Degradation progress of Portland cement mortar under the coupled effects of multiple corrosive ions and drying-wetting cycles

Concrete structures serving in the marine tidal zone usually suffer the most severe degradations due to multiple ions attacks and drying-wetting cycles. This study systematically investigates the influence of multiple ions presented in the seawater on the degradation process of Portland cement mortar under drying-wetting cycles. The interactions between sulfate (SO₄ ^2-) and magnesium (Mg²⁺) on the chloride diffusion and damage behaviors of cement mortar were determined. The results show that the SO₄ ^2- and SO₄ ^2-+ Mg²⁺ in composite solution firstly decrease the chloride ingress and then increase the chloride diffusion, while the presence of Mg²⁺ increases the chloride diffusion of mortar samples regardless of the exposure to the NaCl + MgCl₂ or NaCl + MgCl₂+Na₂SO₄ solution, and rich Mg²⁺ is found at the outer layer. The existence of SO₄ ^2- and SO₄ ^2-+ Mg²⁺ in multiple ions solutions accelerates the degradation progress, whereas the single Mg²⁺ has negligible effect on the damages of samples. The phase compositions highlight that the Mg²⁺+SO₄ ^2- in chloride solution increases consumption of portlandite and causes the decalcification of C–S–H, thus aggravating the degradations of mortar. Moreover, the microstructure analysis reveals that the presence of Mg²⁺+SO₄ ^2- coarsens pore structure of matrix and induces a higher porosity, which further accelerates the ingress of corrosive ions.