Plaster of Paris is a white powder which on mixing with water converts into hard

This is exactly reverse from preparation of “pop” 
$\underset{\text{pop}}{{\text{CaSO}}_{\text{4}}.\frac{1}{2}{\text{H}}_{\text{2}}\text{O}}+\underset{\text{Water}}{\frac{1}{2}{\text{H}}_{\text{2}}\text{O}}\to \underset{\text{Hard\hspace{0.17em}"Gypsum"}}{{\text{CaSO}}_{\text{4}}.2{\text{H}}_{\text{2}}\text{O}}$