How is the hydration state of a chemical related to its adsorption of water?
Surprisingly enough, hemihydrate reacts quickly with water to produce dehydrate (gypsum). It is due to the fact that hemihydrate is produced by heating gypsum to a high temperature. This heating produces a structure with plenty of holes or voids corresponding to the previous positions of the water molecules. Water molecules can easily fill these holes if hemihydrate is mixed with water. Anhydrous calcium sulfate is a mineral (anhydrite) that does not react easily with water. The structure of anhydrous calcium sulfate is compact and does not contain any holes between its atoms to include easily water molecules.