On wiki I found
https://en.wikipedia.org/wiki/Calcium_sulfate#Other_uses
[quote=wikipedia]
Up to the 1970s, commercial quantities of sulfuric acid were produced in Whitehaven (Cumbria, UK) from anhydrous calcium sulfate. Upon being mixed
with shale or marl, and roasted, the sulfate liberates sulfur trioxide gas, a precursor in sulfuric acid production, the reaction also produces
calcium silicate, a mineral phase essential in cement clinker production.[14]
CaSO4 + SiO2 → CaSiO3 + SO3
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