Settling of plaster of Paris takes place due to

Settling of plaster of Paris takes place due to hydration.
Since plaster of Paris is calcium sulphate which has formula $\mathit{CaS}{O}_4.½H_{2}O$. It readily absorbs moisture(water) from the environment. If water of crystallization is added to it, it forms gypsum material, and its reaction is given as:
$\mathit{CaS}{O}_4.½H_{2}O + 1½H_2\mathit{O }\to \mathit{ CaS}{O}_4.2H_{2}O$

$P\mathit{laster}\mathit{ }\mathit{of}\mathit{ }P\mathit{aris}$              $W\mathit{ater}$                   $G\mathit{ypsum}$