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Calcium sulfate is an inorganic compound with the formula CaSO4. It occurs naturally as the mineral gypsum and in a number of hydrates. One form, known as plaster of Paris, is used for building and as a desiccant. The other, gypsum, is used in the production of construction materials such as stucco, division panels, ceiling tablets and wall plaster. Calcium sulfate can also be produced as a byproduct of other reactions or by synthetic processes. It is soluble in water but insoluble in alcohol and other solvents.
At low temperatures, gypsum, or calcium sulfate dihydrate, CaSO4 * 2H2O, is the stable form of calcium sulfate; it is insoluble at high temperature. Gypsum can be dehydrated to produce anhydrous calcium sulfate by heating it. This reaction is known as the calcination of gypsum.
Figure 7 shows the polymorphism of CaSO4. The b-form calcium sulfate hemihydrate, CaSO4
Besides its use as a raw material in the manufacture of building products, gypsum is also widely used in the paper industry. As a filler, it makes paper thicker and brighter so that it is easier to write and draw on. It is also used as a retardant in the drying process of paints and coatings. Another useful application is in the refining of zinc where it is used to co-precipitate heavy metals such as barium from solution of zinc sulfate.
