## 3 Results and Discussion  
### Recycled Concrete Aggregates (HR and CR)  
#### 3.1.1 Recycled Aggregate Test Results  
With the sand equivalent test, the calculation of SE value gives an estimation of the percentage of fines (aggregate particles that pass the 0.063 mm sieve). For higher SE values, the percentage of fines decreases. The results of the test are given in Figure 5. The higher concentration of fines was observed for CR, which can be explained by the high w/c ratio of concrete returns, which reduces the mechanical properties of the cement paste attached on the aggregates and thus produces more fines during crushing. On the other hand, in HR aggregates, the fines concentration is relatively low since the hardened cement paste is of high strength.  
After identifying the content of fines in the recycled aggregates, it was considered necessary to determine if any clay particles exist in these fines and also measure the particle size distribution of aggregates. The results of the methylene blue test that was conducted for this purpose can be seen in Figure 6. It was observed that, for CR aggregates, a higher amount of injected dye solution is required, which is an indication of clay concentration. On the contrary, the concentration of clay in HR fines is insignificant. Since CR is 100% recycled concrete, the existence of clay can be attributed to storing and handling these aggregates in the facility of RMC.  
Figure 5: Results of sand equivalent test. L: crushed limestone; CR: crushed hardened concrete from returned concrete; HR: crushed hardened concrete from test specimens of at least 40 MPa compressive strength.  
Table 3 shows the results of water absorption (WA\({}_{24}\)), particle bulk density (p\({}_{a}\)), particle dry density (p\({}_{\rm rd}\)), particle saturated-surface-dry density (p\({}_{\rm ssd}\)) and Los Angeles coefficient (LA) tests. CR seems to absorb more water compared to HR, which is an indication of lower quality. However, the densities of the two aggregates do not differ essentially. The LA coefficient of CR was lower, but comparable to HR, and both aggregates have acceptable resistance to fragmentation according to the EN 12620 categorization. The mineralogical compositions of the raw materials CR and HR are given in Figures 7 and 8. The presence of calcite and quartz in both materials is related to the binder attached on the aggregates, as well as tri-calcium-silicate compounds (C\({}_{3}\)S) traces. The C\({}_{3}\)S in CR was attributed to unhydrated forms of cement. Additionally, portlandite content (P) originating from the hydration of cement paste was found in HR.  
The durability test results for CR and HR regarding mass loss (wt %) after 10 cycles of freezing-thawing are given in Figure 9. HR aggregates showed better results compared to CR. The mass loss of both aggregates is less than the upper category limit of 40% according to EN 12620.  
The sieved representative samples (<0.75 mm) of CR and HR were collected in order to quantify their water-soluble sulfate salts, utilizing ionic chromatography. Sulfate salts were found in a low proportion in the mate and HR, respectively.  
Although both CR and HR originate from hardened concrete, with a recycled concrete content of 90% or higher, it seems clear that HR is a higher quality aggregate, as expressed by the water absorption values, the fines quality characterization and the durability testing.