Influence of accessory minerals on the thermal activation and reactivity of bentonites

Calcined clays are promising Supplementary Cementitious Materials (SCMs), a key strategy to produce low-carbon cements and to decrease the carbon footprint of this industry. However, although the calcination of kaolinitic clays is relatively well understood, the thermal transformation of smectite-rich bentonitic clays remains less explored, particularly regarding the role of accessory minerals. Thus, this study investigated the calcination of four natural bentonitic clays with distinct mineralogical compositions, aiming to assess the contribution of accessory phases to the transformation mechanisms during thermal treatment. The clays were calcined at different temperatures and analysed using a multiscale characterization approach to monitor phase evolution, dehydroxylation, crystallinity loss and recrystallization, as well as changes in local Al and Si environments. Overall, this research highlights that the calcination and reactivity behaviour of bentonites intended for SCM applications is associated with the combined influence of clay mineral activation, silica polymorph bearing mineral assemblages, physical properties and local structural modifications.