MICROSTRUCTURAL EVOLUTION IN FIRED KAOLINITE

On heating individual platelike pseudohexagonal particles of kaolinite undergo dehydroxylation forming metakaolin at about 550 degrees C. Be tween 900 and 1000 degrees C, nucleation of nanometre scale gamma-Al2O 3 spinel crystals occurs while the original hexagonal particle morphol ogy is retained. At higher temperatures (1200 degrees C) cristobalite and elongated mullite crystals nucleate, and eventually at 1300 degree s C the particles lose their face-on pseudohexagonal habit and 'ball u p'. Dense kaolinite, clap derived bodies show similar phase and morpho logy evolution: after 3 h at 900 degrees C, elongated edge-on, and mor e equiaxed face-on, particles of metakaolin are observed. These closel y packed platelike, amorphous particles retain the individual kaolinit e particle morphology. Light and dark contrast in dense bodies appears to arise from phase separation into alumina and silica rich regions. Between 900 and 1000 degrees C nanometre crystals of gamma-Al2O3 spine l nucleate within these regions and, after 3 h at 1200 degrees C, rand omly orientated mullite and cristobalite crystals nucleate from the am orphous phase.