Calcium sulfate

Calcium sulfate (or calcium sulphate) is an inorganic salt with the chemical formula CaSO
₄. It occurs in several hydrated forms; the anhydrous state (known as anhydrite) is a white crystalline solid often found in evaporite deposits. Its dihydrate form is the mineral gypsum, which may be dehydrated to produce bassanite, the hemihydrate state. Gypsum occurs in nature as crystals (selenite) or fibrous masses (satin spar), typically colorless to white, though impurities can impart other hues. All forms of calcium sulfate are sparingly soluble in water and cause permanent hardness when dissolved therein.

Calcium sulfate occurs at three levels of hydration with different crystallographic structures: anhydrous, dihydrate, and hemihydrate.

The anhydrous CaSO
₄ (anhydrite) crystallizes as an tightly-bound orthohombic lattice with space group Pnma, in which each Ca²⁺
is 8-coordinated, or surrounded, by 8 oxygen atoms from tetrahedral SO²⁻
₄. It is similar in topology to zircon.

The dihydrate CaSO
₄·2H
₂O (gypsum) forms a monoclinic crystal with space group C2/c. Its structure consists of alternating layers: one with Ca²⁺
coordinated with tetrahedral SO²⁻
₄ and another with interstitial water molecules.

The hemihydrate CaSO
₄·⁠1/2⁠H
₂O (bassanite) is also known as plaster of Paris. Specific hemihydrates are sometimes distinguished between α-hemihydrate and β-hemihydrate.

The main use of calcium sulfate is to produce plaster of Paris and stucco. These applications exploit the fact that calcium sulfate which has been powdered and calcined forms a moldable paste upon hydration and hardens as crystalline calcium sulfate dihydrate. It is also convenient that calcium sulfate is poorly soluble in water and does not readily dissolve in contact with water after its solidification.

With judicious heating, gypsum converts to the partially dehydrated mineral called bassanite or plaster of Paris. This material has the formula CaSO₄·(nH₂O), where 0.5 ≤ n ≤ 0.8. Temperatures between 100 and 150 °C (212–302 °F) are required to drive off the water within its structure. The details of the temperature and time depend on ambient humidity. Temperatures as high as 170 °C (338 °F) are used in industrial calcination, but at these temperatures γ-anhydrite begins to form. 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 equation for the partial dehydration is:

The endothermic property of this reaction is relevant to the performance of drywall, conferring fire resistance to 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. But at higher temperatures, calcium sulfate will release oxygen and act as an oxidizing agent. This property is used in aluminothermy. 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: