Tricalcium Aluminate, 3CaO.Al₂O₃

Campbell regards Tricalcium Aluminate, 3CaO.Al₂O₃, as a metastable saturated solution of lime in the compound 5CaO.3Al₂O₃, or as 5CaO.3Al₂O₃, with 4 molecules of calcium oxide of crystallisation, rather than as a stable compound, but the balance of evidence seems to be in favour of the latter view.

Tricalcium aluminate decomposes with partial fusion at 1535°±2° C., and is therefore best obtained by crystallising from a glass of the same composition a little below this temperature. It does not possess hydraulic properties.

Phillips showed that a colloidal solution of tricalcium aluminate can be obtained, but more readily in lime-water than in pure water. It is a positively charged colloid, not coagulated by heat, but by freezing. It has a high adsorptive power and resembles the sols of ferric, aluminium, and chromic oxides.

A hydrated compound, 3CaO.Al₂O₃.6H₂O, is obtained as a crystalline precipitate by the action of hot lime-water on potassium aluminate.