Young's modulus of kaolinite-illite mixtures during firing

Abstract Kaolin (more than 80% of kaolinite) and illitic clay (80% of illite) were used to prepare mixtures with 0–100 mass% of kaolin. Extruded and dried samples were analyzed using thermogravimetry, thermodilatometry and dynamical thermomechanical analysis in the temperature range of 20 °C → 1100 °C → 20 °C, 5 °C/min. The mass loss caused by the release of physically bound water decreased linearly as the amount of kaolin in the mixture increased, from 3.3% mass loss in the samples with 100% illite to 2% mass loss in the samples with 100% kaolin. The mass loss caused by dehydroxylation increased linearly as the amount of kaolin increased, from 3.7% mass loss (samples with 100% of illite) to 10% mass loss (samples with 100% kaolin). The final firing shrinkage depended on the mass fraction of kaolin according to a quadratic function, ranging from 5.8% (samples with 0% kaolin) to 2.4% (samples with 100% kaolin), with the minimum 2% in mixtures with 70 mass% of kaolin. The initial values of Young's modulus varied linearly between 5 GPa (100% illite) and 8 GPa (100% kaolin). Young's modulus of all mixtures increased significantly at low temperatures (up to 300 °C), which was caused by the release of physically bound water. The effect of dehydroxylation on Young's modulus was insignificant, except for samples with 90 to 100 mass% kaolin in the mixture. The final values of Young's modulus depended on the amount of kaolin: from 38.3 GPa for 0% of kaolin to 16.4 GPa for 100% of kaolin. The behavior of Young's modulus during cooling at temperatures below 750 °C (transformation temperature of the glassy phase) was determined by the creation of cracks between the glassy phase and quartz grains. Cracking decreased Young's modulus and continued until the room temperature was reached. The relationships between physical quantities (mass loss, relative thermal expansion, and Young's modulus) and the mass fraction of kaolin identified experimentally were then compared with quantities calculated using the mixture rule. The experimental and calculated values were close; the differences between them were not larger than 3%. The results imply that the physical and chemical processes run in kaolin and illitic clay separately, without mutually influencing each other.