*3.3. Water Retention Characteristics Test*

Figure 13 shows the water retention characteristic curve of the XG-silt specimens. Figure 13a,b show the drying and wetting curves, respectively, and Figure 14 shows the drying–wetting hysteresis curve. Figure 13a,b show that the drying curves all shift to the upper right with increasing XG content, which reflects an increase in the water retention capacity of the treated specimens. Especially in the range of low suction, the water retention capacity of the silt significantly increases with increasing XG content. Because XG carries carboxylic acid (–COOH) and hydroxyl (–OH) with a negative charge, it can interact with cations in the soil particles to generate a capillary force, thus absorbing and retaining more water [46].

**Figure 13.** Water retention curves of silt with different XG contents: (**a**) drying process; (**b**) wetting process.

**Figure 14.** Drying–wetting hysteresis curve of silt with different XG contents.

Figure 14 shows that the water retention capacity of the treated specimens during the drying process is stronger than that during the wetting process, which is related to the water retention hysteretic effect of the soil during drying [47–49]. The hysteretic circle area between the drying and wetting curves of the treated specimens also gradually increases with increasing XG content. The drying and wetting curves of specimens tend to be consistent in the high-suction range [50].

The SEM and mercury intrusion test results show that more biofilms covered the surface of the soil particles with increasing XG content, and the bridging connections are found in the silt pores, which gradually filled with XG. The water retention capacity of the treated silt increased due to the strong water-retaining properties of XG [25,26,29,51], and the hysteretic circle area between the drying and wetting curves also gradually increased. Previous experimental studies show that more water can be retained in soil pores after treatment with XG, which is consistent with the water retention characteristics test results of this study. Additionally, XG has a stronger water retention capacity than other polymers, which can improve the survival rate of vegetation in severely arid areas [19,52].
