Hydrothermal-Assisted Synthesis of Mullite Ceramics from Industrial Byproducts: Phase and Microstructure Evolution

Abstract

This study presents a sustainable route for synthesizing high-performance mullite ceramics using aluminum dross and kaolin, with a focus on enhancing precursor reactivity through hydrothermal pretreatment. Three compositions were evaluated: MC (commercial alumina + kaolin), MD (untreated dross + kaolin) and MH (hydrothermally treated dross + kaolin). Hydrothermal treatment at 200 °C for 24 h effectively removed soluble impurities (e.g., NaCl, AlN) and converted non-oxide phases into reactive Al₂O₃ (e.g., 2AlN+3H₂O → Al₂O₃+2NH₃), increasing the alumina content from 70.252 to 71.803 wt%. X-ray diffraction revealed enhanced mullite crystallinity in MD and MH, attributed to fluxing oxides such as Fe₂O₃ and MgO that promoted liquid-phase sintering and reduced activation energy. Scanning electron microscopy confirmed that MH exhibited a denser microstructure with well-developed mullite whiskers (~12.25 µm). At 1,500 °C, MH achieved superior mechanical properties with compressive strength of 110.48 MPa, porosity of 2.90% and bulk density of 2.30 g/cm³, surpassing both MD (79.01 MPa, 3.10%, 1.92 g/cm³) and MC (23.42 MPa, 11.38%, 1.61 g/cm³). These results demonstrate that hydrothermal pretreatment significantly improves phase compatibility and densification. Beyond material performance, this approach offers a scalable pathway for valorizing industrial waste, advancing circular economy practices and supporting environmentally friendly ceramic manufacturing.

HIGHLIGHTS

• Effect of hydrothermal pretreatment of aluminum dross and kaolin on morphology and mechanical properties of mullite.
• MH sample shows superior properties: 2.90 % apparent porosity, 2.30 g/cm³ bulk density, 110.48 MPa compressive strength.
• Sustainable, scalable approach for converting industrial waste into high-performance ceramics.

GRAPHICAL ABSTRACT