Durability performance of cast-in-situ mortar containing iron tailings sand under synergistic internal-external sulfate, chloride, and magnesium attack

This study investigated the degradation behavior of cast-in-situ mortar incorporating iron ore tailings (IOTs) as a partial replacement for river sand under combined attack involving sulfate, magnesium, and chloride ions (distilled water, internal attack, external attack, and internal–external attack). Throughout the exposure period, changes in appearance, size, mass, and mechanical properties were monitored; meanwhile, microstructural, mineralogical, and pore structure evolution were characterized using scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), thermogravimetry/differential thermogravimetry (TG/DTG), and nuclear magnetic resonance (NMR). The results indicate that the incorporation of 25% IOTs demonstrates optimal mechanical properties and attack resistance. Internal attack more significantly impairs the mechanical improvement from 25% IOTs. The compressive strength increases by 17.65% under external attack alone, in contrast to less than 10% for internal or combined attack. At 50% IOTs content, the improvement effect is significantly reduced compared to that at the 25% content.