What is "calcium sulfite"

Calcium sulfite, or calcium sulphite, is a chemical compound, the calcium salt of sulfurous acid with the formula CaSO. In presence of moisture it commonly forms the hemihydrate CaSO.0.5HO. Its uses as the pure compound are limited, such as a food additive, but it is produced in large quantities during flue gas desulfurization (FGD) with lime, which then exposed to air converts to common calcium sulfate (CaSO, more exactly the gypsum form, CaSO.2HO) by oxygen scavenging, this route of CaSO4 production via CaSO3 intermediate being the main source of drywall wallboard.