Discover

Calcium sulfate
Calcium sulphate hemihydrate
General
Other names	Plaster of Paris
Molecular formula	CaSO4
Molar mass	136.142 g/mol (''anhydrous'')
Appearance	white solid
CAS number	[7778-18-9] (''anhydrous'')
Properties
Density and phase	2.96 g/cm3, solid
Solubility in water	0.24 g/100 ml (20 °C)
Melting point	1460°C (''anhydrous'')
Refractive Index	1.57
Mohs hardness	3.5 (anhydrite), 2.0 (gypsum)
Structure
Coordination geometry	?
Crystal structure	orthorhombic
'Thermodynamic data'
Standard enthalpy of formation ΔfH°solid	-1434.5 kJ/mol
Standard molar entropy S°solid	? J.K–1.mol–1
Heat of fusion	205.668
Specific Heat Capacity	0.732324 J/g-°C (anhydrous)
'Safety data'
PEL-TWA (OSHA)	15 mg/m3
RTECS number	WS6920000
NFPA 704
Supplementary data page
Structure & properties	''n'', ''εr'', etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other cations	Magnesium sulfate Strontium sulfate
Related desiccants	Calcium chloride Magnesium sulfate
Related compounds	Plaster of Paris Gypsum
Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa)

'Calcium sulfate' is a common laboratory and industrial chemical. In the form of γ-anhydrite (a nearly anhydrous form), it is sold as a laboratory desiccant. The hemihydrate (CaSO₄.~0.5H₂O) is better known as 'plaster of Paris', while the dihydrate (CaSO₄.2H₂O) occurs naturally as 'gypsum'. The anhydrous form occurs naturally as β-anhydrite. Depending on the method of calcination of calcium sulfate dihydrate specific hemihydrates are sometimes distinguished: alpha-hemihydrate and beta-hemihydrate^[1]. They appear to differ only in crystal size. Alpha-hemihydrate crystals are more prismatic than beta-hemihydrate crystals and when mixed with water form a much stronger and harder superstructure.
Natural, unrefined calcium sulfate is a translucent, crystalline white rock.

Contents

Commercial production and recovery

Dehydration reactions

Uses

See also

References

External links

Commercial production and recovery

The main sources of calcium sulfate are the naturally-occurring gypsum and anhydrite which occur at many locations world-wide as evaporites. These may be extracted by open-cast quarrying or by deep mining. World production of natural gypsum is around 100 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-burning power stations and other processes (e.g. cement manufacture) are scrubbed to reduce their sulfur oxide content, by injecting finely ground limestone or lime. This produces an impure calcium sulfite, which oxidizes on storage to calcium sulfate.

★ In the production of phosphoric acid from phosphate rock, calcium phosphate is treated with sulfuric acid. Calcium sulfate precipitates out.

★ 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 lime to co-precipitate heavy metals such as barium.
The precipitation processes tend to concentrate radioactive elements in the calcium sulfate product. This is particularly the case with the phosphate by-product, since phosphate rocks naturally contain actinides. Over 200 million tonnes per annum^[2] of calcium sulfate waste is produced by the phosphate industry worldwide.
Calcium sulfate can also be recovered and re-used from scrap drywall at construction sites.

Dehydration reactions

Heating gypsum to between 100°C and 150°C (302°F) partially dehydrates the mineral by driving off approximately 75% of the water contained in its chemical structure. The temperature and time needed depend on ambient partial pressure of H₂O. Temperatures as high as 170°C are used in industrial calcination, but at these temperatures γ-anhydrite begins to form.
The reaction for the partial dehydration is:
CaSO₄·2H₂O + heat → CaSO₄·½H₂O + 1½H₂O (steam)
The partially dehydrated mineral is called 'calcium sulfate hemihydrate' or 'calcined gypsum' (commonly known as plaster of Paris) (CaSO₄·nH₂O), where n is in the range 0.5 to 0.8.^[3]
The dehydration (specifically known as calcination) begins at approximately 80°C (176°F), although in dry air, some dehydration will take place already at 50°C. 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 endothermic property of this reaction is exploited by drywall to confer fire resistance on 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.
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:
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O This reaction is exothermic.
This phenomenon 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 possessing nearly-identical crystal structures, containing "channels" that can accommodate variable amounts of water, or other small molecules such as methanol.

Uses

Expanding on gypsum's role as a building material, at least two teams of researchers are currently designing robots that will build houses out of gypsum and concrete. Robo-builder threatens the brickie Robert Booth A team headed by Dr Behrokh Khoshnevis at the University of Southern California and a team headed by Dr Rupert Soar at Loughborough University’s School of Mechanical and Manufacturing Engineering in England are both working on robots that will spray gypsum or concrete to create the shell of a house. The California team expects that their robot will be able to do so in only 24 hours and plans to build the first prototype shell before April of 2007. The British team's robot, in contrast, is expected to take at least a week to build a house, but possesses more advanced features such as "weaving in ducts for water pipes, electrical wiring and ventilation within the panels of gypsum or concrete." In addition to drastically decreasing the amount of time it takes to build a house, Khoshnevis claims that the automated, two-material construction technology will reduce the cost of building a house to a fifth of today's prices.
Gypsum is also used in the production of the following:

★ Blackboard chalk

★ Cement

★ Drywall

★ Plaster, which has many uses including medical, construction, and artistic.

★ Paint filler

★ Gesso

★ Molds for Casting metals

★ Agricultural soil amendment

★ Solidifying earth (cast earth construction)

★ Tofu coagulation

★ Improving mineral content of brewing water

★ Dietary calcium additives in breads and cereals

★ Delayed release fish feeders (base)

★ Pharmaceuticals

★ Desiccant - anhydrous calcium sulfate (anhydrite) is sold under the brand name Drierite®

★ Burtonisation

See also

★ ''Bathybius haeckeli''

References

1. H F W Taylor, ''Cement Chemistry'', Academic Press, 1990, ISBN 0-12-683900-X, pp 186-187
2. USGS data: world "refined" phosphate rock production is 140 m t: nearly all this is converted to phosphoric acid: 1.7 t of gypsum is produced per t of apatite.
3. Taylor ''ibid'', pp 186-187

External links

★ International Chemical Satefy Card 1215

★ NIOSH Pocket Guide to Chemical Hazards