Alphahemihydrate

Alpha-hemihydrate is a crystalline form of calcium sulfate hemihydrate with the chemical formula CaSO4·0.5H2O. It is produced by calcination of gypsum under hydrothermal conditions, i.e., high temperature and pressure, which yields a dense, strong crystal structure. This contrasts with beta-hemihydrate, which forms when gypsum is heated in open air at atmospheric pressure and tends to be more porous.

The alpha form is characterized by higher density and lower porosity compared with beta-hemihydrate. These characteristics translate into greater mechanical strength and lower set expansion when it hydrates back to gypsum. Because it is less susceptible to water-induced dissolution and achieves higher strength, alpha-hemihydrate is favored in applications requiring durable, high-strength gypsum products.

Common applications include high-strength plasters and dental stones. In dentistry, alpha-hemihydrate is used to produce Type III and Type IV dental stones, which require superior strength, hardness, and dimensional stability. In construction and industry, alpha-hemihydrate can be used for premium plasters and cementitious formulations where improved performance is desirable.

On hydration, alpha-hemihydrate reacts with water to form gypsum (CaSO4·2H2O): CaSO4·0.5H2O + 1.5 H2O → CaSO4·2H2O, releasing setting heat and binding the material. See also calcium sulfate, gypsum, plaster.