## 3 Results and Discussion
### Sem/Xrd  
Calcium carbonate generated from biological treated samples silt A, silt B and silt C was analyzed by XRD, as shown in Figure 6a-c, respectively. Three polymorphs types of calcium carbonate, including calcite, vaterite and aragonite, were identified in three samples by comparison with standard cards. In sample A, the calcium precipitation consisted of 81.7% vaterite, 17.5 aragonite, and 0.8% calcite (Figure 6a). In sample B, the calcium precipitation consisted of 73.8 vaterite, 20.6% aragonite, and 5.6% calcite (Figure 6b) In sample C, the calcium precipitation consists of 60.3% vaterite, 32.3 aragonite, and 7.4% calcite (Figure 6c). This indicates that the calcium polymorph types of the three samples are mainly composed of vaterite and aragonite, and the calcium precipitation polymorph type composition in the samples is not affected by the content of fine particles. Calcite, vaterite and aragonite are the three polymorph types of calcium carbonate, and many researchers have found that calcite is the main calcium polymorph type in the process [24]. There may be two reasons for this change. First, spherical argonite is the primary mineral formed at high levels of hydrolysis (above 18 mM Urea h\({}^{-1}\)) [25]. In addition, calcium sources influence the polymorph type of calcium [20]. Calcite may be formed when CaCl\({}_{2}\) is used, while vaterite and aragonite precipitate easily when Ca(CH\({}_{3}\)COO)\({}_{2}\) is used. This result is consistent with the research results of using Ca(CH\({}_{3}\)COO)\({}_{2}\) as calcium source, and the main calcium polymorph types are vaterite and aragonite [20].  
Figure 5: Effect of different fine particle content on calcium carbonate production.  
At room temperature, calcite is the most stable crystalline phase, while needle-like aragonite and spheroidal aragonite are metastable crystalline phases, which can be transformed into stable crystalline calcite under certain conditions. Different from calcite and needle-like aragonite, spheroidal aragonite has more excellent water solubility, spherical and porous structure and high specific surface product. Few literatures have explored the relationship between the crystal phase of CaCO3 and experimental conditions in the presence of ammonium salts in a gas-liquid system. Studies have shown that under the condition of pH = 11.1, sphearagonite and calcite are generated at the same time, but with the decrease in pH value, the composition of calcite decreases, until pH = 7.9 only sphearagonite crystal phase.  
SEM images of three different fine particle contents of silty soils treated with calcium carbonate precipitation induced by microorganisms were obtained to examine the morphology of samples. After curing by microorganism, obvious mineral precipitation traces can be observed on the surface of silt particles and between pores (Figure 7a-c). At the same time, two different shapes of mineral polymorphs (spherical vaterite and acicular cluster aragonite) can be identified (Figure 7b,c), which are consistent with the corresponding mineral composition revealed by XRD tests. Compared with samples B and C, the calcium polymorph type of sample A is mainly spherical vaterite, and more calcium carbonate polymorphs are deposited on its surface with more uniform size (Figure 7a). With the decrease in fine particle content in silt, the content of acicular cluster aragonite in samples B and C increases (Figure 7b,c). [PERSON] et al. [26] believed that the biological blocking and bioccentation of sand particles caused by calcareous precipitation enhanced the strength of sand particles, in other words, a more obvious effect led to a more compact structure.  
Figure 6: X-ray diffraction results of different silts. (**a**) X-Ray diffraction results of Silt A; (**b**) X-Ray diffraction results of Silt B; (**c**) X-Ray diffraction results of Silt C.