Gypsum Plaster (Plaster Of Paris):
Gypsum plaster, also known as plaster of paris obtained from gypsum, which occurs in form of natural rocks. Gypsum consists of one part of calcium sulphate and two parts of chemically combined water of crystallization.
When gypsum is heated at a certain temperature, water of crystallization is driven off and the powdery product left is called plaster of paris. As soon as water is added to the plaster of paris, it sets immediately without giving time for mixing and application.
When plaster of paris is to be used for plastering the surfaces, its setting time will have to be prolonged which can be done by adding certain salts to it. Addition of some burnt ash and fine sand prolong its setting time. When used for plastering, the setting time of the mortar of plaster of paris and sand is normally 5 minutes. Hence it has to be used very briskly so as to enable its application to the surface before it sets.
Generally dry mix of paris plaster with sand is prepared on an impervious platform. Small quantity of mix is taken in pans and carried to masons. The mason will add water to the dry mix only when it is to be used. This plaster is a very good plaster and gives very smooth finishing. Since it is slightly soluble in water, it cannot be used for externally exposed surfaces.
When raw gypsum is heated from 160 to 170 about 15 water of crystallization is driven off and resulting product is known as hemihydrate calcium sulphate. Hemihydrate is also known as first settle plaster which is actually plaster of paris. The term hemihydrate gypsum plaster is used to indicate plaster of paris with retarders mixed with it.
On further heating to about 200C entire water content of crystallization is driven off and the resulting product is known as gypsum anhydrite or hard burnt plaster. The setting time of gypsum anhydrite is quite large and requires accelerators to be added to shorten its setting time. Anhydrous gypsum plaster indicates gypsum anhydrite added with accelerators.
Advantages Of Gypsum Plaster:
1. It is very good fire resistant and hence a very good heat insulating material. It can successfully protect steel beams, columns, and timber from high temperature.
2. There is no shrinkage while setting and hence it does not develop cracks on heating or setting.
3. It is used for ornamental works like boards and blocks. Gypsum plaster boards are very popularly used for ceilings, internal linings of walls and partition walls. Gypsum plaster boards are very cheap, light in weight, fireproof and easy to work.
4. It does not require curing like conventional plaster which saves water and time during construction.
5. It has good adhesion on fibrous materials.
6. It gives a very good smooth finishing with perfect right angle corners.
7. It is easy to apply.
Disadvantages OF Gypsum Plaster:
1. Gypsum plaster cannot be used in external surfaces as it is slightly soluble in water.
2. It is costly compared to conventional plaster.
3. Storing of gypsum plaster is much more difficult.