Calcium Sulfate

Abstract

AbstractCalcium sulfate, CaSO4, has several forms, ie, calcium sulfate dihydrate (commercially known as gypsum), calcium sulfate anhydrous (anhydrite), calcium sulfate hemihydrate, present in two different structures, α‐hemihydrate and β‐hemihydrate (commercial name of β‐form: stucco or plaster of Paris). It is found abundantly in many area of the world, where it is extracted by mining or quarrying and recently was obtained as a by‐product by numerous industrial applications. In natural deposits, the main form is the dihydrate, some hemihydrate is also present in most areas, although to a lesser extent. Gypsum is also obtained as a by‐product of various chemical processes. The main sources are from processes involving scrubbing gases evolved in burning fuels that contain sulfur (FGD gypsum), and the chemical synthesis of chemicals, such as sulfuric acid, phosphoric acid, titanium dioxide, citric acid, and organic polymers. The hemihydrate is normally produced by heat conversion of the dihydrate. Stucco is the primary constituent used to fabricate products and in formulated plasters used in job or shop‐site applications. About 20–25 million metric tons of calcium sulfate are consumed annually. About 20% of it, in the dihydrate form, is used in fabricated or formulated building materials, Portland cement set regulation, and agricultural soil conditioning, while ∼80% is processed into the commercially usable hemihydrate. The dehydration of gypsum, commonly referred to as calcination in the gypsum industry, is used to prepare hemihydrate, or anhydrite. In addition, calcium sulfate precipitation is relevant in scaling problems, where gypsum formation is an unwanted occurrence, such as in water desalination, industrial water recovery in cooling tower technology, water distillation, and phosphoric acid production.