**3. Results and Discussion**

*3.1. SEM Images of Xanthan Gum-Treated Silt*

Figures at four suitable magnifications (×200, ×2000, ×5000, and ×10,000) were present for microscopic analysis. At ×200 magnification, the macroscopic structure of soil particle distribution can be seen. At the magnification of ×2000, ×5000, and ×10,000, the internal microstructure of soil can be clearly observed. The following are the conclusions based on SEM pictures.

Figure 8a–c show the SEM test results of XG-silt at ×200 magnification. The mass cementation between the silt particles and XG becomes increasingly large with increasing XG content.

(**a**) (**b**) (**c**)

**Figure 8.** SEM pictures of XG-silt at ×200 magnification: (**a**) *mbp*/*ms* = 0%; (**b**) *mbp*/*ms* = 1.0%; (**c**) *mbp*/*ms* = 2.0%.

Figure 9a–c show the SEM images of XG-silt at ×2000 magnification. The pore diameter decreases with increasing XG content.

Figure 10a–c show the SEM images of XG-silt at ×5000 magnification. A comparison shows that more soil particles bonded together with increasing XG content, and the small particles became soil mass with cementation [44]. The gaps and pores of the samples were also gradually filled with XG with increasing XG content, showing observable cementation, and the samples became increasingly dense.

**Figure 9.** SEM pictures of XG-silt at ×2000 magnification: (**a**) *mbp*/*ms* = 0%; (**b**) *mbp*/*ms* = 1.0%; (**c**) *mbp*/*ms* = 2.0%.

**Figure 10.** SEM pictures of XG-silt at ×5000 magnification: (**a**) *mbp*/*ms* = 0%; (**b**) *mbp*/*ms* = 1.0%; (**c**) *mbp*/*ms* = 2.0%.

Figure 11a–c show the SEM images of XG-silt at ×10,000 magnification. Part of the XG formed a biofilm covering the surface of soil particles with increasing XG content, and the other part formed bridge connections (biological polymerization chains) between the aggregate gap [21,37,42]. Additionally, the small particle aggregates became larger with closer connections.

**Figure 11.** SEM pictures of XG-silt at ×10,000 magnification: (**a**) *mbp*/*ms* = 0%; (**b**) *mbp*/*ms* = 1.0%; (**c**) *mbp*/*ms* = 2.0%.

## *3.2. Mercury Intrusion Test*

The pore size distribution of the treated specimens with different XG contents was investigated, which is helpful in analyzing the interaction between the silt and XG. Figure 12a,b show the cumulative pore size and differential pore size, respectively.

**Figure 12.** Mercury intrusion pore size distribution: (**a**) cumulative curves; (**b**) differential curves.

Kodikara et al. [45] found that there are aggregates in the soil, which leads to different pore sizes. The pore size distribution of soil is therefore generally bimodal and manifested as micropore and macropore peaks. Figure 12a shows that the cumulative pore intrusion of the mercury intrusion test specimens decreases from 0.1815 mL/g for XG-free silt to 0.1697 mL/g for silt with 2% XG, which implies that the pore size distribution decreases with increasing XG content. Figure 12b shows when 1% XG is added to pure silt, the large pores (10–80 μm) are gradually connected by XG hydrogels and reduced to medium pores (2–10 μm). Small pores (0.1–2 μm) in the silt are then filled with XG, and their number gradually decreases. The number of large and small pores in silt, therefore, strongly decreases, while the number of medium pores significantly increases. The pore size distribution of silt also turns into unimodal pore size distribution. Upon increasing the XG content from 1% to 2%, the medium pores in the silt are gradually connected by the filling of XG and hydrogels, resulting in a significant reduction in the number of medium pores and a large increase in the number of small pores.
