What’s Calcium Sulphate?

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Calcium sulfate is a salt that is found in abundance in the natural environment and also appears as a by-product of some industrial processes. It is a compound of calcium, sulfur and oxygen, and in its purest form it has the chemical formula CaSO4; this is known as calcium sulfate anhydrous – without water – or the mineral anhydrite. It also comes in a “hydrated” form, known as mineral gypsum, which has the formula CaSO4.2H2O. The different shapes have a wide range of applications, including building materials, desiccants, making casts to treat broken limbs, and creating works of art.

Property

The hydrated compound has low solubility in water reaching a maximum at just over 100°F (38°C), but decreasing at lower or higher temperatures. Moderate heating drives most of the water away, leaving the partially hydrated form as a white powder popularly known as plaster of paris. The hydrated form can be recreated by simply adding water: initially a paste is formed, but this solidifies after a short time as the compound absorbs the water, releasing heat. Stronger heating drives away more water, leaving the anhydrous or anhydrite form.

Event and production

Gypsum and anhydrite are common minerals that can occur in large deposits. They are usually clear or white, but can take on other colors due to impurities, and can be found as crystals or as layers of granular rock. Gypsum deposits were formed by the evaporation of inland seas many millions of years ago: layers of different minerals were deposited in sequence, with those of least solubility forming first as the water slowly disappeared. Where these deposits have been subjected to heat, the water has been driven off, leaving anhydrite. In 2011, gypsum was found in a rock on Mars, indicating that water must have been present sometime in the past.

Natural plaster is sometimes called alabaster, especially when used in sculpture, but this term is also applied to calcium carbonate. The two can be distinguished by treating with hydrochloric acid. The calcium carbonate reacts to produce bubbles of carbon dioxide, while the gypsum is largely unaffected.

Most of the calcium sulfate comes from these deposits, which are mined in many parts of the world. Some is recovered as a by-product of various industrial processes, particularly those involving the treatment of calcium-containing rocks with sulfuric acid. An example is the production of phosphoric acid from calcium phosphate rock. In the laboratory, samples of the pure substance can be prepared by adding sulfuric acid to a solution of a soluble calcium compound; it is precipitated as a fine white powder due to its low solubility.

it is used

Gypsum is a relatively soft mineral that can be easily carved into shapes, and for this reason it has been used extensively in sculpture, where it may be referred to as alabaster. Another common use of this form of calcium sulfate is in construction. Gypsum sheets are often used in the construction of interior walls in homes, due to its fire resistance: it remains relatively cool until most of the water content has been driven off, slowing the progress of a house fire. This mineral is also an essential ingredient in some forms of cement.
Plaster of Paris is widely used in medicine to make plaster casts for broken limbs, as well as in the arts and model making. Anhydrous calcium sulfate is used as a desiccant, to remove moisture where dry conditions are required: commercial forms may contain a compound that changes color in the presence of water to indicate when it is depleted. When this happens, it can be converted back to anhydrous state by heating.

Calcium sulfate is also used in the food industry. It is often added to foods to give a firmer texture and sometimes as a source of calcium. The production of soy proteins such as tofu involves calcium sulfate, which is used as a coagulant to encourage small protein particles to clump together into larger pieces.

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