## 4 Microanalysis
### Results and Analysis
#### 4.2.2 Solidified Soil with Slag and Gypsum Powder  
Figure 5a presents the microscopy images of the solidified soil with slag and gypsum powder. It can be observed that compared with unsolidified soil, the soil structure was compact and that the soil grains varied in shape and size. The contacts between the soil particles were mainly point-point, point-surface, and surface-surface. The surface of the soil particles was non-uniform, and there were many voids. The shape and size of the voids differed, and few overhead voids were observed. Based on Figure 5b, it can be stated that a few crystals are formed on the surface of the grain. The crystals were mainly shaped as strips and fine particles.  
The X-ray powder diffractogram is shown in Figure 6. Combining the EDS and XRD analysis, it can be demonstrated that the main crystalline phase substances were quartz (SiO\({}_{2}\)), albite (Na\({}_{2}\)O\(\cdot\)Al\({}_{2}\)O\({}_{3}\)\(\cdot\)6 SiO\({}_{2}\)), and gypsum (CaSO\({}_{4}\cdot\)2H\({}_{2}\)O). Unlike in the unsolidified soil, CaSO\({}_{4}\cdot\)2H\({}_{2}\)O was the new crystalline substance generated from the hydrolysis of gypsum powder (anhydrous gypsum: CaSO\({}_{4}\)) as follows:  
\[\text{CaSO}_{4}+2\text{H}_{2}\text{O}=\text{CaSO}_{4}\cdot 2\text{H}_{2}\text{O} \tag{1}\]  
CaSO\({}_{4}\cdot\)2H\({}_{2}\)O generally appeared as fine needle-shaped and columnar crystals (as shown in Figure 5b), which contributed toward increasing the compressive strength to a certain extent.  
Figure 5: Microscopy images of solidified soil with slag and gypsum powder. (**a**) 100 \(\upmu\)m scale; (**b**) 10 \(\upmu\)m scale.  
Figure 6: X-ray powder diffractogram of solidified soil with slag and gypsum powder.  
4.2.3. Solidified Soil with Slag and Sodium Silicate  
As shown in Figure 7a, compared with the results for the gypsum powder, a denser structure could be obtained in the soil solidified with slag and sodium silicate. The distribution of massive particles was relatively compact. The contacts between the soil particles were mainly point-surface and surface-surface, and the contact area increased. A certain amount of void and colloidal filler was apparent. In addition, Figure 7b reveals that a certain amount of crystals (strips and needles) formed on the surfaces of the grains, which filled the void and increase the contact between the grains. This was the main reason for the improvement in strength for the solidified soil with slag and sodium silicate.  
The X-ray powder diffractogram is shown in Figure 8. Combining the EDS and XRD analyses, it can be demonstrated that the main substances in the crystalline phase were quartz (SiO\({}_{2}\)), albite (Na\({}_{2}\)O\(\cdot\)Al\({}_{2}\)O\({}_{3}\)\(\cdot\)6 SiO\({}_{2}\)), anorthite (CaO\(\cdot\)Al\({}_{2}\)O\({}_{3}\)\(\cdot\)2 SiO\({}_{2}\)), and CSH (CaO\(\cdot\)SiO\({}_{2}\)\(\cdot\)H\({}_{2}\)O). Quartz and albite were crystalline minerals that pre-existed in the dredger soil, and anorthite and CSH were new reaction products. During the solidification, NaOH from Na\({}_{2}\)SiO\({}_{3}\) hydrolysis increased the pH value of the sample. In the alkaline environment, CaO and other vitreous structural substances in the soil and slag dissolved straightforwardly to yield Ca\({}^{2+}\). Ca\({}^{2+}\) is conveniently replaced with Na\({}^{+}\) from sodium silicate (Na\({}_{2}\)SiO\({}_{3}\)) to form CSH. In this manner, CSH formed continuously, and the OH\({}^{-}\) (alkaline environment) remained in the soil to maintain the circulation of the solidification.  
Figure 8: X-ray powder diffractogram of solidified soil with slag and sodium silicate.  
Figure 7: Microscopy image of solidified soil with slag and sodium silicate. (**a**) 100 \(\upmu\)m scale; (**b**) 10 \(\upmu\)m scale.