Behavior of Kaolinite Pellets at Elevated Temperatures

Abstract

Abstract--Dimensional changes in kaolinite pellets as a function of temperature show two sharp shrinkage "steps," at about 450--550~ and 900-980~ which are roughly comparable in magnitude. Isothermal heat-soaking tests confirm that the rates for both are kinetically controlled. Water vapor inhibits shrinkage at low temperature but promotes shrinkage at high temperature. Both the former reaction, related to dehydroxylation, and the latter reaction, related to "mnilitization," take place at temperatures well below those observed in DTA, TG, and other measurements, indicating that bondbreaking is a necessary prelude to transitions at higher temperatures.Bonding energy changes, as measured by X-ray fluorescence shifts, were used in interpreting the phenomena involved. The relative absence of bonding energy changes in the aluminum until the range 950-1100~ and the presence of such changes in the silicon, suggest that high-temperature energy release is probably related to segregation or crystallization of silica, rather than of an aluminum-containing phase. Caution must be used in interpreting bonding energy changes, and in distinguishing kinetic and thermodynamic contributions to dynamic phenomena.

Are the intensities and durations of small-scale pottery firings sufficient to completely dehydroxylate clays? Testing a key assumption underlying ceramic rehydroxylation dating

“…This pattern led Pampuch (1966) to infer that up to 12% of the OH originally present in kaolinite would remain even after the transition to metakaolinite. Furthermore, Flank (1979) showed that DHX continued to occur for up to 100 minutes during temperature ramps to 1000 °C and for up to 400 minutes at temperatures of 400 o C to 900 °C. Traditional pottery firings, as observed in ethnographic and ethnoarchaeological settings can vary considerably from materials science firings in controlled laboratory conditions.…”

Application of TMA and DTA for study of the crystallization behaviour of SiO2 in the thermal transformation of kaolinite

“…The apparent elimination of this exotherm on alkali treatment of metakaolinite with a view to extracting the free silica prior to DTA supported the I]-quartz hypothesis. However, the actual reason behind the removal of the 980~ peak was the formation of a zeolitic phase [5] by the direct reaction between metakaolinite and NaOH solution, in contrast with the observation of Flank [6]. Furthermore, silica could not be extracted by NaOH due to the non-availability of free SiO2 in the metakaolinite structure.…”