hemihydratet

Hemihydrate is a chemical form in which a substance contains half the amount of water of crystallization found in its corresponding dihydrate. The term is most commonly applied to calcium sulfate hemihydrate, CaSO4·0.5H2O, which is widely known as plaster of Paris.

Calcium sulfate hemihydrate is produced by controlled dehydration (calcination) of gypsum, CaSO4·2H2O, at approximately 120–180°C. The resulting solid is a fine white powder that is reactive with water. The exact water content can vary slightly depending on the heating conditions, but the defining feature is that only half a mole of water remains per mole of calcium sulfate.

When calcium sulfate hemihydrate is mixed with water, it rehydrates to form the dihydrate and sets into a hard, rigid mass. The typical setting reaction is CaSO4·0.5H2O + 1.5 H2O → CaSO4·2H2O, which releases heat as the crystalline structure reorganizes. The setting rate depends on water amount, temperature, and additives.

Uses of plaster of Paris include medical casts and immobilization, dental and orthopedic applications, art and sculpture, and mold-making in industries such as sculpture and construction. In dentistry and orthopedics, specially formulated grades of calcium sulfate hemihydrate provide controlled setting properties and improved strength.

While calcium sulfate is the classic example, the term hemihydrate can apply to other salts that exist in a half-water-hydration form. Handling requires dry conditions to prevent premature hydration and inhalation precautions for dust.