# Effect of Particle Size on Mechanical Property of Bio-Treated Sand Foundation
## 3 Results and Discussion
### Mineralogical and Crystal Morphology Analysis  
The XRD analysis and SEM observation provided trustworthy evidences on the precipitation of calcium carbonate from MICP mineralization. Figure 10a,b illustrate XRD spectrum peaks intensity of diffraction angle for untreated and bio-treated sand specimen, respectively. The main components of the tested sand column were quartz (SiO\({}_{2}\)) and calcite (CaCO\({}_{3}\)), respectively. The calcium bicarbonate Ca(HCO\({}_{3}\))\({}_{2}\) and ammonium chloride (NH\({}_{4}\)Cl) are the other products that could potentially form, besides calcium carbonate. Quartz was the main mineral component in the original sand, and calcite was a new cementitious chemical composition obtained after bio-treatment with MICP technology, which was also the fundamental reason why the bio-treated column gained a certain mechanical strength [7; 9]. This was done to confirm that the crystals observed in the images were actually CaCO\({}_{3}\) precipitated on the silica sand and to detect whether there were any other elements in the column samples after the precipitation process. Based on the performance of the above macroscopic mechanical tests, lower concentration of cementation reagent and more injection cycles resulted in samples with more pores filled, higher calcium carbonate content and greater mechanical strength. This indicated that the concentration of cementation reagent had a potential effect on the microstructure of sand column. Differences in the distribution of calcium carbonate crystals (sand particle size: 300-600 \(\upmu\)m) were observed obviously on SEM image, as shown in Figure 11a-f. Each sample was photographed several times in order to exclude the randomness of experimental observation. As shown in Figure 11a,b, the original sand particles had a smooth surface. Both bio-treated specimen at cementation reagent of 0.5 M and 1.5 M showed that the abundant calcite (CaCO\({}_{3}\)) crystals were filled at the contact point and covered the surfaces of the sand particles. Figure 10: (a) XRD spectrum peaks for untreated sand specimen; (b) XRD spectrum peaks for bio-treated sand specimen. Comparatively, the calcite crystal size was smaller, denser, and uniformly distributed on the surface of sand particles for the bio-treated specimen of lower concentration (0.5 M Urea+CaCl\({}_{2}\)), which was visualized qualitatively in Figure 11c,d. In the bio-treated specimen with the higher concentration of cementation reagent (1.5 M Urea+CaCl\({}_{2}\)) in Figure 11e,f, the calcite crystals grew in heaps and gathered on the original crystal, resulting that the crystals were larger in size and more dispersed in distribution. The phenomenon was consistent with the conclusion reported by [PERSON] and [PERSON] [37]. Crystallization from solution can be considered a two-step process. The first step is the phase separation or \"birth\" of new crystals. The second step is the growth of these crystals to larger sizes. These two processes are known as nucleation and crystal growth, respectively [35]. [PERSON] summarized the experimental observation of the average crystal size from supersaturated solution and revealed the dependence of the average crystal size on the relative concentration of supersaturated solution [40,41]. [PERSON] et al. [42] reported that the higher concentration of Ca\({}^{2+}\) and CO\({}_{3}\)\({}^{2-}\) in the reaction solution, the larger average particle size of the precipitate (CaCO\({}_{3}\)). The consumption of CO\({}_{3}\)\({}^{2-}\) mainly acted on the nucleation of calcite, rather than promoting the growth of calcite crystals at low concentration of Ca\({}^{2+}\) and CO\({}_{3}\)\({}^{2-}\). These findings provide insights into the understanding of the inorganic substances in the reactants mentioned above in terms of microscopic observation.