Gypsum is a very soft mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O.
Heating gypsum to between 100°C and 150°C (302°F) partially dehydrates the mineral by driving off exactly 75% of the water contained in its chemical structure. The temperature and time needed depend on ambient partial pressure of H2O. Temperatures as high as 170°C are used in industrial calcination, but at these temperatures the anhydrite begins to be formed. The reaction for the partial dehydration is:
CaSO4·2H2O + heat ? CaSO4·½H2O + 1½H2O (steam)
The partially dehydrated mineral is called calcium sulfate hemihydrate or calcined gypsum (commonly known as plaster of Paris) (CaSO4·½H2O).
The dehydration (specifically known as calcination) begins at approximately 80°C (176°F), although in dry air, some dehydration will take place already at 50°C. The heat energy delivered to the gypsum at this time (the heat of hydration) tends to go into driving off water (as water vapor) rather than increasing the temperature of the mineral, which rises slowly until the water is gone, then increases more rapidly.
The endothermic property of this reaction is exploited by drywall to confer fire resistance on residential and other structures. In a fire the structure behind a sheet of drywall will remain relatively cool as water is lost from the gypsum, thus preventing (or substantially retarding) damage to the framing (through combustion of wood members or loss of strength of steel at high temperatures) and consequent structural collapse.
In contrast to most minerals, which when rehydrated simply form liquid or semi-liquid pastes, or remain powdery, calcined gypsum has an unusual property: when mixed with water at normal (ambient) temperatures, it quickly reverts chemically to the preferred dihydrate form, while physically "setting" to form a rigid and relatively strong gypsum crystal lattice:
CaSO4·½H2O + 1½H2O ? CaSO4·2H2O This reaction is exothermic.
This phenomenon is responsible for the ease with which gypsum can be cast into various shapes including sheets (for drywall), sticks (for blackboard chalk), and molds (to immobilize broken bones, or for metal casting). Mixed with polymers, it has been used as a bone repair cement. Small amounts of calcined gypsum are added to earth to create strong structures directly from cast earth, an alternative to adobe (which loses its strength when wet). The conditions of dehydration can be changed to adjust the porosity of the hemihydrate, resulting in the so-called alpha and beta hemihydrates (which are more or less chemically identical).
The completely water-free form, called anhydrous calcium sulfate (sometimes anhydrite), is produced by further heating to above approximately 180°C (356°F) and has the chemical formula CaSO4. Anhydrite reacts slowly with water to return to the dihydrated state, a property exploited in some commercial desiccants.
Gypsum from New South Wales, AustraliaGypsum occurs in nature as flattened and often twinned crystals and transparent cleavable masses called selenite. It may also occur silky and fibrous, in which case it is commonly called satin spar. Finally it may also be granular or quite compact. In hand-sized samples, it can be anywhere from transparent to opaque. A very fine-grained white or lightly-tinted variety of gypsum is called alabaster, which is prized for ornamental work of various sorts. In arid areas, gypsum can occur in a flower-like form typically opaque with embedded sand grains called desert rose.
Gypsum is a very common mineral, with thick and extensive evaporite beds in association with sedimentary rocks. The largest deposits known occur in strata from the Permian age. Gypsum is deposited in lake and sea water, as well as in hot springs, from volcanic vapors, and sulfate solutions in veins. Hydrothermal anhydrite in veins is commonly hydrated to gypsum by groundwater in near surface exposures. It is often associated with the minerals halite and sulfur.
Fibrous Gypsum from BrazilThe word gypsum is derived from the aorist form of the Greek verb µa?e??e??, "to cook", referring to the burnt or calcined mineral. Because the gypsum from the quarries of the Montmartre district of Paris has long furnished burnt gypsum used for various purposes, this material has been called plaster of Paris.
Commercial quantities of gypsum are found in Germany, Italy, England, Ireland, in British Columbia, Manitoba, Ontario, Nova Scotia and Newfoundland in Canada, and in New York, Michigan, Iowa, Kansas, Arizona, New Mexico, Colorado, Utah and Nevada in the United States. There is also a large mine located at Plaster City, California in Imperial County. There are commercial quantities in East Kutai, Kalimantan.
A growing source of gypsum is from flue gas desulfurization which scrubs the sulfur emissions from fossil-fuel-burning power stations. This is done by using finely ground limestone which reacts with the sulfur dioxide to produce high-purity gypsum as a by-product.
Gypsum can also be recovered and re-used from scrap drywall at construction sites.
Gypsum is also a by-product of the phosphate fertilizer refining process. Phosphate ore is mixed with sulfuric acid to produce phosphoric acid. The phosphoric acid is then ammoniated to form monoammonium phosphate (MAP) and diammonium phosphate (DAP). The resulting waste product is a slurry of calcium sulfate (gypsum), that eventually dries and hardens. In fact, the phosphate industry creates several times more gypsum as a by-product than is produced or required by the commercial gypsum industry. In the case of the phosphate industry in West Central Florida, this gypsum by-product is prohibited from commercial use because of low levels of naturally occurring radioactive material (NORM) existing in the phosphate ore that becomes technically enhanced and deposited into the gypsum. Because of the sheer volume of gypsum produced by this process, there is also a considerable lobbying effort by the gypsum industry not to allow the phosphate byproduct on the market.  The result is huge piles of gypsum waste that are visible along the highways in the vicinity of the Florida phosphate refineries.
Because gypsum dissolves over time in water, gypsum is rarely found in the form of sand. However, the unique conditions of the White Sands National Monument in the US state of New Mexico have created a 710 km² (275 sq mile) expanse of white gypsum sand, enough to supply the construction industry with drywall for 1,000 years. Commercial exploitation of the area, strongly opposed by area residents, was permanently prevented in 1933 when president Herbert Hoover declared the gypsum dunes a protected national monument.
Expanding on gypsum's role as a building material, at least two teams of researchers are currently designing robots that will build houses out of gypsum and concrete. A team headed by Dr Behrokh Khoshnevis at the University of Southern California and a team headed by Dr Rupert Soar at Loughborough University’s School of Mechanical and Manufacturing Engineering in England are both working on robots that will spray gypsum or concrete to create the shell of a house. The California team expects that their robot will be able to do so in only 24 hours and plans to build the first prototype shell before April of 2007. The British team's robot, in contrast, is expected to take at least a week to build a house, but possesses more advanced features such as "weaving in ducts for water pipes, electrical wiring and ventilation within the panels of gypsum or concrete." In addition to drastically decreasing the amount of time it takes to build a house, Khoshnevis claims that the automated, two-material construction technology will reduce the cost of building a house to a fifth of today's prices.
Gypsum is also used in the production of the following:
Plaster, a construction material
Molds for Casting metals
Agricultural soil amendment
Solidifying earth (cast earth construction)
Improving mineral content of brewing water
Dietary calcium additives in breads and cereals
Desiccant - anhydrous calcium sulfate (anhydrite) is sold under the brand name Drierite®