[DoctorBeaver (OP)]

We've got the builders in and they're doing plastering. What's plaster made from & why does it go sticky then hard when mixed with water?

[daveshorts]

Plaster is based on Gypsum CaSO₄, it is found naturally with Water in the crystal structure. CaSO₄.2H₂O  - there are 2 water molecules in the structure for every CaSO₄ this is then cooked in a furnace which drives off the water to produce CaSO₄.0.5H₂O which is the powder.
You then add water which allows it to hydrate again to CaSO₄.2H₂O this has long thin crystals which grow, and cross each other and interlink with each other making it solid.

[DoctorBeaver (OP)]

Thank you, Dave. As always, your answer was erudite & comprehensive.