## 1 Introduction  
Cementitious structures are widely employed in saline regions due to their remarkable construction advantages [1, 2, 3, 4]. In practical engineering applications within such environments, the structures of cementitious materials are inevitably experience damage, primarily through sulfate corrosion and various loading conditions [5, 6, 7]. This corrosion adversely affects the strength, elastic modulus, and other properties of cementitious materials, ultimately reducing in their durability and service life [8, 9]. Consequently, extending the service life of cementitious materials in saline environments has emerged as a critical issue that requires immediate attention [10, 11, 12]. In recent years, numerous researchers have conducted extensive investigations into the durability of cement-based materials in saline environments relevant to practical en gineering. In concrete specimens, as the erosion duration increases under the combined effects of sulfate exposure and dry-wet cycles, noticeable micro-cracks initially develop in their interfacial transition zones [13; 14; 15]. During the early stages of sulfate attack, the durability of cement-based materials correlates positively with their strength; higher-strength materials demonstrate greater longevity. However, at later stages of sulfate attack, porosity becomes the predominant factor influencing durability [16]. Furthermore, when examining ratios for dry-wet cycle durations such as 1:1, 3:1, 5:1, and 7:1, higher ratios facilitate the more rapid transport of sulfate ions into concrete, resulting in exacerbated erosion [17]. Loading further accelerates sulfate-induced deterioration [18]. [PERSON] et al. [6] investigated the sulfate attack on concrete under dynamic bending loads, and the results indicated that the solution concentration and stress level were the two main factors of the sulfate attack event. [PERSON] et al. [19] studied the influence of sulfate corrosion on the axial compressive load of concrete, and the results showed that under continuous compressive load, the compressive strength, ultrasonic velocity, and dynamic elastic modulus all increased with age. With increasing bending loads coupled with sulfate exposure effects, concrete deterioration becomes increasingly pronounced as the bending load intensifies [20]. Similarly, under simultaneous fatigue loading and dry-wet cycles involving sulfates, both factors can expedite sulfur ion migration within concrete, thereby accelerating its degradation process [21; 22]. The behavior of sulfate transport within concrete is typically simulated through both chemical interactions and mechanical coupling effects [23; 24; 25]. Some studies have identified volumetric strain as a primary contributor to crack formation within concrete structures [26]. The four-point bending test serves as a reliable method for validating model findings that indicate that external loads exacerbate concrete deterioration during sulfate attacks [27]. Moreover, the diffusion model for sulfate ions suggests that the corrosion depth increases with elevated concentrations of sulfur solutions concentrations along with increased water-cement ratios and higher coarse aggregate content levels [28]. How sulfate erosion affects the performance characteristics of cementitious materials subjected to bending loads remains an area requiring further investigation. Currently, there is a limited body of research investigating the microscopic transport of sulfates in cement-based materials under the combined influence of bending loads and environmental factors, particularly concerning the quantitative relationship between sulfate attack and porosity. Therefore, it is essential to analyze sulfate ion transport mechanisms attributable to the coupling of bending forces and sulfate exposure. We designed an experiment to investigate the deterioration effect of cement-based materials under the coupling action of 0%, 20%, and 40% of the ultimate bending load and sulfate in dry-wet cycles.