Calcium Sulfate Dihydrate → Calcium Sulfate, Hemihydrate

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Reaction Steps

Freshly mined gypsum is known as landplaster. It is an ore that includes calcium sulfate dihydrate, also known as gypsum, plaster or terra alba. In many deposits, landplaster includes at least 50% by weight calcium sulfate dihydrate. The calcium sulfate dihydrate is calcined at temperatures above 150° C. to drive off part of the water of hydration, forming calcium sulfate hemihydrate, also known as calcined gypsum, stucco, Plaster of Paris, calcium sulfate semihydrate or calcium sulfate half-hydrate. When added to water, calcium sulfate hemihydrate crystallizes with the water to reform the dihydrate. As the reaction proceeds, an interwoven matrix of calcium sulfate dihydrate crystals forms and becomes hard. The reaction is represented by the formula: CaSO4.½H2O+ 3/2H2O→CaSO4.2H2O

Provenance & Evidence