## 3 Results and Discussion
### Compressive Strength
Excessive silica will consume  
Figure 3: Compressive strength of cement mortar doped with only a single-magnitude-particle-size silica. more water, hinder the hydration reaction, and lead to an insufficient hydration reaction in cement. Excessive tiny particles will form a weak interface in the interface area, which will reduce the improvement effect of silica on the interface transition zone (ITZ) and the interface strength. Furthermore, excess silica and amorphous C-S-H gel (relatively low Ca/Si) can also react to form C\({}_{5}\)S\({}_{6}\)H\({}_{5}\) (Tobermorite) [47], further reducing the strength of the specimen. From Figure 3, we can also see that when adding silica of different particle sizes, the optimal dosage is different, but the strength of different cement mortar specimens shows the same trend of change with the curing age. For example, cement mortars with different particle sizes and different dosages of nano-silica all show the same significant advantages in strength. Based on the compressive strength test results of cement mortar with a single silica particle size of a certain magnitude, the compressive strength of cement mortar when silica of different magnitudes of particle sizes is mixed into cement mortar with the self-healing agent is further discussed, and the influence of the self-healing agent and two different magnitudes of silica particle sizes on the strength of cement mortar is studied. Figure 4 shows the strength test results of eight groups of cement mortar specimens mixed with the self-healing agent and silica of different particle sizes at the ages of 3 days, 7 days, and 28 days. In order to ensure the effectiveness of the results, all data results and standard deviation values of experimental data for each group are given in the figure. The standard deviation value ranges from 0.12 Mpa to 1.15 Mpa. As shown in Figure 4, compared with the specimens in the Control-1 group, the specimens in the Control-2 group with the addition of the self-healing agent showed higher strength at different ages, and the difference in strength between the two groups of specimens also showed a trend of first increasing and then decreasing. This shows that bacterial spores will metabolize during the curing process of the mortar specimens, and the resulting calcium carbonate precipitation will fill some pores, making the internal structure of the mortar specimens denser, which can improve the strength [17]. This effect is more obvious in the early stage of curing and gradually decreases in the later stage of curing. This is because as the cement mortar is cured, the bacterial spores gradually stop their metabolic reactions in the absence of water and oxygen and enter a dormant state. Figure 4: Compressive strength of cement mortar mixed with different amounts of silica particle size. Comparing the compressive strength of mortar specimens from the Control-2, N4.5, and M27 groups reveals that, when silica of various particle sizes is added at the optimal dosage, the compressive strength of the cement mortar specimens is greater than that of specimens with only the self-healing agent. This shows that adding silica particles to the cement mortar with the self-healing agent can improve the strength. This is because the addition of silica can optimize the particle size distribution of the cement mortar and fill the pores in the mortar specimens. At the same time, silica reacts with cement to produce C-S-H (calcium silicate hydrate) gel, which can not only fill the pores but also neutralize calcium hydroxide, preventing calcium hydroxide from reducing the strength of the cement mortar. Furthermore, at early curing stages (3 and 7 days), the compressive strength of mortar specimens in the N4.5 group is higher. By the end of curing (28 days), the compressive strength of specimens from the N4.5 and M27 groups is comparable. Nano-sized silica, being finer and having a larger specific surface area than micron-sized silica, exhibits greater reactivity.