Modifications induced by the thermal treatment of kaolin and determination of reactivity of metakaolin

Three commercial powdered kaolins were analyzed in order to determine their chemical and mineralogical compositions, other than physical characteristics such as grain size distribution and specific surface area (SSA). The kaolins were heated at four different temperatures (550 °C, 650 °C, 750 °C, 850 °C) for 2 h. The obtained metakaolins were characterized in order to confirm the conversion of kaolinite into metakaolinite and also to verify the changes in terms of grain size and SSA, because they influence the reactivity of the metakaolins. The reactivity of the thermally treated samples was evaluated based on pozzolanic activity and 27Al MAS NMR spectroscopy. The thermal treatment leads to: a) the de-hydroxylation of kaolinite, b) the aggregation of the particles of metakaolinite, and c) the beginning of sintering of the particles of metakaolinite. The intensity of the three processes influences the reactivity of the metakaolins and depends on the amount of the kaolinite in the natural raw material and the calcination temperature. By considering the results of pozzalanic activity at 650 °C and 750 °C, after 7, 14, and 28 days of curing, and the average grain size at these temperatures, it is concluded that the pozzolanic activity of metakaolins is related to the grain-size. No correlations were noted with kaolinite content, specific surface area or 27Al coordination number.