What is a gypsum product

US state of New Mexico: The “White Sands” desert area was created by evaporation from a shallow sea - the sand is actually pure gypsum. Photo: Teenymeyer / www.pixelio.de

Gypsum has a lot in common with lime. Not only because both substances are usually white-gray in color and are used as binders by the building materials industry, but also because of their chemical composition and geological origin. Gypsum is even often referred to as "sulphate of lime". In fact, both elements are similar even at the atomic level. Limestone (calcium carbonate) consists of calcium, carbon and oxygen atoms. In gypsum, a sulfur atom takes the place of carbon.

From a chemical point of view, gypsum is calcium sulfate (CaSO4). However, the naturally occurring gypsum usually also contains water (H.2O). In this context, one speaks of crystal water because the water molecules are firmly embedded in the gypsum crystals. While a sponge soaks up water and becomes noticeably wetter and softer, it is exactly the other way around with plaster. It chemically binds the water and becomes harder as a result! As a rule, two water molecules per calcium sulfate molecule are stored in the crystal compound. The full chemical formula for conventional gypsum is therefore CaSO4 • 2H2O. In words: calcium sulfate dihydrate. The term hydrate generally stands for water that is bound into a solid crystal body. In nature, however, gypsum also occurs that does not contain any water of crystallization - i.e. made of pure CaSO4 consists. This is what is known as anhydrite.

How did gypsum stone come about?

Just like limestone, gypsum was formed a long time ago through deposition processes on the bottom of primeval seas. However, it is not that old: While the limestone deposits in Germany formed around 600 million years ago, the gypsum deposits are still comparatively young: the oldest are "only" around 240 million years old.

In addition, unlike lime, gypsum is not formed from the remains of marine animals that have accumulated on the sea floor, but rather through evaporation of the water. It is well known that seawater contains salts, and these salts also include the gypsum building block calcium sulphate. Because sulfates are the salts of sulfuric acid. Around 200 million years ago, some primeval seas apparently no longer received enough fresh water. As a result of natural evaporation, their water level therefore increasingly decreased, and the seas slowly dried up. As a result, the remaining water was at some point oversaturated with salts. These then began to crystallize out as a solid.

It is assumed that the calcium sulfate originally crystallized out together with the stored water - i.e. as CaSO4 • 2H2O. This is how the first layers of gypsum were formed, which over time were more and more pressed together by further deposits, so that finally solid gypsum stone was created. Gradually, more layers of deposits followed, until at some point the entire sea had evaporated. In the deeper layers of gypsum, however, the increasing pressure from above had meanwhile led to drainage of the rock. This created the natural anhydrite rock.

Production of building plaster

Natural gypsum is not always white, but sometimes also transparent - like this gypsum crystal from the Freiberg Mineral Museum. Photo: Bärbel Schulz / www.pixelio.de

In Germany, gypsum stone is mined both in opencast and underground mining. In addition, artificially obtained gypsum is also used today as a raw material in large quantities. This so-called FGD gypsum is created as a by-product in the flue gas desulphurization systems of large coal-fired power plants. However, we do not want to go into further depth on this topic at this point. Just this much: FGD gypsum is chemically absolutely identical to natural gypsum.

To manufacture building materials, the crushed and ground gypsum is dried in a kiln. Depending on the firing temperature, different building plasters are created for different building projects. Burnt plaster is a dry, powdery product that is used as a binding agent in plaster or screed, for example. These dry products are mixed with water again before processing.

Among the various artificially produced building plaster, the so-called hemihydrates are the most important. At firing temperatures of up to 150 ° C, so-called alpha hemihydrate is formed, from which the harder gypsum products are made. If you increase the temperature moderately up to 190 ° C, then you get beta hemihydrate, which is better suited for softer plasters. Despite the different hardness, both variants have the same chemical composition: CaSO4 • ½ H.2O.

By further increasing the firing temperature, plasters can be produced with less and less water content. Artificial anhydrite is also produced in this way. This is used, for example, as a binding agent for screed mortar (calcium sulphate screed). In contrast, plaster of paris is mixed from hemihydrates and anhydrite. Plaster of paris, in turn, consists predominantly of the hard alpha hemihydrate.

Plaster hardening

Gypsum plaster and gypsum screed, but also gypsum boards, essentially consist of building plaster (binding agent) and sand. The mixture is stirred with water. The fired building plaster, which previously had to more or less give up their water of crystallization, are downright eager to find H2O take up. The mixing water is partly stored again in the molecular structure of the gypsum. This hardens the plaster of paris again. The firing process is reversed: the binding agent turns back into calcium sulfate dihydrate - that is, into gypsum stone.

About the author Roland Grimm has been a freelance journalist based in Essen since February 2013 and regularly writes specialist articles for Building material knowledge. Before that, he was a specialist editor at the industry magazine for around six years Building materials market and also editor-in-chief and, from 2010, editor-in-chief of the trade journal building materials practice. Contact: freelance [email protected]

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