Study on Concrete Deterioration in Different NaCl-Na₂SO₄ Solutions and the Mechanism of Cl^- Diffusion

The diffusion of sulfate (SO₄^2-) and chloride (Cl^-) ions from rivers, salt lakes and saline soil into reinforced concrete is one of the main factors that contributes to the corrosion of steel reinforcing bars, thus reducing their mechanical properties. This work experimentally investigated the corrosion process involving various concentrations of NaCl-Na₂SO₄ leading to the coupled erosion of concrete. The appearance, weight, and mechanical properties of the concrete were measured throughout the erosion process, and the Cl^- and SO₄^2- contents in concrete were determined using Cl^- rapid testing and spectrophotometry, respectively. Scanning electron microscopy, energy spectrometry, X-ray diffractometry, and mercury porosimetry were also employed to analyze microstructural changes and complex mineral combinations in these samples. The results showed that with higher Na₂SO₄ concentration and longer exposure time, the mass, compressive strength, and relative dynamic elastic modulus gradually increased and large pores gradually transitioned to medium and small pores. When the Na₂SO₄ mass fraction in the salt solution was ≥10 wt%, there was a downward trend in the mechanical properties after exposure for a certain period of time. The Cl^- diffusion rate was thus related to Na₂SO₄ concentration. When the Na₂SO₄ mass fraction in solution was ≤5 wt% and exposure time short, SO₄^2- and cement hydration/corrosion products hindered Cl^- migration. In a concentrated Na₂SO₄ environment (≥10 wt%), the Cl^- diffusion rate was accelerated in the later stages of exposure. These experiments further revealed that the Cl^- migration rate was higher than that of SO₄^2-.