hyd07 - Portland Cement Hydration Dr. Kimberly Kurtis...

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Dr. Kimberly Kurtis School of Civil Engineering Georgia Institute of Technology Atlanta, Georgia Portland Cement Hydration 3CaO•SiO 2 2CaO•SiO 2 3CaO•Al 2 O 3 4CaO•Al 2 O 3 •Fe 2 O 3 CaCO 3 (limestone) 2SiO 2 •Al 2 O 3 (clay, shale) Fe 2 O 3 (iron oxide) SiO 2 (silica sand) ~ 1450 o C Clinker interground Finished cement Cement Composition Gypsum + CaO•SO 3 •2H 2 O Kiln
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Cement Composition + impurities (K, Na, Mg, etc) the presence and substitution (e.g., Na + for Ca 2+ ) of impurities can lead to polymorphism, structural “holes”, and greater reactivity Cement Hydration Hydration - chemical combination of cement and water Two primary mechanisms: Through solution - involves dissolution of anhydrous compounds to their ionic constituents, formation of hydrates in solution, and eventual precipitation due to their low solubility Topochemical - or solid-state hydration - reactions take place directly at the surface of the anhydrous cement compounds without going into solution
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Cement Hydration When water is added to cement, what happens? Dissolution of cement grains Growing ionic concentration in “water” (now a solution) Formation of compounds in solution After reaching a saturation concentration, compounds precipitate out as solids (“hydration products”) In later stages, products form on or very near the surface of the anhydrous cement Al +3 Al +3 Al +3 Ca +2 Ca +2 Ca +2 OH - OH - OH - SiO - SiO - Cement Hydration
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Cement Hydration Taylor, Cement Chemistry Cement Hydration 4% C 4 AF 7% C 3 A 32% C 2 S 54% C 3 S 0% C 4 AF 17% C 3 A 30% C 2 S 52% C 3 S Image Credit: Bentz at NIST
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Cement Hydration Because the hydration rates of the 4 key phases vary considerably, properties like time to stiffening setting time hardening rate will vary with cement composition. Cement Hydration Taylor, Cement Chemistry
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Cement Hydration 0 2 04 06 08 0 1 0 0 Time (days) 0 10 20 30 40 50 Compressive Strength (MPa) 2,000 4,000 6,000 8,000 10,000 0 60 70 Compressive Strength (psi) C 3 A+CSH 2 C 4 AF+CSH 2 C 2 S C 3 S Compressive Strength development in pastes of pure cement compounds ( Mindess et al, 2003 ) ± Hydrates & hardens rapidly ± Responsible for initial set and early strength Tricalcium silicate (alite) C 3 S Properties of Hydrated Cement Compounds
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± Hydrates & hardens slowly ± Contributes to later age strength (beyond 7 days) Dicalcium silicate (belite) C 2 S ± Hydrates & hardens rapidly ± Responsible for initial set and early strength Tricalcium silicate (alite) C 3 S Properties of Hydrated Cement Compounds ± Liberates a large amount of heat during first few days ± Contributes slightly to early strength development ± Cements with low %-ages are more resistant to sulfates Tricalcium aluminate C 3 A ± Hydrates & hardens slowly ± Contributes to later age strength (beyond 7 days) Dicalcium silicate (belite) C 2 S ± Hydrates & hardens rapidly ± Responsible for initial set and early strength Tricalcium silicate (alite) C 3 S Properties of Hydrated Cement Compounds
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± Reduces clinkering temperature ± Hydrates rapidly but contributes little to strength ±
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hyd07 - Portland Cement Hydration Dr. Kimberly Kurtis...

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