Although the original samples were made with the same dry densities, the procedure of sample preparation and extraction may have changed the void ratio of the samples. This may result in different behavior of samples. To verify the influence of the changed void ratios on the mechanical properties, the void ratios at the end of triaxial tests are calculated. The e-lg p curves of the three groups of samples are shown in Fig. 7. The void ratios of the three tailings are similar at the end of triaxial tests. The differences in void ratio are less than 1% under all confining pressures. Therefore, the effect of the differences between the void ratios of the samples on the mechanical properties can be ignored. Moreover, the void ratios for the CTI, CTII and GT samples are 0.613, 0.621 and 0.616, respectively, after sample preparation, indicating that the freezing method for sample preparation will lead to a decrease in the void ratio.
The main minerals, quartz and illite, account for 90%, and the other mineral compositions with small contents are different. In accordance with mineral composition classification, the mineral composition of tailings can be divided into clay minerals, nonclay minerals, and metal minerals. The mineral contents of the tailings are listed in Table 3. The clay mineral contents of CTII, CTI, and GT increase in this order. This is related to their mechnical properties.
Table 3
Mineral contents of the three types of tailings according to mineral category
Influence of clay content on stress-strain characteristics
The stress-strain characteristics of geomaterials include nonlinearity, elastoplasticity and dilatancy, which are affected mainly by the stress level, stress path, and stress history. Fig. 8 presents the stress-strain relationship curves and failure deformation of the tailings samples. The features shown in the stress-strain curves and deformation pattern include the following:
The initial elastic modulus and peak deviatoric stress both increase, and the axial strain corresponding to the rapid increase in deviatoric stress decreases with increasing confining pressure. The initial elastic modulus and shear strength of the tailings are summarized in Table 4. For CTI samples under a confining pressure of 50 kPa, the stress-strain relationship curve is similar to that at 100 kPa. The deviatoric stress first increases with the axial strain and maintains a stable state after reaching the peak deviator stress. Then, a stress drop occurs at an axial strain of 7%, which is strain softening. The homologous stress-strain relation also appeared in GT samples under confining pressures of 200 kPa and 300 kPa. A strain localization-shear band 22,23 can be clearly observed and presents shear slip deformation. Under a confining pressure of 200 kPa, the peak deviator stress is followed by a slow descent of the deviatoric stress. The deviatoric stress drops obviously at a large strain under a confining pressure of 300 kPa. The stress-strain relation of the sample is strain hardening, and there is no strain localization in the specimen, which shows bulging deformation under a confining pressure of 400 kPa. For CTII samples under confining pressures of 50 kPa and 100 kPa, the samples exhibit strain softening. Under a confining pressure of 200 kPa, the deviatoric stress reaches a peak, and then it always maintains a plateau, called a quasi-steady state 24. Under a confining pressure of more than 200 kPa, strain hardening occurs, and the failure of the specimen is less evident with bulging deformation. For GT samples, strain softening occurs under confining pressures of less than 400 kPa and is in a quasi-steady state under a confining pressure of 400 kPa.
Based on the mineral content of various tailings and the above analysis, the stress-strain relationship of the tailings is affected by the clay mineral content. Strain hardening is more likely to occur under high pressure in the undrained shearing test. As the content of clay minerals increases, the confining pressure required for strain hardening increases. With increasing confining pressure, the stress-strain relationship gradually changes from strain softening to strain hardening, and there is a quasi-steady state between them. The more severe the strain softening, the more obvious the shear band. The tailings present slight bulging failure under high confining pressure. The strain localization is more pronounced with an increase in the content of clay minerals. The bulging deformation depresses with the confining pressure, which implies that the tailings will shrink failure after being sheared at a higher confining pressure because of the limitation of rotation and slip of tailings particles. This is consistent with previous research 25.
Table 4
Initial elastic modulus and shear strength of tailings
Tailings types
|
Initial elastic modulus (MPa)
|
Shear strength (kPa)
|
σ = 50 kPa
|
σ = 100 kPa
|
σ = 200 kPa
|
σ = 300 kPa
|
σ = 400 kPa
|
σ = 50 kPa
|
σ = 100 kPa
|
σ = 200 kPa
|
σ = 300 kPa
|
σ = 400 kPa
|
CTI
|
11.2
|
15.1
|
26.7
|
33.8
|
42.5
|
385.4
|
513.8
|
814.7
|
964.8
|
1209.6
|
CTII
|
9.8
|
12.5
|
20.1
|
23.4
|
29.5
|
311.9
|
379.5
|
635.9
|
785.3
|
1064.5
|
GT
|
10.4
|
13.6
|
27.2
|
29.8
|
41.4
|
392.2
|
522.3
|
796.7
|
960.6
|
1173.3
|
3.2 Influence of clay content on stress path
The track formed by the point representing the stress state in the stress space is called the stress path 26. For the consolidated undrained test, the variables used to depict the effective stress path in stress space are the average effective principal stresses \(p'\) for the abscissa and the average deviator stress \(q'\) for the ordinate. They are defined as follows:
$$p'=({\sigma _1}'+{\sigma _3}')/2$$
2
$$q'=({\sigma _1}' - {\sigma _3}')/2$$
3
Figure 9 shows the stress path of all tailings. The stress paths of all tailings are basically parallel to each other under different confining pressures and present linear growth, indicating that the pore pressure coefficient does not change during the shearing process. A decline occurs in all the stress paths after the peak under confining pressures of 200 kPa and 300 kPa, which is mainly determined by strain softening. Only for CTII under a confining pressure of 300 kPa does the stress path fall to the left after the peak. The reason is that the pore pressure has a more prominent influence on the stress path than the deviatoric stress. Therefore, the pore pressure coefficient has a significant influence on the stress path. Fig. 8 shows that the pore pressure coefficients A of all tailings range from 0 to 1/3, indicating that dilatation occurs during shearing. This explains why the deformation pattern of the tailings samples shows bulging. Moreover, it can be clearly found that the pore pressure coefficient of CTII is significantly lower than that of the other two tailings types, which may be due to the higher content of nonclay minerals in CTII tailings, resulting in faster pore pressure dissipation.
The failure principal stress line 27, that is, the Kf line, refers to the straight line below the strength envelope through the vertices of the Mohr circles, which can be obtained by connecting the average peak deviator stress point in stress space under different confining pressures. The Kf line of CTII is at the lowest position according to the value of the average peak deviator stress, and those of CTI and GT are relatively close. This is chiefly dominated by almost the same pore pressure coefficient and content of clay minerals. However, the clay mineral content of GT is slightly higher, its pore pressure coefficient is higher, and its overall Kf line is still at the highest position.
It can be concluded that the higher the clay mineral content, the larger the pore pressure coefficient. This is due to the higher viscosity between clay minerals and water and the agglomeration of clay minerals, which directly leads to a less effective pore structure. This verifies the finding that the pore pressure dissipation is controlled by the effective pore structure 28–30.
3.3 Influence of clay content on shear strength
For samples with strain softening, the peak deviator stress is regarded as the shear strength. For samples with strain hardening, the deviator stress corresponding to an axial strain of 15% is regarded as the shear strength. Fig. 10 shows the shear strength of tailings containing different minerals as a function of confining pressure. According to the Mohr-Coulomb strength criterion, the relation between the confining pressure and the shear strength is the following formula, with a linear relationship.
$${({\sigma _1} - {\sigma _3})_f}=\frac{2}{{1 - \sin \varphi }}(c\cos \varphi +{\sigma _3}\sin \varphi )$$
4
where (σ1-σ3)f is the shear strength, c is the cohesion, φ is the friction angle, σ1 is the axial stress, and σ3 is the confining pressure.
The shear strengths of CTI and GT are basically the same and are higher than those of CTII. This suggests that the effect of clay mineral content on shear strength is similar to that on the failure principal stress line. The clay mineral content directly affects the strength parameters that determine the shear strength. The strength parameters are generally obtained from the strength envelope. There is always pore water pressure during undrained shear. Therefore, the total strength index and effective strength index can be calculated in the corresponding coordinate space. In accordance with Coulomb shear strength theory, the strength envelope can be expressed by Eqs. (5) and (6).
$${\tau _{\text{f}}}={c_{{\text{cu}}}}+\sigma \tan {\varphi _{{\text{cu}}}}$$
5
$${\tau _{\text{f}}}=c'+\sigma \tan \varphi '$$
6
where τf is the shear strength; ccu is the total cohesion; φcu is the total internal friction angle; c' is the effective cohesion; and φ' is the effective internal friction angle.
Figure 11 presents a schematic diagram of the total strength and effective strength envelope. The total strength envelope is constructed through the tangent points of the total Mohr circles. The common tangent of the effective Mohr circles is the effective strength envelope. The strength parameters of the tailings are summarized in Table 4. The cohesion of the three tailings is quite different. Based on this, the relationship between cohesion and clay mineral content can be established and is shown in Fig. 12. The clay mineral content and the cohesion of tailings have a linear relationship. The changes in total cohesion and effective cohesion with increasing clay mineral content are similar, and their straight fitting lines are close to parallel, indicating that the initial pore water pressures of all tailings are approximately the same. However, this result is inconsistent with the previous conclusion that cohesion first increases and then decreases with increasing clay mineral content 31. The reasons for this result are as follows: (1) the clay mineral content of the tailings samples used in this study has not yet reached the optimal content. (2) The cohesive strength may be related not only to the content of clay minerals but also to the species of clay minerals.
It can also be seen from Table 5 that although the internal friction angle first increases and then decreases with the content of clay minerals, the internal friction angles are restricted within a range between 31° and 33°, showing little change, regardless of the effective or total strength index. Generally, the change in the internal friction angle mainly depends on that of the clay mineral type, such as montmorillonite 32 and kaolinite 33. However, it has little correlation with the overall clay mineral content 34.
Table 5
Shear strength parameters of tailings
Tailings
|
\({c_{{\text{cu}}}}\)(kPa)
|
\({\varphi _{{\text{cu}}}}\)(°)
|
\(c'\)(kPa)
|
\(\varphi '\)(°)
|
CTI
|
80.9
|
32.1
|
76.1
|
33
|
CTII
|
53.1
|
31
|
47.5
|
32
|
GT
|
83.1
|
31.5
|
78.2
|
32.8
|
3.4 Microstructure analysis
Scanning electron microscopy (SEM) is a common method for studying the microstructure of geotechnical materials 35. In accordance with the gradation of tailings samples, SEM scanning uses 1000 times magnification to characterize the microscopic surface morphology of the tailings. Energy dispersive X-ray spectrometry (EDS) analysis is conducted at a certain position of the SEM images to determine the change in the tailings energy spectrum, and the corresponding spatial positions are marked 1, 2 and 3.
Figure 13 illustrates the microstructure and energy spectrum of all tailings at 1000 times magnification. The particles of the CTI and GT samples have more obvious bonding, forming clusters, and there are needle-like structures on their surface. The reason for this is that there is a certain cohesion between the clay mineral particles and between the clay mineral and the nonclay mineral particles, and the particles are combined to shape inclusions in the tailings. The surface of CTII particles is smoother, and their particle fragments are mainly layered and flaky. The bond between particles is not strong. The pores between particles are large and relatively loose, and the distribution of clusters formed by clay minerals is more uniform. These results indicate that the higher the content of clay minerals is, the higher the cohesion, which is consistent with the conclusions drawn from previous mechanical strength analyses 36. The EDS analysis results show that the element distribution on the surface of the sample after the test is not much different from that before the test, indicating that the tailings sample has not reacted chemically after the addition of water. This shows that the mechanical properties of tailings mainly depend on the initial clay content.
3.5 Mechanism analysis of clay content effect on mechanical properties
Nitrogen adsorption is an effective method to study the microfabric characteristics of solid materials. Assuming that the nitrogen adsorbed on the surface of particles is a molecular layer, the specific surface area can be expressed by Eq. (7):
$${S_{\text{g}}}=\frac{{N\delta {V_m}}}{{22400w}}$$
7
where N is the Avogadro constant, the number of gas molecules per unit mass is 6.024×1023, and \(\delta\) is the cross-sectional area of a nitrogen molecule. \({V_m}\) is the single layer adsorption volume of nitrogen on the inner surface of the sample pores. is the quantity of the sample tested.
However, in most cases, nitrogen is not adsorbed in a single layer in the pores of the material. Assuming that the adsorption heat of the first layer is a constant and the adsorption heat of other layers is another value, then based on thermodynamic and kinetic analysis, the real volume of nitrogen in the material pores can be calculated by using the BET (Brunauer, Emmett, and Teller) equation 37 38:
$$\frac{P}{{V({P_0} - P)}}=\frac{1}{{{V_m}C}}+\frac{{C - 1}}{{{V_m}C}}\left( {\frac{P}{{{P_0}}}} \right)$$
8
where is the real volume of nitrogen adsorbed in pores of unit mass sample; is nitrogen partial pressure; \({P_0}\) is saturated vapor pressure of nitrogen at liquid nitrogen temperature; is adsorption heat constant, the larger the value, the stronger the adsorption capacity; The range of \(P/{P_0}\) is 0.05~0.35.
Figure 14 shows the BET curves of all talings. According to Eq. (8), the monolayer adsorbate volume \({V_m}\) and the adsorption heat constant can be obtained by the slope and intercept of the curves. Combined with Eq. (7), the specific surface area \({S_{\text{g}}}\) of the material can be obtained. The calculated specific surface areas of CTI, CTII and GT are 3.03, 4.69, and 2.51 m2/g, respectively, indicating that the specific surface area of tailings is negatively related to the content of clay mineral particles due to more bonded particles forming larger aggregates. The calculated adsorption heat constants of CTI, CTII and GT are 330.94, 223.91 and 336.99, respectively. This shows that clay mineral particles have a stronger adsorption capacity, which is associated with the electronic shell on the surface of mineral particles.
Nitrogen has isothermal adsorption characteristics 39. Based on the one-to-one correspondence between the pressure required for capillary condensation when liquid nitrogen enters the pores and the pore size, assuming that the capillary pores on the surface of the material are cylindrical, the pore diameter corresponding to the pressure at which nitrogen is desorbed from the condensed state can be obtained by the Kelvin equation 40:
$${r_k}=\frac{{ - 2\gamma {{\text{V}}_m}}}{{RT\ln (P/{P_0})}}$$
9
where \({r_k}\) is the pore diameter, \(\gamma\) is the surface tension of nitrogen at the boiling point, \({{\text{V}}_m}\) is the molar volume of liquid nitrogen, is the gas constant, and is the boiling point of nitrogen.
The BJH (Barret-Joyner-Halenda) method 41 is a commonly used method for nitrogen adsorption to measure the pore size distribution. In the process of nitrogen desorption, the calculation equation for the change in pore volume in the range of \(P/{P_0}\) from 1 to 0 is as follows:
$$\Delta {V_{pi}}{\text{=}}{\left( {\frac{{\overline {{{r_{pi}}}} }}{{\overline {{{r_{ki}}}} }}} \right)^2}\left( {\Delta {V_{ki}} - 2\Delta {t_i}\mathop \Sigma \limits_{{j=1}}^{{i - 1}} \frac{{\Delta {V_{pj}}}}{{\overline {{{r_{pj}}}} }}} \right)$$
10
where \({V_{pi}}\) is the measured adsorption pore volume corresponding to the pore diameter \({r_{pi}}\) and \(\Delta {V_{ki}}\) is the volume of liquid nitrogen converted from the amount of nitrogen desorbed from the solid surface when the relative pressure drops from \({P_{i - 1}}\) to \({P_i}\). \({\left( {\frac{{\overline {{{r_{pi}}}} }}{{\overline {{{r_{ki}}}} }}} \right)^2}\)is a conversion fraction.
The nitrogen adsorption test results are usually expressed in terms of the calculated cumulative pore volume and the differential of pore volume to the logarithm of pore size for all calculated pore diameters 42:
$${V_{cumulative}}=\sum {\Delta {V_{pi}}}$$
11
$${V_{\log - differential}}=\frac{{d{V_{pi}}}}{{d(\log {r_{pi}})}}$$
12
Figure 15 shows the pore size distribution of tailings samples by the nitrogen adsorption test. It can be seen from Figure 15(a) shows that as the pore size increases, the cumulative pore volume increases rapidly and then tends to flatten, indicating that almost all pore sizes are within the detectable range. For samples with the same initial density, the cumulative pore volume should eventually approach the same value. However, the ultimate cumulative pore volume of tailings decreases with increasing clay mineral content. This result may be because, due to the higher content of clay minerals, capillary water pressure increases during the consolidation phase, providing additional impetus for pore shrinkage.
Figure 15(b) presents the log-differential pore volume of tailings. All samples have a bimodal pore size distribution, with two major pore sizes, which can be divided into macroscopic pores (between aggregation and aggregation) and microscopic pores (aggregated interior). The duplicate porosity feature usually also occurs when the soil samples are compressed at the optimal moisture content 43,44. Furthermore, the two main pore sizes of all tailings samples are similar, indicating that the bimodal pore size distribution mainly depends on the initial gradation of the material. The rise in the proportion of dominant micropores and the reduction in the proportion of dominant macropores are attributed to the better coating of tailings because of the higher clay mineral content.
From Fig. 15, it can be recognized that the shear strength of tailings is greatly related to the clay content. For instance, as the content of clay minerals increases, the proportion of micropores increases. It is generally manifested as the reduction of porosity and the elaboration of pore size, which both will lead to an increase in the shear strength, as evidenced in this study. However, what calls for special attention is that the effect of clay content on the shear strength is very complicated. Further microstructural analysis and chemical research on the material are required. More deeply, the shear strength of soils is determined by many aspects, not only correlated with the fabric of the materials but also the cementing of the particles, the crushing strength of the particles, and the rearrangement of the particles.