In the hardening stage, of plaster of paris, the component formed is

The plaster of paris absorbs water to form orthorhombic calcium sulfate dihydrate which sets to form a hard mass containing monoclinic CaSO₄​.2H₂​O.

$\underset{\mathrm{plaster} \mathrm{of} \mathrm{paris}}{{\mathrm{CaSO}}_4.\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{\mathrm{H}}_2\mathrm{O}}\underset{\mathrm{setting}}{\overset{{\mathrm{H}}_2\mathrm{O}}{\to }}\underset{gypsum orthorhombic dihydrate}{CaS{O}_4.2H_{2}O}\underset{hardening}{\leftarrow }\underset{gypsum monoclinic dihydrate}{CaS{O}_4.2H_{2}O}$