Setting of plaster of Paris involves hydration to yield another hydrate as follows
${{\text{(CaSO}}_{\text{4}}\text{)}}_{\text{2}}\cdot {\text{H}}_{\text{2}}\text{O}\stackrel{\text{+}{\text{3H}}_{\text{2}}\text{O}}{\to }\text{2}[{\text{CaSO}}_{\text{4}}\cdot {\text{2H}}_{\text{2}}\text{O}]$
Hint :When mixed with water, it forms a plastic mass, within about half an hour, sets to a hard solid mass consisting of interlaced crystals of gypsum.