## 3 Analysis and Discussion of Test Results
### XRD and SEM Results
#### 3.4.2 SEM Analysis  
The microstructure of solidified soil specimens with different geopolymer content (0%, 5%, 10%, 15%, 20%, 25%) after curing for 14 d and 28 d is depicted in Figures 15 and 16. The magnification used for these photos is 1000 times. Analyzing the microstructure of solidified soil provides qualitative and quantitative insights into the effectiveness of soil solidification. The variation in soil structure significantly impacts its mechanical properties. Comparing the microstructure of polymer-solidified soil allows for a direct evaluation of the solidification effect of loess under different cycles and polymer content. Figure 15a depicts the microstructure of the remolded loess after a 14-day maintenance period. The remolded soil primarily consists of small particles, which are randomly arranged and in contact with each other. The gaps between the soil particles are irregularly distributed and lack a specific shape. Upon the addition of geopolymers, the hydration products C-S-H and C-A-S-H fill some of the pores between the soil particles. The floc in the red box represents its hydration product. Additionally, unreacted fly ash glass spherical particles are randomly dispersed within the pores of the soil particles, as indicated by the black circles. As the geopolymer content increases, the amount of hydrated flocculated gelatinous substances  
Figure 14: XRD patterns of solidified soil with different polymers.  
Figure 13: XRD pattern of remolded loess.  
also increases, leading to a continuous reduction in the gap between the soil particles. When the geopolymer addition reaches 25%, the experimental results reveal the presence of numerous unhydrated fine fly ash glass ball particles on the surface of the solidified soil. This occurrence can be attributed to the short curing time, where some steel slag-fly ash geopolymers have not fully undergone hydration.  
Figure 16 presents a microstructure diagram of solidified soil mixed with different geopolymers after 28 days of curing. The figure shows that compared to the 14-day  
Figure 16: SEM image of solidified soil with different polymer content after curing period of 28 d. The content of geopolymer: (**a**) 0% (**b**) 5% (**c**) 10% (**d**) 15% (**e**) 20% (**f**) 25%.  
Figure 15: SEM image of solidified soil with different polymer content after curing period of 14 d. The content of geopolymer: (**a**) 0% (**b**) 5% (**c**) 10% (**d**) 15% (**e**) 20% (**f**) 25%.  
maintenance period, the structure of the solidified soil becomes more compact, indicating better hydration of the geopolymer. Additionally, the remaining amount of geopolymer decreases as the maintenance period increases. This is because during the 14-day curing age, most of the silicon-aluminum oxides in the geopolymer raw materials dissolve and the formed hydration gel fills the pores. However, a portion of the geopolymer remains unhydrated, resulting in less full soil particle gaps in the microstructure compared to the 28-day period. On the other hand, during the 28-day curing period, the silicon-aluminum oxides hydrate to a greater extent, and the fly ash balls are enveloped by the generated C-S-H and C-A-S-H gelling substances. This significantly fills the pores, leading to a more compact microstructure and improved mechanical properties. SEM observation confirms that the steel slag-fly ash geopolymer test's macroscopic properties mainly derive from the steel slag-fly ash substance in the geopolymer. Under the alkali excitation of water glass, a depolymerization-condensation reaction occurs, forming an amorphous phase C-S-H and C-A-S-H flocculent gel that combines with the vitreous body in the geopolymer. This flocculent gel material strengthens the binding force, enhancing the compactness and compressive strength of the solidified soil.