Portland Cement Composition

Portland cement is a mixture of calcium silicates and aluminate with specific composition:

Compound	Chemical Formula	Common Name	Role
Tricalcium silicate	$Ca_{3}SiO_{5}$ (or $C_{3}S$ )	Alite	Early strength (days to 28 days), hydrates rapidly
Dicalcium silicate	$Ca_{2}SiO_{4}$ (or $C_{2}S$ )	Belite	Long-term strength (months to years), hydrates slowly
Tricalcium aluminate	$Ca_{3}Al_{2}O_{6}$ (or $C_{3}A$ )	Celite	Very rapid (flash) setting — controlled by gypsum
Gypsum (retarder)	$CaSO_{4}$ · $2H_{2}O$	Gypsum	Added 2-3% to slow setting time

$2Ca_3SiO_5 + 6H_2O \rightarrow 3CaO \cdot 2SiO_2 \cdot 3H_2O + 3Ca(OH)_2$ $2Ca_2SiO_4 + 4H_2O \rightarrow 3CaO \cdot 2SiO_2 \cdot 3H_2O + Ca(OH)_2$

The product 3CaO· $2SiO_{2}$ · $3H_{2}O$ is C-S-H gel — the primary strength-giving phase.

How gypsum retards setting: $Ca_3Al_2O_6 + 3CaSO_4 \cdot 2H_2O + 26H_2O \rightarrow Ca_6Al_2(SO_4)_3(OH)_{12} \cdot 26H_2O\ \ (ettringite)$

Ettringite coating slows $C_{3}A$ hydration, giving workable time.