## 4 Test Results and Discussion
### Enhanced Resistance to Sulfate Ingress  
The result of evaluating the compressive strength of a bio-coating material with bacteria is shown in Figure 7. The compressive strength of the bio-coating material did not show a significant difference regardless of the condition and treating bacterial immobilization. The compressive strength at 7 and 28 days of normal repair without bacteria mixing was 22.1 MPa and 38.9 MPa, respectively. The compressive strength of the specimen by SSM at 28 days was 40.7 MPa, showing the highest strength. The compressive strength at 7 days  
Figure 5: Photos of the chemical composition analysis equipment: (**a**) XRD equipment; and (**b**) TGA equipment. Figure 6: Effect of immobilization techniques on the population of bacteria in expanded vermiculite. was 22.6 MPa, indicating that the compressive strength was higher than that of normal repair without bacteria mixing at all ages. The compressive strength of the specimen by NPM was 39.8 MPa, which was slightly lower than that of the SSM specimen, however, the difference was negligible at approximately 2%. The compressive strength of this specimen at 7 days of age was 21.4 MPa, which is similar to that of the SSM specimen. Figure 8 shows the changes in the compressive strength of the same specimens by days after immersion in the 5% sulfuric acid solution. The compressive strength of the cylindrical concrete without the bio-coating material before being immersed in sulfuric acid solution was 35 MPa. The compressive strength measurements of the sulfuric acid solution immersion showed that the results of all specimens showed a continuously decreasing trend after sulfuric acid solution immersion due to chemical erosion. The decreasing rate of compressive strength showed the largest change in the case of normal repair without bacteria, from 9% at 7 days to 14% at 28 days. The rate of the decrease in the specimen using repairs with _Rhodobacter capsulatus_ by SSM was 8% at 28 days of sulfuric acid solution immersion. The rate of the decrease in the concrete specimen with the bacteria immobilized by NPM was 1% at 7 days and 5% at 28 days of sulfuric acid solution immersion, indicating the smallest decrease in strength by sulfate deterioration among all the specimens. This indicates that the performance of the repair treated with NPM was maintained for a long time under the sulfate ingress condition, demonstrating the effectiveness of the NPM. Figure 8: Compressive strength variation of the coated concretes immersed in 5% sulfuric acid solution. Figure 7: Compressive strength development of bio-coating materials. The mass variation in the concrete coated with bio-coating material after immersion in sulfuric acid solution also showed a similar tendency to the variation in compressive strength, as shown in Figure 9. The decrease in the mass of the concrete after 28 days of immersion was the highest at 7% in the concrete coated with the normal repair. The decrease in the mass of the SSM specimen and the NPM specimen after 28 days of immersion was 5% and 2%, respectively, showing that the repair layer prepared by creating a negative pressure environment to serve as an effective protection material against sulfate ingress. The volume expansion due to ettrinigite and gypsum--which are hydrates of deterioration formed through the reaction of cement hydrates such as Ca(OH)\({}_{2}\) and CSH gel with sulfuric acid (H\({}_{2}\)SO\({}_{4}\))--causes the cement matrix to crack as well as a decrease in the compressive strength of the concrete [13; 17]. The decrease in the compressive strength and mass of the concrete exposed to sulfuric acid can be significantly improved by applying repairs with slime coating with bacteria. This is because, as shown in Figure 10, the reaction between the cement hydrates and H\({}_{2}\)SO\({}_{4}\) was inhibited due to the block effect of the deterioration factors of bacterial slime formed on the surface of the repairs [19; 20].