## 4 Results of the Calculations
\begin{array}{c}\textbf{C}_{\text{b}}\left(C_{\text{f}(2)}\right)\\ \textbf{(mol/m^{3})}\end{array} \) & \( \begin{array}{c}\textbf{C}_{\text{b}}\left(C_{\text{f}(2)}\right)\\ \textbf{(mol/m^{3})}\end{array} \) & \( \begin{array}{c}\textbf{C}_{\text{b}}\left(C_{\text{f}(3)}\right)\\ \textbf{(mol^{2}/m^{6})}\end{array} \) & \( \begin{array}{c}\textbf{C}_{\text{b}}\left(C_{\text{f}(3)}\right)\\ \textbf{(\%)}\end{array} \) \\ \hline C4 & \( \begin{array}{c}\textbf{9.313}\times\\ \textbf{3}\textbf{(mol/m^{2})}\end{array} \) & \( \begin{array}{c}\textbf{C}_{\text{f}(1)}\\ \textbf{(mol/m^{3})}\end{array} \) & \( \begin{array}{c}\textbf{C}_{\text{b}}\left(C_{\text{f}(2)}\right)\\ \textbf{(mol/m^{3})}\end{array} \) & \( \begin{array}{c}\textbf{C}_{\text{b}}\left(C_{\text{f}(2)}\right)\\ \textbf{(mol/m^{3})}\end{array} \) & \( \begin{array}{c}\textbf{C}_{\text{b}}\left(C_{\text{f}(2)}\right)\\ \textbf{(mol/m^{3})}\end{array} \) & \( \begin{array}{c}\textbf{C}_{\text{b}}\left(C_{\text{f}(2)}\right)\\ \textbf{(mol/m^{2})}\end{array} \) & \(\begin{array}{c}\textbf{C}_{\text{b}}\left(C_{\text{f}(3)}\right)\\ \textbf{(Figure 4 shows the determined courses of chloride binding isotherms in the case of data from Table 5 for the linear approximation (Figure 4a) and data from Table 7 for the spline approximation (Figure 4b). For both descriptions of the problem, it can be clearly stated that concretes C3 and C5, in which the blast furnace and pozzolanic cements were used, have an order greater capacity to bind chlorides than concretes C2 and C4 with the ash and low-alkali normal early strength Portland cements, respectively. It can also be concluded that for concretes C3 and C5, the concentration of bound chlorides in the concrete skeleton is comparable to the concentration of free chlorides in the pore liquid. Moreover, the description of isotherms by the spline function (Figure 4b) gives much more information on the \"skin effect\" in accordance with the assumptions made in Section 3. In all C2-C5 concretes for the last section of isotherms, i.e., for free chloride concentrations comparable to the maximum concentration, we can observe increased chloride binding compared to the first two sections of the isotherms. Due to the fact that it concerns the experimental data after 6 months of the diffusion process under the constant boundary conditions of the first kind, its most intense part is at the same time located at the outer surface of the samples. It also indicates that there is much more hardened cement paste in this subsurface layer compared to the bulk parts of samples that it has a noticeable effect in binding chlorides. It is related to the apparent diffusion coefficient decrease in the  
Figure 4: Chloride binding isotherms for concretes C2-C5 based on the results presented in: (**a**) Table 5 for the linear approximation, (**b**) Table 7 for the approximation with spline function. subsurface layer compared to the effective one by about an order for concretes C2-C4 and by as much as two orders for C5 concrete (Table 7). Figure 5 illustrates the changes in the apparent chloride diffusion coefficients depending on the depth of the samples after 180 days from the beginning of the chloride diffusion process, which enables a more accurate estimation of the extent of the \"skin effect\". At this moment, we can distinguish three zones with different advancement of the chloride binding process:  
1. The first zone is located to a depth of approx. 1-2 mm from the outer surface of the samples. Apparent chloride diffusivity is significantly reduced compared to effective diffusivity. This is the result of a significantly increased proportion of the cement matrix in relation to the aggregate compared to the bulk part of concrete, and thus increased chloride binding. 2. The second zone is located at a depth of about from 1-2 mm to 3-5 mm from the outer surface of the samples.