High temperature phase evolution of Bolivian kaolinitic-illitic clays heated to 1250 degrees C

The thermal behaviour of two types of clays collected from different locations in Bolivia has been studied. The clays contain kaolinite, illite, quartz and small amounts of microcline. The phase evolutions have been characterized from room temperature to 1250 degrees C. For both clays, kaolinite is completely transformed into metakaolinite when heated up to 650 degrees C. During further heating to 1050 degrees C, illite undergoes total dehydroxylation. Mullite is formed in the temperature interval of 1050-1150 degrees C and its formation rate is dependent on the amount of K and Fe present in the clays. The clay with higher amounts of K (3.2 mass %) and Fe (5.6 mass%) has an onset temperature for sintering at about 900 degrees C and an onset temperature for liquid formation at 1080 degrees C. This is about 50 degrees C lower onset temperature for sintering and 94 degrees C lower onset temperature for liquid formation when compared with the clay with lower amounts of K (2.3 mass %) and Fe (1.6 mass %).