Gypsum is an air-hardening materials mainly consisting of calcium sulfate. And its products have many excellent characters, commonly used in construction. There are various kinds of gypsum binding materials, such as building gypsum, high-strength gypsum, anliydrite cement, and high-temperature-calcined gypsum.
Characteristics of Building Gypsum
Compared with other binding materials, building gypsum has the following characteristics:
1. Fast Setting and Hardening
The setting time of building gypsum changes with the calcination temperature, grinding rate and impurity content. Generally, mixed with water, its initial setting needs just a few minutes at room temperature, and its final setting is also within 30min. Under the natural dry indoor conditions, total hardening needs about one week. The setting time can be adjusted according to requirements. If the time needs to be postponed, delayed coagulant can be added to reduce the solubility and the solution rate of building gypsum, such as sulfite alcohol wastewater, bone glue activated by borax or lime, hide glue, and protein glue; if it needs to be accelerated, accelerator can be added, such as sodium chloride, silicon sodium fluoride, sodium sulfate, and magnesium sulfate.
In the hardening process, the volume of building gypsum just expands a little, and there won’t be any cracks. Thus, it can be used alone without any extenders, and can also be casted into construction members and decorative patterns with accurate size and smooth and compact surface.
3. Big Porosity
After hardening, the porosity of building gypsum can reach 50%-60%, so its products are light, insulating, and sound-absorbing. But these products have low strength and large water absorption due to big porosity.
4. Poor Water Resistance
Building gypsum has low softening coefficient (about 0.2-0.3) and poor water resistance. Absorbing water, it.wil1 break up with the freeze of water. Thus, its water resistance and frost resistance are poor, not used outdoors.
5. Good Fire Resistance
The main component of building gypsum after hardcning is CaS04*2H20.When it contacts with fire, the evaporation of crystal water will absorb heat and generate anhydrous gypsum which has good thermal insulation. The thicker its products are, the better their fire resistance will be.
6. Large Plastic Deformation
Gypsum and its products have an obvious performance of plastic deformation.Creep becomes more serious especially under bending load. Thus, it is not used for load-bearing structures normally. If it is used, some necessary measures need to be taken.