## 3 Results and Discussion
### Effects on the AE and Failure Behavior
#### 3.3.2 Acoustic Behavior with Stress  
Figure 6 shows the relationship between AE behavior and the stress of samples with different chemical erosion times under uniaxial compression conditions. According to the literature [27], rocks show four stages of changes in acoustic emissivity and slope of stress curves: initial crack closure, new microcracks sprouting, microcracks expanding and merging, the emergence of macroscopic cracks, and finally, destruction. In our study, except for Figure 6d, where the slopes showed significant changes, the slopes for the rest of the erosion time showed fewer changes. This may be because the material we used was artificially made and already contained a cementitious matrix chemically attacked by sulfate ions and hydrogen ions, and the microcracks had already appeared before loading. This made the slopes more similar in the second and third stages, and the slope change was insignificant. For a better discussion, we divided the whole into four stages using simple slope calculations. As seen in Figure 6, the AE-stress relationship curves of the specimens show a clear demarcation and keep changing, indicating that the difference in the chemical ion erosion time significantly impacts the mechanical response of the CGB to the external load. The results show that the deformation behaviors of the specimens under loading change with the chemical ion erosion time. It is important to note that visible damage refers to the extension of macroscopic cracks. When the erosion time is less than 60 days (Figure 6b,c), the microcrack initiation stage (II) of the specimen is prolonged, and the crack damage zone (IV) is more compressed compared to that of the initial sample (Figure 6a). These results indicate that chemical erosion plays a positive role in the initial immersion stage on the mechanical response of CGB to external loading. With the increase in the chemical ion erosion time (Figure 6d-f), the crack initiation stage (II) of the specimen decreases, and the macroscopic crack damage zone (IV) becomes wider. It indicates that the deformation of the specimen under less stress after entering the damage stage is quickly transformed from microcrack closure to microcrack extension, the macroscopic crack damage zone is enlarged, and the acoustic emissivity rises sharply. The results reflect that for a longer chemical erosion time, the internal structure of CGB deteriorates, and its mechanical response to external load becomes unstable. The degree of chemical ion erosion significantly affects the generation and subsequent extension of macroscopic cracks. Therefore, these results demonstrate that the chemical ion erosion time predominantly affects the acoustic emission behavior of the specimens under uniaxial compressive loading. Figure 5: The relationship between AE count and cumulative AE count of samples with different erosion times under uniaxial compression. The stress threshold determines a specimen's occurrence, coalescence, and the extension of microcracks [28]. This section explores the effect of chemical attack time on the stress thresholds for each deformation stage of the CGB. Figure 6 shows the stress thresholds at each displacement point during loading. The stress thresholds are normalized by the peak stress to quantitatively analyze the changes after different erosion times, as shown in Figure 7. As observed in the figure, the stress thresholds for microcrack extension and macrocrack initiation increase, and then decrease with the increase in the chemical  
Figure 6: Acoustic emission behavior as a function of stress for specimens with different erosion times under uniaxial compression conditions. I—initial crack closure; II—new microcracks sprouting; III—microcracks expanding and merging; IV—the emergence of macroscopic cracks. ion erosion time. For instance, the normalized stress threshold for microcrack extension increases from 0.71 to 0.85, and the normalized stress threshold for macrorack initiation increases from 0.9 to 0.96 after 60 days of erosion.