Evolution of kaolinite morphology upon exfoliation and dissolution: Evidence for nanoscale layer thinning in metakaolin

The use of calcined clays as a supplementary cementitious material has been a subject of intensive research due to their high availability and their potential to be used as a low-CO₂ alternative to Portland cement. Once calcined, kaolinite transforms into metakaolin, which can dissolve and react under alkaline conditions. This reactivity has been studied in terms of dissolution kinetics, but not much is known about the evolution of clay morphology upon dissolution. Here, we apply quantitative imaging approaches to statistically quantify the extent of morphological changes that occur in dissolving kaolinite and metakaolin at multiple scales. Firstly, at the micro-scale, we successfully capture in situ exfoliation of clay particles in an alkaline solution using a novel experimental setup. We find a noticeable difference in the maximum relative Feret diameters (2.4 for kaolinite vs 1.5 for metakaolin) as well as the pattern in which these clays break apart (exfoliation for kaolinite vs disintegration for metakaolin). Secondly, at the nano-scale, we report ex-situ measurements and comparisons of individual layer thicknesses before and after dissolution. For the first time, to our knowledge, we report that kaolinite undergoes an average thinning of ~15 nm (from 110 nm to 95 nm) upon calcination to metakaolin, and consequently, metakaolin layers undergo an evident thinning of ~20 nm (from 95 nm to 75 nm) upon dissolution. These new quantitative results on morphological changes in 1:1 clays upon dissolution could pave towards a fundamental understanding of clay reactivity as well as widespread usage in cementitious systems.