Synergistic effects of metakaolin and dolomite on alkali-activated slag exposed to chloride and sulfate solutions

Abstract

This study investigates the synergistic effects of metakaolin and dolomite on the chemical degradation and phase alteration of alkali-activated slag (AAS) binders exposed to various chloride and/or sulfate-containing solutions. The results indicate that the combined substitution of dolomite and metakaolin promotes gel precipitation and enhances the compressive strength of AAS at a fixed replacement level of 40%. Moreover, the AAS binders show strong stability and limited sensitivity to phase alteration when exposed to NaCl solution, Na2SO4 solution, and artificial seawater. The phase composition and microstructure of the exposed AAS remain largely unchanged in these solutions, except for the trace ettringite precipitation. The exposure to MgSO4 solution shows more severe deterioration on the AAS binders while the incorporation of metakaolin in AAS reduces the Ca/Al ratio of C-A-S-H, rendering better durability against MgSO4 attack. Although the incorporation of metakaolin inhibits the formation of hydrotalcite in AAS, the aluminum dissolved from metakaolin promotes Friedel's salt formation in chloride-bearing solutions, which partially compensates for the weakened chloride binding capacity by reduced hydrotalcite formation. However, the potential release of carbonate ions from dolomite dissolution might occupy the hydrotalcite interlayer, leading to reduced chloride binding capacity in AAS with high dolomite content.