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Four porous corundum‐spinel aggregates were prepared from magnesia and α‐Al2O3 micropowder via the Kirkendall effect. Effects of the particle size of magnesia (PSM, 0–74 μm, 0–15 μm, and 0–44 μm) and firing temperature (1100–1600°C) on the phase compositions, microstructures, pore characteristics of the prepared porous aggregates were investigated. As the firing temperature increased, the PSM influenced the reaction rate of magnesia and alumina, and the sintering process, leading to remarkable changes in the phase compositions, porosity, and microstructure with a negligible impact on the pore size distribution. As the PSM decreased from 0–74 μm to 0–15 μm, the apparent porosity initially increased and then decreased. Relatively compact cyclic spinels formed around the Kirkendall pores in aggregates with 0–74 μm and 0–44 μm magnesia added after fired at 1600°C, as the expansion of spinel formation was inhibited by the high strength of the alumina matrix. Finally, the optimized product is an aggregate with 0–44 μm magnesia added, which exhibits a high total porosity of 32.4%, a low bulk density of 2.69 g/cm3, and a thermal conductivity of 2.178 W/(m·K) at 1000°C, as well as a high compressive strength of 302.5 MPa and a small median pore size of.46 μm. This work provides a promising technique for preparing porous‐corundum‐based aggregates. [ABSTRACT FROM AUTHOR]