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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 percent of CaSO4.2H2O can be used for building purposes. A less pure material needs some purification before use.
When Gypsum is heated at 160-165 degree Celsius, it loses about 14.7 percent 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 percent 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 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.
Wood lath is usually made up of narrow, thin strips of wood with spaces in between. The plasterer applies a slight pressure to push the wet plaster through the spaces. The plaster slumps down on the inside of the wall, forming plaster "keys." These keys hold the plaster in place.
Metal lath, patented in England in 1797, began to be used in parts of the United States toward the end of the 19th century. The steel making up the metal lath contained many more spaces than wood lath had contained. These spaces increased the number of keys.