# Experimental Investigation and Mechanism of Fly Ash/Slag-Based Geopolymer-Stabilized Soft Soil+
## 3 Response Surface Analysis
### Mechanistic Analysis of Geopolymer-Stabilized Soft Soil
#### 3.5.2 Microstructural Observations  
The morphology of the hydration products, the microscopic morphology, and the pore structure of the geopolymer-stabilized soft soils were observed via SEM. Figure 11 presents the SEM images of the FA, slag, and undisturbed soil. As can be seen from Figure 11, the FA particles are spherical and the slag particles are angular with irregular shapes and sharp edges. The scalp clay minerals in the soft soil exhibit obvious edge-edge structures, and the particles are not compactly connected and have more pores, which is the reason for the poor mechanical properties of the undisturbed soil. Figure 12b,c show that the addition of the geopolymer significantly improved the soil's structure, and the particles became more tightly connected and less porous compared to the original soil. In addition, numerous white needle-like and flocculent cementation products can be seen attached to the soil particles. The flocculent white gelling products are mainly the C-S-H and N-A-S-H gelling substances generated from the SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\) in FA and slag through reactions with the compound activator [33; 34; 35]. These products wrap the soil particles and make the soil denser, reducing its porosity. In addition, the ettrinigte needle-like hydration products can be seen in the SEM image of the geopolymer soil. The main reason why ettrinigte could have grown in large quantities is that the soil particles were relatively loose.  
Figure 12a,b present the SEM images of the geopolymer soil specimens with activator moduli of 0.8 and 1.0, respectively. The geopolymer soil specimen with a modulus of 0.8 is more dense and less porous than that with a modulus of 1.0, and the surfaces of the soil particles are covered with more white colloidal material. A large amount of white colloidal hydration products can be seen in Figure 12a, which indicates that a modulus of  
Figure 11: SEM images of the FA, slag (GGBFS), and undisturbed soft soil.  
Figure 12: Electron microscope scans of the soft soil stabilized using different mixture ratios. (**a**) Activator modulus = 0.8, alkali content = 7%, slag = 30% geopolymer soil, (**b**) activator modulus = 1.0, alkali content = 7%, slag content= 30% ground polymer soil, and (**c**) activator modulus = 0.6, alkali content = 6%, slag = 50% geopolymer soil.  
0.8 provides a suitable alkaline environment for the geopolymer reaction to occur to a more adequate extent, producing more colloidal products that effectively fill the spaces between the soil particles. Figure 12a,c present the SEM images of the geopolymer soil specimens with slag contents of 30% and 50%, respectively. More unreacted slag particles can be seen in Figure 12c, which shows that the geopolymer-stabilized soil particles become more closely connected and the pore space gradually decreases with increasing slag content [15]. This is due to the fact that the slag provides more Ca\({}^{2+}\) to participate in the geopolymerization reaction, which generates more C-S-H and N-A-S-H, while the exotherm of the slag in the hydration reaction promotes the geopolymerization reaction [22; 34].