Microstructure and properties of microporous MgO–Al ₂ O ₃ refractory aggregates from Mg(OH) ₂ and Al(OH) ₃

Abstract In this work, microporous MgO–Al ₂ O ₃ refractory aggregates were prepared with Mg(OH) ₂ and Al(OH) ₃ via the in situ decomposition synthesis method. The effect of Al(OH) ₃ addition on the microstructure and properties of microporous MgO–Al ₂ O ₃ refractory aggregates was investigated with scanning electron microscope and mercury intrusion porosimetry. The results indicated that the improved green density of the samples and the reaction sintering accelerated the mass transport rate with adding Al(OH) ₃ from 0 to 4.0 wt%. Besides, a small amount of Al ³⁺ diffused into porous MgO particles, accelerating the merging and growth of nanopores in the porous MgO particles. The intra‐particle pore size and the densification degree of microparticles were increased, and thus the strength of the samples was improved. Due to the formation of Kirkendall voids by interdiffusion of Mg ²⁺ and Al ³⁺ , the inter‐particle pore size increased. Adding Al(OH) ₃ from 4.0 to 17.6 wt%, the Kirkendall voids weakened the mass transport rate and improved the inter‐particle pore size. The merging and growth of nanopores in the MgO particles were limited, resulting in the reduced intra‐particle pore size and increased intra‐particle pore number. The densification degree of microparticles was reduced, and thus the strength of the samples decreased. At the Al(OH) ₃ addition of 4.0 wt%, microporous MgO–Al ₂ O ₃ refractory aggregates had the best comprehensive properties, a bulk density of 2.48 g/cm ³ , an apparent porosity of 29.4%, a median pore size of 1.6 μm with 42.2 vol% nanopores and the thermal conductivity of 4.0 W/(m K).