## 3 Results and Discussion  
### Microbial Growth Curve and Selection  
_Sporosarcina pasteurii_ DSM 33 is a commonly used microorganism for biomineralization, and its activity and concentration should be monitored at various time intervals during its growth process. The study selected the optimal time period during the growth curve to maintain an effective biomineralization process. The microbial growth was monitored over a 72-h period in this experiment. Sample conductivity and OD600 values were measured separately for L-21\(\sim\)10 at 1 h intervals for the first 12 h, 2 h intervals for the 12 th-24 th, 4 h intervals for the 24 th-48 th, and 12 h intervals for the last 24 h. Each data point is the mean value of the sample data in 10 tubes, as shown in Figure 1.  
Figure 1: Microbial 72 h growth curve. Figure 1 indicates that during the 72-h culture of microorganisms, there is a linear correlation between OD\({}_{600}\) and conductivity, and both values progressively increase from the initial stage to 48 h. At the early stage of bacterial growth, the conductivity and OD600 values of the bacterial solution grow slowly, and the microbial activity is low. At around 4 h bacterial growth enters the logarithmic phase and activity increases significantly; at 7 h, it enters the stable growth phase and microbial activity continues to rise; at 48 h, incubation activity reaches its maximum; it then enters the bacterial growth decline phase and activity starts to decrease. The microbial activity reached its maximum at 48 h before diminishing gradually. The reason for the above-mentioned phenomenon has been related to the amount of microbial inoculum and the content of nutrients in the culture solution. At the point when microbial conductivity reached its peak, the highest decomposition rate of CO\({}_{3}^{2-}\) resulted from urea. At this stage, an increased quantity of Ca\({}^{2+}\) was introduced, and more precipitates were produced. Due to microorganism loss resulting from growth conditions during remediation, it is advisable to use microorganisms cultured for 48 h during the remediation test. Comparison of Compressive Strength and Chloride Ion Migration Coefficient before and after Restoration  
The compressive strength of the original group, the 500 \({}^{\circ}\)C damage, and the repaired specimens were compared with provide a more intuitive macro-repair effect, as shown in Figure 2.  
The results from Figure 2 showed that, on average, the compressive strength of cement mortar sand specimens after 500 \({}^{\circ}\)C high-temperature damage was 36.3% lower than that of the undamaged specimens. However, the compressive strength of the repaired specimens was 22.8% higher than that of the damaged specimens and recovered to 78.2% of the undamaged specimens. This was due to the CaCO\({}_{3}\) crystals generated by microbial mineralization, which filled the cracks and pores caused by high-temperature damage. The microbial mineralization product calcium carbonate has a certain strength and adhesion, and the particle size is smaller than the defect size of the cementitious sand specimen, which can fill the defect effectively. This increased the cohesive force of the specimen, ultimately improving its compressive strength. Figure 3 compares the chloride migration parameters of the specimens before and after repair. Figure 2: Comparison of pressure resistance. Figure 3 shows that there was a 16.9% reduction in the chloride ion migration parameters of the repaired specimens, compared with the unrepaired ones. This happened because the mineralization products filled up high-temperature damaged micro-cracks and micro-pores, reducing the diffusion path of chloride ions and thereby enhancing the anti-penetration ability of the repaired specimen to chloride ions. The chloride ion migration parameters of the repaired specimens were significantly different from the undamaged original specimens.