The endothermic property of this reaction is relevant to the performance of drywall, conferring fire resistance to residential and other structures. The equation for the partial dehydration is: 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. Temperatures as high as 170 ☌ (338 ☏) are used in industrial calcination, but at these temperatures γ-anhydrite begins to form. The details of the temperature and time depend on ambient humidity. Temperatures between 100 and 150 ☌ (212–302 ☏) are required to drive off the water within its structure. With judicious heating, gypsum converts to the partially dehydrated mineral called bassanite or plaster of Paris. It is also convenient that calcium sulfate is poorly soluble in water and does not readily dissolve in contact with water after its solidification. 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. The main use of calcium sulfate is to produce plaster of Paris and stucco. Specific hemihydrates are sometimes distinguished: α-hemihydrate and β-hemihydrate. ĢO ( bassanite): hemihydrate, also known as plaster of Paris. The structure is related to that of zirconium orthosilicate (zircon): Ca 2+ĢO ( gypsum and selenite (mineral)): dihydrate. The compound exists in three levels of hydration corresponding to different crystallographic structures and to minerals:Ĥ ( anhydrite): anhydrous state. Hydration states and crystallographic structures Calcium sulfate causes permanent hardness in water. All forms are white solids that are poorly soluble in water. One particular hydrate is better known as plaster of Paris, and another occurs naturally as the mineral gypsum. In the form of γ- anhydrite (the anhydrous form), it is used as a desiccant. Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO 4 and related hydrates.
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