## 3 Results and Discussion
### Chlorides and Sulfate Oxidation Profiles and pH
Citrate, oxalate and silicate promote pH values above 13 [15] in fly ash-based hybrid binders, while sulfate activation in slag-based ones give values between 12.6 and 13 [23]. Carbonate activation of fly ash hybrid binders reports pH values up to 13.4 [15]. The pH of the outermost surfaces of the samples is clearly lower than in their bulk, due to the influence of a very thin carbonation layer that appears due to the reaction with the environmental CO\({}_{2}\)[28]. Figure 10: pH values for hybrid mortars as determined using a pH electrode: (**a**) after the curing process and (**b**) after the curing and drying processes at 40% RH. Striped bars plot the pH values of the mortars measured at the surface, the full color bars plot the pH at the mortar center, and the checkered bars plot the pH values measured at 3–4 mm from the surface. The drying process (Figure 10b) hardly affects the pHs of the surfaces, as carbonation cannot progress due to the fast water elimination in this region. The pH values of the non-oxidized bulk region in the hybrid mortars also remain similar to that of the bulk before the drying step (Figure 10a). Moreover, the oxidized regions (whose thicknesses were negligible just after curing) are thick enough after drying to allow samples to be obtained for determining the pH value. In Figure 10b, it can be seen that the pH values are somewhat lower in the oxidated region than in the non-oxidated one, as reaction 1 suggested. However, if the pH of OPC mortar is measured at a certain distance from the surface, similar to that of the oxidized region in dried hybrid mortars, it can be observed that slightly lower pH values than those in the center of the sample can be measured. Hence, the CO\({}_{2}\) from the air is able to cause a pH decrease of a similar order of magnitude at that distance from the surface during the test carried out. The most porous HN-8% (Figure 4a) seems to undergo the most marked pH decrease (Figure 10). In summary, the quite similar pH values obtained for OPC and hybrid mortars (Figure 9), together with the similar chloride diffusion resistances found (or even somewhat better if HN mortars are considered) (Figure 8), suggest that the hybrid mortars can be a good option for carbon steel-reinforced structures if a negative interference of sulfur oxidation reaction is definitively discarded in further research, as longer exposures to dry environments would allow the progress of the oxidation front. Obviously, the potential negative effect that the limited alkaline of the hybrid mortars (Figure 6) could have on the development of the corrosive attack of the carbon steel bars in structures exposed in certain conditions is another point that merits further research. On the other hand, the studied hybrid formulations cannot be adequate for applications that can suffer intense wear. Moreover, the exposure to continuous or cyclic mechanical stresses [73] would allow the progress of the cracks. On the other hand, there are other factors that deserve further research: exposure to dry-wet cycles could cause the progress of the microcracks due to the cyclic drying stresses in the pores [74], or detailed chemical analysis about how aging can affect the phases comprised in the binders.