Modeling of phase relations and thermodynamics in the Mg(OH)2 + MgSO4 + H2O system with implications on magnesium hydroxide sulfate cement

Temperature-dependent thermodynamic models for Mg(OH)(2) + H2O and Mg(OH)(2) + MgSO4 + H2O systems were developed using the CALPHAD approach. Six magnesium hydroxide sulfate (MHS) hydrates, i.e. 5-1-2, 3-1-8, 1-2-2, 1-2-3, 1-1-5, and 5-1-7, were included in the ternary model and their thermodynamic properties were determined as functions of temperature. The reliability of the solubility data, solubility products and thermochemical data of brucite were evaluated by considering the internally thermodynamic consistency of them. Ternary solubilities of brucite, 5-1-2 and 3-1-8 phases reported in literature were generally evaluated for the development a temperature-dependent ternary model. The models were applied to simulate the phase relations of the Mg(OH)(2) + MgSO4 + H2O systems and the hydration products of MHS cement. However, the model is limited due to the lack of reliable solubility and thermodynamic data in the system. Only with the appearance of sufficient and reliable experimental data, the model can really be adapted or evaluated. Provisionally, according to the simulations, brucite, MgSO4 center dot 7H(2)O, MgSO4 center dot 6H(2)O, MgSO4 center dot H2O and the 5-1-2 phase were identified as stable MHS phases, while 3-1-8, 5-1-7, 1-1-5, 1-2-3 and 1-2-2 were metastable. A preferable proportion is MgO:MgSO4:H2O molar ratio 5:1:12, from which a cement stone with composition of 88.2 wt% '3-1-8' + 11.8 wt % brucite or 100 wt% '5-1-7' could be produced, respectively. However, it was noticeable that both of the two cement stones were thermodynamically metastable. The long-term performance of them would suffer from the phase transition to stable state. Moreover, thermodynamically stable 5-1.2 phase is also only stable in presence of MgSO4-containing solution. Therefore, the poor weather resistant of the material as conventional cement is inherent.