Physical and mechanical properties of Nano-SiO₂-treated loess

Abstract

Improving the performance of loess significantly protects it against failure and degradation, and is important for rammed earth and infrastructure constructed with loess materials. The physical and mechanical properties of nano-SiO₂-treated loess were tested with different contents and curing days—including unconfined compressive strength (UCS) and splitting tensile strength (STS)—and their corresponding water content, density, and void ratio. I paid close attention to the homogeneity during sample preparation—that is, sample quality—based on the UCS test. I then analyzed their relation to physical and mechanical properties to gain a better understanding. The results show that the UCS test is a valuable method for examining the quality of sample preparation. UCS, STS, and density increased, and the water content and void ratio decreased with increasing content and curing days due to nano-SiO₂ addition. The improvement of mechanical strength is related to the ratio of water content to nano-SiO₂ content and curing period rather than the physical properties and additive contents of treated loess. These findings reveal that nano-SiO₂ can be an effective stabilizing agent for loess improvement, which has important implications for geohazard mitigation and engineering management in the Chinese loess area.

Highlights

1 Introduction

Nano-SiO₂ is known for its excellent properties, small particle size, large surface area, and high activity. It is thus widely used as a composite material in civil engineering. Recently, nano-SiO₂ has also been used in soil improvement in geotechnical engineering due to its eco-friendly characteristics and stabilizing effects.

Previous research has shown that nanoparticles added to soils can enhance their strength, stiffness, and durability and can decrease their collapsibility, permeability, moisture, and liquefaction risk (Gallagher, 2000; Hung, 2007; Taha and Taha, 2012; Kim et al., 2014; Ng and Coo, 2014; Ren and Hu, 2014; Ali Zomorodian et al., 2017; Ghasabkolaei et al., 2017; Kong et al., 2018). These changes can generally be attributed to the structural modification of soils. Meanwhile, no changes in mineral composition have been observed in nanoparticle-treated clay (Taha and Taha, 2012; Kong et al., 2018). Previous research has mostly used different nano-clays and nano-metals as additives, and few attempted to use nano-SiO₂ to improve soils. However, little effort has been made to determine the effect of nano-SiO₂ on the physical and mechanical properties of treated loess (Ren and Hu, 2014; Kong et al., 2018). Meanwhile, these physical and mechanical properties have afforded a direct index to assess the improved performance of treated loess. However, their quantitative relationship has not been given enough attention.

In the research of soil improvement, the behaviors of treated soils commonly use laboratory-prepared samples. Hence, prepared samples with uniform density (i.e., homogeneity) are a fundamental problem in assessing test results and analyzing experimental data. Generally, there are two ways to solve the problem of soil mechanism research. One is to enhance homogeneity during sample preparation using new techniques and apparatuses, such as thermal cycling and apparatuses with three-point temperature control (Ma et al., 2015; Nadimi and Fonseca, 2016). Another is to examine the homogeneity of the prepared samples using different methods. This kind of research has gained more attention than sample preparation. Researchers generally used image techniques, including scanning electron microscopy (SEM), X-rays, computed tomography (CT), and digital cameras, to observe the uniformity of samples (Kuo and Frost, 1996; Frost and Park, 2003; Wong, 2003; Yamamuro and Wood, 2004; Hattab et al., 2013). Some have used basic properties, such as water content and void ratio at different sample locations, to analyze sample quality (Frost and Yang, 2003; Ma et al., 2015). In addition, Metelková et al. (2012) used nitrogen adsorption to obtain micropore characteristics for examining sample homogeneity. These methods are influenced by sampling strategy and data, and most are expensive, labor-intensive, and time-consuming. Hence, triaxial shear tests have become another direct method; these examine multiple prepared samples to observe their strain–stress curves (Kuganenthira et al., 1996; Kuo and Frost, 1996; Frost and Park, 2003; Frost and Yang, 2003; Yamamuro and Wood, 2004; Fener and Yesiller, 2013; Ma et al., 2015; Nadimi and Fonseca, 2016; Ng et al., 2017). In soil improvement research, the usual method is to perform UCS tests three times for the average value of treated samples. Nevertheless, less attention has been paid to examining the prepared sample’s homogeneity with additives (Metelková et al., 2012).

This study examined the effect of nano-SiO₂ on the physical and mechanical properties of loess treated at different contents and curing days. It examined the changes in the UCS and STS untreated and treated loesses, their corresponding water content, density, and void ratio, and the relationship between physical and mechanical properties. The homogeneity of the treated samples was examined using strain–stress curves in the UCS test. Finally, the relationship between the physical and mechanical properties of nano-SiO₂ treated loess is analyzed and their changes discussed to understand the cause of improvement of nano-SiO₂-treated loess.

2 Material and methods

2.1 Tested materials

The tested loess is widely deposited in the Malan formation in the Chinese loess plateau. The loess was taken from Lanzhou City, Gansu Province, China. The additive is a commercially available nano-SiO₂. Their basic properties are listed in Table 1.

The particle size distribution and structures of the loess and nano-SiO₂ are shown in Figures 1 and 2. It can be seen that nano-SiO₂ exists as aggregate similar to loess. The mean particle size of the nano-SiO₂ aggregate is approximately 20 μm. The particle size of nano-SiO₂ seems unreasonable , but the result is reasonable due to the presence of nano-SiO₂ aggregates rather than single particles (Figure 2b). Hence, it is necessary to analyze the results of nanoparticle size by a laser diffraction instrument.

FIGURE 1

FIGURE 2

The X-ray diffractions for loess and nano-SiO₂ are shown in Figure 3. The loess is mainly composed of quartz, feldspar, and calcite, including minor chlorite, dolomite, and illite (Figure 3a). The nano-SiO₂ is purely quartz in composition (Figure 3b).

FIGURE 3

2.2 Tested methods

2.2.1 Sample preparation

The loess was first air-dried at room temperature and sifted using a 0.5-mm aperture sieve. The air-dried samples were further dried in an oven at 105°C. Distilled water was added to oven-dried loess to obtain a desired water content of 15%. Then, 0%, 0.2%, 0.4%, 0.8%, 1.0%, 1.5%, and 2.0% of nano-SiO₂ dry weight was thoroughly mixed with the wet loess by hand. The added sequence of water and nano-SiO₂ powder is crucial for uniform mixing. The mixture was kept moisture-uniform and sealed for 24 h in a plastic package. The samples were compacted in five layers inside a cylindrical steel holder with an internal diameter of 50 mm and a length of 100 mm. The sample reached a dry density of 1.47 g/cm³ with the help of a hydraulic jack. They were then sealed in a plastic film and cured at room temperature for 7 days, 14 days, 28 days, and 60 days.

2.2.2 Physical property

Water content and wet density were weighed after curing the prepared cylindrical samples at different nano-SiO₂ contents and curing days. There was a negligible change in the volume of all the cylindrical samples during curing. Bulk wet density was calculated from the cylindrical volume and wet sample weight, and the change in water content was calculated by sample weight before and after curing. The void ratio was then calculated, . All nano-SiO₂-treated loesses used an average specific gravity (G_s) of 2.70 due to a slight difference in test results. There was no apparent change in the specific gravity of the treated loess compared with untreated loess (G_s = 2.71). This may be due to the very light particle density of nano-SiO₂.

2.2.3 Unconfined compressive strength

Unconfined compressive strength (UCS) tests were performed using an automatic loading machine with a maximum loading capacity of 100 kN. The loading speed during all the tests was 0.1 mm/min. The UCS test was also used to check the homogeneity of sample preparation—that is, the sample quality. To this end, one group of all the specific samples were loaded in the same compaction direction during sample preparation. A second group of two specific samples were inversely loaded. In the series, one had the same loading direction with compaction direction during sample preparation, while the other was performed at opposite loading with its compaction direction. The average value of the specific samples was used in data analysis.

2.2.4 Splitting tensile strength

Splitting tensile strength (STS) is an indirect tensile strength test made by a mold (Figure 4B). The mold design was suggested by Tang and Graham (2000). The tensile mold consisted of two separate semi-circular steel supports with a lower inner diameter of 50 mm and an upper inner diameter of 40 mm. The gap offers a deformation space for the mold-holding sample. The STS test is similar to the UCS test with the same loading speed (i.e., 0.1 mm/min), but the mold-holding samples were horizontally placed on the loading plate.

FIGURE 4

In the STS test of the soil samples, the loading force constantly increased with increasing strain (Figure 4). It significantly differed from the rock samples, and their loading force sharply decreased once the samples fractured. Based on observation and analysis, I propose a method for evaluating the STS value which is determined at point F in the stress–strain curve (Figure 4). The F point essentially occurs when the sample breaks with a typical dimple fracture in materials science (Figure 4B). In fact, the fractured surface already exists at F point. However, the loading force shows a continually slow increase due to compaction.

3 Results

3.1 Homogeneity of the sample

Figure 5 shows the stress–strain curves in special samples with the same loading direction as sample preparation. It can be seen that they have almost the same stress–strain curves and, correspondingly, almost identical strength. The sample preparation has a negligible effect on strength, although the change in strain with loading exists to a certain variance. It also meets the requirements of data analysis.

FIGURE 5

Figure 6 shows the stress–strain curves of two nearly identical samples with inverse loading direction. The solid line (S1) represents the same loading direction as the sample preparation, and the dotted line (S2) represents the opposite loading direction. To all tested samples, the peak of S1 is always greater than that of S2, while the strain location at the peak of S1 occurs later than those at the peak of S2. After this, there is an apparent drop in the strain–stress curve of S2. This may be because the upper layer is slightly looser than the lower layer of the prepared samples. Moreover, the trend of the strain–stress curve is similar. After fracture, the strength of S1 declines uniformly, but the sample of S2 still came through densification. Therefore, the homogeneity of the samples is high, with only very slight anisotropy in the whole sample.

FIGURE 6

The tested results reveal that the compaction technique used in the research can produce high-quality samples. Meanwhile, the UCS test is not only a simple and low-cost method for examining the quality of the sample preparation but can also afford direct test results for data analysis. In addition, the special properties of nano-SiO₂, such as intense activity and high free energy, facilitate particle dispersion, likely promoting the homogeneity of the sample. It is interesting to note the added sequence of water and nano-SiO₂ during sample preparation. According to my experience, dry nano-SiO₂ added into curing wet soil is a better choice to guarantee sample homogeneity. Otherwise, it will cause moisture accumulation, making it easy to produce large aggregates in the mixing process.

3.2 UCS

Figure 7 shows the effect of nano-SiO₂ content and curing days on the UCS of untreated and treated loesses. It can be seen that the UCS of treated loess increases with the increasing nano-SiO₂ content and curing days. Moreover, the effect of curing days on UCS is more striking than that of nano-SiO₂ content. There is a striking increase in UCS when nano-SiO₂ content is more than 0.2% (Figure 7c). The apparent increase in UCS is also observed after 28 days. The strength of nano-SiO₂ content of 0.2% is approximately 325 kPa at 60 days, which is approximately three times higher than that of nano-SiO₂ free loess.

FIGURE 7

The test results reveal that curing days significantly improve the mechanical properties of nano-SiO₂ treated loess. As shown in Figure 8, even at low nano-SiO₂ content, UCS increases to different curing days. For short-term curing periods—7 and 14 days—the ductile behavior is integrally maintained compared with longer curing periods—28 days and 60 days. All of the treated samples became much more brittle than the untreated samples.

FIGURE 8

3.3 STS

Figure 9 presents the stress–strain curves of the splitting tensile strength of treated loess with different contents of nano-SiO₂ and curing days. The splitting tensile strength of the treated loess was generally higher than the untreated loess with the same curing days. At the same curing time, the splitting tensile strength of treated loess increases with the content of nano-SiO₂. At the same additive content of nano-SiO₂, the STS increases significantly with curing days, especially at 28 and 60 days. This is different in the USC of treated loess. The STS did not improve substantially under a longer curing period when the loess was treated with a low additive content nano-SiO₂.

FIGURE 9

In the stress–strain curve of the splitting tensile test (Figure 9), the axial strain of both the untreated and the treated loess increases rapidly with the stress in the initial stage. After failure, the curve shows a slow upward trend with the axial strain, which is the compaction progress along the fracture surface. In the short curing period of 7 and 14 days, the failure points of the untreated and treated loess are primarily close to the position at 1% of strain. However, after the long curing period of 28 and 60 days, the failure point of the untreated and treated loesses is between 1% and 2%. After the failure point is exceeded, the compaction trend is weakened. Thus, the ability of untreated and treated loess to withstand splitting is enhanced with nano-SiO₂, which is more obvious under a longer curing period.

Figure 10 summarizes the STS variation of all untreated and treated loesses under different nano-SiO₂ content and curing days. The STS of treated loess increases with increasing curing periods at the same additive content of nano-SiO₂, while the strength of all samples increases linearly with increased nano-SiO₂ content. This differs from that of the UCS of treated loess, which shows a variation of nonlinear increase at the long curing periods of 28 and 60 days.

FIGURE 10

3.4 Physical properties

Figure 11 shows the physical properties of the untreated and treated loesses with nano-SiO₂ content subjected to different curing periods, including the water content, wet density, and void ratio of all samples. All the samples have the same initial conditions: water content of 15.0%, wet density of 1.72 g/cm³, and void ratio of 0.81. After curing for 7 days, the water content and density of the treated loess reduced slightly. With the increase of nano-SiO₂ content, the water content and density are slightly lower, but not obviously so. With curing periods of 14 days and 28 days, the water content and density of the treated loess show a progressive decrease. The water content reduces to 11.8% and 10.8%, and the density reduces to 1.66 g/cm³ and 1.64 g/cm³, respectively. However, the water content of treated loess decreased sharply to approximately 7.5% after a curing period of 60 days, while the density drops to 1.59 g/cm³. At the same time, the dry density results show that all samples maintained the dry density of 1.47 g/cm³ in the initially designed state. This result indicates that the decrease in density is mainly due to the decreased water content in treated loess. Hence, the void ratio of the treated loess increased slightly with the increase of the curing periods (Figure 11C). When the water content in the sample decreases, the suction between particles increases, resulting in closer contact with particles.

FIGURE 11

As shown in Figure 12, under the same curing days, the nano-SiO₂-treated loess has almost identical water content consumption and density reduction to the untreated loess. In addition, the dry density of treated loess did not change, including a slight decrease in void ratio. This means that the reduction of water content in treated loess is the same as in untreated loess.

FIGURE 12

4 Analysis and discussion

4.1 The relationship of mechanical properties

Figure 13 shows the relationship between untreated loess and nano-SiO₂-treated loess unconfined compressive strength (UCS) and split tensile strength (STS). From Figure 13, it is evident that UCS and STS show a relationship of exponential function and their correlation coefficient is close to 0.9. The exponential relation is because the UCS of treated loess presents a nonlinear relation under the long curing periods. In contrast, the STS of treated loess still presents a linear relationship under the high curing period.

FIGURE 13

Previous research has found that the improvement of mechanical strength of nano-SiO₂-treated loess can be attributed to coarser particles and denser contacts (Kong et al., 2018). According to this research, this is due to nano-SiO₂-induced structural modification, causing the formation of aggregate and more uniform aggregate. Some researchers have found that the aggregate can remarkably modify the mechanical behavior of the loess due to the structural adjustment (Zhang et al., 2013; Duan et al., 2024). Thus, the multiscale standpoint should help explain the improvement mechanism of nano-SiO₂-treated loess. Moreover, the solidification has contributed little to the increase in strength due to reduced water content. They also concluded that the changes in nano-SiO₂-treated loess result from physical effects rather than chemical activities (Kong et al., 2018). This finding is consistent with those found in tests on clays treated with other nanoparticles (Taha and Taha, 2012). This differs entirely from chemical additives such as lime, fly ash, and cement (Metelková et al., 2012; Pei et al., 2015; Zhang et al., 2017; Zhang et al., 2018). These chemical additives generally produce short-term modification and long-term stabilization. Modification typically results in decreased water content and increased dry density, while stabilization results from a pozzolanic reaction, causing the formation of coarse aggregates and new minerals. As a result, chemical additive-treated loess has a noticeable strength enhancement. It should be noted that adding a small amount of nano-SiO₂ can also markedly enhance mechanical strength. Hence, nano-SiO₂ can be used as a potentially cost-effective material in soil improvement.

4.2 The relationship of physical-mechanical properties

Figure 14 shows the relationship of ratios of after-curing water content to nano-SiO₂ content (W/C) against the UCS and STS of nano-SiO₂ treated loess at different curing days. The W/C ratios exist in the power function relationship with the UCS of treated loess with different nano-SiO₂ contents after various curing days (Figure 14a). This may be mainly because the water content of treated loess does not decrease with the increase of nano-SiO₂ content but does decrease with the increase of curing days. As shown in Figure 11a, the water content is almost the same under each curing period.

FIGURE 14

The W/C ratios of treated loess have a strong power function relationship with STS (Figure 14b). This may be mainly because the STS of treated loess continuously increases with increasing nano-SiO₂ content and curing days. Under long curing periods, the STS of untreated and treated loess observably increases because the nano-SiO₂ consumes a large amount of water in the sample, which significantly correlate with the lower water content (Figure 12a).

There is no apparent relationship between the after-curing water content of the UCS and STS of treated loess. A relatively close relationship exists between the W/C ratio and mechanical strengths. This finding is consistent with those obtained in other studies of cement-treated clays (Boutouil and Levacher, 2005; Lee et al., 2005; Lorenzo and Bergado, 2006). In addition, there is for chemical additives, such as cement and lime-treated soils, a close relationship between the after-curing void ratio to additive dosage and mechanical strengths (Lorenzo and Bergado, 2006; Consoli et al., 2010). Nevertheless, nano-SiO₂ treated loess does not have such a linkage due to the almost constant void ratio. In addition, the consumed water is strongly related to the curing days of nano-SiO₂ treated loess (Figure 15). For cement treated tailings and sludge, the consumed water is entirely related to the cement content (Boutouil and Levacher, 2005; Koohestani, 2017).

FIGURE 15

These significant differences may be derived from different reactive processes and stabilized mechanisms induced by chemical additives and nanoparticles. As suggested previously (Taha and Taha, 2012; Changizi and Haddad, 2015; Kong et al., 2018), nanoparticles are physical modification processes lacking chemical reactions and cemented mineral generation. Thus, the ratio of after-curing water content to nano-SiO₂ content (W/C) may be a fundamental parameter for assessing the mechanical strengths of nanoparticle-treated soils. Hence, there is a need for further research on the basic parameters in nanoparticle treated soils.

5 Conclusion

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