Building gypsum has the following properties:
- Fast setting and hardening
Building gypsum has fast setting and hardening speed. Usually, it achieves the initial setting within a few minutes at room temperature after mixing with water, and reaches the final setting within 30min. The setting time can be adjusted by adding retarders or coagulants.
- Adjusting the humidity
The hydration product of gypsum is dihydrate gypsum, while the dehydration temperature of dihydrate gypsum is low, about 120 ℃. When the air humidity is low, the dihydrate gypsum can release a portion of crystal water to produce the hemihydrate gypsum and increase the ambient humidity. When the air humidity is high, the hemihydrate gypsum can then absorb the moisture from the environment to produce the dihydrate gypsum, while reducing the environmental humidity and playing a regulatory role in the ambient humidity.
- Good fireproof performance
The hydrate of gypsum after hardening is dihydrate gypsum containing water, which contains an amount of crystal water equivalent to about 21 percent of the total mass. At the normal temperature, the water of crystallization is stable. When the temperature reaches above 100 ℃, the water of crystallization begins to decompose and produce a layer of water vapor curtain on the surface facing the fire source, playing a role in preventing the spread of flame and increase of temperature.
- No shrinkage
In the process of setting and hardening, gypsum has a slight expansion of volume and no cracks will occur when hardened.
- Light weight
In the hydration of building gypsum, the theoretical water requirement accounts for only 18 .6 % the mass of hemihydrate gypsum. But actually, in order to make the gypsum slurry with certain plasticity, it is usually necessary to add 60%-80% of water, and the excess moisture will be gradually evaporated, so that a large number of pores will be left in the hardened gypsum. Generally the porosity is about 50%-60%. Therefore, the building gypsum after hardening is light in the weight and low in the strength, but low in the thermal conductivity and good in the sound absorption.
- Poor resistance to water, frost and heat
After hardening, building gypsum has strong moisture absorption and water absorption. In the humid environment, the adhesive force between crystals will be weakened and the strength will be significantly decreased, and in water crystals will even dissolve and cause damage. If gypsum suffers from cold after absorbing water, then the water within the pores will freeze, produce the volume expansion, and destroy the gypsum body after hardening. So the water resistance and frost resistance of gypsum are poor. In addition, in the environment of too high temperature (above 65 ℃), the dihydrate gypsum will get dehydrated and decomposed, resulting in the decrease of strength. Therefore, building gypsum should not be used in the humid environment or at too high temperature.
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