3.1. Mechanical Properties
The basic mechanical properties of CG are very important to the design of embankments. Therefore, compressive modulus
E, cohesion
c, and fraction
φ, were measured by compressive testing and unconsolidated undrained triaxial test under unsoaked and soaked conditions. In the experiments, a total of 24 samples were used and repeated three times per experiment, with the average value of the value of each mechanical parameter as the final value of each mechanical parameter (show in
Table 2).
Table 2 shows that the basic properties of the CG changed after the CG samples were treated by the acidic, neutral, and alkaline solution. To be specific, compressive modulus increased from 73.1 MPa under raw state to 158.3 MPa, 142.5 MPa, and 146.2 MPa—respectively, under acidity, neutral, and alkalinity conditions—increased by 116.6%, 94.9%, and 100%. In addition, their cohesion
c dropped from 15.1 kPa in an untreated state to 8.0 kPa and 9.7 kPa, respectively, under immersed in acidity and neutral solutions, while increased to 71.3 kPa under soaked in alkaline solution. In terms of samples soaked in acidity, the friction angle φ rose 12.5%, while in neutral solutions, the internal friction angle
φ almost no increase, nevertheless, dropped in alkaline solution. Consequently, the acidic solution contributes to compressive modulus and
φ, while the alkaline solution is beneficial to increasing the compressive modulus and
c.
3.3. Mineral Composition Analysis
The mineral composition of coal gangue affected the filling body’s strength. To identify the change in the mineral composition caused by the solutions with different acidities, an XRD diffractometer was used to measure the mineral diffraction map of the coal gangue. The mineral components of coal gangue before and after immersion are presented in
Figure 2.
According to XRD testing, the calculation results of substances in the coal gangue samples by Jade Software are shown in
Table 4.
Comparative analysis with the standard powder diffraction data for various substances provided by the Powder Diffraction Federation International Data Center (JCPDS—ICDD), the hydro-chemical environment has significantly changed the mineral composition of the coal gangue (in
Figure 2). The main characteristic peaks are located at 20°, 21°, 27°, 37°, and 39°. After soaking in acidic, neutral, and alkaline solutions, the diffraction peaks and peak shapes of CG samples changed slightly. The main mineral composition for every sample found by the k-value method of jade software are quartz (SiO
2), kaolinite (Al
2O
3-2SiO
2-2H
2O), plagioclase (NaAlSi
3O
8-CaAl
2Si
2O
8), illite, and mica. By comparing with the control standard sample-CG, raw-CG contain more quartz and less kaolinite. For raw-CG samples, the content of quartz is 45.4%. In terms of CG samples treated by acidic, neutral, and alkali solution, SiO
2 of the sample increased by 11.7%, 17.6%, and 15.9%, respectively, mica, kaolinite, and plagioclase was all reduced, when compared with raw-CG. Some diffused peaks appear in the alkali-immersed sample at 2θ = 24°, 28°, and 33°. It could be an amorphous product silica-alumina gel according to the chemical composition of the sample [
43].
In order to verify the analysis results of XRD, the XRF test method to detect the oxide composition for comparative analysis, the results of the analysis can be found in
Table 5.
From
Table 5, it can be seen that after immersion of acid–base solution, the content of SiO
2 in the coal gangue has increased, while the relative reduction of oxides such as Al
2O
3, Fe
2O
3, CaO, etc., which proved the consistency of the results of the decomposition of mica, kaolinite, and plagioclase analyzed in XRD.
3.4. Morphology Analysis
The morphology of CG samples with untreated and treated by various acidity solutions was investigated using XRF and SEM. Results are displayed in
Table 6 and
Figure 3.
As can be seen from
Table 6, in raw-CG, pH = 4.5-CG, pH = 7.2-CG, and pH = 8.5-CG, Si/Al ratio are 1.36, 1.4, 1.2, and 1.39, respectively. The ratio of silicon aluminum has increased, and other soluble elements are relatively reduced, which is conducive to the stability of coal gangue.
It is shown in
Figure 3, the surface of sample particles is rough and irregular. After acidic solution treatment (in
Figure 3b), kaolinite and plagioclase dissociated into small unevenly distributed crystals with particle size from 20 µm to 40 µm and mainly small particles. The distribution of minerals in pH = 7.2-CG samples is relatively uneven, compared to that of pH = 4.5-CG samples, there is no significant dispersion between grains. Mineral particles in pH = 8.5-CG sample are evenly distributed, and among mineral particles were full of cotton-like amorphous gelatinous hydration substances, which produced during the soaking process so that surface morphology looks relatively dense which is consistent with the measurement result of
c.
3.5. Elements Analysis
The elemental composition in labelled regions (in
Figure 3) were measured by EDS. The main elements content presented in
Figure 4.
As shown in
Figure 4, the peak strength of each element varies widely from sample to sample, and the content of each element are listed in
Table 7.
According to
Table 7, the proportion of C, Si, Al, O, in CG samples are higher than that of Fe, Ca, Mg, K, Na, and other elements. In raw-CG, pH = 4.5-CG, pH = 7.2-CG, and pH = 8.5-CG, Si/Al ratio is 1.6, 3.3, 2.2, and 3.5, respectively, proving that after the solution is soaked, the Al element is easier to dissolved in the solution, and the final generator is Si element as the main substance. The content of Si increased in the coal gangue samples and the other elements decreased after acid solution treatment. It could be concluded that a large amount of dispersed grains in SEM image are associated with an increasing in the Si content detected by EDS. The lower the content of alumina and decomposing substances in coal gangue, the higher its strength [
13]. Therefore, the acid solution treatment is conducive to the formation of Si and reduction of Al, and after acid immersion, the large particulate matter in the coal zircon is dispersed into the small angular particles of each cluster, which increases the surface area of the particles, also enhancing the compressive modulus and
φ of coal gangue. For alkali-soaked samples, EDS applied a certain depth to the gel material adhering to the coal gangue particles in
Figure 3d in order to verify the diffuse peak material present in the XRD pattern and the gel material composition in the SEM. The surface sweep, combined with the elements in
Table 7, was finally determined to be an aluminosilicate gel, which can adhere to the surface of coal gangue particles or fill inter-granular pores to obtain a more compact and stable structure, leading to a significant improvement of mechanical properties.
3.6. Effects of Solution Acidicity on the Mechnical Propeties of Coal Gangue
At the microscopic level, the changes caused by the interactions between the different acidity solutions and coal gangue result in changes in the mineral composition and mechanical properties of the samples. The reasons for these changes were determined from analyses based on the variety of the pH, ions dissolution from the coal gangue, and changes in the mineral composition of the samples.
After soaking in acetic acid solution, the CG sample contacts with acid solution to produce chemical reaction, which causes many metal cations to escape. Moreover, the original mineral chemical bonds are broken, resulting in the formation of new material SiO
2 aggregates, forming uneven grains (
Figure 3b). Because the acid substances in CG escaped from the CG samples after soaking in the water solution, which provided the acid water environment for the CG. In an acidic environment, kaolinite and plagioclase on the sample surface react with the H
+ ions in the chemical solution were decomposed by acid, and the ion exchange of Al
3+ and Ca
2+ took place, leading to spaces between several fragments in the CG (
Figure 3c). The reaction chemical Equations (1) and (2) are [
44,
45]
As shown in Equation (1), the reaction of acid with kaolinite and plagioclase formed SiO2, Al3+, and Ca2+ exchanged by H+ ions. In XRD and EDS measurement results, it is consistent that the content of silica increases after treatment with acid solution. However, these of treatment with water and alkaline solution are different. That is, XRD shows an increase, but a decrease in EDS, which may be related to the difference points of EDS measurement. In addition, acidic solution is contributing to compressive modulus and φ.
OH
− ions in alkali solution can break the Al–O–Al and Si–O–Si network on the surface of CG. [Si
4]
4− and [AlO
4]
5− formed, respectively, and then synthesized to a three-dimensional polymeric silica-alumina gel [
46] according to Equations (3) and (4) [
47].
Thus, polymeric silicon aluminum salt gel can be formed in the gap of CG, so that other substances in CG are difficult to escape, so the viscosity of the sample increases (
Figure 3d), and the density is improved. In immersion tests, Si
4+ and Al
3+ ion leaching depends on the surface reaction of OH
− aluminosilicate material, in other words, the OH
- ions in the alkali metal Na
+ electrostatic interaction, electrostatic repulsion, and the Si
4+ and Al
3+ ion release. The surface of the coal gangue particle has more active Al
2O
3 and SiO
2, which completely react with the solution OH
− ions. As a result, the surface of the active substance decreases as the reaction progresses further, and the reaction depth continues to increase [
48].
According to Equations (3) and (4), silicon aluminum ions are connected to each other to form new materials to reduce the content of the determination, which is consistent with the results of XRD and EDS. Meanwhile, polymeric silicon aluminum salt gel, a very sticky substance started to bond coal gangue particles, generated by alkaline treatment. Therefore, resulted in the cohesion c of coal gangue samples treated with alkaline solution increased by 3.72 times.