtain the raw materials from mining the limestone, clay or shale core

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Raw materials preparation

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Raw Materials Preparation – Dry Process

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Grinding and Blending of DRY Materials  Cheapest Process  Less clinker quality 

Raw Materials Preparation – Wet Process

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Grinding and Blending of SLURRIED materials Expensive Process due to fuel required to evaporate the water More uniform clinker quality

4 Stages of the cement manufacture Raw Material Preparation Burning Process Cement Kiln

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Burning Process   

Preheated blend sent to Rotary kiln Blend kiln-burned to partial fusion @ 1500oC (2700oF) Complex series of reaction take place in the kiln whereby the raw materials are converted to “CLINKER” Schlumberger Private

Cement Kiln

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Inside of Kiln

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4 Stages of the cement manufacture Raw Material Preparation Burning Process Cement Kiln

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Reaction Zones in a Rotary Cement Kiln

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Reaction Zones in a Rotary Cement Kiln

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VI Cooling C3A and C4AF crystallization occurs as liquid phase disappears

4 Stages of the cement manufacture Raw Material Preparation Burning Process Cement Kiln

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Reaction Zones in a Rotary Cement Kiln

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Clinker Components

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Clinker Components CLINKER - Grind and Heat Treat in kiln at 1500o C : Tricalcium Silicate (Alite)

C2S C3A C4AF

: Dicalcium Silicate (Belite) : Tricalcium Aluminate : Tetracalcium Aluminoferrite

Ca + Mg Oxides, Ca(OH)2, CaCO3, Na2NO4. etc

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C3S

Cement Clinker Grain Structure C3S: Major Component 

55-65% early CS

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25-35% for D, E, F retarded cement

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15-25% content

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Later CS development

C3A: Lowest concentration, hydrates rapidly 

3 - 8 % content

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< 3% content for HSR

C4AF: Interstitial phase, highly reactive 

8 - 12% content

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C2S: Second most abundant, but very slow reaction

4 Stages of the cement manufacture Raw Material Preparation Burning Process Schlumberger Private

Cooling Process

Clinker Cooling System

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Clinker Cooling System

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Cooling Process - Slow Clinker quality VERY dependent on cooling rate Cooling Rate: Slow (4-5oC/min) C3A and C4AF develop a high degree of crystalinity

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C3S and C2S crystals become highly ordered

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Free Magnesia forms periclase crystals

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Less hydraulically active with large particle size

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Quicker early strength development (3-7 days)

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Lower long term strength (28 days)

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Lower sulphate resistance

Result

HIGHER DEGREE OF UNSOUNDNESS

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Cooling Process - Fast

Result:  More hydraulically active with relatively small particle size  Lower early strength development (3-7 days)  Higher long term strength (28 days)  More sulphate resistance  Free MgO is less active UNSOUNDNESS IS LESS LIKELY

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Cooling Rate: Fast (18-20oC/min)  C3A and C4AF remains in glassy phase  C3S and C2S remain less ordered  Free magnesia remain in glassy phase

4 Stages of the cement manufacture Raw Material Preparation Burning Process

Grinding Process

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Cooling Process

Finish Mill Grinding

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Cement Kiln, Grinding and Storage

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Grinding, Storage and Distribution

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The addition of Gypsum occurs

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The particle size distribution is determined also in this stage

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Finally, Portland Cement is completed

Gypsum Addition GYPSUM 

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(CaSO4.1/2H2O)

Pulverize Mixture and Blend to give PSD

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Ca.SO4.2H2O 3 to 5 % Concentration Blend Of Gypsum – Calcium Sulphate Hemihydrate – Anhydrate (CaSO4)

Particle Size Distribution (PSD) Also known as Fineness of Cement Determines the Cement Reactivity

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Slurry Rheology

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Development of Compressive Strength

Thickening Time Measured 3 different ways 

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Wagner

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Blaine

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BET

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Effects of this stage on Cement Properties GRINDING PROCESS AND GYPSUM ADDITION Increased Proportion of cement reacts Rate of Hydration Increased Higher Early Strength Development

GYPSUM

Controlled Addition Difficult Calcium Sulphate Hemi-Hydrate formation leads to shorter Hydration times.

Too Little gives Gelation Problems (rapid C3A)

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FINER GRIND

Cement Clinker Grain Structure

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Effects of Major Clinker Components SILICATE PHASE 

C3S, TRICALCIUM SILICATE

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C2S, DICALCIUM SILICATE – Formed from CaO and SiO2 – Hydrates very slowly with lowest heat of hydration – Affects long term compressive strength

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– Major component in cement (formed from CaO and SiO2) – Hydrates more rapidly than C2S (control setting time) – Contributes to all stages of strength development (esp. early)

C3S AND C2S Hydration

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Effects of Major Clinker Components ALUMINATE PHASE 

C3A, TRICALCIUM ALUMINATE Formed from CaO and Al2O3 Hydrates rapidly and produces highest heat of hydration Plays an important role in: • • •

Early strength development Rheological behavior of slurry Controlling setting and thickening time

– Hydrated C3A is readily attacked by sulphate 

C4AF, TETRACALCIUM ALUMINOFERRITE – – – –

Formed from CaO, Al2O3, and Fe2O3 Gives color to cement Little effect on set properties Produces low heat of hydration

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– – –

Formation of Cement Hydrates Schlumberger Private

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Hydration of Portland Cement 60

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Hydration of Portland Cement

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PRE-INDUCTION PERIOD

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Pre-induction period lasts 30 sec - 4 mins (just after cement mixing)

Highly exothermic reaction

Initial C-S-H gel layer is formed over the C3S surfaces

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Ettringite is formed and precipitates onto the C3A surfaces

Hydration reactions reduced because: 

C3A inhibited by ettringite formation

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C3S inhibited by C-S-H gel layer

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C2S and C4AF naturally slow reactions

Ca(OH)2 (Portlandite) concentration starts increasing

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Hydration of Portland Cement

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INDUCTION PERIOD

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Hydration activity low and heat liberation falls Ca2+ and OH- ion concentration increasing to saturation with respect to Ca(OH)2

Period ends with increase in hydration activity and heat generation  

Change C-S-H gel layer allows hydration of C3S to continue Gypsum used up so C3A hydration resumes quickly

Only small percentage of C3S hydrated during this period

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Ca2+ and OH- ion critical supersaturation level reached, Ca(OH)2 starts to precipitate

Hydration of Portland Cement

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ACCELERATION PERIOD

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Most rapid hydration occurs (Still most C3S available for reaction) Solid Ca(OH)2 crystallizes from solution Cohesive network is formed, strength is developed DECELERATION PERIOD System porosity decreases as more hydrates formed Diffusion of water through hydrate layers is reduced Transport of ionic species is hindered Hydration rate decelerates

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Hydrates interlink and grow, decreasing porosity

Hydration of Portland Cement

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DIFFUSION PERIOD Hydration continues very slowly Reducing rate of heat evolution Thickening of hydration products around cement phases = Porosity Reduction No major structural changes occur Total hydration is never attained under ambient conditions due to Portlandite engulfing C3S grains Period indefinite under ambient conditions 20

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Hydrated C3S Grains (Acceleration Period) Schlumberger Private

Hydrated C3S Grains (Deceleration Period) Schlumberger Private

Physical & Chemical properties of Cement

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Volume changes during setting (p) Temperature Effects (p) Flash & False set (c) Effects of Aging (p) Influence of Alkalis (c) Influence of Surface Area (p) Sulfate Resistance (c) Strength Retrogression (c)

Volume changes during setting Absolute Volume (Chemical Shrinkage) – It results from the reaction with water – 4 -6 % by volume reduction Schlumberger Private

Bulk Volume

– It can increase or decrease – If it decreases, is very low < 0.2% by volume – If it increases, is due to a porosity increase in the cement matrix

Flash & False Set Flash set

Uncontrolled C3A hydration (low or no gypsum)

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High clinker activity

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Pseudo Set

False set 

Precipitation of “secondary gypsum” due to dehydration of gypsum and higher solubility of products formed

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Slurry gelation inevitable

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Reversible by vigorous agitation

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Aging Effects A Result Of: Long Storage Periods

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High Humidity

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Hot Temperatures (False Set risk)

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Carbonation

Affect Cement By: 

Increased Thickening Time

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Decreased Compressive Strength

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Decreased Heat Of Hydration

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Increased Slurry Viscosity

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Unpredictable Slurry Viscosity

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Bulk Handling More Difficult

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Influence of Surface Area Also called fineness Influence the following Cement reactivity

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Slurry rheology

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Thickening time

Compressive strength Measured 3 different ways 

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Wagner

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Blaine

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BET

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Sulfate Resistance

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Expansion – Loss of Compressive Strength

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Cracking – Damage to tubulars

Low C3A content are less likely to sulfate attack 

HSR (High sulfate resistant C3A < 3%)

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MSR (Medium sulfate resistant 3%