Microstructure, thermal expansion, hardness and thermodynamic parameters of cordierite materials synthesized from Algerian natural clay minerals and magnesia

Low-cost, dimensionally stable, and hard cordierite ceramic materials were prepared by reaction sintering two Algerian natural clay minerals and synthetic magnesia. The microstructure and hardness of the developed materials were characterized by a scanning electron microscope and a hardness tester, respectively. Differential thermal analysis, dilatometry, and Raman spectroscopy were used to analyze the transformation of phases and sintering behavior. The coefficient of thermal expansion (α) was determined from dilatometry experiments. The microstructure of DT00M sample synthesized from stoichiometric powder mixture (clay minerals and synthetic magnesia) consisted of cordierite only. Whereas cordierite, magnesium silicate, and sapphirine phases were present in DT04M and DT08M samples prepared from non-stoichiometric powder mixtures containing excess magnesia of 16 and 20 wt.%, respectively. The values of the activation energy (E_a) and frequency factor (A), for cordierite crystals, varied from 577 to 951 kJ/mol, and 1.54 × 10¹⁸ to 1.98 × 10³⁰ S⁻¹, respectively. The kinetic parameter n for the formation of cordierite had values between 2 and 3. While the Gibbs free energy (ΔG^#), enthalpy (ΔH^#), and entropy (ΔS^#) values were found to be in the range 431–483 kJ/mol, 564–938 kJ/mol, and 70–313 J/mol, respectively. Samples sintered at 1300 °C for 2 h showed higher values of hardness compared with those sintered at 1250 °C. The DT04M sample had the highest hardness value of 9.45 GPa, demonstrating an increase of 12.5% with respect to monolithic cordierite (DT00M). In the temperature range 100–1300 °C, DT04M and DT08M samples showed better dimensional stability compared to monolithic cordierite. The DT08M sample showed the lowest thermal expansion (α = 2.32 × 10⁻⁶/°C), demonstrating a decrease of 31.3% with respect to monolithic cordierite.