Gypsum plasters

  The earliest known use of gypsum plaster was in Anatolia (part of modern-day Turkey) and Syria in about 6000 BC, and in about 3700 BC, the Egyptians used gypsum plaster in the inside of the pyramids. The building industry is the major consumer of gypsum plasters. Gypsum, CaSO₄.2H₂O, is mined on a large scale worldwide, and is calcined to form the b-hemihydrate, CaSO₄.¹/₂H₂O. The hemihydrate is referred to as plaster of Paris, the name being derived from Montmartre in Paris where gypsum was quarried. Hydration of the hemihydrate with a carefully controlled amount of H₂O initially gives a slurry which hardens as CaSO₄.2H₂O crystallizes. Crystals are needle-like and it is their intergrowth that provides gypsum with its strength and suitability for the building trade. Calcined gypsum which is stored for long periods may age by absorbing water, and this affects the rehydration process. The setting process of gypsum plasters may be accelerated or slowed down by suitable additives, e.g. <0.1% of citric acid is sufficient to retard the crystallization process. Gypsum plasters suitable for applying to walls have been developed so that additives are already present with the hemihydrate. Building contractors commonly use prefabricated gypsum plasterboards and tiles. Plasterboards are fabricated by pouring a hemihydrate– water–additive slurry onto cardboard sheets ≈ 0.5mm thick. After completing the lamination by applying a second sheet of cardboard, the plasterboard is dried. The incorporation of fibreglass into plasterboards is also possible, giving fibreboard products. An advantage of gypsum plasterboards as partition walls is their degree of fire resistance. In 2000, 108 Mt of gypsum was produced worldwide.     Within the US in 2000, 22.9Mt of prefabricated gypsum products were sold or used in the US, and the chart below shows the distribution of products making up this total.  The average new home in the US contains ≥570m² of gypsum plasterboard.