Influence of CO2-Mineralised Alkaline Waste Materials on the Chloride Ingress Resistance of Sustainable Cementitious Materials

Using CO2-mineralised alkaline waste as a supplementary cementitious material has the potential to advance sustainable construction practices. This paper evaluates the effect of carbonating three fine industrial waste materials—construction and demolition waste, ladle furnace slag, and biomass ash—on the durability performance of blended cements in which they replace 7% and 20% of ordinary Portland cement. The performance of carbonated waste was compared with that of their non-carbonated counterparts through chloride ingress resistance, a key parameter governing reinforcement corrosion. The results show that prior carbonation can significantly influence chloride transport properties, although the magnitude and direction of the effect depend strongly on the waste type and replacement level. Mortars with carbonated construction and demolition waste or ladle furnace slag generally reduced the chloride diffusion coefficient compared with the corresponding non-carbonated materials. This behaviour was associated with greater portlandite consumption and enhanced chloride-binding capacity. Nevertheless, none of the blended systems outperformed the ordinary Portland cement (OPC ) in terms of chloride diffusion resistance. Conversely, biomass ash at 20% replacement led to markedly reduced performance, with the estimated time to steel depassivation decreasing to 1–2 years under the exposure conditions considered. These findings indicate that prior carbonation may improve the suitability of some wastes for cementitious applications relative to their non-carbonated counterparts, although the resulting chloride resistance remains lower than that of OPC. The benefits are of waste carbonation are therefore material-dependent and should be assessed on a case-by-case basis.