Influence of metakaolin and limestone on chloride binding of slag activated by mixed magnesium oxide and sodium hydroxide

Abstract

The layered double hydroxides (LDH) formed in alkali-activated slag (AAS), i.e. hydrotalcite and AFm phases, play a considerable role in its chloride binding behaviours. In addition to slag chemistry, the formation of LDH and chloride binding capacity of AAS are highly influenced by the activator composition and incorporation of other precursors. Considering their potential in enhancing LDH formation, this study investigates the influence of metakaolin and limestone incorporation on the phase assemblages, composition, and chloride binding capacity of AAS activated by mixed reactive magnesium oxide (MgO)-sodium hydroxide (NaOH). The results show that the chloride binding capacity of AAS is increased by up to 50% with 20% metakaolin or limestone substitution. However, different mechanisms for the binding enhancement are proposed for these two blended AAS systems. For metakaolin-blended AAS, the hydrotalcite formation is probably limited by the mobility of the dissolved Mg during MgO hydration and its interaction with dissolved Al from metakaolin; thus the increased chloride binding is primarily attributed to the sorption by C-A-S-H and formation of chloride-bearing chabazite. For limestone-blended AAS, carbonate-AFm phase is identified which enhances the formation of Friedel's salt and its polymorphs, thus showing increased binding capacity. These findings suggest that the enhanced chloride resistance of AAS materials could be achieved with the use of combinational raw precursors if appropriately designed.