CaSO4

Calcium sulfate, with the formula CaSO4, is an inorganic salt composed of calcium cations and sulfate anions. In nature it occurs mainly as the minerals gypsum (CaSO4·2H2O) and anhydrite (CaSO4). A related hydrated form, the hemihydrate CaSO4·0.5H2O, forms by partial dehydration of gypsum and is commonly produced as plaster of Paris. Gypsum also appears in varieties such as alabaster and selenite.

In industry, calcium sulfate is typically obtained from natural gypsum or as a byproduct of flue-gas desulfurization. Heating gypsum converts it to the hemihydrate (plaster of Paris); when mixed with water, plaster rehydrates and cures to a hard matrix, a property exploited in casts, molds, and wall plasters. Further heating yields the anhydrous form, CaSO4, which has different setting and hydration behavior. The dihydrate and hemihydrate forms have distinct handling properties and applications.

Calcium sulfate is sparingly soluble in water, dissolving to form Ca2+ and SO4^2− ions. Its solid forms provide useful physical properties: gypsum is soft and readily moldable, while anhydrite is harder and less prone to hydration. Common applications include construction materials (plasters, drywall cores), dental and medical materials (impression compounds, bone graft substitutes), and as a desiccant or filler in various products.

Safety and environment: calcium sulfate is generally of low toxicity and is noncombustible. It is considered relatively inert under normal conditions, though ingestion in large quantities can have laxative effects due to sulfate content. In geology and industry, its stability and low reactivity contribute to its prominence in evaporite deposits and building materials.