Impact of calcination time on metakaolin morphology and enhancement of pozzolanic reactivity: Insights from in situ TEM

Abstract

Metakaolin reactivity is a function of both the calcination temperature and time. While significant literature exists on temperature-induced changes, studies on the time parameter are limited. Especially, what happens at the particle scale during calcination is unknown. Here, we shed light on this issue by observing kaolin particles on a live heated stage via in situ TEM. We find that individual kaolin particles undergo a layer thinning of ~15 nm as well as an area reduction of 2–5 %. R³ tests reveal an enhancement in reactivity as a function of calcination time (10 to 40 min of calcination leads to an increase of 850 to 950 J/g in 7d cumulative heat). These two phenomena - the morphological changes and the reactivity test results follow a similar exponential trend, which saturates at ~40 min. These results suggest that prolonged calcination times impact particle morphology, which has a strong correlation with clay reactivity.