| Article ID | Journal | Published Year | Pages | File Type |
|---|---|---|---|---|
| 1695122 | Applied Clay Science | 2013 | 9 Pages |
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.
► De-hydroxylation of kaolinite provokes a change of the Al coordination number. ► Thermal treatment provokes the formation of drastically bigger granules. ► The less expensive treatment is at a temperature of 600–650 °C. ► The grain size of metakaolin is an indirect evaluation of pozzolanic reactivity. ► Pozzolanic reactivity is not correlated with SSA and 27Al coordination number.
