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Practically found in the whole world, Plaster occurs abundantly in Brazil in cretáceos lands of sea formation, over all in the states of the Ceará, Great River of the North, Piauí and Pernambuco.
Plaster is a composed sulphate mineral basically of hidratado calcium (CaSO4.2H2O) and the hemidrato gotten for the calcination of this (CaSO4. ½ H2O). It crystallizes in the monoclinical system, forming crystals of different thicknesses, habitually selenita calls. Plaster still can be found under the form of added to granulate, when it receives the name from alabaster, or in fibrosos lodes, with the spar-of-satin name, in virtue of the silky brightness. The pulverulenta form, not crystallized, receives the name from crude gypsum. Its hardness is equal the 2, in the scale of Mohs. In general it has white color, but diverse impurities can give acinzentado, yellowish, pink or brown aspect to it.
When if terço of its weight in water humidifies Plaster with about one, forms a plastic mass that hardens in about ten minutes and suffers expansion, for what is used in the confection of molds with well defined forms. The USA Plaster also in construction, for finishing of I tow it and the ceiling of the habitations and, modernly, in the confection of degradations and thick partition walls, together with cardboard.
Plaster also enters in the composition of the cement portland, in the ratio of two the three percent, as delaying agent of handles. He is still used in ortopédicos devices, works of prótese dental and other fields where if it makes necessary the confection of special molds and fôrmas.
Not calcined, Plaster is used as fundente, fertilizer and as wadding in the paper production and fabrics. It surrounds three-quarter of the world-wide production are calcined, to a temperature between 120 and 165°C, for hemidratado Plaster production, that forms with water a mixture of extreme plasticity, used in molding, casting, ceramics and folder of teeth, beyond serving of construction material as estuque, decorative cement of Keene, roofing tiles and blocks.
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