Phase formation behaviour in alkali activation of clay mixtures

Alkali-activated soils have potential as low carbon, low cost construction materials. There is a lack of fundamental understanding around how soil composition influences alkali activation behaviour, especially for uncalcined soils. The types and relative amounts of clay minerals can vary greatly throughout soils across the world. Since clays are typically the dominant reactive aluminosilicate constituent in soils, it is desirable to understand how the types and relative amounts of clay minerals influence reaction products in alkali activation. In this study, mixtures of kaolinite, montmorillonite and illite precursors were activated with sodium hydroxide solutions. By comparing with extrapolations of cross-characterisation from the behaviour of individual clays, it was shown that phase formation behaviour deviated from an ideal rule of mixtures model. Instead, there was a hierarchy between the clays in determining the reaction products: kaolinite and montmorillonite dominated illite in this regard. This study demonstrates that the viability of a given soil for alkali activation depends not only on the total amount of clay, but the types and relative amounts of clay minerals present. In order to unlock the potential of alkali-activated soils, more understanding is needed of the role of the different components in soil.