Gypsum and its products

GYPSUM AND ITS PRODUCTS

Gypsum is a hydrated sulphate of calcium. Crude gypsum occurs in nature in the form of rock gypsum or gypsite. Gypsum also contains silica carbonate of lime or magnesia and iron oxide. Pure gypsum contains 79 % of calcium and 21% of water. Properties Of Gypsum

• It is white in color

• Its physical forms are crystalline, fibrous and massive

• Its specific gravity is 3.1

• It is a soft material, hardness number being two.

• It is soluble in sulphuric acid

• Its solubility in water is very important. The setting time is 30 minutes.

1. USES OF GYPSUM

• Manufacture of cement to increase its setting time.

• Manufacture of gypsum boards, gypsum plaster and plaster boards.

1. Gypsum Boards

Gypsum boards are made by mixing gypsum with asphalt. It is an ideal substitute for plywood for partitioning and paneling. The advantages of gypsum boards are

• It is easy to nail, screw and cut

• Boards are neatly cut and joined to obtain a finish of continuous surface. Dry construction eliminates the messy plastering.

• It is strong and durable.

 • It is a light weight material and hence load on structure is reduced considerably.

• Painting, veneering and wall papering are possible. • It can give fire protection for up to 2 hours.

• Its sound insulating property is excellent.

• It is cheaper than plywood.

2.  Gypsum Plaster

Gypsum plaster is also known as Plaster of Paris. It is produced by heating ground gypsum to a temperature of 170° C. At this temperature, two-thirds of its heat of hydration is driven out. Plaster of Paris sets immediately if water is added. To gain time for mixing, placing and finishing, retarders like clay, gum, starch and sugar are added. Such a mixture is known as hemihydrate plaster.

If gypsum is further heated to about 200° C, the entire water content is driven out. Such a product is called anhydrate gypsum. However, its setting time is too long. To reduce setting time, accelerators like alum, potassium sulphate or raw gypsum are added.

Properties of Gypsum Plaster

• It has good adhesion on fibrous materials.

• Its setting is by the process of crystallization

• It suffers very little shrinkage after drying

• It is fire resistant and is hence used as insulating material to protect wood and steel columns

• It is made more insulated against sound and heat by adding sawdust or granulated cork Uses of Gypsum Plaster

• It is used for ornamental castings

• For base course of plaster, it is mixed with lime putty. However, it cannot be used in damp condition, as it is slightly soluble in water.

• Gypsum plaster boards are used for linings, partition walls and ceilings.