## 3 Results and Discussion
### Acoustic Emission Results in the Process of Uniaxial Compression Test  
Figure 5 shows the acoustic emission waveforms of 8 channels of B-14-80% sample during the compression test. According to the waveforms, the whole test process can be divided into 3 stages: initial stage, stable stage, and active stage, which corresponds to the micro-crack formation, micro-crack expansion, crack connection, and failure of concrete sample. In this work, the acoustic emission results of A-80% samples, B-20%, B-50%, and B-80% at different pre-loading stages are presented and discussed. The testing results of other samples and further discussions are available on request from the corresponding author. Figure 6 presents the counts, stress, and energy development in terms of loading time of group A samples for different pre-loading stages. Figure 6a reveals that under the condition of uniaxial compression, the AE ringing counts of A-3-80% concrete appeared in two peak stages, at around 20 s and 110 s loading time, which corresponds to the initial and active stages. Figure 6b indicates that a large proportion of energy appears in the initial stage (0-20 s). For concrete at 3 d, the cement hydration has just entered a stable development period, the hydration is insufficient, and the concrete is still unstable. At 3 d, pre-loading is carried out, and the external force applied not only compacts the shrinkage cracks of the concrete but also the original micropores, microscopic gaps, and defects in the concrete are gradually compacted under the action of the external load so that the internal microstructure changes. Due to the pre-loading degree of 80% at 3 d compressive strength, the internal cracks in the concrete developed from the interface cracks between the aggregate and the mortar to the interior of the mortar. A large number of cracks were not completely repaired following the curing process, and they penetrated again at the initial stage of loading. Thus, in Figure 6b, a large proportion of energy appears at the initial stage. The unrepaired cracks reduced the final effective bearing area of the samples and made the energy intensity in the later stage. Figure 4: The relative compressive ratio results between pre-loading samples and virgin samples. Figure 6c-f shows the acoustic emission results of A-7-80% and A-14-80% samples. The ringing counts also showed 2 peaks at around 20 s (initial stage) and 110 s (active stage) loading time, which is similar to the results of A-3-80% samples. However, compared with A-3-80% samples, more counts were observed in the active stage. In Figure 6b, only one peak of the energy development of the A-3-80% sample in the initial stage was observed. However, the energy development in Figure 6d,f showed 2 major peaks, which correspond to the initial and active stages. This is because, at 7 d and 14 d, the hydration degree is sufficiently high to resist the 80% pre-loading degree. There were only a small amount of cracks generated during the pre-loading process. On the other hand, there were still enough un-hydrated cement particles. Thus, in later curing proceedings, the hydration products healed the cracks generated in the pre-loading process. Therefore, when the concrete was subjected to axial compression at 270 days, the energy transition moved backward, and a \"double peak\" phenomenon appeared in the energy history diagram. The acoustic emission ringing counts of the A-28-80% samples under uniaxial compression have two peak stages, as shown in Figure 6g. The stable stage is short, which is shorter than that of the A-7-80% and A-14-80% samples. Figure 6h reveals that most of the energy for A-28-80% samples was concentrated in the initial stage. Only limited energy was released in the stable and active stages. This is because when the concrete is cured for 28 days, the cement hydration is sufficient, and the concrete is stable. At this time, the pre-loading was carried out, and some cracks were generated.