For the magnesium hydroxides prepared from aqueous solutions of magnesium salts such as chloride, nitrate and sulfate by reaction with alkali at various conditions, the process of their thermal decompositions has been investigated by thermal analysis (TGA and DTA), X-ray diffraction study and infrared spectroscopy. As a result, it is found that the thermal decomosition of of magnesium hydroxide progresses in the following sequence : Mg (OH) ₂-380°C-→MgO (distorted cubic) -600°C-→MgO (cubic).

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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.

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