Towards the Mullitization of Mullite Precursor Gel Synthesised from Kaolin Derived-Sodium Metasilicate and Sodium Aluminate

A novel synthesis technique was developed for producing mullite nanopowder from kaolinite clay. Kaolin was converted to metakaolin, reduced into sodium silicate and aluminate, and reassembled at the molecular level through controlled precipitation to form an aluminosilicate hydrogel with intentionally high aluminum content. The hydrogel was sintered at 950, 1050, and 1150 °C for 30 min. X-ray Fluorescence spectroscopy (XRF) confirmed enrichment in alumina, while Fourier Transform Infrared spectroscopy (FTIR) revealed progressive dehydroxylation and the emergence of mullite-specific bonds. X-ray Diffraction (XRD) indicated the onset of mullite crystallization at 950 °C and nearly complete transformation at 1150 °C, achieving ~99 % phase purity. Scanning Electron Microscopy (SEM) showed a clear morphological evolution from porous, amorphous gel particles to faceted grains at 950 °C, and well-interlocked mullite grains (2–4 μm) with minimal porosity at 1150 °C. Energy Dispersive X-ray spectroscopy (EDX) confirmed the elemental progression toward the theoretical Al/Si ratio for stoichiometric mullite. These findings demonstrate that mullite nanopowder with high phase purity and desirable microstructural features can be synthesized at 1150 °C from kaolin.