Effects of some orthophosphates on the hydration of calcium sulfate hemihydrate

The effects of some orthophosphates on the rate of hydration of calcium sulfate hemihydrate have been determined in terms of two basic parameters, viz. α0, which is a measure of seeding in the system, and k, which is a measure of the rate of transfer of calcium sulfate from the lattice of the hemihydrate to that of the dihydrate. Measurements were made of pH and the activities of calcium ions. The only dispersed, undissolved phosphate to affect the reaction was secondary calcium phosphate dihydrate (CaHPO4,2H2O) which is structurally similar to gypsum and appeared to increase nucleation. The actions of the soluble orthophosphates, primary calcium phosphate Ca(H2PO4)2,H2O and the secondary sodium phosphate Na2HPO4, were studied in detail. The effects were various and depended upon the amount added, the pH and residues from incongruent dissolution. Attempts to explain inhibitions in terms of concentrations of the species, H2PO4-, HPO42-, CaH2PO4+ and CaHPO4O were unsuccessful.