Durability performance of iron ore tailings sand cast-in-situ concrete subjected to internal-external combined sulfate-magnesium attack

The degradation mechanism of cast-in-situ concrete using iron ore tailings (IOT) to replace river sand in complex sulfate and magnesium combined environments was investigated. Internal corrosion was also considered in this study. Specimens with river sand replaced by IOT were prepared and immersed in different corrosion solutions. Physical properties such as size and mass changes were recorded during immersion. Mechanical properties were investigated using compressive strength, flexural strength tests. Porosity of specimens were tested by NMR at different corrosion stages. Microstructure and mineral changes were analyzed by SEM, XRD and TG tests. Furthermore, the comprehensive economic benefits were quantified by comparing the carbon emissions of IOT at different replacement rates. Results demonstrate that in concrete immersed in sulfate- and magnesium-rich environments, Mg 2 + significantly retards SO 4 2- diffusion. Microstructural characterization reveals that the pre-incorporated Mg 2+ promotes the precipitation of magnesium-based corrosion products, which accumulate within the pore network, thereby impeding sulfate transport and mitigating corrosion rates. Regarding the effect of IOT replacement, at a 25 % replacement rate, the optimized aggregate gradation and enhanced pore-filling capacity contributed to a maximum reduction of 74.49 % in pore volume compared to its initial state prior to corrosion. Under identical corrosive conditions, this optimization results in a maximum increase of 12.89 % in compressive strength and 22.22 % in flexural strength compared to concrete without IOT. This study identifies 25 % as the optimal IOT replacement rate under combined sulfate-magnesium attack and provides new insights into the durability evolution mechanisms of concrete under combined internal-external corrosion conditions.