Size-dependent hydration mechanism and kinetics for reactive MgO and Al₂O₃ powders with respect to the calcia-free hydraulic binder systems designed for refractory castables

This work investigates the time-dependent hydration behavior of 1:1 molar ratio mixture of reactive α-Al₂O₃ nano- or micro-powder and reactive MgO in relation to the development of new hydraulic binder systems for in situ spinel refractory corundum castables. Mg–Al–CO₃ hydrotalcite-like phases formed in the cementitious matrices cured in the corresponding curing condition up to 280 days of age at 50 °C were subjected to morphological, chemical and structural characterization using XRD, FT-IR, NMR, SEM and TEM techniques. The observations indicate that both the short-term cured MgO–nano-Al₂O₃ cementitious matrix and the long-term cured one containing micro-Al₂O₃ contain mainly OH–(Mg₃) hydroxyl group, whereas the long-term cured MgO–nano-Al₂O₃ cementitious matrix is dominated by OH–(Mg₂Al) hydroxyl group. The thermal stability of hydrotalcite-like compounds (Mg_0.667Al_0.333(OH)₂(CO₃)_0.167(H₂O)_0.5, Mg₆Al₂CO₃(OH)₁₆·4H₂O) (HTlcs) through combined DTA–TGA–EGA–MS analyses shows that HTlcs undergo dehydration first and then dehydroxylation overlapping with decarbonization processes. According to EGA–MS and FT-IR measurements, the Mg–Al cementitious matrices were progressively carbonated over curing time and then interlayer water molecules were preferably coordinated by interlayer CO₃ ²⁻ ions that exist in a complex network of hydrogen bonds between octahedral layers. In the near-fully-hydrated binder paste, dehydration of crystalline water molecules occurs at 262 °C and the thermal stability of hydroxyl groups in HTlcs is OH–(Mg₂Al) (422 °C) > OH–(Mg₃) (337 °C) at a ramping rate of 10 °C/min. The basic information provided by the microcalorimetric measurement is such that the starting grain size significantly affects the heat flow curve. From the SEM and TEM images, the hydrates are found to be hexagonal in shape.