Gypsum (calcium sulfate dihydrate)

Introduction and application

The chemical formula of Gypsum is CaSO4 ∙ 2H2O (Calcium Sulfate Dihydrate). There are a large number of uses for gypsum as a building material. Gypsum is a dry powder that is mixed with water to form a paste which then hardens. It remains quite soft after drying, and can easily be manipulated with metal tools. Calcium sulfate half-hydrate (CaSO4 ∙ ½H2O) react with water to Calcium Sulfate Dihydrate and backwards Calcium Sulfate Dihydrate
separates Water under heating and exchange to Calcium Sulfate half-hydrate. The fissility of CaSO4 ∙ 2H2O  crystalline is caused by CaSO4-double layers (in each layer are Ca2+- and SO4 2-Ionic’s alternately side by side) with relative low hydrogen bridges.

Analysis using STA

The double-step dehydration of calcium sulphate-dihydrate appears between 100°C and 300°C. The first step is the forming of half-hydrate (CaSO4 ∙ 2H2O to CaSO4 ∙ ½H2O). A further dehydration forms the Anhydrate (CaSO4 ∙ ½H2O to CaSO4). With an exothermal effect at approx. 340°C converts the Anhydrate to β-calcium sulphate. The exothermal effect in the curve at approx. 1220°C is the conversion of β-calcium sulphate to α-calcium sulfate. The sulfate decomposition can be seen in a further mass loss at temperatures above 1250°C. Calcium sulfate converts into calcium oxide. The melting of a eutectic mixture of calcium sulphate and calcium oxide is the peak at 1380°C.