Calcium sulfate
Calcium sulfate is an inorganic salt with the chemical formula. It occurs in several hydrated forms; the anhydrous state 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 or fibrous masses, 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.
Hydration states
Calcium sulfate occurs at three levels of hydration with different crystallographic structures: anhydrous, dihydrate, and hemihydrate.The anhydrous 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 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 is also known as plaster of Paris. Specific hemihydrates are sometimes distinguished between α-hemihydrate and β-hemihydrate.
Image:CaSO4.tif|thumb|left|The structure of hemihydrate calcium sulfate is a network of Ca-O-S bonds. Color code: red, green, yellow.
Uses
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.Hydration and dehydration reactions
With judicious heating, gypsum converts to the partially dehydrated mineral called bassanite or plaster of Paris. This material has the formula CaSO4·, where 0.5 ≤ n ≤ 0.8. Temperatures between 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 are used in industrial calcination, but at these temperatures γ-anhydrite begins to form. The heat energy delivered to the gypsum at this time tends to go into driving off water 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 damage to the framing 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 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, sticks, and molds. 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. The conditions of dehydration can be changed to adjust the porosity of the hemihydrate, resulting in the so-called α- and β-hemihydrates.
On heating to, the nearly water-free form, called γ-anhydrite is produced. γ-Anhydrite slowly reabsorbs water to return to the dihydrate state, a property exploited in some commercial desiccants. On heating above 250 °C, the completely anhydrous form called β-anhydrite or "natural" anhydrite is formed. Natural anhydrite does not absorb 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.
Food industry
The calcium sulfate hydrates are used as a coagulant in products such as tofu.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.
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 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 or pink due to impregnation with cobalt 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. 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.Some component reactions pertaining to calcium sulfate:
Production and occurrence
The main sources of calcium sulfate are naturally occurring gypsum and anhydrite, which occur at many locations worldwide as evaporites. These may be extracted by open-cast quarrying or by deep mining. World production of natural gypsum is around 127 million tonnes per annum.In addition to natural sources, calcium sulfate is produced as a by-product in a number of processes:
- In flue-gas desulfurization, exhaust gases from fossil-fuel power stations and other processes are scrubbed to reduce their sulfur dioxide content, by injecting finely ground limestone:
- In the production of phosphoric acid from phosphate rock, calcium phosphate is treated with sulfuric acid and calcium sulfate precipitates. The product, called phosphogypsum is often contaminated with impurities making its use uneconomic.
- In the production of hydrogen fluoride, calcium fluoride is treated with sulfuric acid, precipitating calcium sulfate.
- In the refining of zinc, solutions of zinc sulfate are treated with hydrated lime to co-precipitate heavy metals such as barium.
- Calcium sulfate can also be recovered and re-used from scrap drywall at construction sites.
Calcium sulfate is also a common component of fouling deposits in industrial heat exchangers, because its solubility decreases with increasing temperature.