Calcium Sulphate

Calcium sulphate is an inorganic white solid with the formula CaSO4. It occurs naturally as a mineral and is also produced synthetically, often as a by-product (e.g. from desulphurisation). The compound exists in three levels of hydration: anhydrous (as mineral anhydrite), dihydrate (gypsum or as mineral selenite) and hemihydrate (Bassanite or plaster of Paris).All forms are poorly soluble in water and cause permanent hardness in water.

Synonyms: Gypsum, E516, Plaster of Paris, Anhydrite, Selenite, Alabaster
Chemical Formula: CaSO4
CAS Number: 7778-18-9 (anhydrous), 10034-76-1 (hemihydrate), 10101-41-4 (dihydrate)

Characteristics

Molar Weight: 172.17 g/mol (dihydrate)

Melting Point: 1,460°C (anhydrous)

Flash Point: non flammable

Density: 2.32 g/cm³ (dihydrate)

Forms: Powder, White, Solid

Uses and Applications

Key applications

Food additive
Oil and Gas
Agricultural
Pharmaceuticals
Animal feed
Cosmetic products
Building & Construction products

Industries

General information

Calcium sulphate belongs to the group of calcium compounds and sulphates. Its chemical formula is CaSO4. Calcium sulphate dihydrate is widely used and is well known under the name gypsum.

The substance occurs naturally in the form of the minerals anhydrite (anhydrous form), gypsum (dihydrate) and bassanite (hemihydrate) in evaporites - a chemical sedimentary rock in marine or lake basins.

Calcium sulphate dihydrate can be traditionally obtained by mining gypsum deposits. If calcium sulphate is produced industrially, it is obtained by precipitation from sulphuric acid water with milk of lime or limestone.

However, the deliberate production of calcium sulphate, especially its dihydrate gypsum, in large quantities is not necessary, as it is produced as a by-product in numerous chemical processes. For example, in the production of citric acid or in the food industry in the production of tartaric acid. In addition, calcium sulphate is produced as a dihydrate in wastewater treatment processes during the neutralisation of process wastewater containing sulphate or sulphuric acid pickling. It is also produced in large quantities during flue gas desulphurisation.

Properties of calcium sulphate

Calcium sulphate is a colourless and odourless solid in crystal form. It is poorly soluble in water and decomposes at a temperature of about 1,450 °C. Calcium oxide and sulphur are formed. This produces calcium oxide and sulphur trioxide.

Calcium sulphate is hygroscopic, i.e. it absorbs moisture. Below 66 °C, calcium sulphate crystallises from an aqueous solution as monoclinic dihydrate (gypsum). If the dihydrate is heated, it partially releases its crystallisation water and the hemihydrate is formed. The anhydrous form occurs in nature as anhydrite.

Calcium sulphate in the construction industry

The main application of calcium sulphate is the production of building materials. It is an extremely relevant raw material for the construction industry, where different hydrations of calcium sulphate - dihydrate, hemihydrate and anhydrite - are used. Its dihydrate gypsum (the water crystalline form) is used for gypsum wallboards for the construction of partition walls as well as for gypsum board for drywall construction.

Gypsum plasters and the above-mentioned gypsum plasterboards are used to ensure a surface suitable for painting and wallpapering on rough and uneven masonry. Calcium sulphate is also used as a base material for tiles, various plasters, mortars, fillers, moulds and dry screeds, as well as a filler.

For example, building or stucco plaster is used in renovation work to repair minor damage such as cracks, holes and cable slots on walls. Plaster can also be used to dowel in wood and other building components. Hollow floor substructures are also made of plaster, or cast anhydrite. Plaster base elements for electrical installations in unfinished walls are fixed with plaster. Mixing gypsum with lime produces so-called gypsum lime. It is used for plaster, masonry and stucco work. In contrast to pure stucco plaster, it can be worked and shaped for longer before it hardens.

Since set gypsum is water-soluble to a certain degree, gypsum building materials are used exclusively for interior finishing. In addition, gypsum is only used to a limited extent in wet areas and basements due to its hygroscopic properties - because it absorbs moisture, the risk of discolouration and fungal attack would be too high. The prominent role of gypsum for the building industry is also reflected in the name given to the profession of plasterer, which is now known as stucco plasterer.

Gypsum also plays an outstanding role in the field of fire protection: it is light and offers high fire resistance. The effect is due to the dihydrate's water of crystallisation, which evaporates in a fire and forms a protective vapour curtain.

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Calcium sulphate in the food industry

Since 1995, calcium sulphate has been approved in the food industry under the name E 516 in the EU as a food additive without any quantitative limit. It is used as an acidity regulator, carrier - for example for food colouring -, firming agent and flour treatment agent as well as a complexing agent and stabiliser.

In baking mixtures, calcium sulphate has the task of regulating acidity, stabilising and firming the dough. In bread, it also promotes the proliferation of yeast cultures. In canned food, calcium sulphate serves as a firming agent for tomatoes, potatoes, carrots, beans and peppers, for example. Calcium sulphate serves as a coagulant in the production of tofu. It makes it cut-resistant.

The body absorbs calcium sulphate very easily. It is therefore added to food supplements as a source of calcium. It is also added to food - especially for children, for example in cornflakes - to advertise "extra calcium". The water hardness in spring, drinking and tap water is determined by the calcium sulphate, calcium chloride and corresponding magnesium salts dissolved in it.

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Calcium sulphate in medicine

In dentistry and oral surgery, calcium sulphate is used for the regeneration of bone and tissue. Sinus lifts (jaw reconstruction by thickening the bony floor of the maxillary sinus) can be performed and dental implants can be inserted. Denture and tooth models are made with plaster.

Also in medicine, bone fractures are fixed with the help of a plaster cast, which is first wrapped around the fractured area when moist and forms a stable support after hardening.

Calcium sulphate in the laboratory

In chemistry laboratories, anhydrous calcium sulphate is a popular drying agent for organic solvents because it is cheap and versatile.

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Calcium sulphate in the paper industry

Among other things, the paper industry uses gypsum as a filler, which can be present in paper in proportions of up to 30 per cent. The reason: most fillers are cheaper than the pulp used, so that production costs can be reduced. Fillers also serve to increase the degree of whiteness. They also influence the basis weight and absorbency of the paper - relevant aspects for print quality. The industry also often produces paper coated with calcium sulphate, which makes it shinier and more tear-resistant.