Building gypsum is divided into three grades: excellent, first grade and qualified.
The density of building gypsum is 2.502.80g/cm3, and the bulk density is 800-1100kg/m3. Building gypsum has the following characteristics:
(1) Large porosity (about 50%-60%) and low strength. The theoretical water requirement of gypsum is 18.6%, but in order to make gypsum have the necessary plasticity, the water is usually added up to 60%-80%, leaving a lot of pores after the evaporation of water, so the porosity is very large. The strength of first-class gypsum after hardening is about 5-8MPa for 1 day, and the maximum strength can reach 8-12MPa for 7 days
(2) fast setting and hardening. It can set within 3 to 5 minutes, and the final setting should not exceed 30 minutes. It is necessary to add retarding agent in application. For example, shed sand, sodium potassium tartrate, wring acanthic acid, polyvinyl alcohol, lime activated bone glue or skin glue.
(3) The volume expands slightly after hardening, and the expansion rate is about 1%.
(4) poor water resistance and frost resistance. Architectural gypsum has a strong hygroscopic property after hardening. In wet environment, the bond between crystals is weakened and the strength is significantly reduced. The water in the pores freezes, causing the gypsum to crack.
(5) Good fire resistance. In case of fire, the crystalline water of dihydrous gypsum evaporates and absorbs heat. The anhydrous gypsum generated on the surface is a good insulator.
