## 2 Materials and Methods  
### Materials  
An iron tailing from Minas Gerais, Brazil, classified as a silty sand according to the Unified Classification System [24], was used in this work. The grain size distribution curve is shown in Figure 1, while the main physical parameters are presented in Table 1. The particles unit weight (\(\gamma_{S}\)) is quite high (29.5 kN/m\({}^{3}\)) when compared to natural soils due to the significant iron content. The precursor for the alkali activation reactions was a fly ash (FA) classified as type F, according to ASTM standard C 618 [24], due to its low calcium content. Its grain size distribution curve is also shown in Figure 1 for comparison purposes. The alkaline activator is often made via the combination of sodium hydroxide (SH) and sodium silicate. However, in this work, only SH was used to reduce the carbon footprint. An SH solution of 32% NaOH and 68% water was used, corresponding to a concentration of 11.76 mol/kg, which was diluted for lower concentrations. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \(\gamma_{s}\) (kN/m\({}^{3}\)) & \(\mathbf{D_{50}}\) (mm) & Cu & Cc & \(\mathbf{w_{L}}\) & \(\mathbf{w_{P}}\) \\ \hline
29.51 & 0.086 & 2.70 & 1.01 & NP & NP \\ \hline \hline \end{tabular} NP = non plastic.  
\end{table} 
Table 1: Iron tailings’ parameters. Scanning Electron Microscope (SEM) analyses and X-Ray Diffraction (XRD) analyses were performed on both the iron tailings and the fly ash for microstructural characterization of the starting materials. SEM analyses were performed on a Quanta 400 FEG ESEM (15 kV), made by the FEI company. In this case, the analyses were performed with backscattered electrons detected (BSED) in \(Z\) cont mode. This mode is designed to capture backscattered electrons that are reflected on the sample surface. Information about the different components of the sample is obtained according to the intensity of the colors that appear in the images, and the intensity is directly related to the atomic number (Z). When the incident electron beam interacts with the sample, high-Z elements scatter more electrons back towards the detector than low-Z elements. This results in a contrast in the BSE image, with high-Z regions appearing brighter and low-Z regions appearing darker. This contrast allows for the identification of different phases, inclusions, or compositional variations within the sample. This equipment is equipped with an Energy Dispersive Spectroscopy (EDS) analysis system from the company EDAX with a \"Genesis X4M\" spectrometer, allowing us to obtain the chemical identification of the components present in the sample. XRD analyses were performed, at room temperature, by a PANalytical X'Pert Pro diffractometer, equipped with X'Celerator detector and secondary monochromator in 0/20 Bragg-Brentano geometry. The measurements were carried out using 40 kV and 30 mA, a CuK\(\alpha\) radiation (\(\lambda\alpha\)1 = 1.54060 A and \(\lambda\alpha\)2 = 1.54443 A), 0.017\({}^{\circ}\)/step, 100 s/step, in a 6-100\({}^{\circ}\) 20 angular range. The diffractograms were interpreted, and the obtained phases were identified and quantified using the software HighScore Plus 4.8. Before performing the analysis, the samples were grinded and then assembled in a standard sample holder for powders. Figure 2 shows the iron tailings in their real size (a), the same image with a 100-times magnification from a regular camera (b), and the same image with 4000-times magnification obtained by SEM (c). In the imaged zoomed 4000 times, it is possible to observe the angularity of tailings particles, where the brighter particles are iron oxides. EDS analysis indicated that the iron tailings are mainly composed of iron (Fe) and silicon (Si), with many of its particles coated with aluminum (Al) and iron (Fe) particles, as can be seen in Figures 3 and 4. Figure 1: Iron tailing and fly ash grain size distribution curves. Figure 4: EDS results on the points indicated in Figure 3: (**a**) Z1, (**b**) Z2, and (**c**) Z3.