Abstract
The effects of grinding and firing conditions on CaAl 2Si 2O 8 phase formation by solid-state reaction of kaolinite with CaCO 3 were investigated by differential thermal analysis (DTA)–thermogravimetry (TG), X-ray powder diffraction (XRD) and 29Si and 27Al MAS NMR. Unground and ground samples showed similar crystallization behavior at about 850 °C, and the crystallizing temperature was relatively unaffected by grinding. On the other hand, the crystalline products were strongly influenced by the grinding. Gehlenite (Ca 2Al 2SiO 7) was the dominant phase in the unground samples but layer-structured CaAl 2Si 2O 8 was dominant in the ground samples, together with a small amount of anorthite, which is the stable phase. The amount of anorthite gradually increased with higher firing temperature, the sample fired at 1000 °C being almost completely anorthite. Grinding treatment before firing was effective in accelerating the decomposition of CaCO 3 and extending the temperature range for the formation of CaAl 2Si 2O 8, a phase with local structure similar to that of layered CaAl 2Si 2O 8.
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