Gypsum (calcium sulfate dihydrate)
Introduction and application
Gypsum has the chemical formula CaSO4 √ 2H2O, or calcium sulfate dihydrate. When used as a building material, gypsum has several applications. Dry powder called gypsum is combined with water to create a paste that solidifies. After drying, it stays rather soft and is easily worked with metal instruments. CaSO4 ∙ ½H2O, or calcium sulfate half-hydrate, reacts with water to form calcium sulfate dihydrate and, conversely, separates water upon heating to form calcium sulfate half-hydrate. CaSO4-double layers with relatively low hydrogen bridges—in each layer containing alternating Ca2+- and SO4 2-Ionics side by side—are the reason of the fissility of the CaSO4 ∙ 2H2O crystalline.
Analysis using STA
Between 100°C and 300°C, calcium sulphate-dihydrate undergoes a double-step dehydration process. The formation of a half-hydrate (from CaSO4 ∙ 2H2O to CaSO4 ∙ ½H2O) is the initial stage. Anhydrate is created by additional dehydration (CaSO4 ∙ ½H2O to CaSO4). Approximately 340°C produces an exothermal action that transforms anhydrate to β-calcium sulphate. The transformation of β-calcium sulfate into α-calcium sulfate is the exothermal action shown in the curve at about 1220°C. At temperatures higher than 1250°C, there is an additional mass loss due to the sulfate breakdown. Calcium oxide is produced from calcium sulfate. The peak at 1380°C is the melting of a eutectic combination of calcium oxide and sulfate.