Phase Relations in Magnesium Oxysulfate Cements

Solubilities of MgO in H 2 SO 4 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‐H 2 SO 4 ‐H 2 O at 23°±3°C. The phases MgO, Mg(OH) 2 , 3Mg(OH) 2 MgSO 4 ·8H 2 O (3·1·8), MgSO 4 ·7H 2 O, MgSO 4 ·6H 2 O, and MgSO 4 ·H 2 O were stable at 23°C. The phases Mg(OH) 2 · MgSO 4 ·5H 2 O (1·1·5) and MgSO 4 ·4H 2 O were observed but are considered to be metastable, resulting from overheating during reaction. Invariant liquids were located at 8.2±0.2 MgO, 17.4±0.5 wt% H 2 SO 4 in equilibrium with Mg(OH) 2 and 3·1·8; at 8.6±0.2 MgO, 21.2±0.5 wt% H 2 SO 4 in equilibrium with 318 and MgSO 4 ·7H 2 O; at 6.9±0.2 MgO, 42.4±1.0 wt% H 2 SO 4 in equilibrium with MgSO 4 ·7H 2 O and MgSO 4 ·6H 2 O; and at 6.0±0.5 MgO, 48±2 wt% H 2 SO 4 in equilibrium with MgSO 4 ·6H 2 O and MgSO 4 ·H 2 O. The diagram indicates that cements containing >50 wt% 3·1·8 cannot be made from MgO and MgSO 4 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 which indicate an exaggerated sheet morphology.