Gypsum Plaster                                                    
Gypsum as a Building Material
Gypsum is one of the important and extensively used materials. Some of its well known uses are as a retarder to control the setting time of Portland cement, as a raw material for manufacture of ammonium Sulphate and sulphuric acid, as a dental and surgical plaster and as a filler in paint, paper and rubber industries. But the important use is as a building material where about 90 percent of the yearly output is consumed. Chemically, it is the Sulphate of Calcium with two molecules of water (CaSO4.2H2O). It is a white crystalline substance very sparingly soluble in water. It is soluble in dilute hydrochloric acid but insoluble in sulphuric acid. It contains 79.1 percent calcium Sulphate and 20.9 percent water. The principal gypsum producing regions in India are Bikaner, Jodhpur, Jaisalmer and Barmer districts of Rajasthan which supply about 90 percent of the annual produce of the country. Some of the other important gypsum producing areas are Trichirapalli in Madras; Nellore in Andhra Pradesh; Porbander, Halar, Bhavnagar and Kutch in Gujrat; Dehradun and Garhwal in Uttar Pradesh and Uri in Jammu and Kashmir.

Use of Gypsum in Building Industry

Among the products of gypsum, gypsum plaster and gypsum plaster board find the maximum application in building. The common impurities in gypsum are sand, chalky matter, illminite and gypsum anhydrite. Gypsum containing upto 70% of CaSO4.2H2O can be used for building purposes. A less pure material needs some purification before use.

Gypsum Plaster

When Gypsum is heated at 160-165oCelsius, it loses about 14.7% of its water in the form of steam giving the appearance of boiling of the whole mass. Technically, it is known as the first boil resulting in the formation of hemihydrate of Calcium Sulphate as it still contains about 6% of water. This is known as Plaster of Paris. On heating it further, the remaining water is also driven off. This is known as second boil resulting in the anhydrite of calcium Sulphate, also known as second settle plaster. Plaster of Paris, with small amount of retarder to delay the setting period, is known as retarded hemihydrate gypsum-plaster. Gypsum heated to remove all its combined water results into anhydrous gypsum, which is ground with an accelerator to make anhydrous gypsum plaster. Gypsum Plaster requires small proportions of sand and other aggregates and sets with little change in volume and with negligible shrinkage of drying. It is practically unaffected by bacteria and is light in weight, thus ideally suited for use in multistoried buildings. The plaster sets by natural process of crystallization; therefore, it can be used with ease and without any waste. Since, it dries up quickly the finishing coat can be applied immediately after the undercoat has set, usually the same day. The plaster also shows good adhesion to fibrous materials.

Gypsum as a Fire Resisting Material

The greatest lure of using gypsum in building is its fire resisting quality. A gypsum-plaster resists the onslaught of fire by virtue of its 20.9% of water by weight, which it holds. During conflagration, the weight of crystallization evaporates in the form of steam which condenses to water on reaching the cooler part of the plaster slab and the temperature cannot exceed more than 100degree Celsius until all the water is driven off in the form of steam. Thus there is a very efficient barrier between the passage of heat and the combustible material. Gypsum plaster is therefore widely used as an insulating material for protecting columns and beams of wood or metal from high temperatures. A porous gypsum plaster is an excellent sound absorbing material while dense and hard plasters are efficient reflectors of sound reflecting upto 97% of sound energy incident at their surface.

Limitations of Gypsum Plaster

One of the greatest drawbacks of gypsum-plaster is its solubility in water to the extent of 2 grams per litre. As a result of continued exposure to damp conditions gradual softening of the plaster takes place. It cannot, therefore, be used in humid areas and for external work.

Storage of Gypsum Plaster

Bags containing gypsum plaster must not be in humid surroundings for long periods as it picks up moisture from the atmosphere. Prolonged storage first hastens up the rate of setting and then falling of in strength of the set plaster. Storage for three months of gypsum plaster even in a dry place results in the deterioration of the quality.

Gypsum Plaster Board

Gypsum plasterboards are very popular in many countries. They are commonly used for ceiling construction, for internal lining of wall and for partition walls. They are economical and easy to work and light in weight. They are fire proof and do not expand or contract with change in temperature and humidity. They can take all type of oil paints and wall paper; may be used without finish and can also nailed easily. For making gypsum plaster board, gypsum plaster is treated with water and made into slurry which is filled in the casts smeared with oil to get a clear release of the set which takes almost twenty minutes. Before the poured slurry hardens into a paste of required consistency, fibres (coconut fibre, sisal fibre, mesta fibre, bamboo fibre) are laid into the slurry. The reinforcement fibres are then pressed down and the extended fibres at the edges are folded to give extra reinforcement for nailing. When the slurry becomes sufficiently hard, the board is removed and kept in the sun for drying. Gypsum Hollow tiles and light weight gypsum blocks are made similarly with the exception that some solid rods or cubes are placed in the centre of the mould before filling it with slurry to give hollowness to the finished product.

Gypsum Lath

It is another form of gypsum used in buildings. It is used as base for plaster. It is made of two types, solid and perforated. The fibres covering of lath helps in uniform suction and gives a fairly strong and durable bond with gypsum plaster. Lath can easily be nailed to studding and joints. It also does not stain plaster to create plaster marks.