Investigation of kaolin–quartz mixtures during heating using thermodilatometry and DC thermoconductometry

Abstract

Temperature dependencies of the DC conductivity of mixtures of kaolin and quartz were measured in the temperature range of 20–1200 °C. During heating, several processes take place in the sample—release of the physically bound water (PBW), dehydroxylation of kaolinite, the α → β transition of quartz, and the creation of Al–Si spinel. These processes were studied using differential thermal analysis, thermogravimetry, thermodilatometry, and DC conductivity measurement. At temperatures < 200 °C, H+ and OH− ions are the dominant charge carriers in the DC conductivity due to the release and dissociation of PBW. After the release of PBW, and up to the start of dehydroxylation (~ 500 °C), the DC conductivity is dominated by the transport of Na+ and K+ ions. During dehydroxylation, OH− ions, which are released from the kaolinite lattice, contribute to the DC conductivity. However, the association of OH− ions with mobile alkali metal ions into neutral complexes, as well as depletion of OH− source, results in a deceleration of the increase in the DC conductivity at 500 °C. After the dehydroxylation is completed, alkali metal ions become again the dominant charge carriers. At temperatures above the dehydroxylation region, the DC conductivity slightly decreases with the increasing quartz content. A narrow peak of the DC conductivity observed at 960 °C can be linked to the motion of Al3+ cations into new sites as the metakaolinite collapses. The DC conductivity of different mixtures did not differ significantly. Quartz and grog have lower conductivities than kaolin. Thus, the conduction was determined by the kaolin matrix (60 mass%).