hemihydraatiksi

Hemihydrate is a chemical form in which a hydrate contains half as many water molecules as the corresponding fully hydrated compound. The term is most commonly encountered in connection with calcium sulfate, but it applies to other hydrated salts as well. A typical example is calcium sulfate hemihydrate, CaSO4·0.5H2O, which is produced by calcining gypsum (CaSO4·2H2O) to remove part of its water of crystallization.

Production and properties: Calcium sulfate hemihydrate is formed by heating gypsum at moderate temperatures (around 120–180°C). This dehydration yields two commonly encountered forms: alpha-hemihydrate and beta-hemihydrate. Alpha-hemihydrate is denser and less porous, while beta-hemihydrate is more porous and easier to rehydrate. When hemihydrate is mixed with water, it rehydrates to calcium sulfate dihydrate (gypsum) in an exothermic setting reaction. The typical hydration reaction is CaSO4·0.5H2O + 1.5 H2O → CaSO4·2H2O.

Applications: The most well-known hemihydrate is the plaster of Paris, a finely ground powder used for making casts, models, and molds in construction, art, and dentistry. In medical and dental settings, plaster and related hemihydrates are used for immobilization and impression materials. Gypsum plasters set rapidly, with setting time adjustable by formulation, particle size, and additives.

Other notes: Hemihydrates can occur for other hydrated salts under dehydration, though calcium sulfate hemihydrate is by far the most commercially significant. Safety considerations include dust exposure and handling precautions, as with many inorganic powders.

In summary, hemihydrate describes a half-water form of a hydrate, with calcium sulfate hemihydrate being the best-known example and having widespread industrial and medical applications.