Improving sulphuric acid resistance of slag-based binders by magnesium-modified activator and metakaolin substitution

Alkali-activated slag (AAS) is an appealing alternative to ordinary Portland cement (OPC) in aggressive acid environment considering its relatively low calcium content and potential in retarding aggressive media penetration. This study evaluates the effect of reactive MgO-NaOH mixture as an activator on the performance of AAS with and without metakaolin substitution upon sulphuric acid exposure. The experimental results reveal an improved acid resistance of slag pastes with supplementary magnesium and the improvement is further enhanced with 20% metakaolin substitution. The improved effect by supplementary magnesium is not only attributed to its microstructure refinement for retarding acid penetration; but also contributed by the acid neutralisation capacity of the formed brucite and the potentially positive effect of magnesium on stabilising the AAS binding gel. Stratified silica-rich gels are formed in the metakaolin-substituted AAS, whose formation is primarily attributed to the intrinsic difference of the formed aluminosilicate gels and related to the pH value and the presence of Mg2+. Considering the dense structure of these gels, they might act as protective barriers for the AAS upon acid deterioration.