## 3 Results and Discussion
### Chloride Transport Properties
As a consequence, the effect of drying-wetting cycles and the resultant capillary absorption of chloride penetration became less pronounced as the exposure continuously proceeded. Differences in the \(D_{\text{a}}\) values resulting from varying initial saturation levels decreased in the meantime. Between the binders of 65% slag and OPC at S = 60%, their \(D_{\text{a}}\) values showed a small difference in the first cycles of exposure but a much larger difference after 70 cycles of exposure. This highlights the importance of the saturation level in the chloride penetration process. It should be kept in mind that the significantly positive effect of slag on the resistance to chloride penetration exists primarily when the materials are of high saturation levels. The role of saturation should be carefully taken into consideration in the case of pursuing a sufficient utilization of slag. Figure 10 shows the relationship between bound chloride and free chloride in saturated mortar specimens with different replacement levels of slag after exposure to 70 drying-wetting cycles. The bound chloride content is expressed in mg/g after normalization to the mass of the mortar specimen. As observed, the slag-blended binders clearly had higher amounts of bound chloride than the neat OPC binder, in particular for the case of a high free chloride concentration. The 65% slag binder was found to have had the highest amount of bound chloride. Figure 9: Apparent chloride diffusion coefficient \(D_{\text{a}}\) as a function of drying–wetting cycles for OPC and slag-blended mortars at different saturation levels. The increase in the chloride binding capacity by including slag is attributable in part to the formation of Friedel's salt owing to a higher content of alumina in the raw slag materials. The dilution effects of slag addition on the concentrations of OH\({}^{-}\) and SO\({}_{4}\)\({}^{2-}\) in the pore solution also played an important part in the higher binding of chlorides. Extensive completions between chloride, OH\({}^{-}\) and SO\({}_{4}\)\({}^{2-}\) exist when reacting with alumina phases in cementitious systems. The presence of additional SO\({}_{4}\)\({}^{2-}\) reduces the binding of chlorides in both C-SH and AFm phases in the sense that the C-S-H phases can include more SO\({}_{4}\)\({}^{2-}\) than chlorides [34]. A typical example related to this aspect can be found elsewhere [35]. A higher SO\({}_{4}\)\({}^{2-}\) content, in combination with aluminate-containing phases, leads to an increase in the tendency to transfer AFm to ettrinigite. A preferential reaction of sulfates with alumina-rich phases, such as AFm, was substantiated compared to the formation of chloride containing alumina phases, such as Friedel's salts, as referred to in the work of [PERSON] [36]. The sites for chloride adsorbed on the main hydrates C-S-H are increased as soon as the content of OH\({}^{-}\) is decreased. This has been well documented in previous works [37; 38]. Another important aspect contributing to chloride binding is related to the significant refinement of the pore structure by incorporating slag. As described earlier in this work, the majority of pores are below 20 nm for slag-blended binders, compared to a coarser pore structure of OPC binder, with pore sizes mostly below 60 nm. The pore walls of hydrated cementitious systems are negatively charged, with a compact layer containing alkalis (Ca\({}^{2+}\), Na\({}^{+}\)) and a diffuse layer adsorbed on the pore walls [39]. The presence of the electrical double layer enables the promotion of the chloride binding and, hence, delays the chloride penetration. It should also be mentioned that chloride can be adsorbed in the interlayer of hydtotalcite (Ht), a distinct hydration product from incorporating slag, and the interlayer space is then enlarged because of the larger sizes of the chloride ions compared to the hydroxide ions [40]. As abovementioned, a variety of factors contribute to the chloride binding behavior in the slag-blended binders.