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Calcium sulfate (or calcium sulphate) is an inorganic salt with the chemical formula . It occurs in several Hydrate forms; the anhydrous state (known as anhydrite) is a white crystalline solid often found in Evaporite. 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 Solubility in waterFranz Wirsching "Calcium Sulfate" in Ullmann's Encyclopedia of Industrial Chemistry, 2012 Wiley-VCH, Weinheim. and cause permanent hardness when dissolved therein.
The anhydrous (anhydrite) crystallizes as an tightly-bound orthohombic lattice with space group Pnma, in which each is 8-coordinated, or surrounded, by 8 oxygen atoms from tetrahedral . It is similar in topology to zircon.
The dihydrate (gypsum) forms a monoclinic crystal with space group C2/c. Its structure consists of alternating layers: one with coordinated with tetrahedral and another with interstitial water molecules.
The hemihydrate (bassanite) is also known as plaster of Paris. Specific hemihydrates are sometimes distinguished between α-hemihydrate and β-hemihydrate.Taylor H.F.W. (1990) Cement Chemistry. Academic Press, , pp. 186–187.
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:
This reaction is exothermic and 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 α- and β-hemihydrates (which are more or less chemically identical).
On heating to , the nearly water-free form, called γ-anhydrite (CaSO4· nH2O where n = 0 to 0.05) is produced. γ-Anhydrite reacts slowly with water to return to the dihydrate state, a property exploited in some commercial . On heating above 250 °C, the completely anhydrous form called β-anhydrite or "natural" anhydrite is formed. Natural anhydrite does not react with water, even over geological timescales, unless very finely ground.
The variable composition of the hemihydrate and γ-anhydrite, and their easy inter-conversion, is due to their nearly identical crystal structures containing "channels" that can accommodate variable amounts of water, or other small molecules such as methanol.
For the FDA, it is permitted in cheese and related cheese products; cereal flours, bakery products, frozen desserts, artificial sweeteners for jelly & preserves, condiment vegetables, and condiment tomatoes, and some candies.
It is known in the E number series as E516, and the UN's FAO knows it as a firming agent, a flour treatment agent, a sequestrant, and a leavening agent.
Some component reactions pertaining to calcium sulfate:
In addition to natural sources, calcium sulfate is produced as a by-product in a number of processes:
These precipitation processes tend to concentrate radioactive elements in the calcium sulfate product. This issue is particular with the phosphate by-product, since phosphate ores naturally contain uranium and its such as radium-226, lead-210 and polonium-210. Extraction of uranium from phosphorus ores can be economical on its own depending on prices on the uranium market or the separation of uranium can be mandated by environmental legislation and its sale is used to recover part of the cost of the process.
Calcium sulfate is also a common component of fouling deposits in industrial heat exchangers, because its solubility decreases with increasing temperature (see the specific section on the retrograde solubility).
Dentistry
Calcium sulfate has a long history of use in dentistry. It has been used in bone regeneration as a graft material and graft binder (or extender) and as a barrier in guided bone tissue regeneration. It is a biocompatible material and is completely resorbed following implantation. It does not evoke a significant host response and creates a calcium-rich milieu in the area of implantation.
Desiccant
When sold at the anhydrous state as a desiccant with a color-indicating agent under the name Drierite, it appears blue (anhydrous) or pink (hydrated) due to impregnation with cobalt(II) chloride, which functions as a moisture indicator.
Sulfuric acid production
Up to the 1970s, commercial quantities of sulfuric acid were produced from anhydrous calcium sulfate. Whitehaven Cement Plant Upon being mixed with shale or marl, and roasted at 1400°C, the sulfate liberates sulfur dioxide gas, a precursor to sulfuric acid. The reaction also produces calcium silicate, used in cement clinker production. Anhydrite Process COMMONWEALTH OF AUSTRALIA. DEPARTMENT OF SUPPLY AND SHIPPING. BUREAU OF MINERAL RESOURCES GEOLOGY AND GEOPHYSICS. REPORT NO.1949/44 (Geol. Ser. No. 27) by E.K. Sturmfels THE PRODUCTION OF SULPHURIC ACID AND PORTLAND CEMENT FROM CALCIUM SULPHATE AND ALUMINIUM SILICATES
Production and occurrence
The main sources of calcium sulfate are naturally occurring gypsum and anhydrite, which occur at many locations worldwide as . These may be extracted by open-cast quarrying or by deep mining. World production of natural gypsum is around 127 million tonnes per annum. Gypsum , USGS, 2008
(2025). 9780471484943 ISBN 9780471484943
Related sulfur-trapping methods use lime and some produces an impure calcium sulfite, which oxidizes on storage to calcium sulfate.
Solubility
The solubility of calcium sulfate decreases as temperature increases. This behaviour ("retrograde solubility") is uncommon: dissolution of most of the salts is endothermic and their solubility increases with temperature. The retrograde solubility of calcium sulfate is also responsible for its precipitation in the hottest zone of heating systems and for its contribution to the formation of scale in along with the precipitation of calcium carbonate whose solubility also decreases when Carbon dioxide degasses from hot water or can escape out of the system.
See also
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