Calcium sulfate is a virtually insoluble inorganic compound with the formula CaSO4. It is commonly formed as a by-product of reactions.
A very exothermic thermite reaction occurs with aluminium powder, which is used to 'heat boost' reactions such as a titanium dioxide thermite, where the extra heat is needed to reduce the oxide all the way to titanium metal
Double displacement reactions often are chosen to form calcium sulfate as the precipitation of the solid not only drives the reaction but makes the two formed chemicals easy to separate by simply filtering off the sulfate:
- Ca(OCl)2 + K2SO4 → CaSO4 + 2KOCl
It is generally considered to be insoluble, but does have a very low solubility of 0.21g/100ml @20 degrees (Wikipedia)
Calcium sulfate exists in three hydrates, a dihydrate, a hemihydrate and a anhydrous form. Over 100 degrees the hemihydrate is formed and it looses water to form the anhydrous over 180 degrees. These two forms will slowly absorb water at room temperature to revert back to the dihydrate.
The hemihydrate is easily available as 'plaster of Paris' in hardware stores. The dihydrate is heavily used as 'drywall', a calcium sulfate construction material. A chalk is made from calcium sulfate (a chalk is not made from chalk, who knew) and in many locations it can be found on the ground as the mineral gypsum