Effect of pH and Metal Ions on the Formation of Solid Solution of Calcium Sulfate Dihydrate and Sulfite Hemihydrate

Abstract

Solid solution of CaSO₄·2H₂O and CaSO₃. 1/2H₂O was prepared by the reaction between the mixed solution of Na₂SO₄+Na₂SO₃ (Na₂SO₄ is predominant) and CaCl2 solution as described in the previous paper²⁾. The amount of SO₃^2- ions incorporated into CaSO₄·2H₂O crystal ([SO₃^2-] _inc) was considerably affected by the pH of the reactant solution. That is, [SO₃^2-] _inc was very small at p1-1=4-4. 5 then steeply increased untill pH becomes 5 and showed maximum amount at pH=6. This phenomenon was similar for any reaction temperature, but the higher temperature, the smaller [SO₃2-] inc.
The metal ions such as Me⁺, Sr²⁺, Ba²⁺ presented in the reactant solution gave no significant effect on the amount of [SO₃^2-] inc. On the contrary, Mn²⁺ increased the amount of [SO₃2-] _inc. When K-salts was used instead of Na-salts for the original reactant solution, no difference in the amount of [SO₃^2-] ^inc was indicated. Finally, it was found that incorporation of CaSO₃·1/ 2H₂O into CaSO₄.2H₂O crystals changed the physical properties such as water requirement, setting time and strength of calcined gypsum therefrom.