Calcium sulfate is a calcium salt. It shows up mainly as a food additive in things like tofu and canned vegetables. You can also find it as a dietary supplement.
This form provides elemental calcium. Your body will not absorb it as well as it absorbs calcium carbonate or calcium citrate. Because of that poor absorption, this is not the go-to choice when you want to support bone health. Data on whether taking it with food improves absorption is lacking.