## 3 Results and Discussion  
### Compressive Strength  
Mechanical properties are important reference indicators for cement mortar in engineering applications. Figure 3 shows the compressive strength test results of cement mortar specimens at different curing ages (3 days, 7 days, and 28 days) when only 60-micron silica or only 20-nanometer silica is added to the cement mortar with an internal self-healing agent. 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 0.25 Mpa to 1.07 Mpa. As shown in Figure 3, when the specimens were cured for 2 to 8 days, for the specimens in which only micron-sized silica was added to the cement mortar with an internal self-healing agent, the compressive strength of the specimens in group M3 was the largest, the compressive strength of the specimens in groups M1 and M5 was medium and the values were very close, and the compressive strength of the specimens in group M10 was the lowest. For the specimens in which only nano-silica was added to the cement mortar with an internal self-healing agent, the compressive strength of the specimens in group N0.5 was the largest, the compressive strength of the specimens in group N1 was medium, and the compressive strength of the specimens in group N2 was the lowest. It can be seen from this that when silica of a certain magnitude of particle size is added alone to cement mortar with an internal self-healing agent, there is an optimal dosage of silica within a certain range, and the value of the optimal dosage is related to the magnitude of the particle size. In this experiment, the optimal dosage of 60-micron silica is 3%, and the optimal dosage of 20-nanometer silica is 0.5%. When the dosage of silica is less than the optimal dosage, it plays a smaller role. This is mainly due to the insufficient microscopic filling effect. Although the silica particles are extremely small and can fill the tiny gaps between the cement particles and increase the density of the slurry, the density of the slurry is still limited if the number is not enough to effectively fill all the gaps. In this way, the effect of improving the strength is small. In addition, the effects of the volcanic ash effect, seed effect, and interface effect cannot be ignored. Silica has high activity and can react with calcium hydroxide (Ca(OH)\({}_{2}\)) in cement hydration products to generate more calcium silicate hydrate (C-S-H). However, when the dosage is insufficient, the volcanic ash effect is insufficient, the number of active sites for the reaction and the generated C-S-H is limited, and the strength of the slurry cannot be significantly improved. Silica as a seed can also promote the hydration reaction of cement, and the acceleration effect is related to its concentration. When the dosage is too small, the number of seeds provided is insufficient, the seed effect is not obvious, and the acceleration of the hydration reaction is not obvious, which affects the improvement of early strength. The effect of silica in improving the interface transition zone (ITZ) also depends on the dosage. When the dosage is insufficient, the distribution of tiny particles in the interface zone is uneven, which limits the interface effect and cannot fully improve the density of the interface zone, limiting the improvement of the overall mechanical properties of the material. When the dosage of silica is greater than the optimal dosage, it will also cause the material strength to decrease. The main reason is that the excessive filling effect leads to an increase in porosity. Excessive silica will cause agglomeration in the slurry, resulting in a decrease in density in local areas, increasing porosity, and reducing material strength. In addition, the hydration reaction and interface effect also have an impact at the same time.