## 3 Results and Discussions
### Soil Reinforcement Tests
#### 3.2.5 Microscopic Morphology  
Figures 10 and 11 compare the appearance characteristics of the soil before and after reinforcement observed by optical microscopy and USB hand-held digital microscopy, respectively. After treatment by the method proposed in this paper, areas of the surface of the soil obviously turned white, and the soil particles were filled and cemented by a large number of small white particles. Figure 11: Images of soil under a USB hand-held digital microscope: (**a**) untreated and (**b**) EICP + glutinous rice powder. Figure 10: Images of the soil under an optical microscope: (**a**) untreated and (**b**) EICP + glutinous rice powder. Figure 9: Relationships between UCS and CCC. Figure 12 shows the SEM images of representative samples. The SEM images of the soil improved by EICP with different kinds of organic materials added are similar, taking A4 (Table 2, EICP with skim milk powder) as an example. It is obvious that the intergranular pores of the untreated silt in the Yellow River flood area were large and many noncontact areas existed between the particles (Figure 12a,b). Although some areas formed relatively concentrated calcium carbonate crystals after strengthening by EICP, which filled in the pores in the soil particles and cemented the particles well, most of the calcium carbonate crystals were still single particles dispersed on the surface of the soil particles with equal thickness distributions (Figure 12c). The calcium carbonate mostly comprised unstable vaterite, with only a small part of calcite [1] (Figure 12d). There were still many pores between the particles. This was because the urease [18; 19] used in EICP is a free type and lacks nucleation sites, which had a negative impact on the continuous stacking, bonding, and formation of large calcium carbonate crystals. From Figure 12e,f, there were fewer calcium carbonate crystals in the single particle distribution, more calcium carbonate crystals in concentrated clusters, and the generated calcium carbonate mostly existed in the form of calcite with a size in the range of 5-10 \(\upmu\)m compared with those from traditional EICP. A large amount of calcium carbonate crystals were produced on the surface of the unearthed particles and between the particles of the adjacent soil particles, similar to the behavior of MICP technology, although the urease used in the improved EICP was also free type. It shows that the incorporation of organic material can compensate for the lack of nucleation sites during traditional EICP. Figure 12: _Cont._  
During the test, a few samples exhibited a mildew phenomenon (Figure 13) due to the long curing time (approximately 30 d), which may decrease the strength of reinforced soil. The calcium carbonate crystals formed in this study were mostly spherical and square [50], and flower-like, striped, and rhombic calcium carbonate crystals also appeared under different conditions, such as different temperatures, pH values, and concentrations, in subsequent experiments (Figure 14). The durability of soil treated by EICP containing organic materials and the influence of experimental conditions on the crystal morphology of calcium carbonate still need further investigation. Figure 14: Crystal morphology of calcium carbonate in subsequent tests. Figure 12: SEM images for untreated soil at (**a**) 1000\(\times\) magnification and (**b**) 5000\(\times\) magnification, for EICP-treated soil at (**c**) 1000\(\times\) magnification and (**d**) 5000\(\times\) magnification, and for soil treated by EICP with skim milk powder added at (**e**) 1000\(\times\) magnification and (**f**) 5000\(\times\) magnification. Figure 13: Moldy sample. The untreated silt in the Yellow River flood area contains quartz, sodium feldspar, calcite, muscovite, potassium feldspar, and chlorite. The calcite diffraction peak appeared in the XRD spectrum, and its intensity was relatively low (Figure 15a).