In Situ Spinel Formation in a Smart Nano-Structured Matrix for No-Cement Refractory Castables

The hydration of an equimolar mixture of MgO and Al₂O₃ nano-powders has been proven to be an effective way to synthesize Mg₆Al₂CO₃(OH)₁₆∙4H₂O as a component of a nano-structured matrix and magnesia-alumina spinel precursor for high-performance cement-free corundum-spinel refractory castables. (Mg₃)-OH-brucite sites (417 °C) formed initially within the magnesia-alumina hydrating blended paste were replaced with (Mg₂Al)-OH and (Mg₃)-OH hydrotalcite sites, which were dehydroxylated at 420 °C and 322 °C, respectively. This reorganization was connected with the incorporation of anions and water molecules in the interlayer spacing of hydrotalcite, which was dehydrated at 234 °C. Hence, the thermal decomposition of a nano-structured matrix system containing mainly Mg₆Al₂CO₃(OH)₁₆∙4H₂O consists of a complex sequence of dehydration, dehydroxylation and decarbonization, and this finally leads to the formation of inverse spinel MgAl₂O₄ and periclase MgO through many intermediate stages containing the mixed tetrahedral-octahedral Al phase and MgO-like structure. Hence, the hydraulic bond that primarily existed was replaced by a ceramic bond at a relatively low temperature, i.e., 700 °C, where a spinel was formed. Important changes in oxygen coordination polyhedra around Al³⁺ in the dehydrated-dehydroxylated hydrotalcite occurred between 600 and 1100 °C.