PC Cement Hydration

Introduction Portland Cement Concrete

Continous binder phase

: the cementitious matrix •Binder effect on PCC behavior Affects permeability Affects 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 filler Increase concrete modulus of elasticity

Cementitious Phase

•Portland Cement •Water •Admixtures Liquid Mineral }

Workability & Strength

Cement Manufacture

•Quarrying – Raw materials •Crushing •Grinding •Mixing •Calcinated (1100  C) •Burned (1450  C) •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 set – Strength gain with time – after final

Hydration - Exothermic Reaction

2C 3 S + 11H  C 3 S 2 H 8 + 3CH  H = -500 J/g 2C 2 S + 9H  C 3 S 2 H 8 + CH  H = -250 J/g Calcium silicates (C 3 S or C 2 S) + water  Calcium silicates hydrate (C-S-H) + calcium hydroxide •Amount of CH •CSH depends on proportion of C 3 S and C 2 S - amorphous in nature, is an inexact composition, and is extremely fine (Colloidal).

Tricalcium Aluminate (C

3

A)

C C 3 A + H 2 O + CSH 2 C 3 3 A + H 2 O  A + 3CSH 2 reacts very fast (Gypsum) + 26H  C 6 AS 3  H reacts much slower 32  H = -1350 J/g Tricalcium Aluminate + Gypsum + Water  Ettringite (product #3) Once CSH 2 C 6 AS 3 H 32 is depleted: + 2C 3 A + 4H  3C 4 ASH 12 Ettringite + Tricalcium Aluminate + Water  Monosulfoaluminate (product #4)

Ferrite Phase: C

4

AF

•Forms same reaction as C 3 A but to a lesser degree •Uses small amount of gypsum C 4 AF + 2CH + 14H  C 4 (A,F)H 13 + (A,F)H 3 Ferrite + Calcium Hydroxide + Water  Tetracalcium Hydrate + Ferric Aluminum Hydroxide (product #5) (product #6) like monosulfoaluminate amorphous

Hydration of Portland cement

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Sequence of overlapping chemical reactions

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

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A complex dissolution and precipitation process

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Leading to continuous cement paste stiffening and hardening

Reaction rate: C3A > C3S > C4AF > C2S

Hydration of Portland cement

Reactivity

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Crystal size – Heating rate, burning temp.

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Crystal defects vs. impurities

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polymorphic form – rate of cooling

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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 C 3 S C 2 S C 3 A C 4 AF 58 12 212 69 104 42 311 98 122 59 324 102

Model of CSH

ASTM Types of Portland Cements C3S C2S C3A C4AF Gypsum I 50 25 12 8 5 II 45 30 7 12 5 III 60 15 10 8 5 IV 25 50 5 12 4 V 40 40 4 10 4 Fineness (m2/kg) 350 350 450 300 350 CCS (psi) Heat of Hydration (J/g) 1000 900 2000 450 900 330 250 500 210 250

I II III IV V C 3 S C 2 S 50 25 45 30 60 15 25 50 40 40 C 3 A Fine 12 C 4 AF 8 7 12 10 8 5 12 4 10 350 350 450 300 350

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 plants Blast furnace slag: steel production •lower heat, improved durability • Fine pore structure and lower permeability with same w/c •Improve workability

Pozzolans

2S + 3CH + 7H  C 3 S 2 H 8 •First used by Romans •CSH is of lower CaO content •Low heat and slow strength gain •Similar to increase in C 2 S •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  C 3 (SiA) 2 H 8 (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

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Low W/C ratio

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Proper mixture proportioning

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Use of superplasticizers

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Use of pozzolans

•High degree of hydration 

Good curing