Modifications of Calcium Sulfate

CaS04 2 H2O natural gypsum, dihydrate CaS04 0.5 H2O bassanite, hemihydrate CaS04 anhydrite 

Calcium. sulfate a- and P-hemihydrates have different crystal habits and specific surface areas 

Calcium. sulfate occurs naturally as its dihydrate, (natural gypsum) as anhydrous anhydrite and rarely as its hemihydrate in the form of the mineral bassanite. Only the deposits of natural gypsum and anhydrite are of economic interest. For applications in the construction industry only the hardenable modifications, calcium sulfate hemihydrate (a- and P-form) and anhydrite, which are manufactured by dehydrating the dihydrate, are important. The properties and formation conditions of the different calcium sulfate modifications are given in Table 5.3-11. 

0 gypsum mono- 40 only stable 40 40 hardened gypsum , by  

O bassanite a-hydrate rhombo- metasuible 45 in 80 to 800 very high strength, rapid molding plaster, industrial 

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No economical control method is available for calcium sulfate other than ensuring that the concentration-temperature process path is within the solubility confines of the various crystalline modification of calcium sulfate. The limiting top temperature at which seawater mey be evaporated without calcium sulfate scale deposition is of major design significance. 

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