Comparison of the pozzolanic reactivity for flash and soak calcined clays in portland cement blends

Abstract

The increased attention towards the use of calcined clays as supplementary cementitious materials (SCM’s) has prompted several studies of calcined kaolinite (metakaolin) and to a lesser extent also of heat-treated montmorillonite. However, a major part of these studies does not pay specific attention to the calcination process itself. Furthermore, most studies have been performed on phase-pure clays and such clays are not economically viable for applications as SCM’s at an industrial scale. In this work we investigate the reactivity of SCM’s produced by flash calcination of a natural kaolinite sample and a natural mixed smectite/illite sample. Several key properties of the flash calcined materials are tested and compared to calcined clays obtained by a conventional soak-calcination method. The analyses include characterization of physical properties such as BET surface area, specific density and particle fineness and of the microstructure and degree of reaction by powderXRD as well as 29 Si and 27 Al MAS NMR. Moreover, the pozzolanic reactivity of the SCM’s are investigated by a Chapelle-like test combined with NMR studies of Portland cement—SCM paste samples at different hydration times. Finally, the performance of mortar samples produced with a 35 wt% SCM substitution level is tested with respect to compressive strength and water requirement following the ASTM standard.