Civil Engineering Materials 267Materials TechnologyCement

•Manufacture•Composition•Hydration – setting, strength, heat•Types

Portland cement

In 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.

Main binding agent in concreteActive component – reacts with water to form new compoundsMost costly component of concrete

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

Blended CementsMixtures of OPC and other pozzolans eg fly ash

Cement

Typical composition limits of Portland cementComponents Content (%)

CaO 60-67

SiO2 17-25

Al2O3 3-8

Fe2O3 0.5-0.6

MgO 0.5-4.0

Alkalis (Na2O, K2O) 0.3-1.2

SO3 2.0-3.5

Manufacture of Portland CementInputs

Calcium carbonate (CaCO3) Lime stone

Silica (SiO2) Sand

Alumina (Al2O3) Clay/Shale

Iron Oxide (Fe2O3) Iron ore

Manufacture Mining, transportGrindingCalcining – CaCO3 CaO + CO2Kiln heating to give “clinker”Final grinding with gypsum (CaSO4) – controls hydration rate

Production of Portland Cement

Schematic diagram of rotary kiln.

Chemical constituents of OPCCompound Chemical

formulaBrief formula

Tricalcium silicate (about 50%)(Early strength)

3CaO.SiO2 C3S

Dicalcium silicate (about 25%)(Late strength)

2CaO.SiO2 C2S

Tricalcium aluminate (about 10%)(Early strength – high strength)

3CaO.Al2O3 C3A

Tetracalcium alumino-ferrite (about 10%)(Dark colour)

4CaO.Al2O3.Fe2O3 C4AF

Gypsum (about 5%)CaSO4.2H20

Chemical constituents of OPCCompound Characteristics

C3S Light in colourRapid reaction – evolution of heatEarly strength

C2S Light in colourSlower reactionLate strength

C3A Light in colourRapid reaction – evolution of heatEnhances strength of silicates

C4AF Dark in colour

Gypsum Controls hydration rate

Hydration reactionsSeries of chemical reactions

New compounds - hydratesExothermic reactions – produces heat

Early reactionsC3S and C3A – retarded by gypsum

Forms initial crystalline frameworkCements high in C3S give higher early strength – higher setting temperaturesC3S 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 hydration Calcium silicate grainsWater

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

C-S-H

Hydration reactionC2S + Water ---> C-S-H + Calcium hydroxide +heat

2 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 (Cont’d)

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 reactions

C2S – slower reaction producing less heatFills out crystalline framework and decreases porosityC2S products have higher ultimate compressive strength, but attain strength slowlyCements high in C2S have better chemical resistance

Two stagesStage 1 settingStage 2 hardening

Heat of HydrationCement hydration exothermic heatAmount and rate of heat production

Composition and fineness of cementWater / cement ratioCuring temperature

Temperature affected byThickness of concreteSurface treatment during curing

Heat of HydrationThick concrete elements

Heat not easily dissipatedHeat must be managed externallyMay use low heat cements

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

Setting TimeDepends on

Fineness of cementGypsum content of cementAmount and temperature of waterAmbient temperature

Important forMixing, transportPlacing, compaction, finishingStrength for future construction

Balancerequired

2 to 10 hours

Strength Development

Separate from Setting (hardening) Time

Setting rate is constant for given cement formulationStrength development rate depends on fineness of cementFine cements have greater surface area exposed to water for hydration reactionGain in strength maximum at early ages

Similar

Shrinkage

Drying volume decreaseWetting volume increaseConcrete shrinkage restrained

ReinforcementAggregates with water / cement ratio

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

Types of CementsType GP – General purpose Portland cementType GB – General purpose Blended cementType HE – High Early strength cementType LH – Low Heat cementType SR – Sulphate Resisting cementOff white and white Portland cementsColoured cementsMasonry cementsOil well cementsHigh Alumina Cement (HAC)

Type GP – General Purpose Portland cement

Most common cement in construction

Least expensiveBest understoodDefault cement used in concreteGrey48-65% C3S, 10-30% C2S, 2-11%C3A, 7-17% C4AF

Type GB – General Purpose Blended cement

Can also be used in most forms of construction

Varying %age Portland cement varying propertiesRange of different additives Generally lower rate of strength gain than GPGenerally similar ultimate strength to GP

Type HE – High Early strength cementSpecial cement with high C3S, &/or fine grind

Generates more heat not for thick sectionsGood in cold weatherUseful for early prestress or early form stripGrey50-65% C3S, 7-25% C2S, 6-13%C3A, 7-13% C4AF

Type LH – Low Heat cement

Used in massive concrete – thick sections, high temperatures

Portland cements with high C2S or blended cementsLower strength gain than GP25-30% C3S, 40-45% C2S, 3-6%C3A, 12-17% C4AF

Type SR – Sulphate Resisting cementUsed for ground waters containing sulphates or for Aggregates with sulphates

Lower C3A content enhances sulphate resistance50-60% C3S, 15-25% C2S, 2-5%C3A , 10-15% C4AF

Cement composition

Hypothetical Compound Composition (%)

Type of Portland Cement C3S C2S C3A C4AF

GP 48-65 10-30 2-11 7-17

HE 50-65 7-25 6-13 7-13

LH 25-30 40-45 3-6 12-17

SR 50-60 15-25 2-5 10-15

Strength development summary

Cement paste Impractical- Expense- Shrinkage

Other CementsOff-white and white Portland cements

Low in C4AF

Used for specific architectural requirements

Coloured cementsContain durable inorganic pigments

Masonry CementsFor mortars – high workability, high water retentionUnsuitable for concrete

Other cementsOil-well cements (grouts)

Slurry stays fluid longer and under high temps and pressuresRapid hardening once hydration commencesResistant to sulphur, aggressive water

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

SummaryAggregate propertiesSourceSieve analysis – Grading, Fineness modulus

CementCompositionHydration – setting, strength, heatTypes