Insights to the sulfate resistance and microstructures of alkali-activated metakaolin/slag pastes

In this work, the effects of slag addition on the geopolymerization process and sulfate resistance of alkali-activated cement were studied. The substitution of metakaolin by slag increased the reactivity of precursors, contributing to a higher reaction rate and accelerating geopolymerization process. The mechanical properties and sulfate resistance were significantly improved with the incorporation of slag. The results show that the transformation of gel phases and development of pore structures were the vital reasons for improvement of sulfate resistance. The quantity of C-A-S-H generation increased with the slag content and consequently to a more compact microstructure. The refinement of pore structures came from the higher space-filling capacity of C-A-S-H phase and the filling effect of slag particles. The optimum replacement level of metakaolin was 30 wt%, in terms of sulfate resistance development and the potential damage of ettringite. This work provides a reference for the utilization of granulated blast furnace slag in metakaolin based geopolymer, which is of great value in urging geopolymer towards practical application in harsh environment. •The utilization of slag in alkali-activated metakaolin/slag pastes was realized.•The addition of slag promotes the geopolymerization process and improves the compactness.•The sulfate resistance of geopolymer was enhanced with the incorporation of slag.