Gypsum belongs to the monoclinic crystal system with a high degree of cleavage and is easy to crack into thin slices. Hemihydrate gypsum can be obtained by heating gypsum to 100-200°C and losing part of crystal water. It is an air-hardening cementitious material with two forms, α and β, both of which are rhombic crystals, but with different physical properties. α-type hemihydrate gypsum crystals are good and firm; β-type hemihydrate gypsum is a flaky and cracked crystal, the crystal is very fine, and the specific surface area is much larger than that of α-type hemihydrate gypsum.
When producing gypsum products, α-type hemihydrate gypsum requires less water than β-type gypsum, and the products have higher density and strength. Usually, α-type hemihydrate gypsum, also known as high-strength gypsum, is made by steaming in a saturated steam medium with an autoclave; plaster. Chemical gypsum, a by-product of industry, has the same properties as natural gypsum without much processing. The slurry of hemihydrate gypsum mixed with water re-forms dihydrate gypsum, which rapidly solidifies and hardens during drying to gain strength, but softens when exposed to water.
Gypsum is the main raw material for the production of gypsum cementitious materials and gypsum building products, as well as a retarder for Portland cement. After the gypsum is calcined at 600-800 °C, a small amount of lime and other catalysts are added to grind it together to obtain anhydrite cement (also known as Jin's cement); after calcined at 900-1000 °C and ground, high-temperature calcined gypsum can be obtained. . The products made with these two kinds of gypsum have higher strength than building gypsum products, and the anhydrite binder has better heat insulation, and the high-temperature calcined gypsum has better wear resistance and water resistance.
