calcium sulfate
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Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related
hydrate In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atoms, m ...
s. In the form of γ-
anhydrite Anhydrite, or anhydrous calcium sulfate, is a mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock ...

anhydrite
(the
anhydrous A substance is anhydrous if it contains no water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the flui ...
form), it is used as a
desiccant A desiccant is a hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption (chemistry), absorption or adsorption from the surrounding Natural environment, environment, which is usually at normal ...
. One particular hydrate is better known as
plaster of Paris Plaster is a building material used for the protective or decorative coating of walls and ceilings and for moulding and casting decorative elements. In English "plaster" usually means a material used for the interiors of buildings, while "rend ...
, and another occurs naturally as the mineral
gypsum Gypsum is a soft sulfate mineralThe sulfate minerals are a class of mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the ...

gypsum
. It has many uses in industry. All forms are white solids that are poorly soluble in water.Franz Wirsching "Calcium Sulfate" in Ullmann's Encyclopedia of Industrial Chemistry, 2012 Wiley-VCH, Weinheim. Calcium sulfate causes permanent hardness in water.


Hydration states and crystallographic structures

The compound exists in three levels of hydration corresponding to different crystallographic structures and to different minerals in nature: * CaSO4 (
anhydrite Anhydrite, or anhydrous calcium sulfate, is a mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock ...

anhydrite
): anhydrous state. * CaSO4 · 2 H2O (
gypsum Gypsum is a soft sulfate mineralThe sulfate minerals are a class of mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the ...

gypsum
and
selenite (mineral) Selenite, satin spar, desert rose, gypsum flower are crystal habit In mineralogy Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical mineralogy, optica ...
): dihydrate. * CaSO4 · H2O (
bassanite Bassanite is a calcium sulfate mineral with formula calcium sulfate, CaSO4·water of crystallization, H2O or 2CaSO4·H2O. In other words it has half a water per CaSO4 unit hence its synonym ''calcium sulfate hemihydrate''. Bassanite was first des ...
): hemihydrate, also known as
plaster of Paris Plaster is a building material used for the protective or decorative coating of walls and ceilings and for moulding and casting decorative elements. In English "plaster" usually means a material used for the interiors of buildings, while "rend ...
. Specific hemihydrates are sometimes distinguished: α-hemihydrate and β-hemihydrate.Taylor H.F.W. (1990) ''Cement Chemistry''. Academic Press, , pp. 186-187.


Uses

The main use of calcium sulfate is to produce plaster of Paris and
stucco Stucco or render is a construction material made of Construction aggregate, aggregates, a binder (material), binder, and water. Stucco is applied wet and hardens to a very dense solid. It is used as a decorative coating for walls and ceilings, e ...
. These applications exploit the fact that calcium sulfate which has been powdered and
calcined Calcination refers to heating a solid to high temperatures in absence of air or oxygen, generally for the purpose of removing impurities or volatile substances. However, calcination is also used to mean a thermal treatment process in the absence o ...
forms a moldable paste upon hydration and hardens as crystalline calcium sulfate dihydrate. It is also convenient that calcium sulfate is poorly
soluble Solubility is the property of a solid, liquid or gaseous chemical substance called ''solution, solute'' to dissolve in a solid, liquid or gaseous solvent. The solubility of a substance fundamentally depends on the Physical property, physical an ...

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 Bassanite is a calcium sulfate mineral with formula calcium sulfate, CaSO4·water of crystallization, H2O or 2CaSO4·H2O. In other words it has half a water per CaSO4 unit hence its synonym ''calcium sulfate hemihydrate''. Bassanite was first des ...
or
plaster of Paris Plaster is a building material used for the protective or decorative coating of walls and ceilings and for moulding and casting decorative elements. In English "plaster" usually means a material used for the interiors of buildings, while "rend ...
. This material has the formula CaSO4·(''n''H2O), 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 (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: : CaSO4 · 2 H2O   →   CaSO4 · H2O + H2O↑ The
endothermic An endothermic process is any process with an increase in the enthalpy Enthalpy is a property of a thermodynamic system A thermodynamic system is a body of matter and/or radiation, confined in space by walls, with defined permeabilities, whic ...
property of this reaction is relevant to the performance of
drywall Drywall (also known as plasterboard, wallboard, sheet rock, gypsum board, buster board, custard board, or gypsum panel) is a panel made of calcium sulfate wiktionary:dihydrate, dihydrate (gypsum), with or without additives, typically extruded bet ...

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 Combustion, or burning, is a high-temperature exothermic In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, radiation, and physical pro ...
of
wood Wood is a porous and fibrous structural tissue found in the Plant stem, stems and roots of trees and other woody plants. It is an organic materiala natural composite material, composite of cellulose fibers that are strong in tension and embe ...

wood
members or loss of strength of
steel Steel is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appea ...

steel
at high temperatures) and consequent structural collapse. But at higher temperatures, calcium sulfate will release oxygen and act as an
oxidizing agent Image:DOT hazmat class 5.1.svg, 125px, Dangerous goods label for oxidizing agents An oxidizing agent, also known as an oxidant or oxidizer, is a substance that has the ability to oxidize other substances — in other words to electron accep ...

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: :CaSO4 · H2O + H2O   →   CaSO4 · 2 H2O This reaction is
exothermic In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quan ...

exothermic
and is responsible for the ease with which gypsum can be cast into various shapes including sheets (for
drywall Drywall (also known as plasterboard, wallboard, sheet rock, gypsum board, buster board, custard board, or gypsum panel) is a panel made of calcium sulfate wiktionary:dihydrate, dihydrate (gypsum), with or without additives, typically extruded bet ...

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·''n''H2O 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
desiccant A desiccant is a hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption (chemistry), absorption or adsorption from the surrounding Natural environment, environment, which is usually at normal ...
s. On heating above 250 °C, the completely anhydrous form called β-anhydrite or "natural"
anhydrite Anhydrite, or anhydrous calcium sulfate, is a mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock ...

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.


Food industry

The calcium sulfate hydrates are used as a flocculation, 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 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/extender and as a barrier in guided tissue regeneration. It is an unusually 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.


Other uses

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. 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 (cement), clinker production. : CaSO4 + SiO2 → CaSiO3 + SO3


Production and occurrence

The main sources of calcium sulfate are naturally occurring
gypsum Gypsum is a soft sulfate mineralThe sulfate minerals are a class of mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the ...

gypsum
and
anhydrite Anhydrite, or anhydrous calcium sulfate, is a mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock ...

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 (e.g. cement manufacture) are scrubbed to reduce their sulfur oxide content, by injecting finely ground limestone or calcium hydroxide, lime. This produces an impure calcium sulfite, which oxidizes on storage to calcium sulfate. *In the production of phosphoric acid from apatite, phosphate rock, calcium phosphate is treated with sulfuric acid and calcium sulfate precipitates. *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 calcium hydroxide, 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. 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 decay products such as radium-226, lead-210 and polonium-210. 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).


Retrograde solubility

The dissolution of the different crystalline phases of calcium sulfate in water is
exothermic In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quan ...

exothermic
and releases heat (decrease in Enthalpy: ΔH < 0). As an immediate consequence, to proceed, the dissolution reaction needs to evacuate this heat that can be considered as a product of reaction. If the system is cooled, the dissolution equilibrium will evolve towards the right according to the Le Chatelier principle and calcium sulfate will dissolve more easily. The solubility of calcium sulfate increases thus when the temperature decreases. If the temperature of the system is raised, the reaction heat cannot dissipate and the equilibrium will regress towards the left according to Le Chatelier principle. The solubility of calcium sulfate decreases thus when temperature increases. This contra-intuitive solubility behaviour is called retrograde solubility. It is less common than for most of the salts whose dissolution reaction is
endothermic An endothermic process is any process with an increase in the enthalpy Enthalpy is a property of a thermodynamic system A thermodynamic system is a body of matter and/or radiation, confined in space by walls, with defined permeabilities, whic ...
(i.e., the reaction consumes heat: increase in Enthalpy: ΔH > 0) and whose solubility increases with temperature. 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. So, to dissolve higher amounts of calcium sulfate or calcium hydroxide in water, it is necessary to cool down the solution close to its freezing point instead of increasing its 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 Fouling#Precipitation fouling, scale in boilers along with the precipitation of calcium carbonate whose solubility also decreases when Carbon dioxide, CO2 degasses from hot water or can escape out of the system.


On planet Mars

2011 findings by the Opportunity (rover), ''Opportunity'' rover on the planet Mars show a form of calcium sulfate in a vein on the surface. Images suggest the mineral is
gypsum Gypsum is a soft sulfate mineralThe sulfate minerals are a class of mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the ...

gypsum
.


See also

* Calcium sulfate (data page) *Alabaster *Anhydrite *''Bathybius haeckelii'' *Chalk (calcium carbonate) *Gypsum *Plaster#Gypsum plaster, Gypsum plaster *Phosphogypsum *Selenite (mineral) *Flue-gas desulfurization


References


External links


International Chemical Safety Card 1215
{{Authority control Calcium compounds Sulfates Desiccants Food additives Pyrotechnic colorants E-number additives