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