# Steel Corrosion Behavior in Light Weight Fly-Ash Based Alkali Activated Mortars
## 3 Results and Discussion
### Corrosion Behavior of Steel in Solutions  
To deeply investigate the parameters related to binder chemistry, Figure 8 collects the anodic polarization curves recorded in the leachate pore solution (LPS, with a pH of 12.8) obtained from the alkali activated binder tested in this study. It clearly shows that in both cases steel exhibited a passive behavior at E\({}_{\text{CCP}}\) close to \(-0.3\) V vs. SCE for leachate solution and \(-0.5\) V vs. SCE when chloride ions (using 0.2 M NaCl) were added. In leachate solution, a passivity range of 0.9 V was observed between \(-0.3\) V vs. SCE and \(+0.6\) V vs. SCE. Similar results were obtained in different fly-ash based leachate solutions [21], indicating that alkali activation of fly ash allowed comparable steel corrosion protection, even if its source, chemistry and composition are different. At the end of the polarization, the visual inspection did not reveal any corroded surface. On the contrary, when chloride ions were added in the leachate pore solution, a smaller and less stable passive region of 0.5-0.6 V was observed between \(-0.5\) V vs. SCE and \(+0.07\) V vs. SCE. Consequently, higher corrosion current density was also measured: i\({}_{\text{cor}}\) values of 8.8\(\cdot\)10\({}^{-6}\) A/cm\({}^{2}\) in contact with chlorides and 5.0\(\cdot\)10\({}^{-7}\) A/cm\({}^{2}\) in leachate pore solution. pH measurements of the investigated mortars were carried out as a function of time. After 28 days of curing, they exhibited a pH of 12.8. This high alkalinity is maintained if samples are stored in sealed conditions, while samples exposed to laboratory showed a decrease in pH values due to carbonation: pH of 11.2 and 10.9 was measured after 56 days and 365 days, respectively. In order to investigate the effect of decreasing pH of the pore solution in steel corrosion behavior, anodic polarization curves were recorded in NaOH aqueous solution with pH comparable to the ones measured in the real mortars. For this reason, NaOH concentrations of 0.1 M and 0.001 M were selected to reproduce the alkalinity of the pore solution at the end of the mortar curing period (pH \(\sim\) 13) and after one-year exposure at atmosphere (pH \(\sim\) 11). Results, reported in Figure 9a, showed similar shapes of the polarization curves recorded in 0.1 M NaOH solution compared to the ones in LPS. This is mainly because both the electrolytes had the same pH. However, different i\({}_{\text{cor}}\) values were obtained: higher current densities of 2.6\(\cdot\)10\({}^{-6}\) A/cm\({}^{2}\) (without chloride ions) and 2.3\(\cdot\)10\({}^{-5}\) A/cm\({}^{2}\) (with chloride ions) were measured in 0.1 M NaOH solution. This result indicates that the presence of other elements in the leachate pore solution (e.g., silicates) could act as corrosion inhibitor and enhanced the steel corrosion resistance [21]. Lastly, lowering the pH down to 10.9 (by 0.001 M NaOH solution) induced a strong change in both the polarization curves (Figure 9b): the passive region disappeared meaning that this environment is not able to passivate the steel. However, similar i\({}_{\text{cor}}\) values were recorded for this NaOH concentration of about 3.0\(\cdot\)10\({}^{-6}\) A/cm\({}^{2}\) (without chloride ions) and 3.2\(\cdot\)10\({}^{-5}\) A/cm\({}^{2}\) (with chloride ions). These results highlight the necessity to have a highly alkaline environment close to steel-mortar interface to maintain steel in the passive state. Figure 8: Anodic polarization curves of sand blasted steel (S) in leachate pore solution without (in black) and with 0.2 M NaCl (in grey). To avoid lowering of the pore solution pH, a possible solution could be the application of a post-treatment (e.g., sealing) on the LWM layer, that could mitigate the carbonation of the alkali activated binder exposed to atmosphere, and slow down the ingress of aggressive species, as chloride ions. A particular attention should be paid on the choice of the kind of treatment since it should exhibit passive fire protection properties as well as the 20-mm thick LWM layer.