Gypsum contains a significant amount of water in its natural state.
Gypsum (CaSO42H20) formed as mineral deposits when part of the Earth's seawater evaporated and receded millions of years ago. As the chemical formula suggests, gypsum contains calcium sulfate and water. Manufacturers of commercial gypsum remove much of natural gypsum's inherent water by crushing the rock into powder and heating the powder to remove its water molecules.
Benefits
Dehydration of gypsum allows the gypsum to become rehydrated by the consumer later, when it becomes a malleable material for use in construction and crafts. After the rehydrated gypsum dries and sets, it becomes a rock-hard substance.
Hemi-Hydrate
Gypsum hemi-hydrate, or plaster of Paris, is often used in arts and crafts projects.
Most gypsum dehydration processes form a gypsum hemi-hydrate with the chemical composition CaSO4 1/2H20. Hemi-hydrate contains some of the water inherent in natural gypsum, but still allows rehydration to occur. Manufacturers commonly call gypsum hemi-hydrate plaster of Paris.
Anhydrite
Further crushing and heating of gypsum produces gypsum anhydrite. Anhydrite contains almost none of the water found in natural gypsum. Although slightly stronger than hemi-hydrate when rehydrated, anhydrite gypsum requires more energy to manufacture, resulting in a more-costly, less-popular product.
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