Calcium sulfate hemihydrate

Calcium sulfate hemihydrate (CAS 10034-76-1) is an inorganic calcium salt commercially known under trade names such as Prestia 23 and P 23, and commonly referred to as dried gypsum or plaster of Paris. It is produced by partially dehydrating gypsum and carries the molecular formula Ca.H2O4S.1/2H2O.

In building and construction, it sets rapidly upon rehydration, forming interlocking gypsum crystals that provide structural rigidity in plasters, wallboards, and self-levelling compounds. In papermaking, it acts as a coagulant and filler, improving sheet formation and surface retention. Pharmaceutical manufacturers use it as a tablet binder and diluent, where its controlled setting behaviour supports consistent dosage form production.

Calcium sulfate hemihydrate is available in pharmaceutical and technical grades as a powder or granule. It is subject to REACH registration and may conform to Ph.Eur. or FCC monographs depending on the intended application. Packaging formats typically include 25 kg bags and bulk quantities.