2.1 Materials

Kaolin deposits may often have unsuitable geotechnical properties [7, 22]. Hence, a natural kaolinitic soil obtained from the northwest Iran, was chosen as raw material. In Table 1, the physical and mechanical properties of the used soil are summarized after performing the identification tests in accordance with ASTM, 2006 [23]. The soil sample was classified as CL based on the ASTM-2487 method. The mineral identification with XRD revealed that the used soil contained high amounts of clay content. As can be seen in Figure 1(d), its mineral content was mainly kaolinite (Kao.) (>70%) and quartz. The performed chemical experiments also showed that the main exchangeable cation of the used soil was Na⁺. As presented in Figure 1(a), the soil sample was dried and passed through a #200 mesh before use. It is worth noting that the soil used had low mechanical strength and a very high compression index, making it suitable to be classified as a soft clayey soil. The slag (GGBS) used in this study was obtained from Esfahan Steel Co., Esfahan, Iran. The GGBS was also dried, ground and sieved (<75 μm) to obtain fine uniform powder with particle size distribution curves similar to those of the kaolinite grains. The primary chemical compositions of the soil sample, GGBS and NM were determined based on the results of the X-ray fluorescence test as presented in Table 2.

Figure 1全屏查看

A view of (a) studied soil, (b) polypropylene fiber, (c) nano-metakaolin, and (d) XRD pattern of soil sample

Table 3 shows the obtained results from the particle size distribution (PSD) experiments for the studied soil sample (Kao.), GGBS and NM. The tests were performed using laser diffraction method by means of a laser particle size analyzer modeled Analysette 22 NanoTec-Fritsch. The particle sizes of Kao. and GGBS were in line with those reported in previous studies (e.g., [24] and [7]).

The results presented also indicated that the effective sizes (i.e., D₁₀, D₃₀ and D₆₀, representing the particle sizes at which 10%, 30% and 60% are smaller, respectively) of the used NM were 151, 343 and 780 nm, respectively, showing that it could be classified as a nano-scale material. Moreover, its specific surface area and specific gravity were found to be about 151.7 m/g² and 2.75, respectively. The total amount of SiO₂, Al₂O₃ and ‏Fe₂O₃ in the NM was 80.04%, which satisfied the minimum requirement of 70% recommended in American Standard ASTM C618 for pozzolanic materials. The properties of NM indicated high pozzolanic activity and a high pore refinement effect [25, 26].

The polypropylene fiber (see Figure 1(b)) with properties listed in Table 4 and an optimal average length of 12 mm [6, 9, 19] was utilized to reinforce the composite samples. To prepare the slag-based geopolymer material, hydrated lime (Ca(OH)₂), sodium silicate (Na₂SiO₃) and sodium hydroxide (NaOH) with a purity of more than 90% from Merck Co. Darmstadt, Germany were used.

2.2 Mixture proportions of the stabilized soil samples

As summarized in Table 5, the binders including lime alone (as traditional additive for the stabilization treatment of soft clayey soil), LG (lime activated ground granulated blast-furnace slag), LGNM (LG mixture having various replacement levels of lime with NM) and LGNMF (LG mixture at optimal contents of NM and polypropylene fiber) at various amounts (5%, 10%, 15% and 20% by weight of the binders in dry mass) were prepared to modify the soft clayey soil sample. In so doing, following the previous studies [7, 27, 28], in which the slag to activator ratio was often set at 3:1 and also considering that the aim of the present research was to reduce the consumption of lime as much as possible, the ratio of slag to activator was set to 4:1 in the LG binary composite.

In the LGNM blend, according to the research performed by AKBARI et al [11], since after 40% replacement of nanoparticles, the strength of the samples would decrease, thus 0%, 20% and 40% lime were replaced with the NM particles. For the LGNMF mixture, on the other hand, the wet soil and binders (i.e., LG binary composite with the optimum amount of NM) were first mixed, followed by adding an optimal fiber content of 1 wt%, as recommended by other researchers [9, 29].

2.3 Sample preparation and testing of soil specimens

As mentioned above, different primary homogenized amendments (inc. lime alone, LG, LGNM, and LGNMF) were firstly prepared, and then, the required dosages of the resulting materials were added to the kaolin soil and thoroughly mixed to obtain a well-blended mixture. Afterwards, deionized water (equal to the optimum moisture content measured by compaction testing) was sprayed onto the specimens and mixed evenly to obtain a uniform system in each sample. Subsequently, all the prepared samples were separately compacted statically (at 95% of the maximum dry density) into two cylindrical steel molds: one (35 mm in diameter and 70 mm in length) for the unconfined compression strength (UCS) test and the other (50 mm in diameter and 20 mm in length) for the consolidation test. Furthermore, to provide GGBS-based geopolymer (GP), 10 mol/L alkali solutions with two types of activators including Na₂SiO₃ and NaOH at ratios of 5:1, 4:1, 3:1, 2:1 and 1:1 were produced and then were separately added to the GP mixtures. Next, all the prepared samples were sealed in plastic bags and kept (up to 90 d) in the humidity chamber (with a relative humidity of (95±2)%) at various temperatures of curing (inc. 20 and 40 ℃). The temperature of 40 ℃ was chosen to assess the effect of high field air temperatures which may occur in the hot regions of the world. A series of macro and micro scale tests were then conducted on the prepared samples to evaluate the responses of the binders. The schematic diagram of soil production and macro/micro experimental procedures are shown in Figure 2. After adequate curing, the oedometer experiments were performed in accordance with ASTM D-2435. The UCS of specimens was also determined based on ASTM D-2166. This test was performed under a constant strain rate of 1.2 mm/min. The energy absorption capacity, E_u, is a parameter for evaluation of the brittleness/ductility of the samples, as suggested in Refs. [7, 30]. In the present study, the energy absorption capacity was calculated by integrating the area under stress-strain curve up to the failure strain. In order to monitor the physicochemical interactions among the clay surfaces and the used binders, the electrical conductivity (E_C) and pH experiments were performed using the method described by GOODARZI et al [31]. To this end, suspensions of the soil-binder with varying amounts of the studied binders were first provided at a ratio of 1:20 (soil:water) and were then equilibrated. The E_C and pH values of those soil-water mixtures were determined after 2 h to 90 d of curing period. To clarify the mechanisms involved in the hydration reactions rate and the process of geopolymerization, the soil microstructure was also evaluated by conducting scanning electron microscope (SEM) and X-ray powder diffraction (XRD) tests. For performing the SEM experiments, the remaining soil specimens from the mechanical test were firstly crushed and cleaned to obtain freshly exposed sections, and then coated with a gold film to make the surfaces more conductive and avoid electrostatic charging. The internal morphology of the prepared specimens was viewed using a SEM instrument (Model: VEGA-TESCAN). The XRD tests were also carried out to characterize the phase composition of stabilized samples. In so doing, the desired pieces of representative treated soils used for the UCS test were further ground and sieved (<75 μm) to obtain fine powder before examination. These powder samples were then analyzed using the Bruker D8 X-ray diffractometer with Cu-Kα radiation operated at 40 kV and 40 mA. The specimens were scanned over a range of 2θ from 4° to 60°.

Figure 2全屏查看

Scheme of specimens production and experimental procedures (macro/micro level tests)

It is worth pointing out that in order to validate the results more robustly and accurately, all the geo-mechanical experiments were done in at least triplicate for ensuring the repeatability of the results and the average values obtained were used in the study. However, due to the sufficient accuracy in the process of preparing the soil samples and performing the tests, very little difference was found in the obtained results. Based on this fact, the calculated standard deviation (SD) values for UCS and consolidation tests were in the range of 0.01 to 0.51 and those for the E_C and pH tests were in the range of 0.1 to 0.5, indicating that samples preparation procedures and their testing had been accurate.

3.1 Effects of lime and LGNM composite on the mechanical strength of soil samples

Figure 3 shows the effect of different quantities of lime alone and LGNM composite on the strength of soil samples cured for 7 to 90 d at various temperatures. It can be observed that at the ambient condition of 20 ℃ (Figure 3(a)), the treated soil specimens usually exhibit an increase in the UCS values in response to increases in binder dosage and age of curing. In all the cases, the samples with 20% binder have the highest UCS values and those with 5% binder have the lowest UCS values. Basically, the strength improvement of a chemically stabilized soil might originate from the filling of void spaces within the soil matrix and the bonding of particles through the formation of new cementing phases [1, 3, 6, 11, 27, 32, 33]. In fact, an increase in dosages of lime and/or LGNM blend can lead to a physicochemical rise in the concentration of multivalent cations (e.g., Ca²⁺ and Al³⁺) as well as the hydroxyl ions (OH^-). Consequently, the pozzolanic reactions tend to take place in the system, leading to the generation of cementitious products including CSH and CAH [34-36]. In this context, the amended samples become denser and stronger and their UCS values improve accordingly. Besides, over time, the amendments and the clay lumps may undergo a prolonged pozzolanic process, facilitating the continued formation of cementitious products. This process continuously strengthens the bond among the clay particles and leads to the development of a more robust skeleton matrix [37-40], thus, gradually ameliorating the soil strength, as shown in Figure 3.

Figure 3全屏查看

UCS variation of the stabilized samples cured for 7, 28 and 90 d: (a) At 20 ℃; (b) At 40 ℃

With the increase in curing temperature (i.e., from 20 ℃ in Figure 3(a) to 40 ℃ in Figure 3(b)), the strengths of the specimens appear to increase rapidly (i.e., take a shorter time to reach their maximum strength), providing evidence that the effectiveness of the soil-additive mixture can be remarkably affected by the curing temperature, enabling the system to achieve higher rates of pozzolanic activity [31, 34]. This phenomenon leads to the formation of additional cementing gels and hence the stabilized soils achieve higher levels of strength within shorter curing times. By way of comparison, after 28 d of curing, the UCS values of soils treaded with 20% LG20NM and cured at 40 ℃ are nearly 2.3 times higher than those obtained for the samples cured at 20 ℃. This can be explained by the fact that an increase in the curing temperature may encourage and accelerate the process of pozzolanic activity (Eqs. (2) and (3)), leading to a significant increase in the generation and development of cementitious phases in the soil matrix [7, 33], as will be further discussed in the analysis of E_C and pH test results. As stated, those new hydration gels can not only establish permanent bindings between the clay particles but also improve soil packing (i.e., a denser structure), which is associated with higher degrees of solidification, ultimately contributing to a further increase in the soil strength.

As illustrated in Figure 3, it can be concluded that while there is a small difference between the behavior of LGNM treated samples containing various replacement ratios of NM (0 and 40%), the samples with 20% NM replacement show more significant increase in the strength than the samples having 0 and 40% NM replacement, indicating that the optimal performance for the LGNM composite may be obtained when 20% of lime is replaced with NM. In other words, the strength of LGNM-stabilized soils reaches its pick value in 20% NM, beyond which, the strength of LGNM-stabilized soils begins to decrease. In fact, when 20% of lime is replaced with nano-material, the active constituents such as silicates and aluminates present in NM reach a balance with lime during the pozzolanic process; and thus, the NM can be entirely consumed to produce hydration cementitious gels, thereby improving the overall mechanical behavior. However, when the lime replacement exceeds 20%, the content of activator (i.e., lime) becomes insufficient for the complete consumption of the nano-material during the pozzolanic reaction. As a result, a part of NM remains unconsumed, which will make the interparticle bonds weaker. This can result in lower strength of the stabilized soil, as shown in Figure 3. A similar trend can be observed in stabilized clay soils treated with ca-based stabilizers (e.g., lime and/or cement) mixed with other pozzolanic materials such as zeolite, fly ash and metakaolin [11, 36, 41-43].

From Figure 3, it can be seen that with the same level of lime, the LGNM mixture affects the strength more significantly than it does in the samples treated with lime alone, in such a way that the strength of 20LG20NM sample (containing 3.2% lime) almost equals the strength of the mixture of soil+7% lime. The findings demonstrate that adding NM to the soil-LG mixture can lead to a significant decrease (by nearly 2.2 times) in the required amount of lime in improving the mechanical behavior of clayey soils, indicating the high potential of nano slag-based geopolymer in enhancing the performance of conventional lime-treatment approach. The reason behind this behavior may be attributed to the optimized consumption of lime in the soil-binder system at the presence of LGNM composite. In fact, a part of binder in the case of stabilization with the lime alone (i.e., the conventional approach) may be consumed during the short-term reactions (e.g., cation exchange, flocculation and agglomeration). While such reaction mechanisms may show very little influence on the soil strength, the flocculated particles might wrap the surface of binder grains. This may hamper the material ionization (Eq. (1)), hinder the process of the pozzolanic reactions (Eqs. (2) and (3)), and consequently result in an increase in the required amount of lime to ensure a successful treatment. Another reason that can diminish the availability of lime in the conventional system is known as carbonation, which may take place between atmospheric CO₂ and lime. It might deter the strong cementitious pozzolanic reactions, thereby preventing the UCS enhancement [34]. In contrast, lime in LGNM treated soil samples can mainly be consumed by the growth of cementing phases (Eqs. (2) and (3)) due to its direct interaction with GGBS and the high pozzolanic action of NM particles, thereby forming more permanent bindings among the clay particles, which can develop the soil resistance against external loadings. Moreover, the presence of NM can improve the pores morphology of treated soils. This is because the ultra-fine particles in the NM can make it a very efficient filler, which will uniformly fill the available soil micro-pores and promote the formation of hydration products of the system [44-46]. Those nano particles may also contribute to the enhanced dispersibility of the new hydration gels in the matrix, providing important refinement to the porous network of the soil. Moreover, the application of nano-materials in the soil-lime mixture may serve as a catalyst, commonly referred to as the nucleation effect [47]. It will result in improved rate of pozzolanic activities, and thus generate more connections among the clay lamellae and stronger interparticle bonding through cementing phases [48]. Thus, it seems that with the same level of lime, samples with LGNM composite show higher UCS values. On the other hand, it is widely recognized that the optimum amount of binder to achieve successful stabilization is a function of the site characteristics and the project objectives. Based on this fact and considering the American Concrete Institute (ACI) recommendations (UCS2068 kPa) [49], it seems that at the room temperature, adding 20% LGNM can overcome the problems associated with the soft soil and the solidified soil samples can be accepted as a pavement material. However, in hot conditions (40 ℃), the application of 10% of this composite can satisfy the ACI regulation limit. In the present study to further validate the obtained results, the regression results between the UCS of lime treated soil samples and binder dosage (BD) at different curing time and temperatures were depicted in Figure 4, where R²0.94, would indicate the accuracy of test results.

Figure 4全屏查看

The regression between the UCS and binder dosage (BD) at different curing times and temperatures

Overall, it can be concluded that with the same amount of lime, the LGNM, as a nano one-part geopolymer, affects the soil solidification more significantly than it does in the case of the samples treated with lime alone, resulting in a remarkable decrease (by nearly 2.2 times) in the required amount of lime for improving the soil mechanical performance. This can be attributed to the optimized consumption of lime in the former system. In fact, in the case of conventional lime treatment, a part of the binder may be consumed during short-term reactions (e.g., cation exchange and flocculation and carbonation), which may have a little influence on the solidification process [11, 34, 42]. Under these conditions, the flocculated particles could potentially coat the surface of the binder grains, impeding the progression of pozzolanic reactions, causing an increase in the dosage of lime to achieve successful treatment [47]. In contrast, lime in the LGNM mixture can be mainly consumed for generating cementitious gels due to its direct interaction with the slag or NM, thereby promoting the formation of permanent bonds among the clay particles and thus enhancing the soil resistance to external loads. In addition, the presence of ultra-fine NM particles in the soil-lime mixture not only can considerably improve the binding abilities in the matrix through physical filling effects and nucleation process but also may contribute to the enhanced dispersibility of new hydration gels in the matrix, leading to significant improvements in mechanical performance.

3.2 Effects of LGNM composite on volume change behavior of soil samples

To assess the effects of nanomodification with NM on the volume change behavior of the samples treated with LG binary system, the oedometer experiments were also performed. To this end, the changes in the compression index (C_C) of the samples before and after interaction with the additives were calculated, the results of which are presented in Figure 5. Based on the presented results in this figure, it is evident that in all the treatment conditions, the C_C values decreased significantly with an increase in the binder dosages. This can be apparently attributed to the substitution of the existing comparatively weaker and monovalent cations on the clay surfaces with multivalent ions (e.g., Al³⁺ and Ca²⁺, see Table 2) of the additives. Such a replacement, resulting from the solubility or partial solubility of the agents in the pore fluid, can ultimately create appropriate conditions for reducing the charge density on the surface of the clay fractions and increasing the inter-particular attractive forces (IPAF). This phenomenon can dramatically reduce the thickness of diffuse double layer (DDL). The DDL compression will cause an induced pre-consolidation influence [7, 28, 31] which in turn decreases the overall soil compressibility. Simultaneously, in another phase, the high ionic strength after the addition of amendments may lead to an increase in the osmotic pressure, which can suppress the repulsive forces throughout the clay lamellae system [2, 31, 34]. Once the IPAF overcomes the inter-particle repulsion, the soil particles are pushed closer and this rearrangement can augment the contact area between them. These processes might flocculate the soil into larger lumps that may not have sufficient opportunity for hydration. It will also reduce the soil’s water-holding capacity, potentially causing the soil skeleton to shrink [50]. In other words, the lower ability of these formed lumps to adsorb and hold water makes them less prone to volume instability resulting in lower C_C values, as depicted in Figure 5. Subsequently, after the second level reactions (i.e., pozzolanic reactions), those modified (flocculated) clay particles would once again be further blended through the cementitious compounds such as CSH, CAH and CASH [3, 5, 7, 9, 33]. The induced elevated friction forces among the clay surfaces, due to both physical and chemical reactions mentioned earlier can lead to the drainage of the soil moisture, causing a decrease in the soil compressibility [34]. In fact, the flocculated structure resulting from short-term reactions and permanent bindings among the clay particles by hydration products tend to further reduce the settlement potential of soil associated with consolidation.

Figure 5全屏查看

Effect of LGNM properties on the volume change behavior of the stabilized soil sample: (a) Soils cured for 7-d at 20 ℃; (b) Soils cured for 7-d at 40 ℃; (c) Soils cured for 28-d at 20 ℃; (d) Soils cured for 28-d at 40 ℃

On the other hand, as previously mentioned, a large amount of lime in the LGNM treated soil samples can be consumed by the growth of a closely packed and significantly high-strength calcium silicate hydrate phase due to the high pozzolanic reactivity of NM with lime [12, 25, 51]. As a result, the generation of additional, stronger cementing phases under the LGNM treatment may not only create a more pozzolanic stabilized matrix but also help in ameliorating the compactness of micro-pores (i.e., forming a denser internal microstructure in the stabilized samples after the introduction of the optimal amount of NM to the system). This can significantly improve the soil resistance to structural reorganization [6] and thus can be considered as another important factor in more reducing the C_C values upon the LGNM application. The presence of NM particles can also physically improve the pores morphology of soil-additive system. Indeed, the ultra-fine particles in this nano-scale material (having about 50% nano particles, <500 nm) can make them very efficient fillers. In this context, adding NM can uniformly fill the available soil micro-pores, leading to a dramatic decrease in both the proportion and size of soil, which in turn contributes to gain in strength [35, 48]. Moreover, the inclusion of these nano particles in the soil-binder mixture may contribute to the enhanced dispersibility of the newly formed hydration gels in the matrix, providing an important refinement to the porous network of the soil [6]. Hence, the number of pore spaces in the LGNM solidified samples can dramatically decrease, resulting in a much denser internal structure. This optimized fabric makes the soil stiffer, and thus the LGNM treatment significantly modifies the soil’s volume change behavior, providing evidence that the application of slag coupled with NM can serve as a suitable agent for enhancing the efficacy of the traditional lime-stabilization method. It is worth noting that such an improvement in the soil geo-mechanical properties through the enhancement of interfacial friction and internal pores in the matrix using nano components (e.g., nano-zeolite and nano-silica) has also been reported in the previous studies [11, 52].

In line with the UCS test results, the stabilized soil specimens cured at 40 ℃ (particularly after 7-d) experience lower volumetric changes under external loading compared to the same specimens treated at 20 ℃ curing condition. This might be attributed to the accelerated generation and propagation of cementing products in the system cured at elevated temperatures due to an increase in the rate of hydration reactions. This observation aligns well with the conclusions drawn by MA et al [36] in the case of cement/metakaolin-modified clay. In fact, the temperature rise during the curing process tends to accelerate the pozzolanic actions and thus impart additional cementing phases inside the matrix, which may play an effective role in further occupying the soil pores [7, 11, 31, 34]. It will increase both the micro-scale compactness of the matrix, and its interparticle bonding ability (see SEM micrographs). This process can strengthen the soil grains, giving rise to promoted cementation and thus enhanced solidification performance of the system. Under these conditions, the re-arrangement of particles and the subsequent deformations in the soil mass due to increased external loads will be minimized [1, 2], which is, in turn, conducive to an enhancement in the soil compression performance.

3.3 Effects of adding fibers on the geo-mechanical performance of LGNM treated soil samples

It was expected that the composites with LGNM would be stronger and stiffer than the treated soil with LG alone. However, the general pattern observed in the stress-strain relationship curves of LGNM-stabilized soils in Figures 6(a) and (b) indicated that while the peak strength grew higher with the increase of the LGNM dosage (The optimum state of the NM replacement (i.e., NM=20%), which was obtained from Figures 3 and 5, was considered for this section) and curing temperature, the specimens could not significantly maintain the stress with increases in the axial strain. This indicates that the ductility behavior of the untreated soil (without any agents) reduces, and illustrates a gradual brittle behavior (i.e., a strain-softening response) as the amount of binder raises up to 20%. As the failure modes of some representative specimens in Figure 6 indicate, the soft soils treated with a one-part GP material (i.e., lime and GGBS in conjunction with NM) failed with the emergence of numerous cracks under increasing applied stress, implying that they might show a brittle pattern similar to the documented performance of conventional lime-treatment methods in previous studies [9, 11, 34]. The appearance of multiple wide vertical cracks was more evident in the stabilized sample cured at a higher level of binder, corroborating an increase in the brittleness trend under axial loadings. Such an alteration in failure modes (i.e., a decline in the deformability of the stabilized soil samples) is very desirable for the long-term stability of geotechnical structures (e.g., shallow foundations and transportation layers) [35, 45]. Therefore, the fiber was added to the soil-binder mixtures in order to decrease their cracking potential during the load application and to overcome the observed drawback (i.e., the high brittleness) of the stabilized GP material.

Figure 6全屏查看

Stress-strain curves of samples stabilized with LGNM composite by 20% substitution of lime with NM after 28 d of curing: (a) At 20 ℃; (b) At 40 ℃

Based on the results illustrated in Figures 7(a) and (b), adding fiber to the soil samples could strongly help in modifying the soil failure response with a gradual post-peak pattern, indicating that these composites tend to exhibit a ductile manner. Incorporating fibers into the system can also improve the peak strength by approximately 1.1 times more than that achieved with stabilization alone, showing a further strength gain upon adding the fiber. In other words, the application of fiber can complement the effectiveness of GP stabilizer in improving the mechanical properties of soft soil, potentially preventing the brittle failure modes associated with higher binder dosages. In fact, it can be seen from Figure 6 that the post-peak stress in the samples without fiber could diminish rapidly. This indicates that the treatment process can reduce the resistance to deformation, causing the soil to fail at a relatively low strain, and leading to a rapid decline in bearing capacity over a short period of time. In contrast, mixtures containing fibers typically exhibited a ductile behavior and a residual stress appeared following the failure, suggesting that the fiber-reinforcement can make a gradual failure. As shown in Figure 7, the post-peak stress (i.e., the residual strength) of all the composites having 1% fiber is significantly larger than that of composites without fiber (Figure 6), suggesting that use of fiber would prohibit a sudden rupture, which is evident in the image of failed mode in Figure 7. Similar patterns have also been reported in the previous studies [6, 19, 21, 43]. This can be very beneficial for the long-term stability of geotechnical structures (e.g., flexible pavements, the ductile behavior could effectively mitigate cracking during the pavement construction, curing and operation). To explain the reason for the better performance of fiber reinforced samples, it can be argued that fibers might act as bridges around the failure planes which can effectively help in transferring the applied loads to fibers and reducing the stress level on the soil particles due to the high-tension capacity of the polymer [53]. Such an enhancement during the fiber-reinforcement process can suitably contribute towards minimizing the depth of stress influence in the system. This, in turn, will mitigate and/or completely impede the formation of cracks, and restrict the induced deformation in the stabilized soil experiencing the applied force. Indeed, while the unreinforced soil samples experience wide and deep failure cracks when the applied stress reaches a peak, the fibers could serve as a spatial three-dimensional mesh to bond the soil particles into a well interlocked matrix and create an integrated system [6, 9, 19, 54]. This can lead to a significant reduction in the dimension of the cracks formed under external loads and thus prevent the propagation of generated failure planes within the matrix, resulting in an improvement in the stability of products under more intense stress.

Figure 7全屏查看

Stress-strain curves of samples stabilized with LGNMF composite by 20% substitution of lime with NM after 28 d of curing: (a) At 20 ℃; (b) At 40 ℃

Another explanation can be that the presence of cementitious compounds on the fibers could enhance the interfacial adhesion between fiber surfaces and the matrix (see the SEM images) and thus restrict their relative movement. Such a phenomenon would make fibers more effective as tensile reinforcing members, thereby developing the soil geo-mechanical behavior. In fact, the role of fibers in enhancing the geo-mechanical performance of stabilized clay can be mainly attributed to two mechanisms. First, based on the outstanding tension capacity of the PP-fibers, it may be assumed that fibers tend to bridge the formed gaps during the application of load, thereby creating a firm grip in the system which leads to a decline in the level of stress on the soil particles [21, 29, 30, 47]. By attenuating the initiation and expansion of cracks, fiber addition restricts soil deformation. This seriously affects the bonding skeleton of the matrix, dramatically reducing the brittleness trend, as can be seen in the stress-strain curves with relatively smoother post-peak responses. This indicates improved resistance to deformation after the application of fiber. Second, the presence of high content of cementitious compounds in LGNM blend might ameliorate the fiber-clay interfaces, restricting their relative movement. This contributes to a more integrated structure within the matrix when subjected to loads, resulting in a gradual failure response of specimens treated with LGNMF composite. Given that at low levels of cementitious phases, the bonding in matrix might be weak, the interfacial adhesion of the fiber surfaces with the soil particles can be minimal. As a result, the fiber net (i.e., a spatial three-dimensional mesh) will not be efficiently formed, leading to poor mechanical improvements [9, 11, 34]. To further clarify the contribution of fibers in improving the geo-mechanical behavior of the tested composites, the change in the energy absorption capacity (E_u) of the specimens was measured and presented in Figures 8 and 9. For this purpose, the area under the stress-strain curve up to the peak points in Figures 6 and 7 was considered as the energy absorption capacity (E_u) value of the stabilized soils. Furthermore, to determine the hardness of the mixtures and calculate the secant modulus (E₅₀) using the diagram in the aforementioned figure, the slope of a straight line drawn from the origin to the middle of the highest stress point was computed and presented in Figures 8 and 9. It is evident from Figure 6 that with an increase in the binder proportion, the slope of the stress-strain curve becomes sharper. This gradual sharpening signifies an increase in E₅₀, indicating that addition of binder can increase the energy absorption capacity of soil through improved particle interlocking mechanisms, as previously discussed.

Figure 8全屏查看

Variations of (E_u) and (E₅₀) of samples stabilized with LGNM composite by 20% substitution of lime with NM after 28 d of curing: (a) At 20 ℃; (b) At 40 ℃

Figure 9全屏查看

Variations of (E_u) and (E₅₀) in samples stabilized with LGNMF composite by 20% substitution of lime with NM after 28 d of curing: (a) At 20 ℃; (b) At 40 ℃

Based on the results presented in Figure 8, the E_u of the fiber-reinforced samples was higher than that of the samples without fibers due to the expansion of the area below the stress-strain curve (see Figure 7). The E_u values for the samples with 20% LG20NM and 20% LG20NMF were about 20.6 and 36 kJ/m³, respectively. These results underscore the increased energy required to deform and ultimately cause failure in the soil-fiber structure compared to the unreinforced system, a finding in agreement with the results obtained by WU et al [55] and GOODARZI et al [28], who reported a remarkable increase in the energy absorption capacity of cement treated soil in the presence of fiber. Thus, it can be argued that the strain of the samples containing fibers is higher than the samples without fibers, which will cause an increase in E₅₀, indicating the lack of growth of brittleness due to the presence of fibers. In other words, the fiber inclusion can change the failure mode of LGNM stabilized soil from a brittle pattern with distinct cracks (see Figure 6) to a ductile failure pattern featuring multiple micro-crack (see Figure 7) due to the bridging effect of fibers. Additionally, Figure 9 indicates that the elevated curing temperatures can play a prominent positive role in enhancing soil fracture energy, aligning with prior research [7, 9, 34]. This enhancement may stem from the increased formation of cementitious phase in this series of samples, fostering stronger fiber-clay interfaces and consequently bolstering resistance against external loading as highlighted in studies [56-60].

3.4 Effect of alkaline solution properties on UCS of GP-stabilized clay

Figure 10 illustrates the influence of varying proportions of alkali activator (AA) on enhancing the mechanical properties of the soil treated by LG20NM composite. The figure clearly indicates a rising trend in UCS of LGNM-treated soils in response to the Na₂SiO₃:NaOH ratio. For instance, after 28 d of curing, the UCS value of soil sample stabilized with 20% LG20NM and AA solution (Figure 10(b)) is about 5 MPa, representing a 1.25-fold increase compared to the strength achieved with LGNM treatment alone as depicted in Figure 10(a). In fact, adding the AA solution to the mixture of soil-LGNM can lead to the further dissolution of slag/NM/clay-based aluminosilicate during the process of geopolymerization, that can steadily produce amorphous 3D network silico-aluminate structures via polycondensation reactions. This may not only help in filling the void spaces in the soil (i.e., reducing pore volumes) but also contributes to the additional generation of cementing phases in the system, yielding a higher strength. As can be seen, the effect of Na₂SiO₃ on increasing UCS is more pronounced than that of NaOH alone. In other words, when NaOH alone is used as AA solution, the UCS of the samples reflects a minimum value; however, an increase in the Na₂SiO₃ level may lead to a greater increase in the soil strength.

Figure 10全屏查看

UCS variation of treated samples cured for 28 d at 40 ℃: (a) Sample with LGNM composite alone; (b) Sample with LGNM composite and alkaline activating solution

In general, the geopolymer material can be produced by mixing GP precursor and alkaline activator (AA) [2, 5]. Based on the previous studies [7], AA has a prominent role in the process of polymerization and its typical solutions are usually composed of NaOH and/or Na₂SiO₃. Indeed, the application of NaOH alone (particularly at high dosages) may negatively affect the geopolymerization reaction rate [61], and thus hinder the generation of aluminosilicate gels, resulting in a decrease in the UCS values as shown in Figure 10(b). Therefore, using the alkaline solution with a permutation of NaOH and alkali metal (e.g., Na₂SiO₃) is often recommended to solve this issue in the GP synthesis and expedite the kinetics of polymerization [62], leading to increased strength improvement. The enhanced performance of the slag-treated clay upon adding the AA solution aligns well with previous findings reported by SALIMI and GHORBANI [7] and GHAFFARY et al [63]. As shown in Figure 10, the highest UCS for sample having AA solution is attained at the Na₂SiO₃:NaOH ratio of 3:1; whereas, further increase in Na₂SiO₃ content leads to a decrease back in the soil strength, likely because of some negative effects on the release of alumina and silica from the precursor owing to the lower dosages of hydroxide ions in the soil pore fluid (i.e., pH of the system not high enough). Moreover, the high level of Na₂SiO₃ in the activating solution may also have hampered structural formation [64], resulting in a mechanical degradation on the soil matrix. It is worth noting that similar studies [65, 66], in the absence of NM recommend a lower optimal ratio of Na₂SiO₃:NaOH to achieve the best geo-mechanical performance. This may be attributed to the fact that the LGNM treatment needs a less concentrated alkaline solution to produce the GP material due to the high pozzolanic activity of NM particles.

3.5 Assessing the interactions between the soil particles and binder through the pH and E_C tests

Once the soil-binder pH reaches approximately 12.4, silica and alumina are released from the soil particles and are then combined with hydroxide ions to form Al(OH)₃ and Si(OH)₂. Afterward, such elements can be combined with free calcium ions in the soil pore fluid to produce the hydration products such as CSH and CAH gels. These newly formed pozzolanic materials can act like glue and stick the particles together [4, 19, 27], thereby significantly increasing the binding effect between the clay particles that might result in the improvement of geo-mechanical performance of soil, as clearly shown in Figures 3 and 5. Hence, to further clarify the interactions between the clay particles and the used binders, the pH of those composites was measured and presented in Figure 11.

Figure 11全屏查看

The pH variations of natural and stabilized kaolinite: (a) Samples with lime cured at 20 ℃; (b) Samples with lime cured at 40 ℃; (c) Samples with LG20NM cured at 20 ℃; (d) Samples with LG20NM cured at 40 ℃

As shown in this figure, the pH value of the soil containing 5% lime at 20 ℃ after 3 d of curing was approximately 12.45. This implies that such a treatment can create an appropriate alkaline environment for the aforesaid pozzolanic reactions [28]. It was found that the proportion of binders (L and/or LGNM) increased the pH of the samples at all curing time due to the higher presence of hydroxide ions (OH^-). On the contrary, after adequate curing time, the pH value of the soil-binder mixtures began to decline, a phenomenon that could be attributed to the consumption of solutes and a decrease in the concentration of hydroxide ions during the pozzolanic reactions [7]. Comparing the results in Figures 11(a) and (c) reveals that in samples stabilized with lime alone, the pH levels are slightly greater than that obtained for the sample stabilized with LGNM blend. This observation can be linked to the further pozzolanic activity of the latter system, which is in line with the macro test results. In fact, lime in the LGNM treated samples may be preferentially consumed for generating cementitious gels due to its direct interaction with the slag or NM, and therefore, the reduced presence of free hydroxide ions in the system with LGNM leads to a more pronounced decrease in soil pH values.

Besides, it can be concluded from Figures 11(c) and (d) that the maximum pH value pertained to the samples with 20% LG20NM, which would reach 12.53 at 20 ℃ after 3 d of curing and would fall to 11.96 at 40 ℃, indicating a reduction in the pH levels with the increase in curing temperatures. In this case, the high curing temperature can effectively accelerate the rate of pozzolanic operations, leading to a remarkable increase in the quantity of cementing gels generated [7, 31, 67]. It can accelerate the consumption of additives and thus lead to a reduction in the rate OH^- ions distributed in the system, thereby decreasing the pH values of the specimens. As a result, the stabilized samples cured at 40 ℃ exhibit a lower pH level as compared to the soil samples experiencing the ambient condition (20 ℃). Figure 12 shows the changes in E_C trend in the LGNM-treated specimens at curing conditions of 20 and 40 ℃ after 2 h to 90 d (2160 h) of curing. As stated before, immediately after the addition of binders to soil samples, the concentration of solutes in the pore fluid of the soil increases due to short-term reactions, resulting in a dramatic increase in the E_C levels. This tends to provide a denser fabric due to an increase in the osmotic pressure, causing the enhanced geo-mechanical behavior of the stabilized clay soils. Figure 12 also illustrates a decrease in the E_C values of soil specimens as the curing time progresses. This trend is apparently due to the fact that the additives are gradually consumed during the prolonged pozzolanic process, which may consume the additive, thereby leading to a reduction in the ion concentration and thus a decline in the E_C levels.

Figure 12全屏查看

The E_C variations of natural and stabilized kaolin: (a) Samples with LG20NM cured at 20 ℃; (b) Samples with LG20NM cured at 40 ℃

More importantly, the E_C values in the samples cured at 40 ℃ were lower than those obtained for the samples processed at 20 ℃. From Figure 12, the highest E_C value was attained for the composite sample treated with 20LG20NM, which would reach 5.17 mS/cm after one day of curing at 20 ℃, and would reach 4.08 mS/cm at 40 ℃. Indeed, it is well documented that the pozzolanic process is temperature dependent and the E_C values are significantly affected by the concentrations of dissolved ions in the soil-additive mixture [9, 31]. On the other hand, as previously described, the elevated temperature can drastically accelerate the rate of pozzolana (slag/NM/clay-based aluminosilicate) dissolution, so does the progress of hydration reactions [11]. In other words, it can be argued that the elevated temperature may serve as a catalyst for long-term reactions, which can ultimately lead to a better mechanical performance as shown in Figures 3 and 5. Hence, the formation and development of cementing gels in the soil matrix can follow a raising trend with increase in the curing temperature [7, 68]. As a result, the level of cations and anions in the system decreases owing to the further consumption of the amendments upon the treatment at a higher curing temperature, causing a notable decrease in the electrical conductivity of the soil-binder mixtures. Therefore, the soil samples cured at 40 ℃, which reflect higher pozzolanic activity, and thus a lower rate of free ions in the pore fluid, display smaller E_C rates than those cured at 20 ℃, as clearly confirmed in Figure 12. This finding is well consistent with the consolidation test results, in which the soil samples cured at 40 ℃ exhibited reduced compressibility potential due to increased pozzolanic activity and the subsequent formation of hydration products.

3.6 Effects of composite and geopolymer binders on micro-structural behavior of soil samples

In conjunction with the mechanical and chemical experiments, to corroborate the findings from the macro scale tests which indicated that the higher stability of stabilized soil in the presence of LGNM composite could mainly be due to an intensified pozzolanic process, the formation of more cementitious products, and further refinement of pore structure, the representative samples were subjected to SEM and XRD tests. The SEM micrographs of the 28-d stabilized samples with 20LG20NM and 20LG20NMF and the raw soil (without any binder) are displayed in Figures 13(a)-(d). The obtained results reveal that the untreated soil (Figure 13(a)) has generally a non-integrated structure and the clay grains are completely separated from each other. Such a poor arrangement may provide unfavorable engineering properties like low strength and high compressibility, as shown in Figures 3 and 5. In addition, the SEM micrographs provide visual evidence for a relatively aggregated structure and the presence of new cementitious materials after the addition of LGNM blend (Figure 13(b)) and a slag-base geopolymer containing 20LG20NM+Na₂SiO₃:NaOH solution in a 3:1 ratio (Figure 13(c)). As stated in the last section, the observed improved structure of stabilized specimens can be linked to both physical reactions (e.g., cation exchange and flocculation/aggregation) and pozzolanic activity. As illustrated in Figure 13(c), the appearance of a more compact and denser micro-structure indicates an increased filling effect of the nano slag-base geopolymer due to enhanced pozzolanic activities and the formation of additional cementitious gels which might have resulted from utilization of such a treatment, leading to the development of amorphous 3D network silico-aluminate phases [3, 10]. This would reflect the effective binding ability in the soil matrix under the application of LGNM geopolymer. The mechanism behind such a behavior could be mainly ascribed not only to the more generation of cementing phases in the LGNM system, but also to the microstructural nano-modifications at the presence of NM. In fact, as addressed before, the integration of nanomaterials into the lime stabilized soil can act as catalyst for short-term reactions through physical filling and as catalyst for pozzolanic process through the nucleation effect, both of which are effective strategies to improve the soil porous network and ameliorate the binding of clay particles. Consequently, a further improvement in the engineering properties of those specimens may occur. These results are in complete consistency with the large-scale tests. It should also be noted that adding fibers (Figure 12(d)) to the stabilized specimen would enhance the particle interlockings and thereby improve the soil geo-mechanical performance, as clearly confirmed by the results of UCS tests.

Figure 13全屏查看

SEM images: (a) K; (b) K+20LG20NM cured for 28 d at 40 ℃; (c) Soil-slag geopolymer containing 20LG20NM+Na₂SiO₃:NaOH solution at ratio of 3:1; (d) Reinforced sample treated with the 20LG20NM

Generally, it is evident from Figure 13(d) that the inclusion of fiber tends to enhance the cohesion within the soil mass, implying that fibers can act as mechanical reinforcements which do not modify/alter the innate characteristics of clay particles. The fibers have the ability to bridge the voids within the stabilized composite which in turn improve soil’s bonding skeleton, and reduce stress level on the soil particles under external loads. Besides, the observed cementitious compounds formed on the surface of fibers tend to augment the adhesion at the fiber-clay interface (i.e., a more interface action). This can mechanically interlock the soil particles and fibers, which will ultimately prohibit their relative movement and thus generates an adhesive matrix under increased stress levels. These refinements combined with the interweaving of fibers enable the matrix to reflect a more integrated structure, which restrains the stabilized soils from undergoing considerable deformation, resulting in a gradual failure response as shown in Figure 7. It is worth noting that as the purpose of presenting SEM images in this study was mainly to compare the morphological features of the selected samples in different conditions before and after the stabilization process, it seemed necessary to use the same image scales for the ease of comparisons. On the other hand, after assessing various magnification levels to enable concurrent observation of the arrangement of kaolinite particles, the presence of cement nanostructures and their distribution on the fibers, we found the appropriate magnification to fall within the range of 3000× to 3500×, consistent with previous stabilization studies (e.g., ROZBAHANI et al [6], VAKILI et al [19], PATERIYA et al [44], SAFA et al [69]). The XRD patterns in Figure 14 are related to (a) the natural soil sample, (b) the soil-lime mixture, (c) the soil stabilized with 20% LG20NM, and (d) the soil-slag geopolymer mixture containing 20% LG20NM + Na₂SiO₃:NaOH solution at a ratio of 3:1. As can be seen in this figure, in all the stabilized soil samples, while the peak intensities of the clay mineral dramatically decrease, the peak intensities of some new hydration products such as CSH and CAH increase. In general, the observed decrease in the kaolinite reflexes might be connected to the coupled influences of short- and long-term reactions on the micro-structural behavior of soil, as explained in Section 3.2. In fact, the generation of an integrated structure along with the dissolution of clay mineral during the pozzolanic process (see Eqs. (2) and (3)) as well as the soil particles wrapped with the cementitious gels, can lead to a reduction in the reflection of radiation [6, 28], which ultimately would lessen the peak intensity of clay minerals compared to the untreated soil sample. An analysis of the formation of hydrated calcium silicate in LGNM-containing sample (compared to the sample with lime alone) can confirm the positive effects of LGNM composite on intensifying particle cementation.

Figure 14全屏查看

XRD patterns: (a) K; (b) K+5% L cured for 28-d at 40 ℃; (c) K+20LG20NM cured for 28-d at 40 ℃; (d) Soil-slag geopolymer containing 20LG20NM+Na₂SiO₃:NaOH solution at ratio of 3:1

Based on the presented results in Figure 14, the amount of CSH gel in the LGNM-stabilized sample is significantly higher (up to 30%) than that for the sample treated with lime alone. Herein, the direct contact of lime with the active aluminosilicate phases in the slag and NM (see Table 2) may have drastically accelerated the rate of pozzolanic reactions, which in turn triggers the formation of more cement nanostructure phases in the soil even more than the conventional stabilization approach (i.e., the application of lime alone). This is because a portion of the additive in the former system is used for carbonation and short-term reactions like cation exchange and flocculation/aggregation [2, 34, 68]. As a result, more lime is required to complete the process of soil-additive interaction. In other words, a further increase in the peak intensity of CSH and CAH can mainly reflect a higher pozzolanic activity within the sample stabilized with the LGNM blend (Figure 14(c)) as compared to the soil amended with the lime alone (Figure 14(c)). The generated new cementing phases can block some available voids in the pore structure and help in interlocking the soil particles [2, 3, 4, 19]. This micro-structural reorganization can improve the mechanical behavior of solidified clayey soils [1, 7], resulting in a higher UCS coupled with a notable decrease in the C_C values upon the LGNM application, as confirmed in the last sections. Besides, as can be seen from Figure 14(d), the activation of LGNM blend with the AA solution leads to an additional increase in the generation of cementitious gels like CSH and CAH. This can be attributed to an intensified dissolution of slag/NM/clay-based aluminosilicate during the geopolymerization process, which produces amorphous 3D network silico-aluminate structures through polycondensation reactions. This may help in filling the void spaces in the soil, reducing pore volumes, and enhancing binding in the soil mass, thereby contributing to increased strength levels, as confirmed in Figure 10.

In order to further improve the impact of binders on modifying the internal microstructure of composites, the change in the permeability of soil samples was also determined through oedometer experiments, with the results presented in Figure 15. It is essential to note that soil permeability plays a vital role in ensuring the long-term stability and effective performance of geotechnical structures such as earthen dams and clay liners in landfills.

Figure 15全屏查看

Variation of the permeability coefficient (k) of the soils treated with various levels of binders and curing periods: (a) Samples with the LG alone; (b) Samples with the LG20NM composite

Based on the presented results in Figure 15, the permeability coefficient, k, depends on the type and level of amendments. Generally, in all the treated samples, the permeability of soil samples decreased as the amount of binder increased. The reason for the lower permeable nature of these specimens can mainly be ascribed to the generation of further hydration products like CSH and CAH, which could gradually block off the available soil micro-pores, resulting in a reduction in the proportion and size of pores. This will allow lower water to pass through the soil matrix, leading to a continual reduction in the k value of the stabilized specimens as the dosage of binders increases. Besides, as previously mentioned, over time, the binders and the clay particles may undergo prolonged pozzolanic reactions, which tend to ameliorate the generation of viscous cementitious gels, a phenomenon which continuously facilitates the binding in the soil matrix and helps in interlocking the clay particles. This can profoundly fill the voids within the soil matrix, reducing easy drainage of water [37]. The results in Figure 15 also demonstrate that at the same dosage of binder, the inclusion of optimum amount of NM into the LG matrix would cause a significant decrease in the soil permeability. For instance, the k value of soil treated with 20% LGNM will approximately be 200 times lower than that obtained through the application of 20% LG alone after 90 days of curing. In fact, the extremely small particles of NM can act as fillers [12, 13], leading to a notable reduction in the porosity within the soil structure and therefore resulting in a reduction in soil permeability. Besides, the presence of nanomaterials (i.e., NM) augments the pozzolanic operation in the system through the nucleation effect, which accelerates the rate of hydration reactions, as previously addressed. This leads to a more significant reduction in the permeability of the soil/LGNM mixture by facilitating the generation of additional cementitious compounds within the soil mass (see Figure 14). These compounds gradually occupy the void pores and thus lead to reductions in their distribution, and in their size, tending to create fewer flow paths [7, 31]. As a result, the LGNM solidified clay exhibits a significantly lower permeability coefficient value, which agrees well with the observed modification of pores network within the stabilized samples (see the SEM images in Figure 13). Under these conditions, the transport of water and ions into and from the treated material will decrease, indicating a significant improvement on the compatibility of LGNM system against aggressive environments.

In sum, as previously discussed, the production process of traditional soil stabilizing agents (e.g., cement and lime) faces many challenges, since it consumes large amounts of raw feedstocks and nonrenewable energy resources; moreover, it severely contributes to greenhouse gas (mainly CO₂) emissions [8, 69]. Nowadays, alkali-activated materials (AAMs) are suggested as potential multifaceted supersedes for such binders due to their cost-effectiveness and minimal environmental impacts [70-72]. In general, two approaches are recognized for manufacturing this relatively new class of viable composite materials including (i) two-part (TP) and (ii) one-part (OP) or “just add water” [71-74]. The two-part alkali-activated materials (TP-AAMs) consist of aluminosilicate-rich precursors mixed with aqueous solutions of high-alkaline activator (AA). However, there are some obstacles (e.g., they are not user friendly, can have harmful effects on human skin, and require concentrated alkali solutions) that prohibit the large-scale utilization of TP-AAMs, making them risky to workers during handling and mixing as well as leading to an increase in the cost and energy requirements. Another challenge that may restrict the widespread use of TP-AAMs is their brittleness behavior and low tensile strength [75]. The concerns with such disadvantages have put pressure on the development of OP-AAMs or one-part geopolymer (as a sub-group of AAMs), which are used like lime or cement. In fact, OP-AAMs technology (in which the aluminosilicate precursors are dry mixed with the solid alkaline activator and then water is added to the system) is a promising alternative in overcoming the difficulties associated with the conventional TP-AAMs and is expected to reveal much lower levels of carbon footprint than those associated with the application of cement and lime alone [76]. Currently, in order to produce a more eco-sustainable OP-AAM (as the latest generation of amendments), there is great interest to use various supplementary cementitious industrial slags like GGBS to benefit from their attributes in enhancing stabilization process and to transform the low-value materials into useful products (waste recycling) and to alleviate reliance on natural precursors and the demand to consume energy in producing the traditional binders [27, 33, 51]. Furthermore, in the production of OP-AAMs or geopolymeric dry mixtures, employing mechanochemical activation by incorporating highly reactive silica or alumina sources (such as nanomaterials) along with fibers can present an innovative approach to enhance the binding capability and ductility of the matrix [76-80]. This may offer immense potentials to manage the consumption of AA component and thus mitigate its adverse influences. Therefore, it can be considered as a more attractive and eco-friendly technique to soil stabilizing by AAMs that, unfortunately, has been poorly documented particularly in the case of stabilization of weak soils under ambient conditions and/or elevated curing temperatures. Consequently, the present study was performed to enhance the soft clayey soil treatment effects using a novel mechanochemical activated slag-based GP by the optimized inclusion of nano-admixture (i.e., NM) and PP fiber upon different curing regimes. The obtained results revealed that the LGNMF composite may feature a broader range of economic, technical and environmental benefits compared to the traditional soil stabilizing agents.