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 (SO42-) and magnesium (Mg2+) on the chloride diffusion and damage behaviors of cement mortar were determined. The results show that the SO42- and SO42- + 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 SO42- and SO42- + 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++SO42- 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++SO(4)(2-)coarsens pore structure of matrix and induces a higher porosity, which further accelerates the ingress of corrosive ions.