Thermodynamics of cordierite formation during firing of water-permeable ceramics

Abstract

The analysis of the influence of an increase in firing temperature on the thermodynamic possibilities of solid-phase chemical reactions in the MgO–Al2O3–SiO2 system in the direction of cordierite formation has been conducted. The thermodynamic direction of solid-phase reactions in the MgO–Al2O3–SiO2 system allows determining rational firing temperatures of materials containing cordierite for the manufacture of products for various purposes. During low-temperature firing (up to 1200 K) of finely dispersed masses based on talc-clay-alumina compositions, materials with high open porosity but low strength characteristics can be obtained. This is because the reactive sintering mainly occurs due to reactions forming intermediates for cordierite synthesis and incomplete processes of destruction in natural mineral raw materials. In the firing temperature range of 1200–1500 K, the thermodynamic probability of reactions immediately via several cordierite formation mechanisms increases, which should limit the growth of individual crystals while increasing their quantity. This should positively affect the strength and heat resistance of materials due to low values of the thermal coefficient of linear expansion. Firing temperatures of 1500–1659 K correspond to the limits for cordierite formation and contribute to the consolidation of the material with a change in the nature of porosity from open to closed. Exceeding firing temperatures above 1659 K results in the thermodynamic instability of the cordierite-corundum phase combination and can lead to the formation of defects due to the appearance of a significant amount of melt in the phase composition of the material.