## 3 Results and Discussion
### Microscopic Analysis  
#### 3.4.1 X-Ray Diffraction  
As shown in Figure 7, this figure shows the XRD results of the self-healing product at the healing age of 28 days. There are eight groups of samples in the spectrum, of which Control-1 is the control sample, and the other samples are samples with different microbial addition amounts. From Figure 6, we can see that only the Control-1 group did not have a peak for calcium carbonate, and the test results of the other seven groups all showed obvious peaks for calcium carbonate. The main crystalline phases that appeared were quartz (Q), with the main peaks appearing at 26.6\({}^{\circ}\) (20), 20.9\({}^{\circ}\), 50.1\({}^{\circ}\), and other positions; plagioclase (P), with the main peaks appearing at 22\({}^{\circ}\), 28\({}^{\circ}\), 42\({}^{\circ}\), and other positions; and calcite (C), with the main peaks appearing at 29.4\({}^{\circ}\), 39.4\({}^{\circ}\), 47.4\({}^{\circ}\), and other positions.  
The quartz (Q) and plagioclase (P) peaks in the control group (Control-1) are higher in intensity and more stable, which indicates the crystal phase composition of the matrix when no other substances are added. In the added groups (N4.5, M27, and their mixed addition groups), the peak intensities of quartz (Q) and plagioclase (P) can be seen to have changed. In the N4.5 + M27 sample, multiple calcite (C) peaks appeared, especially near 29.4\({}^{\circ}\), indicating that calcite was generated in the healing product. When cracks appear in the cement mortar, Bacillus pseudoformal is activated by water and begins to multiply in the cracks, converting nutrients (such as calcium sources and carbon sources) into calcium carbonate (CaCO\({}_{3}\)) [49]. The calcite (C) peak of the N4.5 + M27 sample in the XRD spectrum shows the generation and precipitation of calcium carbonate, which confirms this view.  
From Figure 7, we can see that the peak intensity of quartz (Q) in the N4.5 sample is significantly greater than that in the M27 sample. This indicates that the content or crystallinity of quartz in the N4.5 sample is higher than that in the M27 sample. This difference is due to the different effects of nano-silica and micro-silica on the formation of quartz during the self-healing process. Nano-silica can more effectively promote the formation or crystallization of quartz, resulting in a higher XRD peak. This further verifies that nano-silica has a higher specific surface area and activity than micro-silica, and can provide more reaction interfaces. During the cement hydration process, nano-silica reacts  
Figure 7: X-ray diffraction (XRD) analysis results of the self-healing products in specimens at the age of 28 days.  
first with calcium hydroxide to generate more hydrated calcium silicate (C-S-H), which enhances the density of the material. The slower reaction of micro-silica can continuously provide a silicon source for hydration and continue to increase the strength of the specimen. Nano-silica can also act as a crystal nucleus to promote the precipitation of calcium carbonate. In samples doped with nano-silica (such as N4.5 + M27), the calcite (C) peak intensity is higher, indicating that nano-silica has a promoting effect on the formation of calcium carbonate.  
In addition, silica of different particle sizes can fill different pores in the cement matrix and improve the density and strength of the material. A composite admixture can provide a variety of crystal nuclei, promote more calcium carbonate precipitation, and improve the self-healing effect. From the XRD spectrum, it can be seen that the calcite peak is more significant in the composite admixture samples (such as N4.5+M27), indicating that the synergistic effect of a composite admixture makes the self-healing effect better.