## 3 Results and Discussion
### Chlorides and Sulfate Oxidation Profiles and pH
Previous literature [69] proposes that the oxidation of sulfides in concrete takes place as reaction 1 describes:  
Footnote 2: The inclusion of the oxidation layer in hybrid mortars after drying and images of the cross sections of the samples of the different hybrid mortars. \[\text{Fe}_{1-\text{x}}\text{S}\left(\text{s}\right)+\left(2-\text{x}/2\right) \text{O}_{2}+2\text{x}\,\text{OH}^{-}\rightarrow\left(1-\text{x}\right)\text {Fe}^{2+}+\text{SO}_{4}\text{${}^{2-}$}+\text{x}\,\text{H}_{2}\text{O} \tag{1}\]  
This reaction changes the color of the mortar, as can be seen in Table 4. The interphase observed in mortars with slag between the outer gray region, where the sulfurs have been transformed into sulfates, and the inner green region, where the sulfurs remain unreacted, is called the \"oxidation front\". The measured values for the progress of the oxidation front after drying, as well as the aspect of the cross sections of the samples, are also shown in Table 4. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **HC-3\%** & **HC-6\%** & **HN-4\%** & **HN-8\%** \\ \hline Cross-sectional view of the mortar samples & & & \\ \hline Oxidation depth (mm) & 9.5 \(\pm\) 0.6 & 9.6 \(\pm\) 0.5 & 8.6 \(\pm\) 0.5 & 9.5 \(\pm\) 0.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Thickness of the oxidation layer in hybrid mortars after drying and images of the cross sections of the samples of the different hybrid mortars. The oxidation rate depends strongly on the alkalinity of the pore solutions, which can be considered quite similar for the mortars under study (as will be shown hereafter), and on molecular oxygen availability [70]. This last factor is clearly related to the porosity (Figure 4) and eventual presence of defects (Figure 5) in the mortars. The drying conditions implemented in this study after curing have allowed the oxidation front to quickly progress (Table 4). The data in Table 4 show that the microstructure of HN-4% limits the progress of the oxidation front to a somewhat greater extent than the other hybrid mortars under study. The slowest progress of the oxidation front in HN-4% (Table 4) can be related to the crack absence (Figure 5d) and refined porosity (Figure 4b) in this mortar that can limit the O\({}_{2}\) access to the bulk material during the exposure at 40% RH. The other low porous hybrid mortars studied (HC) have a microcracked structure (Figure 5b,c), and microcracks can act as preferential diffusion path favoring the progress of the oxidation front in the mortars. Reaction 1 implies that the pH could decrease due to the hydroxyl ions consumption that this reaction entails, as this is a key point for durability if reinforced structures are considered [71; 72]. So, the slightly slower progress of the oxidation front observed for HN-4% could perhaps be another positive point for this formulation. The alkalinity of the mortars is a key factor for determining the passivity of the embedded steel. Low pH values are related to worse corrosion performance, leading pHs typical of carbonated concretes to general attack [71]. All the pHs measured in the different regions of the mortars under study are high enough to guarantee the passivity of the steel in the absence of aggressive ions in the pore solution. Moreover, in the presence of corrosive ions such as chlorides, the probability of corrosion of the steel for a given chloride concentration is related to the ratio [Cl\({}^{-}\)]/[OH\({}^{-}\)] [72]. Figure 10 shows the pH values of the HN and HC after the curing and drying processes and compares them with those of OPC in similar conditions. Figure 10a shows that the pH values of the bulk of the mortar samples after curing are clearly higher than 12.6, which is the pH of what the pore solution in equilibrium with the portlandite should have, so other more soluble hydroxides are present [28]. Those pH values are clearly in the range measured in as-cured hybrid binders by other authors, using different activators.