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Concept
Calcium sulfate
Summary
Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related hydrates. In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant. One particular hydrate is better known as plaster of Paris, and another occurs naturally as the mineral gypsum. It has many uses in industry. All forms are white solids that are poorly soluble in water. Calcium sulfate causes permanent hardness in water.
The compound exists in three levels of hydration corresponding to different crystallographic structures and to minerals:
CaSO4 (anhydrite): anhydrous state. The structure is related to that of zirconium orthosilicate (zircon): Ca2+ is 8-coordinate, SO42- is tetrahedral, O is 3-coordinate.
CaSO4·2H2O (gypsum and selenite (mineral)): dihydrate.
CaSO4·1/2H2O (bassanite): hemihydrate, also known as plaster of Paris. Specific hemihydrates are sometimes distinguished: α-hemihydrate and β-hemihydrate.
Gypsum#Uses
The main use of calcium sulfate is to produce plaster of Paris and stucco. These applications exploit the fact that calcium sulfate which has been powdered and calcined forms a moldable paste upon hydration and hardens as crystalline calcium sulfate dihydrate. It is also convenient that calcium sulfate is poorly soluble in water and does not readily dissolve in contact with water after its solidification.
With judicious heating, gypsum converts to the partially dehydrated mineral called bassanite or plaster of Paris. This material has the formula CaSO4·(nH2O), where 0.5 ≤ n ≤ 0.8. Temperatures between are required to drive off the water within its structure. The details of the temperature and time depend on ambient humidity. Temperatures as high as are used in industrial calcination, but at these temperatures γ-anhydrite begins to form. The heat energy delivered to the gypsum at this time (the heat of hydration) tends to go into driving off water (as water vapor) rather than increasing the temperature of the mineral, which rises slowly until the water is gone, then increases more rapidly.
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