DISSOLUTION RATE OF GYPSUM IN AQUEOUS SALT SOLUTIONS

We measured the dissolution rate of gypsum particles (0.5− to 1.0-mm fraction) in distilled water and in aqueous salt solutions of NaCl, MgCl2, Na2SO4, MgSO4, and CaCl2 at salt concentrations of 20, 40, and 60 mmol kg−1. Two gypsum samples were tested, one from a gypsum mine in Egypt and another from New Mexico, United States. We monitored the dissolution rate by measuring Ca concentrations in these solutions with and without shaking. The rate of gypsum dissolution increased with shaking and with increasing concentration of the salt solutions. Neither ion concentration nor species influenced the rate of gypsum dissolution when the rate was normalized by the solubility. The first-order reaction equation was unsuited for describing the dissolution process when Ca concentrations exceeded about 50% of gypsum solubility. The second-order reaction equation described the process well, providing a rate constant independent of ion concentrations or species.