## 3 Analysis of Experimental Results
### Internal Damage Mechanism
#### 3.6.3 Properties on the Micro Scale  
The roughness of the interface of the self-compacting concrete with a higher replacement of RCA was generally higher, as shown in Figure 14. Substances from the outside world, such as sulfate ions, typically penetrate into concrete via the weak links of concrete. It has long been understood that the main weak point in concrete is the interfacial transition zone (ITZ) created between the aggregate and mortar [42]. The surface morphology of concrete ITZ is made up of a group of amorphous spherical particles with certain pore structures between them, according to [PERSON] et al.'s research [39; 43]. In the 3D illustration, the irregular spherical gullies somewhat matched the ITZ inside the concrete, where the aggregate and mortar were visible in contrast to the ITZ smoothness.  
Figure 11: SEM results of RCASCC-50 at the end of 120 wet and dry cycles. (**a**) Erosion products and internal cracks. (**b**) Erosion products fill internal cracks.  
Figure 12: SEM results of RCASCC-25 after the end of 120 wet and dry cycles. (**a**) Pin cluster erosion products. (**b**) EDS energy spectrum point sweep analysis. (**c**) Pin-columnar erosion products. (**d**) EDS energy spectrum point sweep analysis.  
Figure 13: SEM results of RCASCC-100 at the end of 120 wet and dry cycles. (**a**) Surface structure of concrete after erosion. (**b**) Concrete surface erosion products.  
Figure 14: _Cont._  
The graphical results in Figure 14a-e show how the internal ITZ of the self-compacting concrete varies in roughness according to the RCA replacement rate. The specimens with higher RCA replacement rates also clearly had higher roughness levels [41]. The overall was around 30% greater than the specimens with lower replacement rates, which laterally showed that the replacement rate of RCA would have an impact on the roughness of the specimens' internal ITZ. When coupled with cement and sand to harden in the concrete configuration, recycled coarse aggregate forms an ITZ surface that is rougher because it adheres to more old mortar and has a higher shape index during crushing. The primary cause was that, in areas where the ITZ structure was sparse, the concentration of SO42- significantly rose increased substantially with the number of the sulfate dry and wet cycles increased [23]. The swelling erosion products that were produced, including calcium alumina, filled the tiny gaps between the gel components and improved the microstructure of ITZ [43]. The outcome was a denser concrete construction. With the number of wet and dry cycles of sulfate erosion, the crystalline material in the ITZ structure of RCASCC steadily increased, and the pores between the hydration products gradually expanded [43]. The holes between the gel particles widened when more water molecules from the solution  
Figure 14: Changes in the transition zone of RCASCC interface under 120 wet and dry cycles. (**a**) RCASCC-0. (**b**) RCASCC-25. (**c**) RCASCC-50. (**d**) RCASCC-75. (**e**) RCASCC-100.  
entered the ITZ [44; 45]. Later, as the initial cracks in ITZ grew and continued to form inside the concrete, the concrete's internal structure was destroyed.