## 4 Test Results and Discussion
### Chemical Property Evaluation through Surface Analyzing Techniques (XRD and TGA)  
Figure 13 shows the XRD analysis result of the sample collected from the inner concrete surface protected from the repair material after 28 days of immersion in 5% sulfuric acid solution. In the same figure, the XRD pattern of the sample taken from the concrete surface without exposure to sulfuric acid was also indicated as the comparative data (virgin concrete). In all samples, peaks such as gypsum and portlandite, which are cement hydrates, were formed, and the peak of ettrinigite, a reaction compound between gypsum and C\({}_{3}\)A, was also identified. However, the peak intensity of gypsum, a sulfuric acid-deteriorated hydrate that causes the expansion and cracking of the cement structure, was slightly different depending on the coating conditions. In the concrete surface coated with the normal repair without bacteria mixing, the peak intensity of gypsum was high at approximately 11-12\({}^{\circ}\), 31\({}^{\circ}\) and 33\({}^{\circ}\). On the other hand, the peaks of gypsum were not relatively high in the concrete surface coated with slime bacteria-mixed repair (SSM and NPM). In addition, the XRD patterns of these specimens were similar to those of virgin concrete, not deteriorated by sulfuric acid. These results indicate that when the repair mixed with slime-forming bacteria is applied, the base concrete can be reasonably protected from sulfate attack through the effect of inhibiting sulfate ingress by the slime, and deteriorated cement hydrates (gypsum) from sulfuric acid are not formed. Furthermore, considering that the peak intensity of gypsum increases in the sample exposed to sulfuric acid deterioration, it can be expected that the peak intensity of ettrinigite is also increased. However, the XRD patterns for all samples showed a similar peak intensity of ettrinigite regardless of the application of bacteria. This is thought to be because ettrinigite loses stability in a low pH environment such as sulfuric acid solution and is easily converted to gypsum and aluminum sulfate [44]. Figure 14 shows the result of mass variation due to the thermal decomposition of hydrates in the samples taken from the concrete surface below the coating material. The specimens in the figure are of virgin concrete not affected by the sulfuric acid deterioration and the other specimens are those after 4 weeks of 5% sulfuric acid solution test (normal repair, SSM, and NPM). All specimens showed a decrease in mass due to thermal decomposition over various temperature ranges, including 60-250 \({}^{\circ}\)C and 450-500 \({}^{\circ}\)C. Typical cement hydrates--Ca(OH)\({}_{2}\) and calcite--show a mass reduction curve as they undergo thermal decomposition in the temperature ranges of 450-500 \({}^{\circ}\)C and 700-850 \({}^{\circ}\)C, respec  
Figure 13: XRD patterns at the surface layer of coated concrete samples immersed in 5% sulfuric acid solution for 28 days. tively. On the other hand, C-S-H does not show a clear reduction curve of mass variation at a specific temperature. The gypsum formed in the cement matrix was typically present in the form of dihydrate gypsum (CaSO4\(\cdot\)2H2O), and gradually starts to dehydrate at a temperature of 60-95 \({}^{\circ}\)C. The gypsum changes to hemihydrate gypsum (CaSO4\(\cdot\)1/2H2O) through proper dehydration at a temperature of 95-175 \({}^{\circ}\)C. Thereafter, it changes into anhydrous gypsum (CaSO4) at approximately 250 \({}^{\circ}\)C due to continuous dehydration. The thermal decomposition of ettrinigite formed by the reaction of anhydrous gypsum with aluminate (C3A, 3 CaO\(\cdot\)Al2O3) occurs at approximately 88-105 \({}^{\circ}\)C, partially overlapping with the curve of the mass variation of gypsum [43; 45; 46]. In the temperature range of 60-250 \({}^{\circ}\)C, the mass reduction of the normal repair specimen without bacteria mixing was the largest at 11.4%, which was approximately twice that of virgin concrete without sulfuric acid deterioration in this temperature range.