hydration heat in the early stages, improve corrosion resistance [9; 10; 12; 13; 14], and improve concrete's durability performance [15; 16; 17; 18; 19].  
It is generally believed that chloride ion penetration in concrete is one of the most critical causes of steel reinforcement corrosion that may further cause structural failure [20; 21; 22]. Chloride ions are present in many environments, including marine, road de-icing salt, salinized soil, and industrial wastewater environments. Chloride ions penetrate through pores into concrete and accumulate around steel reinforcements, which destroys the passive film on the steel surface and activates steel reinforcement corrosion [23; 24; 25]. For existing reinforced concrete structures, the penetration process of chloride ions is an important parameter in durability life assessment [26]. [PERSON] et al. [24] examined the microstructures of fly ash concrete and normal concrete in a chloride atmosphere environment; the fly ash content was discovered to be sensitive to the chloride binding capacity and diffusion coefficient. [PERSON] et al. [27] found that chloride transportation was affected by drying-wetting cycles and compressive stress ratios--the former significantly increased the chloride ion content, and the latter decreased the chloride ion content. [PERSON] and [PERSON] [28] pointed out that a pore size of \(<\)100 nm has little or no effect on concrete strength and permeability, while a pore size of \(>\)100 nm negatively affects concrete strength and permeability. The permeability of fly ash concrete seems physically different from that of normal concrete. Unlike that in salt-spray environments, concrete in seawater environments has different chloride ion concentrations, in which migration speed of chloride ion varies considerably. The chloride ion distributions in concrete are quite different, which is related to the way that chloride ions penetrate concrete. In fact, chloride ion penetration is a complex process involving extensive transportation mechanisms, such as diffusion under a concentration difference [27], penetration under water pressure [29], and capillary action under a humidity gradient [30]. In a soaking environment, when the concrete is saturated, chloride ions are mainly transferred through diffusion to the concrete. Meanwhile, in a salt-spray environment, the migration of chloride ions is achieved mainly by capillary suction and the diffusion effect. Therefore, the chloride profile of concrete can be quite different in different environments, which may cause different corrosion mechanisms of reinforcement in concrete. Thus, the knowledge of such durability behavior of fly ash concrete remains unclear, even though it is essential for promoting durable material design in construction under water. Due to filling and pozzolanic effects [31; 32; 33], adopting fly ash to partly replace cement can increase chloride ion resistance and lead to the better durability of concrete [34; 35; 36]. Moreover, there are very few references on the distribution of chloride ions in concrete soaked with seawater, thus indicating the demand for a comprehensive study that covers not only the micro-properties and transportation mechanism but also the chloride binding capacity of fly ash concrete.  
The current investigation aimed to systematically examine the effect of fly ash on chloride ion diffusion in concrete in a water soaking environment. Specifically, the effect was quantified by the chloride ion diffusion coefficient and chloride ion deposit amounts at different layers. The study also compared the amount of free and binding chloride ions, as well as binding resistances, under different dosages of fly ash replacement. Through a mercury intrusion method and SEM, the study explored the effects of fly ash on the pore structure and cement hydration products.