Calcium sulfate hemihydrate (CaSO4 • 0.5H2O) is the dehydrated product of gypsum, calcium sulfate dihydrate (CaSO4•2H2O). It is obtained by heating gypsum to approximately 150°C. It is known as gypsum plaster or plaster of Paris (POP). On adding water, it solidifies by an exothermic reaction into hydrated calcium sulphate. It is commonly used to precast and hold parts of ornamental plasterwork placed on ceilings and cornices. Plaster of Paris casts can be used as a treatment of fractures to immobilize the limb. It is also used as a desiccant, and in medicine as a tablet excipient.
