2.1. 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 (
γS) 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.
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 θ/2θ Bragg–Brentano geometry. The measurements were carried out using 40 kV and 30 mA, a CuKα radiation (λα1 = 1.54060 Å and λα2 = 1.54443 Å), 0.017°/step, 100 s/step, in a 6–100° 2θ 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
Figure 3 and
Figure 4.
The XRD analysis identified the main minerals present in the iron tailings: proto-hematite (7.7%), low quartz (91.4%), and berlinite (0.9%), as illustrated in
Figure 5.
Figure 6 shows the fly ash in the real size (a), its image with 80-times magnification from a regular camera (b), and its image with 8,000,000-times magnification obtained by SEM (c).
The SEM micrograph illustrates the typical spherical shape of the fly ash particles (
Figure 6c), resulting from their production method, where they come to a solid state while in suspension. However,
Figure 6c also shows some angular particles, which, based on EDS analyses, could be coal. In fact, the EDS analysis indicated not only the possible presence of coal but also calcium carbonates among fly ash components.
The main minerals present in the fly ash obtained in the XRD analysis were quartz (54.7%) and mullite (45.3%), as can be seen in
Figure 7.
To evaluate the compaction properties of the iron tailings, a Standard Proctor test [
25] was performed, whose compaction curve is presented in
Figure 8. The obtained optimum water content was 14%, and the maximum dry unit weight (
γd) was 18.1 kN/m
3. In the same graph, the curves for different saturation degrees are included, indicating that the wet branch of the compaction curve has approximately a degree of saturation of 70%.
2.2. Description of the Statistical Model
A statistical methodology based on the response surface method (RSM) was used to obtain a suitable mathematical model for predicting the maximum peak strength of the material resulting from different mixtures. The Design of Experiments (DOE) underneath the RSM [
20] minimizes the number of required experiments while providing an efficient tool for data analysis and interpretation, therefore allowing efficiency and economy in the experimental process and scientific objectivity in the resulting conclusions.
Three input variables were considered: ash content (
tFA), liquid content (
tL), and activator concentration (
cSH). The quantity
tFA is given by the mass ratio of fly ash with the iron tailings;
tL is the liquids-to-solids ratio in the mixture given by the mass of sodium hydroxide and water divided by the mass of soil and fly ash, and
cSH is the molal concentration of sodium hydroxide solution. The range of variation for each variable in the DOE was defined based on preliminary tests. These intervals are presented in
Table 2.
Based on these input variables and their range, a set of mixtures to be tested is proposed which constitutes the plan of experiments undertaken and reported in the present paper. The set of mixtures, in a total of 21 runs, is presented in
Table 3 and depicted in
Figure 9. The designed plan is a factorial plan composed of 8 factorial runs plus 4 central runs (
Figure 9a) augmented with 6 axial runs (
Figure 9b) and 3 middle plane runs (
Figure 9c); see [
20].
The output variable considered in this work was the maximum peak strength (qu) obtained in the unconfined compressive strength (UCS) test.
2.3. Specimen Preparation and Testing Procedure
Mine tailings that come from the ore-processing plant are in a slurry, which passes through a filter to reduce the water content to approximately 17 to 23%. For this reason, this work intended to simulate the wet tailings as they come from the filter plant.
Therefore, 15 mL of water was added to the oven-dried tailings for each mixture combination. With the water incorporated into the tailings, fly ash was added and mixed until full homogenization. This process is not only more realistic but has also the advantage of releasing less dust during mixing. Then, sodium hydroxide solution (activator) was added with further homogenization before transferring the final mixture to a sealed plastic container to minimize liquid loss. This mixture preparation methodology is shown in
Figure 10.
Due to the initial added water, the concentration of sodium hydroxide in the solution was diluted, with the actual concentration (
cA) being calculated using Equation (1):
where
mSHs is the mass of solid sodium hydroxide in the activator,
mw is the initial mass of water added, and
mwSH is the mass of water present in the activator.
The specimens were prepared in molds of 36 mm in diameter and 66 mm in height by static compaction in three layers targeting a dry unit weight,
γd, equal to 17.2 kN/m
3, which corresponds to 95% compaction degree compared to the optimum unit weight presented in
Figure 8. As noted by [
26,
27], dry stackings are usually constructed with different compaction levels in order to optimize the execution and follow the production rate, so in most cases, a 100% Proctor density is not achieved.
Demolding was carried out on the same day, using an extractor, and the specimens were subsequently stored in an insulated box for 7 days, before being tested in unconfined compression.
To evaluate if this preparation procedure was providing repeatable specimens, the porosity (
η) of the specimens on the molding day was calculated using Equation (2):
where
γdm is the dry unit weight of the mixture, given by Equation (3);
wS,
wFA, and
wSHs are the dry masses of soil, fly ash, and solid sodium hydroxide in the mixture, respectively;
wT is the total mass of the specimen on the molding day;
γsS,
γsFA, and
γsSHs are the particles unit weight of soil, fly ash, and solid sodium hydroxide; V is the volume of the specimen; and
tL is the liquid content.
The unconfined compression strength test (UCS) performed was based on ASTM D2166-06 [
28] using a 20 kN load cell with a ball joint to ensure vertical load application, and a linear variable differential transformer (LVDT) for vertical strain measurement.
From the UCS test, the stress–strain curve of the specimens was obtained, as well as the values for the maximum peak strength (
qu), given by Equation (4):
where
Fmáx is the peak load registered, and
Acorr is the corrected area due to the radial deformation of the specimen during the compression. This area is calculated by using Equation (5), based on [
29]:
where
D is the specimen diameter, and
εa is the axial deformation in the moment of the maximum strength, given by the ratio of the height variation by the initial high of the specimen (Δ
H/H0).