Raw gypsum (calcium sulfate dihydrate) and anhydrite fall under subheading 252010. Once gypsum is calcined — partially or fully dehydrated by heating — it becomes a plaster (hemihydrate or anhydrous calcium sulfate) and is classified under 252020. The thermal processing step, not the end use, determines which subheading applies.

Heading 2520 covers natural gypsum and anhydrite. Synthetic gypsum produced industrially — such as from flue-gas desulfurization — is generally classified elsewhere, typically under headings for chemical byproducts or calcium sulfate compounds, depending on its form and purity. Importers should seek a binding ruling when the origin of the calcium sulfate is industrial rather than mined.