Synergistic effects of sulfate and magnesium ions on chloride diffusion behaviors of Portland cement mortar

Abstract

This paper aims to identify the synergistic effects of sulfate (SO42−) and magnesium (Mg2+) ions presented in seawater on the chloride diffusion behaviors including chloride binding capacity and apparent chloride diffusion coefficient of Portland cement mortar. Mortar mixtures with different supplement cementitious materials (SCMs) are prepared and exposed to NaCl, NaCl + MgCl2, NaCl + Na2SO4 and NaCl + Na2SO4 + MgCl2 solutions with a fixed chloride concentration. Chloride transportation and binding capacity of Portland cement based mortars are investigated by titration. In order to identify phase assemblages and microstructural evolutions under exposure, samples are examined by applying XRD, TGA, SEM-EDS and MIP. The SO42−+Mg2+ in NaCl solution reduces the binding capacity and increase the chloride diffusion coefficient due to the reduction of pH and aggravation of C-S-H decalcification. Meanwhile, the presence of SO42− in chloride solution enhances the chloride diffusion and refines the pore structure, while the SO42− can form to ettringite (AFt) and results in significant micro-cracking. Furthermore, the effects of SCMs on the chloride diffusion are investigated.