Degradation mechanism of cement mortar exposed to combined sulfate–chloride attack under cyclic wetting–drying condition

The degradation of cement mortar subject to combined chloride ingress and sulfate attack under cyclic wetting–drying condition was studied through microstructural analysis. The specimens were exposed alternatively to a designated salt solution (NaCl, Na₂SO₄, or a mixture of NaCl and Na₂SO₄ solution) and air drying to simulate the natural conditions of tidal zones in the marine environment. Backscattered electron micrographs obtained with scanning electron microscopy showed three distinct layers formed from the surface inwards after exposure. The elemental distribution and phase composition of these layers were studied with energy dispersive spectroscopy through elemental mapping and point analysis, respectively. Phase changes after exposure were identified and compared among layers to understand the interaction between chloride and sulfate ions. The results showed a progressively layered degradation of cement mortar subject to the wetting and drying cycle, including a leaching zone, a deposition layer, and an inner area with relatively unaltered microstructure but with signs of ion penetration. Sulfate and chloride ions had mutual inhibiting effects on each other in cement mortar and the penetration depths of these two types of ions were different. A mechanism of progressively layered degradation of cement mortar under the investigating condition was proposed and discussed.