Phase equilibria in aqueous magnesium chloride, sulfate, chromate and phosphate systems

In the first part of the present study, which is presented in Paper I, solubility of MgO in aqueous HCl solutions at 23° ± 3°C was measured and combined with analyses of neat magnesium oxychloride cements to construct an equilibrium phase diagram for the system MgO-MgCl2-H2O. Invariant liquids in equilibrium with Mg(OH)2 and 5Mg(OH)2·MgCl2·8H2O (5.1.8), with 5·1·8 and 3Mg(OH)2·MgCl2·8H2O (3·1·8), and with 3·1·8 and MgCl2·6H2O were located at 0.8 ± 0.2 MgO, 17.0 ±0.5 MgCl2; at 1.0 ± 0.2 MgO, 22.2 ± 0.5 MgCl2; and at 1.2 ± 0.2 MgO, 34.5 ± 0.5 wt % MgCl2, respectively. Specific gravities, relative humidities, and pH values of solutions saturated with the equilibrium phases were found to be, within experimental error, indistinguishable from those of MgCl2 solutions. Studies of relative reaction rates indicated that the 5.1.8 phase crystallizes most rapidly in the system and that cements near the 3.1.8 composition react rapidly with atmospheric CO2 to produce a chlorocarbonate surface layer. In paper II of the dissertation, solubilities of MgO in H2SO4 solutions were measured and x-ray analyses of over 100 neat cement samples were used to construct an isothermal phase diagram for the system MgO-H2SO4-H2O at 23° ± 3°C. The phases MgO, Mg(OH)2, 3Mg(OH)2·MgSO4·8H2O (3·1·8), MgSO4·7H2O, MgSO4-6H2O, and MgSO4·H2O were found to be stable at 23°C. The phases Mg(OH)2·MgSO4·5H2O (1.1.5) and MgSO4·4H2O were observed but are considered to be metastable, resulting from over-heating during reaction. Invariant liquids were located at 8.2 ± 0.2 MgO, 17.4 ± 0.5 wt.% H2SO4 in equilibrium with Mg(OH)2 and 3·1·8; at 8.6 ± 0.2 MgO, 21.2 ± 0.5 wt.% H2SO4 in equilibrium with 3·1·8 and MgSO4·7H2O; at 6.9 ± 0.2 MgO, 42.4 ±1.0 wt.% H2SO4 in equilibrium with MgSO4·7H2O and MgSO4·6H2O; and at 6.0 ± 0.5 MgO, 48 ± 2 wt.% H2SO4 in equilibrium with MgSO4·6H2O and MgSO4·H2O. The diagram indicates that cements containing more than 50 wt.% 3.1.8 cannot be made from MgO and MgSO4 solutions at 23°C. Steam curing, however, offers some possibilities for increasing binder phase content. Mg(OH)2 formed in the system provides x-ray patterns indicating an exaggerated sheet morphology. For the system MgO-CrO3-H2O, solubility of MgO in CrO3 solution at 23° ± 3°C was measured. Phases developed in various compositions at different time of curing were analyzed by x-ray diffraction. The data, tabulated in Appendix C, do not permit construction of a phase diagram because of obvious non-equilibrium conditions. For the system MgO-P2O5-H2O, reactions between reactive MgO, MgO calcined at 1200°C for 1 hr. and Mg(OH)2 and H3PO4 solutions were studied. Crystalline phases developed were analyzed by x-ray diffraction technique. The data, tabulated in Appendix D, do not permit construction of a phase diagram because of obvious non-equilibrium conditions --Abstract, pages iii-v