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cement

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Civil Engineering Materials 267Materials Technology Cement

Manufacture Composition Hydration setting, strength, heat Types

Portland cementIn Britain in the early part of the nineteenth century hydraulic limestone was used to manufacture Cement > Portland cement, and its name is derived from its similarity to Portland stone, a type of building stone that was quarried on the Isle of Portland in Dorset, England.

CementMain binding agent in concreteActive component reacts with water to form new compounds Most costly component of concrete

Portland Cement (OPC)Finely ground powdered reagents which harden when mixed with water Formula governed by standards

Blended CementsMixtures of OPC and other pozzolans eg fly ash

Typical composition limits of Portland cementComponentsCaO

Content (%)60-67

SiO2Al2O3 Fe2O3 MgO Alkalis (Na2O, K2O) SO3

17-253-8 0.5-0.6 0.5-4.0 0.3-1.2 2.0-3.5

Manufacture of Portland CementInputsCalcium carbonate (CaCO3) Silica (SiO2) Alumina (Al2O3) Iron Oxide (Fe2O3) Lime stone Sand Clay/Shale Iron ore

Manufacture

Mining, transport Grinding Calcining CaCO3 CaO + CO2 Kiln heating to give clinker Final grinding with gypsum (CaSO4) controls hydration rate

Production of Portland Cement

Schematic diagram of rotary kiln.

Chemical constituents of OPCCompoundTricalcium silicate (about 50%) (Early strength) Dicalcium silicate (about 25%) (Late strength) Tricalcium aluminate (about 10%) (Early strength high strength) Tetracalcium alumino-ferrite (about 10%) (Dark colour) Gypsum (about 5%)

Chemical formula Brief formula3CaO.SiO2 2CaO.SiO2 3CaO.Al2O3 4CaO.Al2O3.Fe2O3 C3S C2S C3A C4AF

CaSO4.2H20

Chemical constituents of OPCCompoundC3S C2S

CharacteristicsLight in colour Rapid reaction evolution of heat Early strength Light in colour Slower reaction Late strength

C3AC4AF Gypsum

Light in colour Rapid reaction evolution of heat Enhances strength of silicatesDark in colour Controls hydration rate

Hydration reactionsSeries of chemical reactionsNew compounds - hydrates Exothermic reactions produces heat

Early reactionsC3S and C3A retarded by gypsum Forms initial crystalline framework Cements high in C3S give higher early strength higher setting temperatures C3S less resistant to acids, sulphates

Hydration reactionC3S + Water ---> C-S-H + Calcium hydroxide + heat 2 Ca3SiO5 + 7 H2O ---> 3 CaO.2SiO2.4H2O + 3 Ca(OH)2 + 173.6kJ pH rises over 12 because of the release of (OH)Hydrolysis slows down quickly after it starts, resulting in the decrease in heat evolved. The reaction slowly continues producing Ca- and (OH)- until the system becomes saturated. Ca(OH)2 starts to crystallize. Simultaneously, calcium silicate hydrate begins to form.

Pores in calcium silicate through different stages of hydrationWater Calcium silicate grains

C-S-H

(a) (b) (c) (d)

Hydration has not yet occurred and the pores (empty spaces between grains) are filled with water. Beginning of hydration. Hydration continues. Although empty spaces still exist, they are filled with water and calcium hydroxide. Nearly hardened cement paste. Note that the majority of space is filled with calcium silicate hydrate.

Hydration reactionC2S + Water ---> C-S-H + Calcium hydroxide +heat2 Ca2SiO4 + 5 H2O---> 3 CaO.2SiO2.4H2O + Ca(OH)2 + 58.6 kJ

Tricalcium aluminate and tetracalcium aluminoferrite also react with water. Their hydration chemistry is more complicated as they involve reactions with the gypsum as well.

Rate of heat evolution during the hydration of Portland cement

Rate of heat evolution during the hydration of Portland cement (Contd)Stage I: Hydrolysis of the cement compounds occurs rapidly with a temperature increase of several degrees. Stage II: The evolution of heat slows dramatically in this stage. This period can last from one to three hours. During this period, the concrete is in a plastic state which allows the concrete to be transported and placed without any major difficulty. It is at the end of this stage that initial setting begins. Stages III and IV: Concrete starts to harden and the heat evolution increases due primarily to the hydration of C3S. Stage V is reached after 36 hours. The slow formation of hydrate products (C-S-H) occurs and continues as long as water and unhydrated silicates are present.

Hydration reactionsLater reactionsC2S slower reaction producing less heat Fills out crystalline framework and decreases porosity C2S products have higher ultimate compressive strength, but attain strength slowly Cements high in C2S have better chemical resistance

Two stagesStage 1 setting Stage 2 hardening

Heat of HydrationCement hydration exothermic heat Amount and rate of heat productionComposition and fineness of cement Water / cement ratio Curing temperature

Temperature affected byThickness of concrete Surface treatment during curing

Heat of HydrationThick concrete elementsHeat not easily dissipated Heat must be managed externally May use low heat cements

Rate of strength gainHeat of hydration related to rate of strength gain

Setting TimeDepends onFineness of cement Gypsum content of cement Amount and temperature of water Ambient temperature

2 to 10 hours

Important forMixing, transport Placing, compaction, finishing Strength for future construction

Balance required

Strength Development

Similar

Separate from Setting (hardening) TimeSetting rate is constant for given cement formulation Strength development rate depends on fineness of cement Fine cements have greater surface area exposed to water for hydration reaction Gain in strength maximum at early ages

ShrinkageVariations in the moisture content of cement paste are accompanied by volume changes.

Drying volume decrease Wetting volume increase Concrete shrinkage restrainedReinforcement Aggregates with water / cement ratio

Types of CementsType GP General purpose Portland cement Type GB General purpose Blended cement Type HE High Early strength cement Type LH Low Heat cement Type SR Sulphate Resisting cement Off white and white Portland cements Coloured cements Masonry cements Oil well cements High Alumina Cement (HAC)

Type GP General Purpose Portland cementMost common cement in constructionLeast expensive Best understood Default cement used in concrete Grey 48-65% C3S, 10-30% C2S, 2-11%C3A, 7-17% C4AF

Type GB General Purpose Blended cement

Can also be used in most forms of constructionVarying %age Portland cement varying properties Range of different additives Generally lower rate of strength gain than GP Generally similar ultimate strength to GP

Type HE High Early strength cementSpecial cement with high C3S, &/or fine grindGenerates more heat not for thick sections Good in cold weather Useful for early prestress or early form strip Grey 50-65% C3S, 7-25% C2S, 6-13%C3A, 7-13% C4AF

Type LH Low Heat cementUsed in massive concrete thick sections, high temperaturesPortland cements with high C2S or blended cements Lower strength gain than GP 25-30% C3S, 40-45% C2S, 3-6%C3A, 12-17% C4AF

Type SR Sulphate Resisting cementUsed for ground waters containing sulphates or for Aggregates with sulphatesLower C3A content enhances sulphate resistance 50-60% C3S, 15-25% C2S, 2-5%C3A , 10-15% C4AF

Cement compositionType of Portland Cement GP HE LH SR Hypothetical Compound Composition (%) C3S C2S C3 A C4AF 48-65 10-30 2-11 7-17 50-65 7-25 6-13 7-13 25-30 40-45 3-6 12-17 50-60 15-25 2-5 10-15

Strength development summaryCement paste Impractical - Expense - Shrinkage

Other CementsOff-white and white Portland cementsLow in C4AF Used for specific architectural requirements

Coloured cementsContain durable inorganic pigments

Masonry CementsFor mortars high workability, high water retention Unsuitable for concrete

Other cements

Oil-well cements (grouts)Slurry stays fluid longer and under high temps and pressures Rapid hardening once hydration commences Resistant to sulphur, aggressive water

High Alumina Cements (HAC)Manufactured from Bauxite product High early strength Expansive (low shrinkage) Can lose strength at high temps, humidity

SummaryAggregate properties Source Sieve analysis Grading, Fineness modulus Cement Composition Hydration setting, strength, heat Types

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