Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related hydrates. In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant. One particular hydrate is better known as plaster of Paris, and another occurs naturally as the mineral gypsum. It has many uses in industry. All forms are white solids that are poorly soluble in water. Calcium sulfate causes permanent hardness in water.
3D model (JSmol)
|E number||E516 (acidity regulators, ...)|
|Molar mass||136.14 g/mol (anhydrous)|
145.15 g/mol (hemihydrate)
172.172 g/mol (dihydrate)
|Density||2.96 g/cm3 (anhydrous) |
2.32 g/cm3 (dihydrate)
|Melting point||1,460 °C (2,660 °F; 1,730 K) (anhydrous)|
|0.21g/100ml at 20 °C (anhydrous) |
0.24 g/100ml at 20 °C (dihydrate)
Solubility product (Ksp)
|4.93 × 10−5 mol2L−2 (anhydrous)|
3.14 × 10−5 (dihydrate)
|Solubility in glycerol||slightly soluble (dihydrate)|
|Acidity (pKa)||10.4 (anhydrous) |
|107 J·mol−1·K−1 |
Std enthalpy of
|Safety data sheet||See: data page|
|US health exposure limits (NIOSH):|
|TWA 15 mg/m3 (total) TWA 5 mg/m3 (resp) [for anhydrous form only]|
|TWA 10 mg/m3 (total) TWA 5 mg/m3 (resp) [anhydrous only]|
IDLH (Immediate danger)
|Plaster of Paris|
|Supplementary data page|
|Refractive index (n),|
Dielectric constant (εr), etc.
|UV, IR, NMR, MS|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
The compound exists in three levels of hydration corresponding to different crystallographic structures and to different minerals in the nature:
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 calcium sulfate hemihydrate, calcined gypsum, and plaster of Paris.This material has the formula CaSO4·(nH2O), where 0.5 ≤ n ≤ 0.8. Temperatures between 100 °C and 150 °C (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 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:
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 alpha and beta hemihydrates (which are more or less chemically identical).
On heating to 180 °C, 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 desiccants. 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.
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 production.
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:
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).
The dissolution of the different crystalline phases of calcium sulfate in water is exothermic and release 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 (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 scale in boilers along with the precipitation of calcium carbonate whose solubility also decreases when CO2 degasses from hot water or can escape out of the system.
Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the orthorhombic barium (baryte) and strontium (celestine) sulfates, as might be expected from the chemical formulas. Distinctly developed crystals are somewhat rare, the mineral usually presenting the form of cleavage masses. The Mohs hardness is 3.5, and the specific gravity is 2.9. The color is white, sometimes greyish, bluish, or purple. On the best developed of the three cleavages, the lustre is pearly; on other surfaces it is glassy. When exposed to water, anhydrite readily transforms to the more commonly occurring gypsum, (CaSO4·2H2O) by the absorption of water. This transformation is reversible, with gypsum or calcium sulfate hemihydrate forming anhydrite by heating to around 200 °C (400 °F) under normal atmospheric conditions. Anhydrite is commonly associated with calcite, halite, and sulfides such as galena, chalcopyrite, molybdenite, and pyrite in vein deposits.Bassanite
Bassanite is a calcium sulfate mineral with formula CaSO4·0.5(H2O) or 2CaSO4·H2O. In other words it has half a water per CaSO4 unit hence its synonym hemihydrate.
Bassanite was first described in 1910 for an occurrence on Mount Vesuvius. It was named for Italian paleontologist Francesco Bassani (1853–1916).At Vesuvius it occurs as alterations from gypsum within leucite tephrite and as fumarole deposits. It occurs in dry lake beds in California and Australia. It also occurs interlayered with gypsum in caves.Blackboard
A blackboard (also known as a chalkboard) is a reusable writing surface on which text or drawings are made with sticks of calcium sulfate or calcium carbonate, known, when used for this purpose, as chalk. Blackboards were originally made of smooth, thin sheets of black or dark grey slate stone.Calcium bromate
Calcium bromate, Ca(BrO3)2, is a calcium salt of bromic acid. It is most commonly encountered as the monohydrate, Ca(BrO3)2•H2O.It can be prepared by reacting calcium hydroxide with sodium bromate or calcium sulfate with barium bromate. Above 180 °C, calcium bromate decomposes to form calcium bromide and oxygen. In theory, electrolysis of calcium bromide solution will also yield calcium bromate.
It is used as a bread dough and flour "improver" or conditioner (E number E924b) in some countries.Calcium sulfate (data page)
This page provides supplementary chemical data on calcium sulfate.Calcium sulfite
Calcium sulfite, or calcium sulphite, is a chemical compound, the calcium salt of sulfite with the formula CaSO3·x(H2O). Two crystalline forms are known, the hemihydrate and the tetrahydrate, respectively CaSO3·½(H2O) and CaSO3·4(H2O). All forms are white solids. It is most notable as the product of flue-gas desulfurization.Caliche
Caliche () is a sedimentary rock, a hardened natural cement of calcium carbonate that binds other materials—such as gravel, sand, clay, and silt. It occurs worldwide, in aridisol and mollisol soil orders—generally in arid or semiarid regions, including in central and western Australia, in the Kalahari Desert, in the High Plains of the western USA, in the Sonoran Desert and Mojave Desert, and in Eastern Saudi Arabia Al-Hasa. Caliche is also known as calcrete or kankar (in India). It belongs to the duricrusts. The term caliche is Spanish and is originally from the Latin calx, meaning lime.
Caliche is generally light-colored, but can range from white to light pink to reddish-brown, depending on the impurities present. It generally occurs on or near the surface, but can be found in deeper subsoil deposits, as well. Layers vary from a few inches to feet thick, and multiple layers can exist in a single location.
In northern Chile and Peru, caliche also refers to mineral deposits that include nitrate salts. Caliche can also refer to various claylike deposits in Mexico and Colombia. In addition, it has been used to describe some forms of quartzite, bauxite, kaolinite, laterite, chalcedony, opal, and soda niter.
A similar material, composed of calcium sulfate rather than calcium carbonate, is called gypcrust.Gypcrust
Gypcrete or gypcrust is a hardened layer of soil, consisting of around 95% gypsum (calcium sulfate). Gypcrust is an arid zone duricrust. It can also occur in a semiarid climate in a basin with internal drainage, and is initially developed in a playa as an evaporate. Gypcrete is the arid climate's equivalent to calcrete, which is a duricrust that is unable to generate in very arid climates.Gypsum
Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard chalk and wallboard. A massive fine-grained white or lightly tinted variety of gypsum, called alabaster, has been used for sculpture by many cultures including Ancient Egypt, Mesopotamia, Ancient Rome, the Byzantine Empire and the Nottingham alabasters of Medieval England. Gypsum also crystallizes as beautiful translucent crystals of selenite (mineral). It also forms as an evaporite mineral and as a hydration product of anhydrite.
The Mohs scale of mineral hardness defines hardness value 2 as gypsum based on scratch hardness comparison.List of desiccants
A desiccant is a substance that absorbs water. It is most commonly used to remove humidity that would normally degrade or even destroy products sensitive to moisture.
List of desiccants:
Calcium sulfate (Drierite)
Sulfuric acidMud cracks on Mars
In January 2017, scientists announced the possible discovery of mud cracks in Gale Crater on Mars. The Curiosity Rover imaged what may be the first mud cracks (desiccation cracks) ever found on Mars. They may have been formed from drying mud. The site, called “Old Soaker,” was within an exposure of Murray formation mudstone on lower Mount Sharp.
It is hypothesized that these cracks formed more than 3 billion years ago and then were buried by more sediment. All this material eventually turned into rock. Later wind erosion removed the layers that covered the cracked layer. The cracks were filled with material which was resistant to later erosion. This erosion resistant material formed raised ridges, as some of the surrounding layer was removed.
This is the first sighting of mud cracks. Previously, Curiosity has examined cracks and ridges of different shapes that were made by groundwater carrying minerals, such as calcium sulfate. Cracks for this process were caused by the pressure of overlying sediments fracturing rock.
Gale Crater held ancient lakes that varied in depth and area over time, and sometimes disappeared. Mud cracks show that there were dry times when lakes disappeared. Besides this evidence of mud, Curiosity has found evidence of ancient lakes in older layers and also in younger mudstone. Nathan Stein, a graduate student at the California Institute of Technology led the investigation.Orcein
Orcein, also archil, orchil, lacmus and C.I. Natural Red 28, are names for dyes extracted from several species of lichen, commonly known as "orchella weeds", found in various parts of the world. A major source is the archil lichen, Roccella tinctoria. Orcinol is extracted from such lichens. It is then converted to orcein by ammonia and air. In traditional dye-making methods, urine was used as the ammonia source. If the conversion is carried out in the presence of potassium carbonate, calcium hydroxide, and calcium sulfate (in the form of potash, lime, and gypsum in traditional dye-making methods), the result is litmus, a more complex molecule. The manufacture was described by Cocq in 1812 and in the UK in 1874. Edmund Roberts noted orchilla as a principal export of the Cape Verde islands, superior to the same kind of moss found in Italy or the Canary Islands, that in 1832 was yielding an annual revenue of $200,000. Commercial archil is either a powder (called cudbear) or a paste. It is red in acidic pH and blue in alkaline pH.Phosphogypsum
Phosphogypsum refers to the calcium sulfate hydrate formed as a by-product of the production of fertilizer from phosphate rock. It is mainly composed of gypsum (CaSO4·2H2O). Although gypsum is a widely used material in the construction industry, phosphogypsum is usually not used, but is stored indefinitely because of its weak radioactivity. The long-range storage is controversial.Sidewalk chalk
Sidewalk chalk is typically large colored (and sometimes white or cream) sticks of chalk (calcium sulfate rather than rock chalk, calcium carbonate) mostly used for drawing on pavement or concrete sidewalks. It is sometimes used by children to draw a four square court or a hopscotch board. Blackboard chalk is typically shorter.
There are several different types of sidewalk chalk, typically coming in solid-colored sticks. 3-D sidewalk chalk sets, in which each stick of chalk is created with two particular colors that appear 3-dimensional when viewed through the 3-D glasses that come with the chalk, also exist.Sidewalk chalk can be cheaply homemade.Solubility chart
A solubility chart is a chart with a list of ions and how, when mixed with other ions, they can become precipitates or remain aqueous.
The following chart shows the solubilities of multiple independent and various compounds, in water, at a pressure of 1 atm and at room temperature (approx. 293.15 K). Any box that reads "soluble" results in an aqueous product in which no precipitate has formed, while "slightly soluble" and "insoluble" markings mean that there is a precipitate that will form (usually, this is a solid), however, "slightly soluble" compounds such as calcium sulfate may require heat to form its precipitate. Boxes marked "other" can mean that many different states of products can result. For more detailed information of the exact solubility of the compounds, see the solubility table.
The chemicals have to be exposed to their boiling point to fully dissolve.Spackling paste
In the United States, spackling paste is a putty used to fill holes, small cracks, and other minor surface defects in wood, drywall, and plaster. Typically, spackling is composed of gypsum plaster from hydrated calcium sulfate and glue.Thermoluminescent dosimeter
A thermoluminescent dosimeter, or TLD, is a type of radiation dosimeter. A TLD measures ionizing radiation exposure by measuring the intensity of visible light emitted by a crystal inside the detector when the crystal is heated. The intensity of light emitted is dependent upon the radiation exposure. Materials exhibiting thermoluminescence in response to ionizing radiation include calcium fluoride, lithium fluoride, calcium sulfate, lithium borate, calcium borate, potassium bromide, and feldspar. It was invented in 1954 by Professor Farrington Daniels of the University of Wisconsin-Madison.Tintina (rock)
Tintina is a rock on the surface of Aeolis Palus, between Peace Vallis and Aeolis Mons (Mount Sharp), in Gale crater on the planet Mars. The approximate site coordinates are: 4.59°S 137.44°E / -4.59; 137.44.
The rock was encountered by the Curiosity rover on the way from Bradbury Landing to Glenelg Intrique in January 2013. The rover ran over the rock and broke it. revealing white surface area in the rock. This was the brightest material yet seen by MastCam up to that time.When the broken white area was analyzed with the rover's MastCam, strong signals of mineral hydration, as indicated by a ratio of near infrared reflectance intensities, were found. According to mission scientists, the mineral hydration signals were consistent with hydrated calcium sulfate and a "watery past" on Mars.Winogradsky column
The Winogradsky column is a simple device for culturing a large diversity of microorganisms. Invented in the 1880s by Sergei Winogradsky, the device is a column of pond mud and water mixed with a carbon source such as newspaper (containing cellulose), blackened marshmallows or egg-shells (containing calcium carbonate), and a sulfur source such as gypsum (calcium sulfate) or egg yolk. Incubating the column in sunlight for months results in an aerobic/anaerobic gradient as well as a sulfide gradient. These two gradients promote the growth of different microorganisms such as Clostridium, Desulfovibrio, Chlorobium, Chromatium, Rhodomicrobium, and Beggiatoa, as well as many other species of bacteria, cyanobacteria, and algae.
The column provides numerous gradients, depending on additive nutrients, from which the variety of aforementioned organisms can grow. The aerobic water phase and anaerobic mud or soil phase are one such distinction. Because of oxygen's low solubility in water, the water quickly becomes anoxic towards the interface of the mud and water. Anaerobic phototrophs are still present to a large extent in the mud phase, and there is still capacity for biofilm creation and colony expansion, as shown in the images at right. Algae and other aerobic phototrophs are present along the surface and water of the upper half of the columns. Green growth is often attributed to these organisms.