Pentacalcium Aluminate, 5CaO.3Al₂O₃

Pentacalcium Aluminate, 5CaO.3Al₂O₃ and 3CaO.5Al₂O₃ both exist in a stable and unstable form. The stable forms have respectively the melting-points 1455°±5° C. and 1720°±10° C. The unstable forms are only produced under special conditions of cooling of the fused compounds.

By the action of lime-water on metallic aluminium a hydrated crystalline dicalcium aluminate, Ca₂Al₂O₅.7H₂O, is obtained.

Calcium aluminates are mainly important from their relation to Portland cement and will be mentioned again under that heading.

Several natural calcium alumino-silicates are known, anorthite, CaAl₂(SiO₄)₂, lime garnet, Ca₃Al₂(SiO₄)₃, and several members of the group of minerals known as zeolites which have the general formula M^••Al₂Si_nO2n + 4.mH₂O.