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 (Mg0.667Al0.333(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.