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Calcium Sulphate
The hemihydrate (CaSO4.~0.5H2O) is better known as plaster of Paris,
while the dihydrate (CaSO4.2H2O) occurs naturally as gypsum. The
anhydrous form occurs naturally as β-anhydrite. Depending on the method
of calcination of calcium sulfate dihydrate, specific hemihydrates are
sometimes distinguished: alpha-hemihydrate and beta-hemihydrate.[2]
They appear to differ only in crystal size. Alpha-hemihydrate crystals
are more prismatic than beta-hemihydrate crystals and when mixed with
water form a much stronger and harder superstructure.
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