## 3 Results and Discussion  
### Compressive Strength  
The compressive strength of IRPC with variable replacement rates (0%, 25%, 50%, 75% and 100%) after different sulfate wetting-drying cycles (0, 30 and 60) was displayed in  
Figure 3: Schematic diagram of the pore distribution.  
Figure 4. It can be found that the initial compressive strength of IRPC was excellent. The compressive strength of all groups of IRPC before sulfate attack was more than 40 MPa. After 30 sulfate wetting-drying cycles, the compressive strength of most groups of IRPC increased slightly, except in the case of 100% replacement with RCA\({}_{2}\). The possible reason for this phenomenon is that the hydration reaction of the IRPC matrix was still going on at the initial stage of the sulfate wetting-drying cycle [35]. The secondary hydration products improved the compactness and strength of the IRPC. In addition, the high-temperature drying of the wetting-drying cycles could also promote the hydration reaction of the IRPC matrix [36]. When sulfate enters the concrete, the erosion products, such as gypsum and ettringite, will be generated, and these erosion products will accumulate in the pores and cracks of IRPC, especially in its ITZ [37]. However, at the initial stage of sulfate wetting-drying cycles, the erosion products can fill up the pores and cracks and improve the compactness of IRPC. Therefore, the compressive strength of IRPC increased slightly after 30 sulfate wetting-drying cycles. However, due to the poor performance of RCA\({}_{2}\) and its weak ITZ, the compressive strength of IRPC\({}_{2}\) with a 100% replacement rate decreased slightly after 30 cycles, but the reduction was very small. This indicates that the reduction in strength due to the utilization of RCA is playing the leading role. On the whole, secondary hydration and filling of sulfate erosion products can slightly increase the compressive strength of IRPC at the early stage of sulfate wetting-drying cycles.  
After 60 sulfate wetting-drying cycles, the compressive strength decreased compared with its initial compressive strength. It is worth noting that the decline of compressive strength increased with the growth of the replacement rate. This is because the positive effects mentioned above had peaked when the wetting-drying cycles increased. The volume of sulfate erosion products expanded further, and cracks occurred in the IRPC matrix, especially in ITZ, resulting in the decline of strength. To sum up, the change of compressive strength under sulfate attack can be divided into a strengthening stage and a degradation stage.  
Another point of concern is that there was little difference between IRPC\({}_{1-25}\) and INPC in sulfate resistance at the 25% RCA\({}_{1}\) replacement rate. IRPC\({}_{1-25}\) even had a slightly higher compressive strength than INPC. This might be on account of the good properties of RCA\({}_{1}\), and appropriately attached mortar provided space for the accumulation of sulfate corrosion products. Therefore, the appropriate addition of high-quality RCA will have a positive effect on the sulfate resistance of concrete.  
Figure 4: Compressive strength of IRPC with variable replacement rates after different sulfate wetting-drying cycles.