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Gypsum is a very soft mineral composed of calciumSee also Calcium, New York, United States. potassium calcium scandium Mg Ca Sr Full table General Name, Symbol, Numbercalcium, Ca, 20 Series alkaline earth metal Group, Period, Block 2 (IIA), 4, s Density, Hardness 1550 kg/m3, 1. 75 Appearance silvery whi sulfateSulfate is the IUPAC name for the SO2- ion, consisting of a central sulfur atom single bonded to four tetrahedrally oriented oxygen atoms. This anion has a net negative two electric charge. Almost all ionic compounds with sulfate anions are soluble in wat dihydrate, with the chemical formula CaSO₄·2H₂O.

1 Chemical structure

Heating gypsum above approximately 150°C (302°F) partially dehydrates the mineral, by driving off exactly 75% of the water contained in its chemical structure.

CaSO₄·2H₂O + heat → CaSO₄·½H₂O + 1½H₂O (steam)

The dehydration (specifically known as calcinationCalcination is the process of heating a substance, but below melting point, causing a loss of moisture. The reaction also causes the decomposition of carbonates. An example of this is the calcination of limestone into quicklime using a limekiln.) begins at approximately 80°C (176°F) and the heat energy delivered to the gypsum at this time (the heat of hydration) goes into driving off water (turning it into water vapor), not into increasing the temperature of the mineral. As water is lost, the temperature of the gypsum slowly increases until all the water has been removed, then begins rising normally at a quicker rate. The ability of hydrated gypsum to remain at relatively low temperatures, even if a flame is applied directly to it for a short period of time, is exploited by drywallDrywall (also called gypsum board GWB plasterboard SHEETROCK and Gyproc is a building material consisting of gypsum formed into a flat sheet and sandwiched between two pieces of heavy paper. It can be easily cut to shape with a small saw or even by scorin to confer fire resistance to the wooden frames of houses and other buildings. Even if a fire is impinging directly on a sheet of drywall, the wood frame behind it will remain at a relatively low temperature (until too much water has been lost from the gypsum), preventing the destruction of the wood and the collapse of the structure.

The partially dehydrated mineral is called calcium sulfate hemihydrate or calcined gypsum (though more commonly known as plaster of Paris) and has the chemical formula CaSO₄·½H₂O. Calcined gypsum has an unusual property: when mixed with water at normal (ambient) temperatures, it recombines with the water that was driven off during calcination, and sets to form a strong gypsum crystal lattice:

CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O This reaction is exothermic.

The anhydrous form, called anhydrous calcium sulfate (sometimes anhydriteAnhydrite is a mineral anhydrous calcium sulfate, CaSO. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. Distinctly developed crystals are somewhat rare, the mineral usually pres, 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.

Most minerals, when rehydrated, simply form liquid or semi-liquid pastes, or remain powdery. Gypsum, on the other hand, forms a strong crystal structure immediately upon receiving the water, and this phenomenon is responsible for gypsum's ease of being cast into sheets (for drywall), sticks (for blackboard chalk), molds (to set broken bones, or create molds for metal casting), and other forms. Small amounts of calcined gypsum are added to earth to create strong structures directly from cast earth , an alternative to adobeThis page deals with adobe, the construction material. For information about the software company, see Adobe Systems. Adobe is a building material composed of sandy clay and (usually) straw, which can be cast into bricks or shaped directly into walls usin (which loses its strength when wet).