# Research on Solidification Methods and Stabilization Mechanisms of Sulfate Saline Soils
## 4 Results and Analysis
### Microstructure Analysis of Stabilized Soils
#### 4.3.2 SEM Analysis  
Figure 7a,b show SEM images of sulfate saline soils. The microstructure of the saline soils shows the following characteristics: the skeletal particles are dominated by single particles, and the interstices of the particles are dominated by pores, which are basically unfilled; the soil particles are in point contact before solidification, with little bonding material, large voids, and small contact surfaces. Figure 6: XRD results for saline soil solidification: (**a**) L1, L2, L3 stabilized for 7 d; (**b**) F1, F2, F3 stabilized for 7 d; (**c**) P3 stabilized for 7 d and 28 d; (**d**) L3, F3, P3 stabilized for 28 d.  
Figure 7: (**a,b**) SEM of saline soils. Figure 8a shows an SEM image of L3 cured for 28 d. Trace amounts of bound material produced by the reaction of soil particles with Ca(OH)\({}_{2}\) can be seen in the area indicated by an arrow. Fibrous-like AFt can be seen in the area indicated by white circles, indicating interaction between lime and saline soils. The pores of L3 are filled with a large amount of filler material, which increases the contact area between particles from point to face contact. The structural and overall integrity of saline soils is improved when soil particles are almost completely bound together. Figure 8b,c show SEM images of F3 cured for 28 d. The saline soil surface is covered with FA-hydrated binding material, and hydrated gel material is visible in the area shown in the white circle. In the binder-stabilized saline soils, some granular crystals were observed on the surface of the samples, which were salted to the generated C\(-\)A\(-\)H and C\(-\)S\(-\)H [41]. The resulting C\(-\)A\(-\)H and C\(-\)S\(-\)H gels [42] fill the pores between the soil particles and connect the soil particles together to form a cohesive whole. In saline soils, the bonding mechanism between soil particles shifts to face contact, where the bonding area is larger than point contact. After 28 d of hydration, the relatively low content of amorphous hydrated calcium (aluminum) silicate (C\(-\)(A)\(-\)S\(-\)H) gel [43] in FA reacted with sulfate to produce a small amount of Aft [24], which was not observed in the results of XRD experiments. Various gels encapsulate the saline soil particles and fill the pores, thereby increasing the strength of the specimen. Figure 8d shows an SEM image of P3 cured for 28 d. The PAM gel fills the pores [31] between the saline soils particles, but the particles continue to make point-to-point contact, resulting in an increase in the strength of the saline soils [44]. These compounds fill some of the pores on the surface and above the sample, reducing the porosity and increasing the density. Figure 9 shows SEM images of the stabilized soil after 3 freeze-thaw cycles. [PERSON] carried out a characterization of stabilized soil specimens with a lime content of 3% and 9% after 5 freeze-thaw cycles. It was found that with 9% lime content, not only the surface material was damaged but also some of the particles were directly crushed [41]. In Figure 9a, it can be seen that there is significant cracks in saline soils [40]. No small cracks were observed in L3 (Figure 9b), and the bonding mechanism between the soil particles was not altered, suggesting that the early strength loss of the lime stabilized soil after freeze-thaw cycles was negligible. Figure 9c shows that there are no visible cracks between soil grains in F3 stabilized saline soils. This indicates that the FA-stabilized soil  
Figure 8: SEM images of the three binders stabilized for 28 d: (**a**,**b**) L3; (**c**) F3; (**d**) P3. has less strength loss after three freeze-thaw cycles. The strength of the PAM-stabilized soil with low salinity is remarkable, as shown in Figure 9d, where the film on the soil particles disappears and the connection of the soil particles in point-to-point contact is changed. Figure 10 shows SEM images of stabilized soil subjected to 20 freeze-thaw cycles.