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25.20 ‑ Gypsum; anhydrite; plasters (consisting of calcined gypsum or calcium sulphate) whether or not coloured, with or without small quantities of accelerators or retarders.
Gypsum is a natural hydrated calcium sulphate generally white and friable.
Anhydriteis a natural anhydrous calcium sulphate used in the manufacture of sulphuric acid or of certain types of plaster.
Plastersconsist of gypsum partly or completely dehydrated by calcination.
The characteristic of gypsum is that, when calcined, it loses part of its water thus forming plasters which on being mixed with water set hard. In order that the plasters should not set too quickly small quantities of retarders are often added to the calcined gypsum. For special purposes gypsum is calcined until all its water is lost, and a small quantity of an accelerator such as alum is added (Keene's cement or English cement). Similar plasters are made by adding alum to natural anhydrite. All these prepared plasters remain in this heading.
This heading also covers :(1) Plaster reduced to a floury consistency for use in dressing woven fabrics or surfacing paper. (2) Plaster containing added colouring matter. (3) Plaster which has been specially calcined or finely ground for use in dentistry, whether or not containing small quantities of accelerators or retarders. This heading does not include preparations for use in dentistry with a basis of plaster (heading 34.07).
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