## 3 Results and Discussions
### Compressive Strength of RHA Recycled Aggregate Concrete  
The impact of RHA with different replacement ratios on the compressive strength of recycled concrete is shown in Figure 5. As shown in the figure, the compressive strength of the recycled concrete gradually increased as the hydration reaction continued, and the growth rate shows a trend of first being rapid, then slowing down and finally gradually stabilizing. For instance, the compressive strength of 30% RHA at 7 days, 28 days, 90 days and 360 days was 33.8 MPa, 42.9 MPa, 46.4 MPa and 48.3 MPa, which increased by 69.98%, 18.84%, 7.25% and 3.94%, respectively. The variation in recycled concrete's compressive strength may be because, in the initial stage of curing, the strength growth of the concrete is mainly due to the RHA particle's filling action. The fine RHA particles can fill the pores and microcracks of the paste and interface transition zone, reducing the porosity to a certain extent and increasing the bulk density of the concrete, thereby significantly improving the early strength of concrete.  
In contrast, the strength growth of recycled concrete in the later stages of curing is primarily due to the RHA particle's pozzolanic effect [36]. Although the pozzolanic reaction between RHA and free calcium hydroxide (CH) led to a reduction in the paste porosity [37], the existence of the porous structure of the recycled aggregate attached mortar led to the absorption of large amounts of water by the system and a reduction in the free water content of the matrix, resulting in a weakening of the pozzolanic effect of the RHA particles, thus causing a reduction in the total volume of hydration products, which leads to a slow strength growth of the concrete in the later curing period. This is similar to the results of [PERSON] et al. on the later strength variation of RHA natural aggregate concrete [38]. The strength growth of concrete at different curing ages is caused by a combination of the cement hydration reaction, the filling effect and the pozzolanic effect of RHA particles [39; 40].  
The compressive strength of recycled concrete shows a tendency to first increase and then decrease as the RHA replacement ratio increases. For instance, the compressive strength of 10% RHA, 20% RHA and 30% RHA at 360 days was 51.9 MPa, 52.8 MPa and 48.3 MPa and 102.17%, 103.94% and 95.08% of 0% RHA, respectively. This is probably because the high silica content and high specific surface area of RHA particles, which can contact and react with more CH and water, produce extra C-S-H gels to improve the matrix compactness, thus leading to the improvement of concrete compressive strength. However, when the strength loss due to the decrease in cement content was more significant than the compressive strength increment contributed by the RHA filling effect and pozzolanic effect, this will lead to a reduction in the compressive strength of concrete at a high RHA replacement ratio. According to the variation in the compressive strength of concrete with  
Figure 5: The impact of RHA replacement ratios on the compressive strength of recycled concrete.  
an RHA replacement ratio, it is clear that the optimum replacement ratio of RHA is 20%, which is similar to the report of RHA natural aggregate concrete. For instance, [PERSON] et al. [32] and [PERSON] et al. [41] found that the concrete specimens with a 20% RHA replacement ratio resulted in the highest paste compactness due to the joint action of the filling effect and pozzolanic effect, thus resulting in the optimum mechanical characteristics of concrete.