Gypsum Interpretation Center 2/6

Characteristics of gypsum

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Characteristics of gypsum

Traditional gypsum is characterised by a very different composition to industrialised gypsum as a direct consequence of its production process. Its extraction requires simple methods and tools, and in the past in certain rural areas it was even used solely for the consumption of the owners or neighbours of the area, depending on their needs. This is not the case of this quarry, which has been exclusively dedicated over the centuries to the extraction and industrialisation of this material.  The ease of extraction of the aljez stone, being of low hardness, favoured its production, without requiring significant technical means. This old gypsum quarry, together with the old ovens where the stone was cooked to make it useful as a construction material, is currently only made up of a single bell-shaped cave where the workers carried out their work sheltered from the inclement weather, but originally there were two more caves, one under the interpretation centre and another a few metres from here, which due to circumstances sank to form what is now a “gorge” in the middle of the mountain.

However, this ancient bell-shaped arrangement, which gives its name to the site, “el campanario”, disappeared when the entire roof collapsed. Inside the old cave there are now smaller, but still of considerable size.

Today, the term gypsum is used to refer, on the one hand, to the mineral composed chemically of hydrated calcium sulphate and the sedimentary rock with the same composition, which is called gypsum rock. On the other hand, the powdered product obtained by calcination, grinding and screening of the gypsum stone, composed of anhydrous or semi-hydrated phases, is known as an intermediate phase of the calcium sulphate-water system, which, when kneaded with water, is hardened after a physico-chemical process to arrive at the final result. Therefore, when using the word gypsum we can refer to: The mineral; The sedimentary rock -aljez-; The powdered product obtained after its transformation; and the final hardened result.

Aljez or gypsum stone is a natural sedimentary rock, monomineralic, composed mainly of calcium sulphate crystallised together with water, although it may contain small quantities of other minerals. It forms part of the evaporites, as it has its origin in the drying up, by evaporation, of salt lakes or inland seas without spillways to the sea.

This material has its origin in the thin sheets of salt water that create these natural sedimentary rocks in arid climates such as the one typical of the municipality. Gypsum therefore has a crystalline structure and occurs in many forms and in abundance, but is often accompanied by impurities of clay, sand and other salts such as carbonates, chlorides and other sulphates. In the natural environment it is possible to find three different phases whose composition is based on calcium sulphate, two mineral-hydrated phases: gypsum and basanite and another anhydrous phase, anhydrite. 

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