In this article we are going to discuss about gypsum as a building material, we will also discuss how is gypsum formed, Technical Requirements of Building Gypsum etc.
What is Gypsum?
Gypsum is a common mineral also it is an air-hardening materials mainly consisting of calcium sulfate(CaSO4·2H2O).
And its products have many excellent characters, generally used in construction. There are many types of gypsum binding materials available in market, such as building gypsum, high-strength gypsum, anliydrite cement, and high-temperature-calcined gypsum.
How is gypsum formed
Gypsum is made up of oxygen, sulfur, calcium and water. its a sedimentary rock and a crystalline mineral, It takes the form of crystals which can at times be found projecting from rock – leading to its old English name of the Spear Stone.
As evaporation occurs the sulfur is not protected by the water and oxygen contacts sulfur bonding with it to form a sulfate (SO4 2_). The sulfate then bonds with calcium (Ca) and water (H2O) to create gypsum.
Technical Requirements of Building Gypsum
The building gypsum is white and has a density of 2.6 to 2.75g / cm3 and a bulk density of 800 to 1000kg / m3.
As per GB9776-88 building gypsum can be classified into high-class, first-class and acceptable grades in light of strength, fineness and setting time (see the below table) Among them, bending strength and compressive strength are measured by letting samples contact with water for 2 hours.
| Indicators | Grades | High-class | First-class | Acceptable |
|---|---|---|---|---|
| Strength | Bending Strength (≥) | 2.5 | 2.1 | 1.8 |
| (MPa) | Compressive Strength (≥) | 4.9 | 3.9 | 2.9 |
| Fineness (%) | Sieve Residue of 0.2 mm Square-hole Sieve (≤) | 5.0 | 10.0 | 15.0 |
| Setting Time | Initial Setting Time(≤) | 6 | ||
| (min) | Final Setting Time(≤) | 30 |
Building gypsum is tagged by the order of name, bending strength and
standard number.
For example, the building gypsum of 2.5MPa bending
strength can be tagged as: Building Gypsum 2.5 GB 9776.
Hydration of building gypsum technically requires that water requirements accounts for 18.6% of the weight of semi-hydrate gypsum. But in fact, water often accounts for 60%-80% in order to make the gypsum slurry have certain plasticity.
Excess water gradually evaporates in the hardening process, which leads to a large number of pores left in the hardened gypsum, with the porosity of 50%-60%.
Building gypsum has low strength, small apparent density, low thermal conductivity and high sound absorption after hardening.
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