Setting of Plaster of Paris takes place due to

The Setting of Plaster of Paris occurs due to hydration.
Plaster of Paris is calcium sulphate hemihydrate, with chemical formula $\mathit{CaS}{O}_4.½H_{2}O$. It readily absorbs moisture(water) from the environment. On mixing with water, plaster of paris takes up water of crystallization to form gypsum.

Its reaction is given as -

$\mathit{CaS}{O}_4.½H_{2}O(\mathit{plaster}\mathit{ }\mathit{of}\mathit{ }\mathit{paris})+1½H_{2}O\left(\mathit{water}\right) \to \mathit{CaS}{O}_4.2H_{2}O\left(\mathit{gypsum}\right)$