In an investigation of the resistance of Portland cement to sulphate action a new form of hydrated tricalcium aluminate, with six molecules of water, was prepared by treating pure tricalcium aluminate with steam under pressure and drying the product over quicklime. This hydrate is crystalline and isotropic. It is stable at 100 deg. C., but at 275–300 deg. C. it gives up three-quarters of its water, forming a stable 1.5-hydrate. Some water is retained up to 1000 deg. C. Before the last of the water is expelled decomposition of the aluminate takes place. Recombination occurs at 1000–1100 deg. C.