What is Gypsum?

Michael Pollick

Gypsum is the more common name for a mineral compound called calcium sulphate dihydroxide, or sulphate of lime. It is generally found underground near deposits of limestone or other minerals formed by evaporation. One of the most common forms of raw gypsum is a pure white crystal called alabaster. Another unprocessed type forms in desert terrain and its crystals resemble the petals of a flower. For this reason, many people refer to this form of calcium sulphate as the "desert rose."

A drywall installer applies a finish coating to a gypsum wall.
A drywall installer applies a finish coating to a gypsum wall.

Because the calcium and sulfur molecules in gypsum are chemically bound to water, this material is routinely heated in order to remove 50% to 75% of its original moisture. The resulting powder is called burnt gypsum, although its white or translucent color does not change. This dehydrated form is valued for its ability to solidify almost immediately after water is added to it. It is marketed as the molding agent Plaster of Paris, and is used to make ordinary schoolroom chalk.

Ordinary schoolroom chalk is made from gypsum.
Ordinary schoolroom chalk is made from gypsum.

Another common use for gypsum is the formation of drywall panels. During home construction, these panels are nailed into place to form a finished wall. Gypsum is naturally resistant to fire and heat, which helps it form a barrier between combustible wooden frames and the room itself. These drywall boards can also be cut to size without elaborate tools, making an ideal building material.

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The White Cliffs of Dover are made of limestone, which is often found above gypsum deposits.
The White Cliffs of Dover are made of limestone, which is often found above gypsum deposits.

Gypsum is also used to strengthen soil weakened by too much clay. Gardeners routinely amend weak soil by adding a layer of this material, which does not change the soil's acidity levels. Gypsum-enriched soil can also be used as an alternative to adobe mud, which tends to disintegrate after years of exposure to rainwater.

Because of its hardening properties, gypsum is a popular ingredient in cement mixtures. It is also used as a mild abrasive in some toothpastes, since it is considered a non-toxic substance. On Moh's scale of mineral hardness, the compound is considered a 2, harder than talc but softer than limestone.

Powdered gypsum is typically used to make plaster.
Powdered gypsum is typically used to make plaster.

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Discussion Comments

anon270039

So what is jesmonite?

anon176177

does gypsum affect the health? one customer commented about that. what's the actual truth?

anon39382

Is gypsum wall board available in 60" widths?

anon17865

What is PH of Gypsum? Is it very alkaline?

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