## 3 Results and Discussion
### Influence of Free and Binding Chlorides  
Binding chloride is the solidifying process of chloride ions by cement-based materials. The binding chloride in the cement composite could not transport in water in concrete. Binding chloride can be divided into two types: chemical bound and physical absorption. Chemical bound results from the reaction of chloride and cement, while physical absorption depends on the electrostatic interactions (or van der Waals forces) [46; 47] between ions. Equation (3) shows that chemical bound chloride ions follow the formation of [PERSON]'s salt [48]:  
\[\mathrm{C_{3}A+CaCl_{2}+10H_{2}O\to C_{3}A\cdot CaCl_{2}\cdot 10H_{2}O}, \tag{3}\]  
\(\mathrm{C_{3}A\cdot CaCl_{2}\cdot 10H_{2}O}\) reduces the porosity of concrete, thus increasing the resistance of chloride penetration and reducing the chloride ion binding capacity. According to Table 2, the contents of Al\({}_{2}\)O\({}_{3}\) in the fly ash and cement were 23.95% and 4.62%, respectively. Since Al\({}_{2}\)O\({}_{3}\) is a raw material for salt formation, adding fly ash into the concrete increased Friedel's salt formation, which further improved the chloride binding capacity.  
Figure 5 shows the distribution of total chloride, free chloride, and bound chloride of the concrete samples. The bound chloride amount was much lower than the total and free chloride amounts. Figure 6 shows the relationship between bound chloride and concrete depth, in which the bound chloride amount decreased as concrete depth increased, but then the chloride amount went slightly up. PCFA15 and PCFA30 had much higher bound chloride amounts compared to that without any fly ash addition (PCFA0). The bound chloride content decreased significantly within the 0.0-12.5 mm depth, while it slightly increased to a constant value beyond 12.5 mm. In this case, free chloride could be reduced in the pores of concrete, which deceased the potential risk of rebar corrosion. The free chloride content kept a linear relationship with the total chloride content [49], as shown in Figure 7. Equation (4) introduces the index R to represent the chloride binding capacity:  
\[R=\frac{\partial C_{b}}{\partial C_{f}}, \tag{4}\]  
where \(C_{b}\) is the concentration of the bound chloride and \(C_{f}\) is the concentration of the free chloride.  
Equation (5) gives a linear formulation to represent the linear relationship between the free and total chloride contents:  
\[C_{t}=KC_{f}+b, \tag{5}\]  
where \(C_{t}\) is the concentration of total chloride and \(K\) and \(b\) are constants.  
Substituting Equation (1) into Equation (2) results in Equation (6):  
\[C_{b}=(K-1)C_{f}+b, \tag{6}\]  
Equation (7) introduces chloride binding capacity index \(R\) shown below:  
\[R=K-1, \tag{7}\]  
Based on the calculations of Equation (7), Figure 8 shows the relationship between the chloride bound capacity amount and concrete depth. The chloride binding capacity increased as concrete depth rose. Based on Equation (4), the bound capacity indexes of chloride were 0.1426, 0.1882, and 0.2134,respectively, for PCFA0, PCFA15, and PCFA30. This calculation indicates that the chloride binding capacity improves as fly ash addition increases.  
Figure 5: Distribution of total chloride, free chloride, and bound chloride in concrete samples (by mass%).  
Figure 8: Relation between chloride bound capacity amount and concrete depth.  
Figure 6: Relationship between bound chloride amount and concrete depth (by mass%).  
Figure 7: Relation between free chloride and total chloride depth (by mass%).