## 3 Results and Discussion
### Self-Healing Performance  
To investigate the impact of silica with varying particle sizes on the self-healing performance of cement mortar containing a self-healing agent, eight groups of cement mortar specimens were prepared with silica of different particle sizes. These specimens were pre-cracked, with cracks of approximately 300 microns in width selected for evaluating the self-healing results. The results of self-healing of cracks in eight sets of cement mortar specimens observed at 0, 3, 7, and 28 days using electron microscopy are given in Figure 5. As shown in Figure 5, the cracks in the Control-1 group specimens have no self-healing properties, and the healing of the cracks is very limited. This is because ordinary cement mortar can only repair cracks through the limited further hydration of unhydrated cement, and the repair effect is not obvious. In contrast, all groups of specimens except Control-1 contain self-healing agents. As the curing progresses, the cracks gradually heal and the width decreases over time. It usually takes 28 days to fully heal. This demonstrates that once cracks appear, the bacterial spores carried by the expanded perlite in the cement mortar are activated. These bacteria then produce calcium carbonate through metabolism, which effectively fills the cracks. Adding a self-healing agent with a volume fraction of 10% results in the complete healing of microcracks in the cement mortar. As shown in Figure 5, we can see that compared with the Control-2 group of specimens, the other groups of cement mortar have higher self-healing efficiency and can achieve the healing effect of completely blocking cracks at an earlier healing age. This is because when silica is added to cement mortar under the condition of adding the same self-healing agent, the unreacted silica at the cracks of the cement mortar can react with calcium hydroxide to form C-S-H gel, accelerate the filling of cracks, and improve the healing efficiency of cracks. Among the eight groups of cement mortar specimens, the N4.5-M27 group has the most silica added, the most unreacted silica after curing and forming, and the highest crack healing efficiency, and is the earliest sample group (healing age: 3 days) to show an obvious healing effect. Within a certain range, the amount of silica added is proportional to the self-healing efficiency of cement mortar. This observation verifies the above conclusion. In addition, by comparing the healing results of the N4.5 group specimens and the M27 group specimens, it is not difficult to find that the healing efficiency of the cracks in the M27 group specimens is higher than that in the N4.5 group specimens. This is because the two groups of specimens have different particle sizes of silica added at the optimal dosage. The N4.5 group of specimens is mixed with nano-sized silica, which will undergo a rapid and violent volcanic reaction in the early stage and is more likely to react completely during the entire curing period. The M27 group of specimens is mixed with micron-scale silica. It will continue to undergo the volcanic reaction during the whole curing period, but it is not easy to completely react in the middle curing period. In addition, the amount of incomplete-reaction silica is larger. This comparison result also supports the above conclusion that mixing appropriate amounts of silica of different particle sizes in cement mortar can play a role at different stages and improve the self-healing properties of cement mortar. As shown in Figure 5, we can also see that the control group (Control-1 group) had obvious cracks and no healing after 3 days of healing. On the 7 th day, the cracks were still visible and showed slight signs of healing. On the 28 th day, the healing of the cracks improved, but there were still obvious unheated parts. The microbial addition groups (Control-2, N4.5, M27, N1.125-M6.75, N2.25-M13.5, N3.375-M20.25, N4.5-M27) showed  
Figure 5: Crack healing status of eight groups of specimens at 0, 3, 7, and 28 days.