Alkali-activated slag (AAS) paste: Correlation between durability and microstructural characteristics

Abstract This study was aimed at evaluating the effects of alkali activator solution and utilization of silica fume (SF) on durability and microstructural characteristics of alkali-activated ground granulated blast furnace slag (GGBFS) pastes. The pastes were activated with sodium silicate (SS) blended with NaOH (SH) solution with molar concentrations of 8 M, 12 M, and 16 M and SS:SH ratios of 1.0, 2.5, and 4.0. The same mixes were prepared with SF as partial replacement of GGBFS at a dosage of 10% by weight. The initial and final water absorption as well as the electrical resistivity of the specimens were measured. The optimal paste fabricated with 12 M NaOH solution and SS:SH ratio of 2.5 showed the lowest water absorption and the highest electrical resistivity. Inclusion of SF reduced the final water absorption by 29% and increased the electrical resistivity by 62% compared to the same mix without SF. The observations were confirmed by the results of scanning electron microscopy (SEM) where the optimal paste showed a dense matrix with few pores. Furthermore, the chemical composition of the pastes was studied through energy-dispersive spectrometry (EDS) and X-ray diffraction (XRD) analyses. Moreover, the resistance to aggressive environment conditions of the pastes was examined through exposure to 5% hydrochloric acid and nitric acid up to 90 days. Generally, specimens were more resistant to nitrates. The specimens showed about 5–8% increase in their compressive strength compared to the unconditioned specimens during the first 28 days; however, up to 22% reduction in compressive strength was noticed after 90 days of exposure. It was shown that SS:SH ratio has a higher contribution to acid attack resistance of alkali-activated slag (AAS) pastes.