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.
Its solubility in H2O is 0.2parts/100 at 18.75o. It dehydrates completely >650o. Dry it below 300o to give a solid with estimated pore size ca 38% of volume. Anhydrous CaSO4 (Drierite) has a high affinity for H2O and will absorb 6.6% of its weight of H2O to form the hemihydrate (gypsum). It sets to a hard mass with H2O; hence it should be kept in a tightly sealed container. The solubility of gypsum in H2O is unusual: 0.176% at 0o, 0.209% at 30o, 0.210 at 40o, 0.204 at 50o and 0.200 at 60o. [Hulett J Am Chem Soc 27 49 1905, James & Partington J Chem Soc 107 1019 1915, Namba J Soc Chem Ind 40 2797 1920.]
white or slightly off-white powder or granules