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Anhydrite

Gypsum karstanhydrideanhydrite stone
In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant.
Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO 4.

Gypsum

gypsiferouscalcium sulfate dihydrategypseous
One particular hydrate is better known as plaster of Paris, and another occurs naturally as the mineral gypsum.
Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO 4 ·2H 2 O. It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard/sidewalk chalk, and drywall.

Selenite (mineral)

selenitesatin spargypsum flowers
All varieties of gypsum, including selenite and alabaster, are composed of calcium sulfate dihydrate (meaning that it has two molecules of water), with the chemical formula CaSO 4 ·2H 2 O. Selenite contains no significant selenium, the similarity of the names of the substances coming from the Ancient Greek word for the Moon.

Drywall

plasterboardwallboardsheetrock
The endothermic property of this reaction is relevant to the performance of drywall, conferring fire resistance to residential and other structures.
Drywall (also known as plasterboard, wallboard, sheet rock, gypsum board, buster board, custard board, rusted sword, or gypsum panel) is a panel made of calcium sulfate dihydrate (gypsum), with or without additives, typically extruded between thick sheets of facer and backer paper, used in the construction of interior walls and ceilings.

Desiccant

drying agentdesiccantsdryers
In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant.
Other common desiccants include activated charcoal, calcium sulfate, calcium chloride, and molecular sieves (typically, zeolites).

Bassanite

calcium sulfate hemihydrate
Bassanite is a calcium sulfate mineral with formula CaSO 4 ·1⁄2H 2 O or 2CaSO 4 ·H 2 O. In other words it has half a water per CaSO 4 unit hence its synonym calcium sulfate hemihydrate.

Solubility

solubleinsolublewater-soluble
It is also convenient that calcium sulfate is poorly soluble in water and does not readily dissolve in contact with water after its solidification. 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 boilers along with the precipitation of calcium carbonate whose solubility also decreases when CO 2 degasses from hot water or can escape out of the system.
A few, such as calcium sulfate (gypsum) and cerium(III) sulfate, become less soluble in water as temperature increases.

Tofu

bean curdbeancurdsilken tofu
The calcium sulfate hydrates are used as a coagulant in products such as tofu.
It is high in iron, and can have a high calcium or magnesium content depending on the coagulants (e.g. calcium chloride, calcium sulfate, magnesium sulfate) used in manufacturing.

Fouling

scalescalingfouled
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). 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 boilers along with the precipitation of calcium carbonate whose solubility also decreases when CO 2 degasses from hot water or can escape out of the system.

E number

E-numberEE numbers
It is known in the E number series as E516, and the FAO knows it as a firming agent, a flour treatment agent, a sequestrant, and a leavening agent.

Sulfuric acid

sulphuric acidsulfuricH 2 SO 4
Up to the 1970s, commercial quantities of sulfuric acid were produced in Whitehaven (Cumbria, UK) from anhydrous calcium sulfate. Upon being mixed with shale or marl, and roasted, the sulfate liberates sulfur trioxide gas, a precursor in sulfuric acid production, the reaction also produces calcium silicate, a mineral phase essential in cement clinker production.
This is treated with 93% sulfuric acid to produce calcium sulfate, hydrogen fluoride (HF) and phosphoric acid.

Calcium hydroxide

slaked limelimehydrated lime
Another calcium compound, calcium hydroxide (Ca(OH) 2, portlandite) also exhibits a retrograde solubility for the same thermodynamic reason: because its dissolution reaction is also exothermic and releases heat.
The variably hydrated phases of calcium sulfate (gypsum, bassanite and anhydrite) also exhibit a retrograde solubility for the same reason because their dissolution reactions are exothermic.

Calcium sulfate (data page)

This page provides supplementary chemical data on calcium sulfate.

Phosphoric acid

phosphoricH 3 PO 4 orthophosphoric acid
Calcium sulfate (gypsum) is produced as a by-product and is removed as phosphogypsum.

Phosphogypsum

Phosphogypsum refers to the calcium sulfate hydrate formed as a by-product of the production of fertilizer from phosphate rock.

Sulfur trioxide

SO 3 sulphur trioxidetrioxide
Upon being mixed with shale or marl, and roasted, the sulfate liberates sulfur trioxide gas, a precursor in sulfuric acid production, the reaction also produces calcium silicate, a mineral phase essential in cement clinker production.
It was once produced industrially by heating calcium sulfate with silica.

Mars

MartianCoordinatesplanet Mars
2011 findings by the Opportunity rover on the planet Mars show a form of calcium sulfate in a vein on the surface.
On March 18, 2013, NASA reported evidence from instruments on the Curiosity rover of mineral hydration, likely hydrated calcium sulfate, in several rock samples including the broken fragments of "Tintina" rock and "Sutton Inlier" rock as well as in veins and nodules in other rocks like "Knorr" rock and "Wernicke" rock.

Calcium fluoride

CaF 2 fluoritecalcium difluoride
:CaF 2 + H 2 SO 4 → CaSO 4 (solid) + 2 HF

Calcium carbonate

CaCO 3 calcareouscalcium
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 boilers along with the precipitation of calcium carbonate whose solubility also decreases when CO 2 degasses from hot water or can escape out of the system.
However, modern manufactured chalk is mostly gypsum, hydrated calcium sulfate CaSO 4 ·2H 2 O. Calcium carbonate is a main source for growing Seacrete.

Plaster

plaster of Parisgypsum plasterplaster-of-Paris
One particular hydrate is better known as plaster of Paris, and another occurs naturally as the mineral gypsum.
The hydration of plaster of Paris relies on the reaction of water with the dehydrated or partially hydrated calcium sulfate present in the plaster.

Chalk

chalk pitchalk rockblack chalk
Blackboard chalk manufacture now may use mineral chalk, other mineral sources of calcium carbonate, or the mineral gypsum (calcium sulfate).

Bathybius haeckelii

Urschleimbathybius haecklii
He noticed that it was a precipitate of calcium sulfate from the seawater that had reacted with the preservative liquid (alcohol), forming a gelatinous ooze which clung to particles as if ingesting them.

Hydrate

hydrousdihydratemonohydrate
Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO 4 and related hydrates.

Anhydrous

waterlessfree from the presence of water
In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant.

Stucco

renderedrenderstuccoed
The main use of calcium sulfate is to produce plaster of Paris and stucco.