Thermal phase sequences in gibbsite/kaolinite clay: electron microscopy studies

In Brazil, gibbsite/kaolinite clay is used extensively in the manufacture of refractory products. In this study, electron microscopy was used to characterize the phase sequences occurring during thermal transformation in this type of clay. The clay was powdered, heated on platinum foils to temperatures of 200–1550 °C and program cooled. The heated powders were characterized using transmission electron microscopy, selected area electron diffraction, elemental microanalysis and X-ray diffraction. After heating the gibbsite/kaolinite clay to 200 °C or better at 300 °C, morphological characterization under a transmission electron microscope revealed that the chi-alumina from the gibbsite is identical in size and shape (hexagonal) to the undecomposed kaolinite crystals. The thermal transformations of gibbsite and well-crystallized kaolinite follow independent specific patterns up to 1100 or 1200 °C. Examining the organization of the internal granules formed from the dehydroxylation of gibbsite and kaolinite at increasing temperature (400–800 °C), a morphological difference was observed between the aluminas (chi and kappa) and the pseudomorphic metakaolin crystals. The two reaction sequences (a) metakaolin spinel mullite; (b) kappa-alumina alpha-alumina occur in the 900–1100 °C range and characteristic morphological differences among the crystals are evident, especially in the spinel, which presents elongated particles. Mullite content increases between 1000 and 1550 °C, whereas alpha-alumina increases up to 1300 °C and decreases by 1550 °C, indicating interaction among the several high-temperature phases.