Calcium L-ascorbate dihydrate, commonly called vitamin C calcium salt, is a biologically important antioxidant and a required coenzyme for various hydroxylation reactions and for collagen synthesis.A physiological antioxidant and coenzyme. Calcium L-ascorbate dihydrate, commonly called vitamin C calcium salt, is a biologically important antioxidant and a required coenzyme for various hydroxylation reactions and for collagen synthesis. A physiological antioxidant and coenzyme.