Plaster of Paris constituent (6)

I believe the answer is:

gypsum

(Other definitions for gypsum that I've seen before include "Kind of chalk used to make plaster of Paris" , "Mineral used in making plaster of Paris" , "Chalky material in plaster of Paris" , "Mineral used to make plaster of Paris" , "Alabaster" .)

I've seen this clue in The Sun.

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