Analysis of the ion diffusion mechanism affecting the durability of concrete in saline-alkali soils in Western China

Although significant theoretical and technological advancements have been made in the application of concrete in saline soil regions over the past two decades, newly constructed reinforced concrete structures in these areas still face severe issues of corrosion and degradation. This is due to the complex deterioration environment in saline soil regions, characterized by the combined effects of salt corrosion, dry-wet cycles, and freeze-thaw conditions. The reduced service life of concrete structures in this region is closely related to the diffusion and distribution patterns of high-concentration chloride salts and various corrosive ions within the concrete. These patterns affect the content, transformation, and microstructure of corrosion products , ultimately leading to a shorter service life compared to other environments. This paper simulates the saline-alkali soil environment using solutions of different concentrations of chloride sulfates and magnesium salts, studies the diffusion and distribution patterns of chloride ions and sulfate ions in concrete under this environment, and analyzes the mechanism of action in conjunction with changes in microstructure. The experimental system adopts a dry-wet cycle test that can represent the characteristics of the semi-arid continental climate in Western China. The results show that although the content of free chloride ions and total chloride ions entering the concrete in the saline-alkali soil simulation solution is the lowest, the binding capacity of chloride ions is significantly greater than that of sulfate ions and far exceeds that in other environments. Under the action of high-concentration chlorides alone, the content of chloride ions in concrete is the highest, and the binding capacity of chloride ions also increases with the concentration of chlorides. The content of free sulfate ions and total sulfate ions entering the concrete in the saline-alkali soil simulation solution and their binding capacity are higher than in the control solution. Due to the ability of sulfate ions to hinder the diffusion of chloride ions in concrete, magnesium ions play a hindering role in the early stage and an accelerating role in the later stage. This results in concrete corroded by the saline-alkali soil environment, which has a characteristic of low chloride ion content and high sulfate ion content. The ions in the saline-alkali soil solution that cause concrete damage are Cl - , SO 4 2- , and Mg 2+ . These ions react with the concrete to form Friedel’s salt, Aft and AFm phase calcium aluminate , gypsum, Mg-S-H, and Mg(OH) 2 , among other substances. These corrosion products significantly impact the microstructure of concrete, causing the microstructure of concrete to transition from dense to loose to cracked much earlier than in other environments. • Simulated the saline-sodic soil environment in Western China. • Investigated chloride and sulfate ions' diffusion and distribution patterns from a multi-ion interaction perspective. • Elucidated the mechanism of saline-sodic soil environment on concrete from ion binding capacity and microstructure aspects.