Tricalcium aluminate, 3CaO.Al₂O₃

Tricalcium aluminate, 3CaO.Al₂O₃.

Projection of liquidus surface for the system lime-silica-alumina.

Tricalcium aluminate, 3CaO.Al₂O₃, is an important constituent of Portland cement clinker, which consists mainly (over 90 per cent.) of silica, lime, and alumina. Its constitution has long been the subject of discussion. A complete thermal and optical study of the system lime - silica - alumina has been made by Rankin and Wright, and the results, so far as they bear on the constitution of Portland cement, are shown in fig. Considering only the lime, silica, and alumina content, the variations in the composition of the commercial cement are restricted to the area ABCD. It may be deduced from the diagram by aid of the Phase Rule that a fluid mass of this composition, when slowly cooled until complete solidification has occurred, is converted into a mixture of tricalcium silicate, 3CaO.SiO₂, dicalcium silicate, 2CaO.SiO₂, and tricalcium aluminate, 3CaO.Al₂O₃.

Portland cement clinker should therefore consist essentially of these three compounds if, in its manufacture, the heating be prolonged for a length of time sufficient for equilibrium to be established. This deduction has been verified by experiments carried out in the laboratories of the United States Bureau of Standards. Since, however, complete equilibrium is not reached in its manufacture, the cement also contains lime, CaO, and the aluminate 5CaO.3Al₂O₃ as minor constituents. The iron oxide contained in the cement may be present as calcium ferrite or magnetite, or held in solid solution in the compounds 2CaO.SiO₂ and 5CaO.2Al₂O₃; the magnesia and alkalies are apparently held in solid solution in the compounds 2CaO.SiO₂ and 3CaO.Al₂O₃.

In contact with water, the compounds CaO.Al₂O₃, 3CaO.Al₂O₃, and 5CaO.3Al₂O₃ yield an amorphous hydrate, 3CaO.Al₂O₃.xH₂O, which in time crystallises; except with tricalcium aluminate, amorphous aluminium hydroxide is also formed. When hydrated with aqueous calcium sulphate an additional compound, calcium "sulpho-aluminate." 3CaO.Al₂O₃.3CaSO₄.xH₂O, is formed, which crystallises in long prismatic needles.