Understanding the impact of main seawater ions and leaching on the chloride transport in alkali-activated slag and Portland cement

A growing body of evidence has suggested an aggravated chloride diffusion in concrete exposed to seawater than to pure NaCl solution of the same chloride concentration, indicating the potential impact of other constitutional ions in seawater, e.g., sulphate and magnesium. This study aims to systematically investigate the impact of sulphate, magnesium and leaching, and their interaction on the chloride transport in alkali-activated slag (AAS) and ordinary Portland cement (OPC). The results indicate that the aggravating effect of magnesium on chloride diffusion overwhelms the suppressing effect of sulphate, leading to increased chloride diffusion in both AAS and OPC. Despite without the pore filling effect by ettringite formation as in OPC, the sulphate in seawater also suppresses the chloride transport in AAS, which is likely linked to the electrical coupling effect during multi-ion transport and the pore wall thinning effect by the aggregation of the formed Na-SO4 clusters in the gel pore. The origin of the peaking behaviour is found to be contributed by not only the impact of leaching, but also the effect of sulphate and magnesium on releasing the bound chloride from the reacted phases at the exposure front.