Chloride resistance of alkali-activated slag : mechanism and improvement

Abstract

Alkali-activated slag (AAS) is a promising green alternative to traditional ordinary Portland cement (OPC) with a potentially lower carbon footprint. Despite the reported satisfying mechanical performance, there remain certain unknowns about the durability performance of AAS in aggressive environments, which hinders its industrial acceptance and practical application. This study aims to understand the degradation mechanism of AAS upon chloride attack from complex marine environments and examine different strategies for improving the chloride resistance of AAS. In the first part of this study, the degradation mechanism of AAS upon seawater exposure is studied in comparison to the performance of OPC. To verify the individual and synergistic effects of sulphate and magnesium in seawater, as well as the effect of leaching, on the chloride transport mechanism of AAS, the chloride binding capacity, phase assemblage, microstructure and chloride diffusivity are examined for AAS and OPC binders exposed to different scenarios. The experimental results highlight the important role of pore structure on chloride diffusion from seawater, suggesting that the aggravated chloride diffusion in AAS exposed to seawater than to NaCl solution with the same chloride concentration is mainly due to the pore coarsening effect of magnesium which provokes chloride ingress. While for sulphate, it does not affect the pore structure of AAS upon short-term exposure, suggesting that its retarding effect on chloride diffusion may be attributed to its electrical coupling with chloride ions to maintain the electroneutrality during diffusion and formation of ionic clusters at the electrical double layer (EDL) which can block the fine pore channels. In the second part of this study, by considering the role of binder chemistry in forming and modifying the layered double hydroxides (LDHs) in AAS, i.e. hydrotalcite-like phases and AFm phases, and the interaction between LDHs and chloride ions, different chemical or mineral additives are used for tailoring AAS to pursue enhanced chloride resistance. It is found that the corrosion inhibitive ions (i.e. nitrite and nitrate) can be intercalated in the interlayer of formed hydrotalcite in AAS and released upon chloride exposure, showing a possible routine for preventing their leaching from AAS without compromising the chloride resistance performance. By introducing supplementary mineral sources containing constitutional metal ions of LDHs (e.g. MgO or CaO), the formation of LDHs may enhance, depending on their hydration rate and mobility of the dissolved ions during slag reaction. In sodium carbonate-activated slag, the enhanced formation of LDHs is more pronounced due to its slow reaction compared to the sodium hydroxide-activated slag, contributing to the increased chloride binding capacity. This study contributes to the understanding of the degradation mechanism of traditional and alternative cementitious materials upon seawater exposure, which is particularly important for designing more durable alkali-activated concrete for marine practices and predicting its service life in marine environments. Moreover, knowledge developed from this study can be useful for designing and optimising AAS mixtures with higher sustainability and better durability performance.