When you lay Gyprock on an internal wall, the material inside the Gyprock or Victorboard (these are trademarks), and the plaster, what you lay on cracks and sand interminably, is to a first approx. #CaSO_4*2H_2O#; the stuff is fairly insoluble, and this is why it forms a white slurry that dries over a day or so.
I write #CaSO_4*2H_2O# because I need a definite formula before I can calculate its percentage composition, i.e. its #"hydration"#. Note that I happened to KNOW this formula, calcium commonly forms a #Ca^(2+)# ion, that forms an insoluble precipitate with sulfate dianion, #SO_4^(2-)#; the water solvate is along for the ride.
And then we address the quotient:
#"%water"="mass of water"/"Mass of calcium sulfate dihydrate"xx100%=#
Ain't got no diagram........