Synergistic effects of sulfate, magnesium, and bicarbonate ions in mine water on the hydration, microstructure, and long-term durability of Portland cement paste

Abstract

The use of mine water containing aggressive ions as mixing water in concrete poses a significant threat to the long-term durability of cement-based materials. This study systematically investigates the individual and combined effects of sulfate (SO₄²⁻), magnesium (Mg²⁺), and bicarbonate (HCO₃⁻) ions on the hydration behavior, mechanical performance, and microstructural evolution of Portland cement paste. Cement pastes were prepared using simulated ion-containing solutions and evaluated through compressive strength testing, isothermal calorimetry, XRD, TG-DTG, MIP, and SEM analyses. Results indicate that all three ions accelerate early hydration and strength development, with the ternary ion system exhibiting a 73.5 % increase in 7-day compressive strength (from 28.3 to 49.1 MPa). However, prolonged curing results in strength deterioration, particularly under multi-ion conditions, where compressive strength decreases by 25 % (from 55.0 MPa at 28 days to 41.3 MPa at 180 days). Quantitative XRD analysis shows that ion incorporation significantly alters hydration product composition: after 90 days, AFt content decreases from 10.7–11.8 % to 7–9.1 %, while calcite formation increases from 3.5–7.7 % to 3.9–9.5 %, and CH content rises under high-ion conditions, indicating a shift in hydration equilibrium. MIP results reveal that total porosity remains nearly constant (15.3–17.1 %), but mesopore fractions increase from 9.9–13.3 % to 10.5–14.0 %, suggesting progressive pore refinement but reduced matrix integrity. Microstructural observations confirm that deterioration is associated with excessive AFt formation, transformation of C–S–H into M−S−H, and accumulation of CaCO₃, leading to increased microcracking and loss of compactness. This study provides new insights into the synergistic and time-dependent effects of aggressive ions on cement hydration and durability. The findings not only elucidate the deterioration mechanisms of ion-contaminated systems but also offer theoretical guidance for mix design optimization, development of ion-resistant binders, and pretreatment strategies to ensure the safe and sustainable use of mine water in future concrete infrastructure.