## 3 Results and Discussion
### SEM Analysis  
To further investigate the effect of the aggregates' quality grades on the sulfate attack mechanism of IRPC, an SEM analysis on the ITZ of IRPC was carried out. Figure 9 shows the SEM images of the IRPC prepared from three kinds of aggregates with different quality grades. First of all, Figure 9(a1,a2) displayed the ITZ of INPC before and after the sulfate attack. As can be seen in Figure 9(a1), the initial ITZ of INPC was relatively smooth, and no obvious cracks and pores were observed in the paste. After the sulfate attack, the ITZ of INPC expanded slightly and widened, as shown in Figure 9(a2). The little white spots on the ITZ were the erosion products, which were small in volume. NA possessed the best properties and was regarded as the high-quality RCA. Therefore, INPC prepared from NA possessed the best compressive strength, abrasion resistance and sulfate resistance, which was consistent with the results obtained in macroscopic experiments.  
In addition, the SEM images of IRPC\({}_{1}\) prepared from the medium quality of RCA\({}_{1}\) were revealed in Figure 9(b1,b2). Figure 9(b1) showed that the initial ITZ of IRPC\({}_{1}\) was slightly undulating and uneven, and there were some micro pores in the paste of IRPC\({}_{1}\). After sulfate wetting-drying cycles, the short columnar crystals of gypsum [44] were observed in the ITZ of IRPC\({}_{1}\) in Figure 9(b2). The formation of erosion products, such as gypsum, further expanded the volume of the ITZ and reduced its bonding strength [45], which had an adverse effect on the mechanical properties and abrasion resistance of IRPC.  
Finally, Figure 9(c1,c2) showed the SEM images of IRPC\({}_{2}\) prepared from the low quality of RCA\({}_{2}\). As shown in Figure 9(c1), the initial ITZ of IRPC\({}_{2}\) was rough and uneven on the surface with a straight crack in it. After sulfate attack, large columnar crystals of gypsum and needle-like phase crystals of ettringite [46] were observed in Figure 9(c2). The massive sulfate erosion products made the ITZ expand rapidly in volume and become loose. The ITZ of IRPC\({}_{2}\) was the most cracked and loosest after the sulfate attack. The compressive strength and abrasion resistance of IRPC\({}_{2}\) were the worst among all three types. This was because that the quality grade of RCA\({}_{2}\) was the low-grade, indicating a large amount of attached mortar was attached to the surface of RCA\({}_{2}\). The new ITZ formed between the old attached mortar and the new mortar had a low bonding strength and a large number of cracks and pores [47]. Therefore, after sulfate attack, erosion products  
Figure 9: SEM images of (**a1**) INPC, (**b1**) IRPC\({}_{1-100}\) and (**c1**) IRPC\({}_{2-100}\) before sulfate attack, and (**a2**) INPC, (**b2**) IRPC\({}_{1-100}\) and (**c2**) IRPC\({}_{2-100}\) after 60 sulfate wetting-drying cycles.  
tended to accumulate in these cracks and pores. The volume of sulfate erosion products gradually expanded and made the weak ITZ more fragmented [48], resulting in a further reduction in the strength of IRPC\({}_{2}\).  
However, the accumulation of these sulfate erosion products was not all negative. When the replacement rate of RCA was low, and the quality of RCA was better (attached mortar content of RCA is low), the sulfate erosion products could fill the micro pores and cracks in the ITZ and even in the attached mortar of RCA. This filling effect could improve the compactness of IRPC; thus, improving its strength and abrasion resistance. This phenomenon was consistent with what was observed in the macroscopic experiment.