# Effect of Dry-Wet Cycle Periods on Properties of Concrete under Sulfate Attack
## 1 Introduction  
Sulfate attack is one of the most important factors that affect the durability of concrete, and it is greatly accelerated by dry-wet cycles [1; 2; 3]. Sulfate erosion mainly includes two aspects: physical and chemical erosion [4]. The first occurs through the crystallization of sulfate, producing crystallization pressure and cracks in concrete structures [5], while chemical erosion is mainly caused by expansive products, such as ettrinigite (AFt) and gypsum, produced by the reaction between sulfate and cement hydration products. These products make concrete expand, crack, flake, disintegrate, and increase in permeability, thus providing access for other erosive media (e.g., Cl\({}^{-}\) and CO\({}_{2}\)) to enter the interior of the concrete, resulting in the acceleration of its deterioration. Simultaneously, the erosion process consumes much OH-, leading to the decomposition of the unstable hydrated calcium silicate gel (C-S-H), thereby weakening the bonding effect and reducing the strength of the concrete [6; 7]. In environments presenting dry-wet cycles, such as those exposed to splashes and tides, the sulfate attack of concrete is very complicated [8]. During the wet process, SO\({}_{4}\)\({}^{2-}\) gradually penetrates into the concrete through microcracks and pores and accumulates. During the dry process, the water rapidly evaporates, and the concentration of SO\({}_{4}\)\({}^{-}\) in the concrete pores gradually increases, while Na\({}_{2}\)SO\({}_{4}\) crystallizes [9]. The SO\({}_{4}\)\({}^{2-}\) reacts with the cement hydration products, which increases the rate of expansion and cracking, eventually accelerating the deterioration of the concrete [2]. [PERSON] et al. [10] and [PERSON] et al. [11] analyzed the effects of sulfate attack and dry-wet cycles on cement-based materials, showing that dry-wet cycles significantly influenced the sulfate attack process. In recent years, many researchers have studied new materials or methods to enhance the ability of concrete to resist sulfate attack under dry-wet cycles. Some have studied the effects of different active admixtures (fly ash, slag, and silica fume) on concrete under sulfate attack in dry-wet cycles, finding the active admixtures to have positive effects on crack repair and the performance of concrete [3; 12; 13]. [PERSON] et al. found that a biofilm coating could effectively improve the resistance of concrete to sulfate attack [14]. Previous studies have shown the coupling of external load and sulfate attack under dry-wet cycles to negatively impact concrete. [PERSON] et al. focused on the sulfate attack of concrete under a combination of bending, fatigue loads, and dry-wet cycles; the results showed that the loads and dry-wet cycles expedite the transport of sulfate ions in the concrete, accelerating the corrosion with sulfate [15]. The degradation of cement-based materials under the coupled effects of multiple corrosive ions and dry-wet cycles is another research direction. [PERSON] et al. studied the effects on the transition zone of the concrete interface under dry-wet cycles with forms of sulfate attack, and the results suggest that the interface roughness increased with exposure time [2]. However, the dry-wet cycling regimes used in different studies on the erosion of concrete under the influence of sulfate attack and dry-wet cycles are unclear. This leads to differences between various studies, which is not conducive to integration and systematization for this area of research. Over the past few years, some scholars have begun preliminary research into and exploration of the dry-wet cycle system. For example, [PERSON] et al. divided the dry-wet cycle into two processes: drying and wetting. They studied the transfer of moisture in concrete and found that the drying time determines the depth of concrete deterioration [16] and the dry-wet cycle period significantly affects the depth of the wetting and drying process inside the concrete. [PERSON] et al. [8] and [PERSON] et al.