## 3 Results and Discussion
### Evaluation of Self-Healing Performance as a Function of Temperature and pH
#### 3.2.2 Chemical Evaluation of Self-Healing Precipitates  
TG-DTA was carried out to quantitatively analyze the types and amounts of Ca(OH)\({}_{2}\) and CaCO\({}_{3}\) precipitated by self-healing on the surface and interior of cracks. The cracked section was almost severed from the cement matrix, as illustrated in Figure 14, and TG-DTA specimens were only collected from the cracked section of each test body.  
Figure 13: X-ray CT images of cracks before and after self-healing at different pH values and a constant temperature of 40 \({}^{\circ}\)C: (**a**) CM series; (**b**) CN series.  
Figures 15 and 16 show the Ca(OH)\({}_{2}\) and CaCO\({}_{3}\) content in the cracked section before self-healing (step A) and after self-healing (step B) as functions of the self-healing temperature (20, 40, and 60 \({}^{\circ}\)C; constant pH of 9.0) and pH (9.0, 10.5, and 12.0; constant temperature of 40 \({}^{\circ}\)C).  
Figure 16: Ca(OH)\({}_{2}\) and CaCO\({}_{3}\) content after self-healing at different pH values (pH = 9.0, 10.5, 12.0) and a constant temperature of 40 \({}^{\circ}\)C.  
Figure 14: Preparation of specimens for thermogravimetric-differential thermal analysis (TG-DTA).  
Figure 15: Ca(OH)\({}_{2}\) and CaCO\({}_{3}\) content after self-healing at different temperatures (\(T\) = 20, 40, 60 \({}^{\circ}\)C) and a constant pH of 9.0.  
As can be seen in Figure 15, when the self-healing temperature was set at 20 \({}^{\circ}\)C, the Ca(OH)\({}_{2}\) and CaCO\({}_{3}\) content changed by about 20% in CM-20 (which was immersed in a Ca(OH)\({}_{2}\) solution with CO\({}_{2}\) microbubbles) and by about 25% in CN-20 (which was immersed in a Ca(OH)\({}_{2}\) solution with CO\({}_{2}\) nanobubbles). When the self-healing temperature was set at 40 \({}^{\circ}\)C, the Ca(OH)\({}_{2}\) and CaCO\({}_{3}\) content changed by about 25% in CM-40 and by about 30% in CN-40. After self-healing at 60 \({}^{\circ}\)C, the Ca(OH)\({}_{2}\) and CaCO\({}_{3}\) content changed by about 15% in CM-60 and by about 20% in CN-60. In a parallel experiment, the pH of the Ca(OH)\({}_{2}\) solution was varied (9.0, 10.5, and 12.0), while the temperature was kept constant at 40 \({}^{\circ}\)C. As shown in Figure 16, the Ca(OH)\({}_{2}\) and CaCO\({}_{3}\) content changed by about 25% in CM-9.0 (which was immersed in a Ca(OH)\({}_{2}\) solution with CO\({}_{2}\) microbubbles) and by about 30% in CN-9.0 (which was immersed in a Ca(OH)\({}_{2}\) solution with CO\({}_{2}\) nanobubbles) when self-healing took place at a pH of 9.0. When the self-healing pH was set at 10.5, the Ca(OH)\({}_{2}\) and CaCO\({}_{3}\) content changed by about 15% in CM-10.5 and by about 20% in CN-10.5. After self-healing took place at a pH of 12.0, the Ca(OH)\({}_{2}\) and CaCO\({}_{3}\) content changed by about 20% in CM-12.0 and by about 25% in CN-12.0. Overall, the amount of Ca(OH)\({}_{2}\) after self-healing (step B) was lower than that before self-healing (step A), whereas the amount of CaCO\({}_{3}\) increased, regardless of the temperature and pH of the Ca(OH)\({}_{2}\) solution that supplied the CO\({}_{3}\)\({}^{2-}\) ions for self-healing. This suggests that the Ca\({}^{2+}\) ions from Ca(OH)\({}_{2}\) accumulated not only on the surface of the cement paste, but also in the cracked section, and reacted with CO\({}_{3}\)\({}^{2-}\) ions to produce a large amount of CaCO\({}_{3}\) via self-healing [1]. In addition, the CN series of test bodies had a somewhat higher CaCO\({}_{3}\) content than that in the CM series. When the pH was adjusted to remain constant at 9.0, the amount of CaCO\({}_{3}\) generated decreased with a temperature in the order of 40 \(>\) 20 \(>\) 60 \({}^{\circ}\)C; when the temperature was adjusted to remain constant at 40 \({}^{\circ}\)C, the amount of CaCO\({}_{3}\) generated decreased with a pH in the order of 9.0 \(>\) 12.0 \(>\) 10.5. This means that the filling rate of self-healing precipitates was the highest, i.e., self-healing was the most effective at 40 \({}^{\circ}\)C and a pH of 9.0 [24,25].