3.1 Macroscopic mechanical characteristics
3.1.1 Stress-strain response of triaxial compression test
(a) Influence of admixtures on the stress-strain curves
The stress-strain types of the soil samples with different confining pressures and admixtures volume fractions are presented in Fig. 7. It can be seen that these samples behave as softening and plastic characteristics in the presence of confining pressure. The plastic behavior is reflected in Group A and Group B samples. The softening characteristic is observed particularly in the Group C samples. This softening characteristic is present slightly in the Group D and Group E samples. For higher confining pressure (400 kPa), these samples have a tendency to develop with plastic characteristics. One important reason is that with the increase of confining pressure, the inhibiting function on the radial deformation of the sample is enhanced, which directly causes to the transformation of the curve feature (Farnam, Moosavi et al. 2010).
For Fig. 7(b), at any volume fraction of red Pisha sandstone content, the plastic behaviors of Group B samples are not changed compared with sample A. According to Fig.7(c), the softening characteristic showed a significant improvement by increasing carbide slag content. With increasing carbide slag content in a constant red Pisha sandstone content, these plastic behaviors have a tendency to change to the softening behaviors in Fig.5 (d) and (e). It is remarkable that a transition from softening to plastic behavior can be predicted between 5 to 10 wt.% carbide slag in Fig.7 (d).
(b) Strength characteristics
Peak strength versus red Pisha sandstone and carbide slag content of soil samples are plotted in Fig.8 (a). Note that the maximum deviatoric stress observed during the triaxial test on each specimen was recorded as the peak strength in softening behavior curves. And For the plastic behavior curves or hardening behavior curves, the deviatoric stress at 15% axial strain was determined as the peak strength. As illustrated in the figure, the peak deviatoric stress was influenced by both the content of red Pisha sandstone and carbide slag. To analyze the overall trend of strength change, the strength change ratio a is introduced:
Where qi is the peak strength of Group B to Group E samples; qA is the peak strength of sample A.
In Fig.8 (b), the relationship between strength change ratio and admixtures and volume fractions is shown. For Group B samples, by increasing red Pisha sandstone content in a constant confining pressure condition, the strengths no obvious change. Only adding red Pisha sandstone up to 15wt.% volume fraction of silt clay is even smaller than the strength of the plain silt clay. For the Group C samples which are containing only carbide slag, the strengths increase obviously with the increase of carbide slag volume. Take the sample with the confining pressure of 200 kPa as an example, the strengths of the samples in Group C are increased by about 124%, 143%, and 189% compared with sample A, respectively. When carbide slag and red Pisha sandstone are mixed into the soil at the same time, the peak strength of sample E3 is best. The strength change ratio is 136% under 200 kPa confining pressure.
Failure patterns of these samples under the triaxial compression test are shown in Fig.9. Since the failure modes were approximately similar in all the confining pressure ranges, the pictures for the 200 kPa confining pressure test are depicted. As shown in this figure, sample A (plain silty clay) and Group B samples are plastic deformations. The deformation of group B samples is slightly less than that of sample A. The Group C samples are obviously brittle deformation. The shear failures were observed on a distinguished diagonal plane above the soil samples. And these samples also show other multiple shear cracks during loading. With the increase of the content of carbide slag (from 10wt.% to 20 wt.%), the failure mode becomes more obvious. In addition, no obvious shear failure surface was formed on Group D and Group E samples, and there was a slight bulge and shear in the middle of the samples. This indicates that adding red Pisha sandstone may lead to a more ductile behavior and seems to be sufficient to prevent crack propagation and lateral expansion. As a result, the failure mode of these samples is changed from brittle failure mode to plastic failure mode.
From what has been discussed above, it is clearly found that the red Pisha sandstone plays a less significant role in triaxial properties with increasing confining pressure to RPS-stabilized soil. There is little change in strength and deformation. For CS-stabilized silty clays, an increase in volume fraction of carbide slag increases to strength in soil. However, the deformation which showed a distinguished diagonal plane and multiple shear cracks were obviously aggravated. For RPS-CS stabilized silty clays, the strength was increased and a combination of shear and bulge mode of failure was observed. Meanwhile, the degree of deformation does not vary much.
Therefore, it can be considered that the samples of CS-stabilized silty clay (C1,10wt.% CS) and RPS-CS stabilized silty clay (E3, 15wt.% RPS and 15wt.% CS) have a better improvement effect. In the following, only these specimens were subjected to freeze-thaw cycles test, PSD test, XRD test, and SEM test.
3.1.2 The ability of freezing and thawing resistance (C1 and E3)
(a) Influence of freeze-thaw cycles on strength characteristics
Triaxial compression results of C1 (CS-stabilized silty clay) and E3 (RPS-CS stabilized silty clay) under different freeze-thaw cycles and confining pressure conditions are shown in Fig.10. The confining pressure has little effect on the curve type, and all of them show softening behavior. The freeze-thaw cycles' have an obvious effect on the stress-strain behavior. For CS-stabilized silty clays, the peak deviatoric points gradually move to the right with the increase of freeze-thaw cycles. However, the number of freeze-thaw cycles has a slight effect on the residual stress. The characteristics of RPS-CS stabilized silty clays are basically opposite to those of CS-stabilized silty clays. The peak points did not move with the increase of freeze-thaw cycles. The residual stress decrease with the increase of freeze-thaw cycles under 100 kPa confining pressure. When the confining pressure values are 200 kPa and 400 kPa, the residual stresses are basically the same values under freeze-thaw cycles ≤10. The residual stresses have a tendency to decrease under freeze-thaw cycles ＞10. Thus, it can be considered that the 5 freeze-thaw cycles are the critical value of soil stability for RPS-CS stabilized silty clay.
The variation of the peak strength with the confining pressure under different freeze-thaw conditions is summarized in Fig. 11(a). It can be seen that the peak strength of CS-stabilized silty clays tends to decrease with increasing freeze-thaw cycles; while the freeze-thaw cycles have little influence on the peak strength of RPS-CS stabilized silty clays. Comparing with the lower confining pressure, the sample at higher confining pressure has a larger loss on the peak strength. Fig.11(b) shows the relationship between the strength loss ratio and the freeze-thaw cycles, respectively. The maximum strength loss ratio of CS-stabilized silty clays is 0.137 under the confining pressure of 400 kPa, the freeze-thaw cycles of 20. The minimum strength loss ratio is 0.018 under the confining pressure of 100 kPa, the freeze-thaw cycles of 5. The maximum strength loss ratio of RPS-CS stabilized silty clays is 0.047 under the 100 kPa confining pressure, the 15 freeze-thaw cycles. The minimum strength loss ratio is 0.004 under the 400 kPa confining pressure, the 5 freeze-thaw cycles. On the whole, the strength loss ratio of CS-stabilized silty clays is greater than that of RPS-CS stabilized silty clays.
Since the failure modes were approximately similar in all the confining pressure ranges, failure patterns of CS-stabilized silty clays and RPS-CS stabilized silty clays under the 200 kPa confining pressure test are depicted in Fig.12. For CS-stabilized silty clays, shear failure was observed on a diagonal plane which is typical of the brittle materials at the top of the figure. After freeze-thaw cycles, the shear plane angle of the sample decreases, but the distinguished shear plane is still obvious. And these samples also show other multiple shear cracks during the freeze-thaw process. For RPS-CS stabilized silty clays, the failure mechanism is bulging and shearing at the bottom of the picture. And all shear planes are smaller than those of CS-stabilized silty clays. Therefore, it is clearly found that the RPS-CS stabilized silty clays have smaller deformation during the freeze-thaw process.
3.1.3 Shear indexes
According to the Mohr-coulomb theory, the internal friction angle (j) and cohesive force (c) of these samples are calculated, as presented in Fig.13. It can be seen that with the increase of freeze-thaw cycles, the internal friction angle of CS-stabilized silty clays decreases gradually (from 35° to 30°), while the internal friction angle of RPS-CS stabilized silty clays has little change. This suggests that the effect of freeze-thaw cycles on the internal friction angle of CS-stabilized silty clays is more obvious. It is known from the cohesive force of RPS-CS stabilized silty clays gradually decreases with the increase of freeze-thaw cycles. The cohesive force of CS-stabilized silty clays shows a trend of decreasing first and then increasing. And, the change range of cohesion force of RPS-CS stabilized silty clays is smaller.
With the view of the quantitative point, the conclusion that the freeze-thaw cycles have a greater effect on cohesive force and internal friction angle of CS-stabilized silty clays than that of RPS-CS stabilized silty clays is verified again (Li, Li et al. 2018). This further explains the deformation of RPS-CS stabilized silty clays is less than CS-stabilized silty clays after multiple freeze-thaw cycles, combined with Fig. 12.
3.2 Microscopic reinforcement mechanism
3.2.1 PSD test results
(a) Variation of particle size distribution
Fig.14 presents the particle size gradation curves for CS-stabilized silty clays and RPS-CS stabilized silty clays under different freeze-thaw cycles, respectively. It can be summarized that during the freeze-thaw cycle process, the particle size becomes small.
At presented, in Fig.14(a), a larger peak exists in the 50mm -100 mm and a smaller peak exists in the 200mm -500mm for CS-stabilized silty clay without freeze-thaw cycle. The first peak moves to the left and decreases under 5 freeze-thaw cycles. And the second wave peak also decreases, but there is basically no moved leftward and rightward. When the freeze-thaw cycle is more than 10 times, the first peak starts to increase and the second peak gradually disappears. Eventually, there is only one peak on each of the curves. Fig.14(b) and Table 4 shows that for RPS-CS stabilized silty clay without freeze-thaw cycle, two obvious peaks appear on the curve in the particle size range of 50mm -100mm and 400mm -600mm, respectively. The first peaks gradually increase and the second peaks gradually disappear with the increase of freeze-thaw cycles. At the same time, these peaks do not shift from side to side. Ultimately, the clay-sized particle percentage increased from 0.607% to 2.715% for CS-stabilized silty clay, which is higher than those of RPS-CS stabilized silty clay (from 1.027% to 0.607%) after 20 freeze-thaw cycles. The silt-sized particle percentage increases substantially from 48.176% to 61.974% and from 39.152% to 58.127%, respectively. And RPS-CS stabilized silty clay has more sand-size particles than CS-stabilized silty clay.
Some experts believed that the short-term flocculation formation and long-term pozzolanic reactions result in changes in clay-sized and sand-sized particle percentages in the stabilized soils (Tran, Cui et al. 2014). The flocculation contributes to the agglomeration of fine particles and the synthesized pozzolanic products would coat the surface of soil particles, both making a relatively high fraction of coarse-grained particles (Kampala, Horpibulsuk et al. 2013, Tran, Cui et al. 2014, Jiang, Du et al. 2015).
Table 4 Results of PSD test.
(2mm to 74mm) (%)
(b) Grading coefficient
The grading coefficients of all samples are shown in Table 5. When the uniformity coefficient (Cu) is greater than or equal to 5, the soil sample is inhomogeneous soil. And when the curvature coefficient (Cc) is in the range of 1 and 3, the grain size distribution of soil samples is good. It can be seen from the table that the Cu of CS-stabilized silty clays increases at first and then decreases with the increase of freeze-thaw cycles. The number of freeze-thaw cycles is greater than 10 times, and the Cu basically remains unchanged (＞5) and the non-uniformity is significant. The Cc is in the range of 1 and 1.31, and the particle size distribution is good. The Cu of RPS-CS stabilized silty clays decreases with the increase of freeze-thaw cycles. When freeze-thaw is greater than 10 times, the particles are relatively uniform (Cu＜5). The Cc is in the range of 1 and 1.13, and the particle grading is a little worse than that of CS-stabilized silty clay.
Table 5 Grading coefficients of samples
|Characteristic particle size(mm)
Note: D10、D30、D60 are the effective particle sizes corresponding to the cumulative percentage of particle size of 10%, 30%, and 60%; Cu is the non-uniformity coefficient and Cc is the curvature coefficient.
(c) The relationship between D50 and strength
Chen et al. pointed out that the sand-sized particles' content was directly related to strength (Chen, Li et al. 2020). The more sand particles in the samples, the larger the pores are. And more sufficient the chemical reaction of the admixture to improve the soil, the more obvious the effect of improving its strength is. The relationship between strength and particle size under freeze-thaw cycles is shown in Fig.15. It can be seen that the D50 and strength of RPS-CS stabilized silty clays are larger than that of CS-stabilized silty clays during the freeze-thaw process. The experimental results are consistent with previous research results. The specimens with more sand-sized particles can provide better pore connectivity, the pozzolanic reaction can reach a higher degree, consequently, this sample has a stronger cementing property between the soil particles. Thus, from the view of particle size distribution, it can be concluded that compared to the CS-stabilized silty clay, the red Pisha sandstone mixed with carbide slag will be yield at higher stress.
3.2.2 XRD results
The crystalline phases determined by XRD analysis were shown in Fig.16. Unlike the case in sample A, the annite, albite, clinochlore, and calcite diffraction peaks in CS-stabilized silty clay (C1) without freeze-thaw cycle became weaker in Fig.16(a). However, new diffraction peaks (biotite, dolomite, kornerupine, ettringite, and portlandite) were observed after the pozzolanic reaction. This indicated that when silty clay was activated by 10 wt% CS, the structure of some mineral phases, which have poor water erosion resistance and strength, was destroyed to yield active Si and Al. It is noted that when the freeze-thaw cycle is greater than 5, some minerals (dolomite and ettringite) diffraction peaks disappeared and other diffraction peaks became weaker. In Fig.16(b), when 15wt% RPS and 15wt% CS were used as the mineral additives, RPS-CS stabilized silty clays, did not produce any new diffraction peak after the pozzolanic reaction, compared with RPS-CS stabilized silty clay. But, with the increase of freeze-thaw cycles, the ettringite diffraction peaks do not tend to decrease, unlike CS-stabilized silty clay. Other diffraction peaks do not change with the increase of freeze-thaw cycles. At the same time, it can be seen that calcite is present in these samples. The formation of calcite is attributed to the air exposure of soil during the curing period and subsequent carbonation of CS and hydrated products.
In addition, some experts found that the active Si and Al can generate produced amorphous gels(Kinuthia, Wild et al. 1999, Du, Jiang et al. 2014). This is because the pozzolanic reactions between portlandite (Ca(OH)2) and reactive SiO2 and Al2O3 in the soil matrix are reacted, which can be expressed by the following:
During the pozzolanic reactions, alkaline portlandite is gradually consumed and transformed to less alkaline secondary cementitious products (C-S-H gels and C-A-S-H gels) with good water erosion resistance and high strength. For the CS-stabilized silty clays, formation of C-S-H is detected at 2q of 21.5°, 29.3°, 36.2°, and 49.1°. The XRD results of the RPS-CS stabilized silty clays reveals the formation of C-S-H (21.5°, 27.5°, 28.3°, 29.3°,31°, 36.2°,49.1° and 50.9°). As C-S-H is the secondary cementitious product formed in the pozzolanic reactions, the identified peaks of C-S-H and the presence of portlandite in the CS-stabilized silty clay and RPS-CS stabilized silty clay after 20 freeze-thaw cycles confirm the progress of pozzolanic reactions is not completely over.
3.2.3 SEM results
The scanning electron microscopy was taken at 15,000 magnifications, as shown in Fig.17 and Fig.18. According to particle distribution characteristics (Fig.14 and Table 4), the ratio of large-size particles in RPS-CS stabilized silty clay is greater than that in CS-stabilized silty clay without freezing and thawing cycle. Thus, the soil particles in CS-stabilized silty clay are easily spotted, which are loosely surrounded and bonded by the CSH gel matrix in Fig. 17a. Meanwhile, the sample had a poor microstructure with many pores. For the RPS-CS stabilized silty clay, CSH gels are only around several large-size soil particles in Fig. 18a. The particle shape was irregular and the diamond horn of the particle is prominent. The contact mode between particles is point contact, indicating a very limited hydration reaction occurred. The differences can exist due to the surface area and hydrophilia in CS-stabilized silty clay are stronger than that of RPS-CS stabilized silty clay that it was because the former contains more clay particles (Yi, Gu et al. 2014). The hydration reaction of CS-stabilized silty clay is earlier than that of RPS-CS stabilized silty clay, and the uneven flocculation and agglomeration products are formed in the early curing time.
With the increase of freeze-thaw cycles (from 5 to 20), the contact mode of soil particles and hydration products in CS-stabilized silty clays gradually transition from edge-to-face to face-to-face in Fig.17(b)-17(d). The CSH gels are basically wrapped around the soil particles after 20 freeze-thaw cycles. Apart from the rounded and flocculent CSH particles, reticular microstructures were displayed in the sample. However, some soil particles could be seen clearly, and the size of the particles was heterogeneous. For the RPS-CS stabilized silty clay, when the freeze-thaw cycle is 5 times, the content of CSH gels increased and slowly adhered to the surface of soil particles rather than just around the soil particles in Fig.17(b). With the increase of freeze-thaw cycles, large amounts of small-sized rounded and plate-like CSH fill most of the clay pores (Fig.17(c) and Fig.17(d)). After 20 freeze-thaw cycles, the distribution of the gel among the particles seems to be fairly homogeneous. The pores between the soil particles have become mostly filled with gels, which altered the microstructure and produced a denser and less permeable material.
An earlier study showed that CSH gels control the final properties of the binding matrix and the mechanical behavior (shear strength, stiffness, and compressibility) of the stabilized soil (Corrêa-Silva, Miranda et al. 2020). The different microstructure and binding ability of CSH gels might reflect the different strengths of soils. The strength of samples decreases as the CSH gels shape more amorphous and binding ability decreases. Thus, it can be seen that the morphologies of CSH gels in Fig.17e and Fig.18e are different; the former is more dispersed, more amorphous, and less uniform. These may be the reasons why the CS-stabilized silty clay has no advantage in strength over that of the RPS-CS stabilized silty clay after 20 freeze-thaw cycles.