Solid solution of calcium sulfite hemihydrate and calcium sulfate.

Abstract

Formation of solid solution of calcium sulfite hemihydrate and calcium sulfate and the solid solubility limit thereof were investigated. In the products prepared by coprecipitation of calcium sulfite hemihydrate and calcium sulfate from aqueous solutions containing sulfate and sulfite ions in different ratios, presence of sulfate which can not be removed by washing with water and is assumed to be incorporated into CaSO3⋅1/2H2O crystal lattice was confirmed. The products were verified to be solid solution of calcium sulfite hemihydrate and calcium sulfate by means of differential thermal analysis, thermogravimetry, IR spectroscopy and X-ray diffraction analysis. Solid solubility of sulfate in CaSO3⋅1/2H2O was indicated to be dependent on reaction temperature, and it was, in the present investigations, about 9mol% at 15°C, increasing with temperature to the maximum value of about 19.5mol% at 83°C and higher. It was found that sulfate ions up to about 9mol% substituted in good order for sulfite ions of CaSO3⋅1/2H2O to form solid solution accompanying small changes in lattice parameters, and that sulfate ions of more than 9mol% caused disorder of sulfite crystal. In addition, from the results of examination of thermal properties and content of combined water of this solid solution, it was considered that the state of forming solid solution changed at the solid solubility of about 9mol%.