Question

Why p.o.p (plaster of Paris) once added with water and hardened cannot be turned back into calcium sulphate?

Answer 1

becuz it is a chemical change

Answer 2

Yes you can recycle plaster (or gypsum)... and it is done on almost all the excess or recycled plaster board and other plaster products.

I am a little vauge on all the exact chemisty of it.... but basically it amounts to running the plaster through a crusher and then heating it in a ball mill..... to drive out the WATER of HYDRATION.... this is "chemically combined water"... not "soaked in water" like a sponge.

Plaster can more or less be recycled indefinitely......

If I was just experimenting, I'd do it, just to find out... 

IF I was using lots of it, regularly, I'd be setting up a small mill to break up / grind and heat the plaster.