Insights into the evolution of carbonate-bearing kaolin during sintering revealed by in situ hyperspectral Raman imaging

Sintering reactions of clay bodies have previously been studied by numerous experiments that involve quenching of the sintered ceramic bodies to room temperature and analyzing the reaction product by different analytical techniques. In this study, green bodies containing quartz, alkali feldspar, kaolinite, and calcite, were progressively fired in air at various temperatures from room temperature to about 1060 degrees C. For the first time, mineral reactions and textural relationships were studied in situ as a function of temperature and time with a spatial resolution of a few micrometers by confocal hyperspectral Raman imaging. Gehlenite, wollastonite, and pseudowollastonite could unambiguously be identified as newly formed phases during sintering, and their textural evolution could be followed with temperature and time. From 800 degrees C onwards wollastonite formed at the direct contact to gehlenite, whereby at temperatures higher than 990 degrees C wollastonite seems to be gradually replacing gehlenite. The crystallization of pseudowollastonite was observed already similar to 290 degrees C below the accepted critical temperature (similar to 1125 degrees C) for the wollastonite-to-pseudowollastonite transformation, suggesting that pseudowollastonite can form metastably. The results of this study demonstrate that hyperspectral Raman imaging is a powerful method to study in situ phase transitions and recrystallization processes at grain boundaries during high-temperature sintering of ceramic materials.