PC Cement Hydration

Introduction

Portland Cement ConcreteContinous binder phase: the cementitious matrix

•Binder effect on PCC behaviorAffects permeabilityAffects strength

Dispersed particulate phase: the aggregates•Coarse: #4 to 1½”•Fine: #100 to #4•Aggregates have a major effect on PCC behavior

Serve as a fillerIncrease concrete modulus of elasticity

Cementitious Phase

•Portland Cement•Water•Admixtures

LiquidMineral

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Workability & Strength

Cement Manufacture

•Quarrying – Raw materials •Crushing•Grinding •Mixing•Calcinated (1100C)•Burned (1450C)•Clinker is produced (10 mm size)•Inter-ground with 5% gypsum (1-100 m)

- most reactive ( <50 m)

Hydration process

Hydration process

Setting – Solidification of the plastic cement paste

•Initial set – beginning of solidification – Paste become unworkable – loss in consistency - not < 45 min.

•Final set – Time taken to solidify completely – Not > 375min.

Hardening – Strength gain with time – after final set

2C3S + 11H C3S2H8 + 3CH H = -500 J/g

2C2S + 9H C3S2H8 + CH H = -250 J/g

Calcium silicates (C3S or C2S) + water Calcium silicates hydrate (C-S-H) + calcium hydroxide

•Amount of CH depends on proportion of C3S and C2S•CSH - amorphous in nature, is an inexact composition,and is extremely fine (Colloidal).

Hydration - Exothermic Reaction

Tricalcium Aluminate (C3A)C3A + H2O reacts very fastC3A + H2O + CSH2 (Gypsum) reacts much slowerC3A + 3CSH2 + 26H C6AS3H32 H = -1350 J/g

Tricalcium Aluminate + Gypsum + Water Ettringite (product #3)

Once CSH2 is depleted:C6AS3H32 + 2C3A + 4H 3C4ASH12

Ettringite + Tricalcium Aluminate + Water Monosulfoaluminate (product #4)

Ferrite Phase: C4AF

•Forms same reaction as C3A but to a lesser degree•Uses small amount of gypsum

C4AF + 2CH + 14H C4(A,F)H13 + (A,F)H3

Ferrite + Calcium Hydroxide + Water Tetracalcium Hydrate + Ferric Aluminum Hydroxide

(product #5) (product #6)like monosulfoaluminate amorphous

Hydration of Portland cement

Sequence of overlapping chemical reactions

Hydration reactions of individual clinker mineral proceed simultaneously at differing rates and influence each other

A complex dissolution and precipitation process

Leading to continuous cement paste stiffening and hardening

Reaction rate: C3A > C3S > C4AF > C2S

Hydration of Portland cement

Reactivity

•Crystal size – Heating rate, burning temp.

•Crystal defects vs. impurities

•polymorphic form – rate of cooling

•Fineness

e.g. C3S and C2S with impurities hydrate faster than their pure forms

Heat of hydration (Cal/g)

Compound 3 days 90 days 13 years

C3S 58 104 122

C2S 12 42 59

C3A 212 311 324

C4AF 69 98 102

Model of CSH

ASTM Types of Portland Cements

I II III IV VC3S 50 45 60 25 40C2S 25 30 15 50 40C3A 12 7 10 5 4C4AF 8 12 8 12 10Gypsum 5 5 5 4 4

Fineness350 350 450 300 350(m2/kg)

CCS (psi) 1000 900 2000 450 900

Heat of 330 250 500 210 250Hydration(J/g)

I II III IV V

C3S 50 45 60 25 40

C2S 25 30 15 50 40

C3A 12 7 10 5 4

C4AF 8 12 8 12 10

Fine 350 350 450 300 350

Blended Cements

•20 to 70% of total binding material•Total = Cement & supplementary cementitious material•Most mineral admixtures are industrial by products•Use is economical, ecological, or technical in nature

Fly ash: coal fired power plantsBlast furnace slag: steel production

•lower heat, improved durability• Fine pore structure and lower permeability with same w/c•Improve workability

Pozzolans

2S + 3CH + 7H C3S2H8

•First used by Romans•CSH is of lower CaO content•Low heat and slow strength gain•Similar to increase in C2S•Reactivity based on surface area (silica fume)•Some contain alumina (can present durability problems)•Crystalline compounds (quartz); acts to dilutents•Unburned carbon may affect air entrainment•Can have a wide range of composition and reactivity

Blast Furnace Slag

•Rapidly cooled slags - to prevent crystallization•(CSA)glass + H C3(SiA)2H8 (self - reacting)•Forms alumin substituted CSH•Presence of CH accelerated reaction•Mixed with cement

Porosity and pore structure

Capillary pores

Gel pores

Pore size distribution

2.5 nm

High strength and low permeability concrete

• Low W/C ratioProper mixture proportioningUse of superplasticizersUse of pozzolans

•High degree of hydrationGood curing