The macroscopic mechanical properties and frost resistance durability of concrete are closely related to the changes in the internal pore structure. In this study, the two-dimensional and three-dimensional ICT (Industrial Computerized Tomography) pore characteristics of C30 concrete specimens before and after freezing and thawing in clean water, 5 wt.% NaCl, 5 wt.% CaCl₂, and 5 wt.% CH₃COOK solution environments are obtained through concrete frost resistance durability test and ICT scanning technology. The effects of pore structure changes on concrete frost resistance, durability, and compressive strength mechanical properties after freezing and thawing cycles in different salt solution environments are analyzed. This paper provides new means and ideas for the study of concrete pores. The results show that with the increase in the freezing and thawing times, the concrete porosity, two-dimensional pore area, three-dimensional pore volume, and pore number generally increase in any solution environment, resulting in the loss of concrete compressive strength, mortar spalling, and the decrease in the relative dynamic elastic modulus. Among them, the CH₃COOK solution has the least influence on the concrete pore changes; the NaCl solution has the greatest influence on the change in the concrete internal porosity. The damage of CaCl₂ solution to concrete is second only to the NaCl solution, followed by clean water. The increase in the concrete internal porosity from high to low is NaCl, CaCl₂, clean water, and CH₃COOK. The change in the pore volume of 0.1 to 1 mm³ after the freeze–thaw cycle is the main factor for reducing concrete strength. The test results have certain guiding value for the selection of deicing salt in engineering.

Severe electrochemical corrosion and leaching degradation of underground structures are induced by coupled stray current and groundwater in rail transit systems like subways. The microstructural evolution of fly ash geopolymer (FAG) during accelerated leaching was systematically investigated by simulating a coupled direct current (DC) stray current and soft water environment. Pore solution alkalinity and electrolytic cell OH⁻ concentration were utilized as evaluation indicators, combined with X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP). Results indicate that substantial OH⁻ leaching occurs under the coupled stray-current and soft-water exposure, leading to reduced pore-solution alkalinity and changes consistent with the degradation of the N–A–S–H binding network. Consequently, porosity, most probable pore diameter, and the quantity of harmful pores are increased. However, no obvious changes in the major diffraction peaks associated with quartz and mullite were detected within the resolution of the qualitative XRD analysis. Furthermore, a linearly decreasing trend over time is observed for the coupled leaching rate. Mix proportion analysis demonstrates that FAG leaching resistance is improved by reducing the water-to-binder ratio; specifically, superior gel phase content and pore structure are maintained at a ratio of 0.30. Additionally, cumulative OH⁻ leaching is effectively reduced by decreasing the sodium silicate modulus, with optimal resistance exhibited between 1.0 and 1.2. Concurrently, pore solution alkalinity before and after leaching is significantly elevated by increasing the alkali dosage. This exerts a pronounced inhibitory effect on OH⁻ leaching, thereby substantially enhancing the overall leaching resistance.