Effect of MgO Additive on Properties of Corundum–Mullite Duplex Ceramic Synthesized from High-Alumina Fly Ash

Corundum–mullite duplex ceramics were fabricated via a solid-state reaction technique using high-alumina fly ash and α-alumina powders. The effects of magnesium oxide on bulk density, apparent porosity, compressive strength, and microstructure of the duplex ceramics were investigated by scanning electronic microscopy, mechanical testing, and X-ray diffraction, respectively. Results showed that the prepared ceramics were mainly dominated by mullite and corundum phases, and the mullite was in the form of columns and crosses to form a net-like structure. The bulk density and the compressive strength increased with the increase in MgO addition, while the porosity decreased contrariwise. Optimal performance among tested compositions was observed at 12 wt% MgO addition, yielding a bulk density of 3.012 g·cm−3, a porosity of 8.12%, and a compressive strength of 263 MPa, demonstrating the potential of this composite for high-performance ceramic applications.