To examine the mineralogical changes after the curing, X-ray diffraction (XRD) techniques were performed on the original and treated loess, which was collected from the specified radial point. All samples were scanned using a Philips PW 3710 X-ray diffractometer with Cu-Kα radiation. Data were recorded from 5 to 45 at a rate of 0.05°2θ/s per step. Meanwhile, a thermogravimetric analysis, including mass loss (TG) and its derivative (DTG), was performed up to 1,000°C in oven-dried samples, at a ramp of 10°C/min on a Perkin-Elmer Instrument, Waltham, MA. In this thermogravimetric test, 10 mg of sample was filled into the instrument.

The macro-texture and micro-structure of the original and treated loess were examined. The macroscopic particle size distributions of these samples were determined using the sieve and sedimentation methods (ASTMD422-63, 2007). The micro-morphology and micro-size were observed on the representative samples after metallization with gold powder using a JSM-5600 LV Scanning Electron Microscope (SEM).

The slake test has been recommended to assess the aggregation or cementation of soil treated by stabilization agents Pei et al. (2015), due to directly visual structure stability. The slake test was performed on each desired radius. Cylindrical samples were prepared after the oven-dry test. The four samples on each desired radius were simultaneously soaked in the plate, and water completely covered them. Their stability was recorded accompaning the time by camera. Complete disintegration was indicated by no further breakage of samples. The disintegration rate was calculated by dry weigt of the samples divided by the completely disintegrated time.

Hardness value was determined using an Equotip hardness tester (Piccolo). Previous research has sought to assess the strength of loess stabilized with lime and fly ash piles (Pei et al., 2015). This study found the Equotip to be a convenient quantitative tool for assessing the changes in strength and stiffness of tread soils due to its ease of use and rapid outcomes. This study used the Equotip hardness test on a newly planed soil surface after obtaining the cutting ring samples, which allowed for rapid and multiple measurements at the desired radius. The hardness value was determined as the average value of 30 measurements along each desired radius from the nano-Sio₂ pile edge.

Density and water contents are two of the most commonly determined properties in characterizing the engineering behavior of soil. The preferred test method for the laboratory determination of the moisture content is the oven drying method. Density and water contents are determined as the reduction in the mass of the test specimen after oven drying. The specimens, cut using a circular ring of iron, were 2.8 cm in diameter and 1.5 cm in height at each desired radial point. All specimens were weighed, recorded, and then oven dried at 105°C for 24 h. The mass of the dry sample was recorded. The density and water contents were subsequently calculated, and the average value was used in the analysis.

Liquid and plastic limits were measured using the fall cone test following the Chinese standard procedures GB/T50123 (1999), which define the liquid and plastic limits for penetration as 17 and 2 mm, respectively. The tested samples were taken along each desired radius.

The total SSA and CEC of the samples were measured using a methylene blue spot test method. The test procedure for determining the total SSA of soil was described by Santamarina et al. (2002), and its CEC was calculated using the formula suggested by Çokça and Birand (1993). Chemical properties were measured using a water quality monitor, including pH, total dissolved solids (TDS), electrical conductivity (CE), and oxidation-reduction potential (ORP) of extracts from 1:5 soil to water.

The results of the XRD of the original loess and loess treated with nano-SiO₂ pile at different radial distances are shown in Figure 2. Based on the XRD diffractograms, there is no new reflection; only intensity is enhanced. Sharp changes occurred in intensity and full width in all treated loess, and the crystalline structure seemed to be quite insensitive after nano-SiO₂ mitigation into loess around the pile. Generally, the changes in reflection shape can be related to a structural adjustment that fabric tends to become more isotropic due to the increase in crystallinity (Choquette et al., 1987; Souli et al., 2013). Thus, only physical structure modification exists while lacking the chemical reaction (Kong et al., 2018). Meanwhile, a slight difference occurred in the mixture of nano-SiO₂ added into the loess; the intensity and full width indicated only slight enhancement in XRD diffractograms (Kong et al., 2018). As a whole, the physical structure modification shows that an isotropic process occurred due to the increase of crystallinity.

The TG-DTA results of this study are presented in Figure 3. Among these results, the TG-DTA curves indicated two noticeable changes in two temperature domains. The first significant drop finished at about 150°C, corresponding to the loss of absorbed water in treated the loess. The second striking change occurred in the temperature range from 600°C to 780°C. The samples influenced by nano-SiO₂ migration showed relatively lower and narrower peaks with lesser mass loss, and the turning point of the curves shifted toward a slightly lower temperature. In the first domain, the loss of the absorbed water of original loess is related to the absorption of migrated nano-SiO₂ due to its large surface area and high activity. Menwhile, it is striking compared to the changes in the second domain. Previous studies have found that more hydration products causing denser structure can produce taller and wider peaks (Wu et al., 2016), and the shift in the turning point toward a lower temperature is derived from a more stable stage for materials (Galindo et al., 2015). Hence, the apparent changes in the second domain contributed to the generation of a more stable structure. Meanwhile, a looser structure existed in the sample with a more significant distance from the nano-SiO₂ pile. These findings also support the XRD results.

The particle size curves of original loess and loess treated with nano-SiO₂ pile are shown in Figure 4. The macrostructure changes suggest a slight increase in the coarse fraction in the treated loess, with samples farther from the nano-SiO₂ pile becoming coarser. As a whole, the loess influenced by the nano-SiO₂ pile has a higher uniformity coefficient than that of the original loess. This finding also supports the idea that the loess involves an isotropic process with coarser particles due to crystallinity.

The SEM micrographs of the loess at 5 and 20 cm radial distances from the nano-SiO₂ pile are shown in Figure 5. The microstructure of the samples closer to the nano-SiO₂ pile is denser and more filling, with a more homogeneous state (Figure 5A), whereas that of the farther example is a looser and larger void space, with greater aggregation and more contact (Figure 5B). This result can be attributed to the degree of nano-SiO₂ migration into the loess. When the loess is close to the nano-SiO₂ pile, the degree of its migration is high; as a result, the nano-SiO₂ filled the pores and adhered to the soil particles (Figure 5A). Meanwhile, low nano-SiO₂ migration occurred in the loess farther from the pile. The few nano-SiO₂ particles mainly adhered to the surface of the soil particles, strengthening the bonding, while also filling the pores between particles. These created a bridge effect to link together soil particles as greater aggregates with larger void space (Figure 5B). The observation from microstructures effectively explains the changes in the particle size curve of the samples at a different radial distance from the nano-SiO₂ pile.

Figure 6 shows the disintegration characteristics of treated loess at different radial distances from the nano-SiO₂ pile with soaking time, and Figure 7 shows disintegration rate of treated loess at different radial distances from the nano-SiO₂ pile. It can be seen that the disintegration rate of the treated loess has an obvious decrease acomparing with original loess, and that the disintegration rate of the treated loess increased with with increasing radius from the piles (Figures 6, 7). These trends are matched with the denser packing from micro-structure observation, as shown in Figure 5. These results reveal that that the treated loess with the nano-SiO₂ pile have stronger cementation of aggregates, leading to more stable structure between aggregates.

The changes in water content, natural density, and void ratio of treated loess with different radial distances from the nano-SiO₂ pile are shown in Figure 8. As indicated, the water content of loess treated with the nano-SiO₂ pile is lower than that of the original loess (Figure 8A), while the natural density of the treated loess is noticeably greater than the original one (Figure 8B). As a result, the void ratio of the treated loess is strikingly low compared to that of the original loess (Figure 8C). On the other hand, as shown in Figure 8, when the treated loess is closer to the nano-SiO₂ pile, the decrease in water content and the void ratio are stronger, and the increase in natural density tapers off as radial distance from the pile increases. Consequently, the loess treated with the nano-SiO₂ pile becomes dryer and denser, essentially producing a solidification process—namely, the water is removed and the densification also occurs in the treated loess (Zhang et al., 2018a). These state changes in the treated loess can attribute to the nano-SiO₂ migration, which has dual roles during the progress. Two direct effects involve adsorption and filling due to the special properties of nano-SiO₂, and one indirect derivative effect is the generation of tighter bonding between aggregates or particles. Meanwhile, the stronger effects in the treated loess are closer to the nano-SiO₂ pile.

The changes in the Atterberg limit of treated loess at different radial distances from the nano-SiO₂ pile are shown in Figure 9. As shown, the nano-SiO₂ migration has almost no effects on liquid limit, plastic limit, or plasticity index. This may be because the treated loess has no essential change in composition, with modification of only its structure and state. The results indirectly support that this is lacking for chemical reactions.

The changes in the hardness of treated loess at different radial distances from the nano-SiO₂ pile are shown in Figure 10. The hardness of the treated loess decreases as the radius from the nano-SiO₂ pile increases, but the hardness values is greater than that of loess treated with the lime and fly ash piles (Pei et al., 2015). In addition, no similar radial cracks occurred in the loess stabilized with the lime pile. This means that there should have a greater mechanical strength in the loess stabilized with the nano-SiO₂ pile than in the untreated loess. The increase in the mechanical strength is related to the nano-SiO₂ migration, which results in structure and state modification of the treated loess. Hence, the mechanical improvement is attributed to the coarsening effect and solidification process of the treated loess (i.e., large aggregate formation) as well as the dryer and denser state with more contact, smaller void space and more stable structure (as shown in Figures 4–8). Furthermore, the weakening strength improvement depends on the degree of nano-SiO₂ migration as the radius from the pile increases.

The changes in SSA and CEC of treated loess at different radial distances from the nano-SiO₂ pile are shown in Figure 11. Both SSA and CEC increase as the radial distance from the nano-SiO₂ pile increases, but all measurements are smaller than the original loess ones. The decrease in SSA and CEC is due to the formation of large aggregates due to the nano-SiO₂ migration. Nevertheless, their increasing trends seem to contradict the results of the particle size distribution (Figure 4). However, the increase in SSA and CEC with increasing radial distance from the nano-SiO₂ pile is a reasonable expectation. The total SSA and CEC depend not only on the particle size of a soil, but also on the structure of the soil (Zhang et al., 2018b). Previous research has found that the formation of larger aggregate clusters can result in a larger void space (Zhang et al., 2013). As observed in microstructural images (Figure 5), the treated loess farther from the nano-SiO₂ pile showed a larger void space in the greater aggregates. Hence, the higher SSA and CEC of the farther treated loess could be attributed to a larger surface contact area due to its structure modification.

The changes in chemical properties (i.e., EC, TDS, pH, and ORP) of treated loess at different radial distances from the nano-SiO₂ pile are shown in Figure 12. As a whole, the EC and TDS of the treated loess with radial distance from the nano-SiO₂ pile, and their pH and ORP, accordingly decrease with increasing radius. It should be noted that the EC and TDS of the treated loess at radii of 5 and 10 cm are very slightly lower compared to the original loess, and the values at radii of 15 and 20 cm are also slightly higher compared to the original loess. The pH and ORP of the treated loess are overall lower than those of the original loess. This trend clearly shows the effect of nano-SiO₂ migration into loess, causing the changes in chemical properties due to the high activity and small size of the nano-SiO₂ particle. The very slight changes in chemical properties illustrate that the loess treated with nano-SiO₂ can keep an almost constant chemical environment. Thus, the nano-SiO₂ particle has the potential to act as an eco-friendly stabilized additive in loess improvement.