How does the crystal structure of MgO influence the hydration rate, phase composition, and performance of clay-based magnesium silicate hydrate cements?

The crystal structure of magnesia (MgO) as a precursor for MgO-based cements (MBCs) is not often considered, which results in variable hydration rates and performance data for this promising class of alternative cements. Current literature reports a wide range of calcination temperatures (500°C-1000 °C) in the preparation of reactive MgO from Mg-rich carbonates or hydroxides, resulting in MgO powders with important differences in morphology, crystallography, and reactivity. This study investigates how the thermochemical conversion of hydrous magnesium carbonates (Mg₅(CO₃)₄(OH)₂•5H₂O) and brucite (Mg(OH)₂) at different temperatures (350°C–600 °C) yields MgO with distinct crystal structures and morphologies that influence hydration pathways in MBCs. Early hydration kinetics reveal that MBCs with hydroxide-derived MgO show increased initial dissolution with rising calcination temperature, while carbonate-derived MgO develops a dormant stage at higher temperatures. Analysis of XRD crystallite size evolution suggests different hydration behaviors: hydroxide-derived MgO shows complete loss of the MgO (111) reflection with concurrent appearance of Mg(OH)₂ (0001) peaks, while carbonate-derived MgO retains the (111) plane with only modest size reduction. These structural differences correlate with mechanical performance, as MBCs formulated with carbonate-derived MgO at 450 °C demonstrate 25 % higher compressive strength and improved water resistance (0.16 % expansion after immersion) compared to reference MBCs. These findings highlight the importance of MgO crystal structure in determining MBC hydration pathways and performance, while demonstrating that lower calcination temperatures can reduce energy consumption while maintaining or improving cement properties.