## 3 Results and Discussion
### Acoustic Emission Results in the Process of Uniaxial Compression Test
However, most cement particles were hydrated in the first 28 d. There was insufficient hydration in the later curing process to heal the cracks. As a result, the cracks generated in pre-loading cannot be effectively healed. Thus, compared with A-7-80% and A-14-80% samples, most of the energy for the A-7-80% samples was released in the initial stage. Figure 5: Acoustic emission waveforms of 8 channels during the whole loading process. It is suggested that for ordinary concrete without SCMs, the load-bearing age should not be earlier than 7 days. Besides, the load should be less than 80% of the current strength. Figure 7 shows the counts, stress, and energy development of group B concrete with different pre-loading stages. Compared with ground A concrete in Figure 6, there are more signals detected in the active stage, especially for B-3-80% and B-28-80% samples. Moreover, the signal intensity in the stable stage for group B concrete was also higher than for group A. The reason is that the addition of SCMs in group B concrete has a higher secondary hydration potential after the pre-loading process. The incorporation of SCMs induced a pozzolanic reaction during the curing process after pre-loading. Thus, the generated hydration products effectively healed the damage from the pre-loading. Therefore, compared with group A concrete, there are more signals detected in stable and active stages for group B concrete. Figure 6: Counts, stress, and energy in terms of loading time of A-80% concrete samples of different pre-loading stages. (**a**) A-3-80% counts and stress development. (**b**) A-3-80% energy development. (**c**) A-7-80% counts and stress development. (**d**) A-7-80% energy development. (**e**) A-14-80% counts and stress development. (**f**) A-14-80% energy development. (**g**) A-28-80% counts and stress development. (**h**) A-28-80% energy development. Figure 7: Counts, stress, and energy in terms of loading time of B-80% concrete samples of different pre-loading stages. (**a**) B-3-80% counts and stress development. (**b**) B-3-80% energy development. (**c**) B-7-80% counts and stress development. (**d**) B-7-80% energy development. (**e**) B-14-80% counts and stress development. (**f**) B-14-80% energy development. (**g**) B-28-80% counts and stress development. (**h**) B-28-80% energy development. Figure 10: The acoustic emission signal correlations of the B-7-80% sample. (**a**) The total energy in terms of amplitude. (**b**) The frequency of the peak in terms of amplitude. (**c**) The ringing counts in terms of amplitude. (**d**) The duration in terms of amplitude. Figure 9: The acoustic emission signal correlations of the A-7-80% sample. (**a**) The total energy in terms of amplitude. (**b**) The frequency of the peak in terms of amplitude. (**c**) The ringing counts in terms of amplitude. (**d**) The duration in terms of amplitude. Figures 9b and 10b show an obvious correlation between peak frequency and amplitude, and there are four concentrated intervals of peak frequency. Combined with the analysis in the previous section, it shows that there are four kinds of acoustic emission sources in the concrete. In Figures 9b and 10b, the peak frequencies are about 38 kHz and 171 kHz. The amplitude ranges are 0-4000 mv and 0-8000 mv, respectively, and a large number of acoustic emission signals are concentrated, indicating that there are two main types of acoustic emission sources for concrete materials, and the peak frequencies are about 38 kHz and 171 kHz, respectively. Figures 9c and 10c reveal that the ringing count and the amplitude have a clear relationship. The ringing count decreases with the increase of amplitude. Power relationships are obtained for group A and group B concrete samples. Compared with group A concrete, the signals of group B concrete with amplitude less than 1000 mv and ringing count less than 500 increased significantly. The relationships between duration and amplitude are shown in Figures 9d and 10d.