Reactivity of Heated Kaolinite from a Combination of Solid State NMR and Chemical Methods

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The effect of the heat-treatment temperature for kaolinite on the types of formed aluminum and silicon sites after dehydroxylation and the relation of these sites to the reactivity of the heated materials are investigated in this work. Kaolinite has been heated to temperatures in the range 500 – 1100 °C in intervals of 50 °C. The reactivity for each sample has been tested in acid media using HF (1 vol.%) and the residues have been stored and dried for further analysis. The heat-treated samples and their corresponding residues are analyzed by solid-state 29Si and 27Al MAS NMR, where comparison of the spectra for the heated clay and the residue provide information about the structurally reactive sites. The chemical methods indicate that kaolinite reaches its maximum reactivity at ~800 °C. The reactivity decreases at higher temperatures as a consequence of the formation of spinel-type and mullite phases, as deduced by 27Al NMR and by the evolution with temperature of the Si/Al ratio for the dissolved phase, determined by ICP analysis. Comparison of normalized 29Si and 27Al NMR spectra for the heated samples and their residues provides the basis for a clear differentiation between different silicon and aluminum environments present in the samples. The 27Al NMR spectra suggest the presence of different tetrahedral aluminum sites and that these sites are correlated with different silicon environments, suggesting that Q3 and Q4 silicon sites coexist in metakaolin.