Gypsum powder (hereinafter referred to as semi-hydrated gypsum powder CaSO4.1/2H20, the same below) is widely used in ceramics forming consumption for its high quality, low cost, good activity after mixing with water, incomparable condensation, water absorption and easy processing properties, and is used to manufacture models of grouting, rolling and cold-forming.
Gypsum powder is prepared by stirring natural gypsum minerals. Natural gypsum is a calcium sulfate mineral CaSO4.2H20 containing two crystalline water. According to differential thermal analysis, there are two endothermic reactions in dihydrate gypsum at low temperature. When the temperature is about 140 C, the crystal water is discharged at the beginning. When the temperature ranges from 150 to 170 C, it is quickly synthesized into hemihydrate gypsum (CaSO4.1/2H20). When the temperature continues to rise to 225 C, all crystalline water will be lost and anhydrous gypsum (CaSO4) will be formed.
Gypsum powder can be divided into two kinds according to the different processing technology and product crystal structure, namely ordinary gypsum powder (also known as beta gypsum powder) and high strength gypsum powder (also known as alpha gypsum powder). Beta gypsum powder is prepared by calcining natural gypsum powder in a convex bottom iron pot at 160 ~170 C under atmospheric pressure; alpha gypsum powder is prepared by placing natural gypsum block material in autoclave, heating and dewatering under autoclave pressure for 3~5 hours at 130 ~1.5~3 atmospheric pressure, then drying in a drying chamber at 150 ~170 ~C and crushing it. The crystalline structure and product properties of the two gypsum powders are different due to their different manufacturing processes. The crystalline structure of beta gypsum powder is regular flake crystal, and there are many holes and cracks in the grain.
The water requirement of the standard consistency is larger than the surface area, the hydration reaction of the model is stronger after mixing gypsum powder with water, the initial setting time is shorter, the porosity and water absorption of the model are higher, but the strength of the model is lower; while the crystal structure of the alpha gypsum powder is a neat needle crystal, smaller than the surface area, the water requirement of the standard consistency is smaller, the porosity and water absorption of the model are smaller, and the reaction of the model after mixing gypsum powder with water is worse. The gypsum paste has good activity, long initial setting time and high model strength. According to the characteristics of these two crystalline gypsum, the grouting forming model is made of ordinary beta gypsum powder, while the rolling forming model and plastic forming model are made of alpha gypsum powder.
In addition to the above two kinds of gypsum powder, mixed gypsum powder composed of the above two kinds of gypsum powder in different proportions (usually 70%-80% of beta gypsum powder and 30%-20% of alpha gypsum powder) is also used in consumption. This kind of gypsum powder is applied to the manufacture of grouting forming models (e.g. sanitary ceramics models) in ceramic consumption because of its good water absorption performance of beta gypsum and high strength of alpha gypsum. In recent years, with the development of the ceramic industry, many super-high strength gypsum powders, such as mother mould gypsum powder, K-type powder, resin gypsum powder and so on, have been derived from high strength alpha gypsum powder by improving processing technology and adding some special additives.