Calcium Sulfate

Abstract

AbstractCalcium sulfate, CaSO4, in mineral form is commonly called and occurs abundantly in many areas of the world. In natural deposits, the main form is the dihydrate. Some anhydrite is also present in most areas, although to a lesser extent. The hemihydrate is normally produced by heat conversion of the dihydrate. The resulting powder is also known as plaster of Paris. Stucco has the greatest commercial significance of these materials. It is the primary constituent used to fabricate products and in formulated plasters used in job or shop‐site applications. About\documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}${23\times 10^{6}{\hskip0.167em}{\hskip0.167em}{\rm{t}}}$\end{document}of gypsum are consumed annually. About 80% is processed into the commercially usable hemihydrate. Uses of gypsum are in fabricated or formulated building materials, Portland cement set regulation, and agricultural soil conditioning. The natural, or mineral, form of gypsum is most widely extracted by mining or quarrying and used commercially. 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, and the chemical synthesis of chemicals, such as sulfuric acid, phosphoric acid, titanium dioxide, citric acid, and organic polymers. Gypsum rock from the mine or quarry is crushed and sized to meet the requirements of future processing or removed for direct marketing of the dihydrate as a cement retarder. Once subjected to a secondary crusher, calcining, and drying, the product is fine‐ground. The majority of fine‐ground dihydrate is used as feed to calcination processes for conversion to hemihydrate. The dehydration of gypsum, commonly referred to as calcination in the gypsum industry, is used to prepare hemihydrate, or anhydrite. Three processing methods are used for the production of α‐hemihydrate. One involves charging lump gypsum rock into a vertical retort and applying steam. After calcination for 5–7 h the hot moist rock is quickly dried and pulverized. Another method has lower water demand. The dihydrate is heated in a water solution containing a metallic salt at pressures not exceeding atmospheric. A third method prepares very low water‐demand α‐hemihydrate by auto‐claving powdered gypsum in a slurry. A crystal‐modifying substance is added to produce large squat crystals. Soluble anhydrite is commercially manufactured in a variety of forms by low temperature dehydration of gypsum. Crude gypsum is the principal form of calcium sulfate shipped in international trade, although by the 1980s an increased volume of fabricated products moved across international borders.