Magnesium oxysulfate cement produced from calcined Mg(OH)2: hydration, microstructure and mechanical properties

This study investigated the hydration behaviour, phase assemblage, microstructure and mechanical performance of magnesium oxysulfate (MOS) cement produced with highly reactive MgO obtained by calcining Mg(OH) 2 (active MgO ≈ 89%). Formulations with MgO:MgSO 4 molar ratios of 5, 6 and 8 were evaluated. MOS pastes were prepared by increasing the MgO content while maintaining a fixed amount and concentration of the MgSO 4 solution. The pastes were characterised by isothermal calorimetry, pH measurements of diluted aqueous suspensions, X-ray diffraction with Rietveld refinement, thermogravimetric analysis and scanning electron microscopy coupled with energy-dispersive spectroscopy. Compressive strength was determined at 7 and 28 days. The main exothermic peak occurred at approximately 11–12 h, and most measurable phase transformations took place within the first 24 h. At 7 days, MOS5 contained approximately 40% of the 5 · 1 · 7 phase and 9.5% brucite, whereas MOS8 contained approximately 27% of the 5 · 1 · 7 phase and 28% brucite. A substantial amorphous fraction of approximately 40–46% persisted after the first day. At 28 days, compressive strength decreased from 59.6 MPa for MOS5 to 49.1 MPa for MOS8. The pH of the diluted aqueous suspensions remained between 9.51 and 10.19 over 91 days. The investigated Mg(OH) 2 -derived MgO system exhibited rapid hydration and early phase stabilisation. Under the adopted formulation conditions, increasing MgO content at a fixed MgSO 4 solution composition was associated with greater brucite formation and lower 5 · 1 · 7 content. The strength trends were consistent with this phase balance, although simultaneous changes in water-to-binder ratio and fresh-state behaviour prevent the isolated effect of the MgO:MgSO 4 ratio from being established.