Dissolution kinetics of calcined kaolinite and montmorillonite in alkaline conditions: Evidence for reactive Al(V) sites

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Calcined clays are attracting significant research attention because of their potential to partially replace CO2-intensive portland cement. This potential depends largely on their reactivity, especially dissolution under alkaline conditions. Identification and characterization of reactive sites in these amorphous calcined clays has so far not been reported. Here, we investigate kaolinite (1:1 clay) and montmorillonite (2:1 clay) calcined at different temperatures under alkaline conditions (0.1 mol/L NaOH). Solution compositions are determined in batch dissolution experiments, whereas 27Al and 29Si MAS and CP/MAS NMR are used to investigate the structure of the undissolved residues to identify sites that are reactive and undergo preferential dissolution. The highest Si and Al dissolution rates for kaolinite are observed for calcination at 700°C, corresponding to the temperature of optimum reactivity, whereas the rates decrease and become increasingly incongruent at higher temperatures. The Si and Al dissolution rates for optimally calcined kaolinite are 4 and 12 times larger than the corresponding rates for optimally calcined montmorillonite, in accord with the much higher reactivity of calcined kaolinite compared to calcined 2:1 clays. This superior performance of kaolinite is explained by novel 27Al NMR results, which show strong evidence for preferential dissolution of highly reactive pentahedral aluminum sites.

OriginalsprogEngelsk
TidsskriftJournal of the American Ceramic Society
Vol/bind102
Nummer12
Sider (fra-til)7720-7734
Antal sider15
ISSN0002-7820
DOI
StatusUdgivet - dec. 2019

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