*3.6. X-ray Diffraction*

Figure 9 shows the X-ray diffraction results of the corrosion product characterization once the electrochemical studies have been completed. The procedure consisted of longitudinally cutting the specimens at immersion times of 0, 1.5, 3, 6, and 9 years to study the

concrete–steel interface. The spectra in Figure 7 show crystalline quartz (SiO2), C-S-H formation (CS), calcium carbonate (C), and beta larnite corresponding to Ca2SiO4. The corrosion products identified on the steel surface correspond to compounds such as brownmillerite related to CaAlFeO, goethite, magnetite, fayalite, iron oxyhydroxides, and hematite. These compounds are related to iron oxides and hydroxides in all the samples analyzed.

**Figure 9.** The X-ray diffraction patterns of the concrete–steel interface.

Additionally, the ray diffraction results for the alkali-activated slag concrete samples experiencing at least 1.5 years or more of saline solution immersion time showed overlapping characteristic diffraction peaks. The same intensity values were observed for the calcium silicate-hydrate phase at 28.52◦ and calcium carbonate at 29.46◦. It is also evident that the samples evaluated after up to three years of immersion in the saline solution showed a higher intensity peak due to the stability of the layer of cementitious material present in the steel–concrete interface. This layer was not removed by the pH present in the concrete. When the concrete samples are subjected to salt immersion, the influx of chloride ions accelerates the removal of the oxide layer. The removal degree of the oxides is directly related to the exposure time of the samples to the solution [41]. However, the passivating layer was determined with EIS measurements (Figure 4). The impedance values indirectly indicate the formation of a protective passivating layer over the steel.

The study determined that the corrosion products found on the steel surface are the phases corresponding to iron oxides, especially the ones corresponding to the fayalite Fe2(SiO4) at 39.17◦ formed in the electrochemical reactions. Magnetite compounds (FeO·Fe2O3, Fe2+Fe3+2O4) with peaks located at 35.19◦; 56.90◦; 62.20◦; 68.37◦; 79.50◦, and 82.58◦, and hematite (α-Fe2O3), with peaks at 51.40◦, 54.91◦, and 71.46◦, were also noted. These products arise from the reaction process, as well as during the manufacture of steel; they are known as the "steel shell", but they offer low anticorrosive protection. The protective layer is mainly formed by iron oxyhydroxides (48.51◦), transforming into a mixture of hydroxides and goethite (42.97◦). Although this iron oxide has excellent chemical stability, it has an irregular morphology and protects the steel against electrochemical attacks. In addition, compounds such as magnetite and hematite increase the thickness of the oxide layer, forming a mechanical protective barrier (small scale) [42,43]. Finally, the thick passive layer formed with low protection compounds (iron oxyhydroxides and goethite), the electrochemical stability of the steel, and the low porosity of the concrete generate more excellent anticorrosive protection of the steels embedded in concretes based on activated slag.
