Cement

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Types

Cement normally consists of a 30:20:1 ratio of calcium oxide, silicon dioxide, and gypsum. Some aluminum oxide can be substituted for silicon dioxide.

Portland Cement

C3S / Alite 50% {{#Chem: 3CaO*SiO2}} C3S + 6H = C-S-H + 3CH

Alite + 6 Water = C-S-H + 3 Portlandite
{{#Chem: 2(CaO)3SiO2 + 6H2O = (CaO)3(SiO)2(OH)3 + 3Ca(OH)2 dH=-173.6}}

+120cal/g very reactive compound, high heat of hydration, high early strength
C2S / Belite 25% {{#Chem: 2CaO*SiO2}} C2S + 4W = C-S-H + CH

Belite + 4 Water = C-S-H + Portlandite
{{#Chem: (CaO)2SiO2 + 4H2O = (CaO)3(SiO)2(OH)3 + CaOH}}

+62cal/g low heat of hydration, slow reaction
C3A / Celite 10% {{#Chem:3CaO*Al2O3}} C3A + CSH2 = Ettringite

Celite + Gypsum = Ettringite
{{#Chem: (CaO)3Al2O3 + CaSO4 = (CaO)3(Al2O3)(CaSO4)3·32H2O}}

+300cal/g problems with sulfate attack, high heat of hydration
C4AF 10% {{#Chem:4CaO*Al2O3*Fe2O3}} Absorbs excess sulfate to prevent it from poisoning other reactions
Gypsum 5% {{#Chem:CaSO4}} used to control the set of cement

These materials form at 1450°C. The C-S-H component is what drives the wet-set to completion and results in the strong cement bond in the finished product.

See Also