Investigations on kaolin mixtures: Impact on mullite formation kinetics and microstructure evolution

This study evaluates Algerian kaolin (Djebel Debbagh (DD1) and Tamazart (KT2)) as potential substitutes for commercial kaolin (Lab) in the production of mullite-based ceramics. Three compositions were prepared by incorporating the appropriate percentage of alumina to each calcined kaolin to achieve stoichiometric mullite precursors. The phase evolution of individual kaolin powders, as well as their mixtures with alumina, depends strongly on the calcination temperature and kaolin impurities. The differential scanning calorimetry combined with thermogravimetric analysis (TGA) showed lower secondary mullite formation temperature for the KT2-based mixture. However, X-ray diffraction revealed a complete mullitization in DD1 mixture. The K₂O hindered cristobalite formation and reduced secondary mullite formation rate. Microstructure analysis showed lath-shaped primary mullite and equi-axed secondary mullite particles. After sintering at 1600°C, The KT2-based sample (M3) exhibited higher density (3.013 g/cm³) and hardness (9.9 GPa), whereas the DD2-based sample (M2) showed moderate densification (2.91 g/cm³) and higher flexural strength (159.42 MPa). Impurities (mainly Fe₂O₃, and K₂O) promoted liquid phase sintering, resulting in greater densification in M3, whereas M2 showed more homogeneous microstructure, refined grains, and lower glassy phase content, contributing to enhanced strength.