Evolution of the Al local environment in calcined kaolinite followed by electron energy loss spectroscopy

Metakaolin has demonstrated significant potential as a partial replacement for clinker in cement, contributing to the development of a new generation of supplementary cementitious materials (SCMs) aimed at reducing CO₂ emissions associated with cement production. The formation of four- and five-fold aluminum ( [IV] / [ V ] Al) due to the dehydroxylation of clay layers during the calcination process was shown to be one of the key elements explaining the increase of their reactivity in cementitious media. Understanding of the reaction mechanisms leading to their formation is thus fundamental to optimize the calcination temperature of the clay and later optimize their contribution in cement reactivity. Nuclear magnetic resonance spectroscopy (NMR) is the main technique for the characterization of the local Al environment but is unsuitable for Fe-containing samples and cannot reveal heterogeneities at the particle level. In contrast, transmission electron microscopy allows investigating the particle morphology, crystallinity and chemical composition and environment in a single experiment. In this study, the potential of electron energy loss spectroscopy (EELS) at the L edge as an alternative to probe the Al coordination in thermally activated kaolinites is investigated. The evolution of the Al EELS spectra profile as a function of the calcination temperature is in agreement with [IV] / [ V ] Al formation. The Al coordination transformation is accompanied by a structural disorder increase. But the hexagonal morphology of kaolinite plates is barely altered, with only a partial dissolution of the edges. Finally, it reveals that at the intermediate 525 °C calcination temperature, the particles do follow different kinetics, but that the dehydroxylation process keeps proceeding homogeneously within individual plates.