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Natural gypsum dihydrate (CaSO4 · 2H2O), also known as gypsum, calcined, ground to get β-type hemihydrate gypsum (2CaSO4 · H2O), the building gypsum, also known as plaster, plaster. If the calcination temperature is 190 ° C can be obtained model gypsum, its fineness and whiteness than building gypsum high. If the gypsum at 400-500 ° C or higher than 800 ° C under calcination, that is, in the gypsum, the condensation, hardening slowly, but after hardening strength, wear resistance and water resistance are better than ordinary building gypsum.

Usually white, colorless, colorless transparent crystal known as through the gypsum, sometimes with impurities from ash, light yellow, light brown and other colors. Streaks white. Transparent. Glass luster, cleavage pearl luster, fibrous aggregates silk luster. Cleavage very complete, and medium, cleavage tablets into the surface angle of 66 and 114 diamond-shaped body. Of brittle. Hardness of 1.5 to 2. Slightly different in different directions. The relative density of 2.3. Polarizing microscope: colorless. Biaxial crystal (+). 2V = 58. Ng = 1.530, Nm = 1.523, Np = 1.521. 2V decreases with increasing temperature, and becomes zero at about 90 ° C.

There are three stages of crystal water discharge: 105 ~ 180 ℃, the first discharge of a water molecule, followed by the immediate discharge of half a water molecule, into calcined gypsum Ca [SO4] 0.5H2O, also known as plaster or hemihydrate gypsum . 200 ~ 220 ℃, the remaining half of the water molecules discharged into type Ⅲ anhydrite Ca [SO4] · εH2O (0.06 <ε <0.11). About 350 ℃, into type Ⅱ gypsum Ca [SO4]. 1120 ℃ and further into type Ⅰ anhydrite. The melting temperature was 1450 ° C.