The chemical formula for gypsum isCaSO₄ ∙2H₂O(calcium sulfate dihydrate). Gypsum has a wide variety of uses as a building material.
Plaster is a dry powder that is mixed with water to form a paste, which then hardens. It remains fairly soft after drying and can be easily worked with metal tools.
Calcium sulfate hemihydrate (CaSO₄ ∙½H₂O) reacts with water to form calcium sulfate dihydrate; conversely, calcium sulfate dihydrate releases water upon heating and is converted back to calcium sulfate hemihydrate.
The cleavability of crystallineCaSO₄ ∙2H₂Ois caused byCaSO₄ bilayers(in each layer,Ca²⁺ andSO₄²⁻ ions are arranged alternately side by side) with relatively weak hydrogen bonds.
The two-step dehydration of calcium sulfate dihydrate occurs between 100 °C and 300 °C. The first step is the formation of a hemihydrate (CaSO₄ ∙2H₂OtoCaSO₄ ∙½H₂O).
A further dehydration reaction forms the anhydrous form ( CaSO₄ ½ H₂O to CaSO₄ ). With an exothermic effect at approximately 340 °C, the anhydrous form converts to β-calcium sulfate. The exothermic effect in the curve at approximately 1220 °C is the conversion of β-calcium sulfate to α-calcium sulfate. The sulfate decomposition is evident in a further mass loss at temperatures above 1250 °C.
Calcium sulfate is converted to calcium oxide. The melting point of a eutectic mixture of calcium sulfate and calcium oxide is 1380 °C.
