## 3 Results and Discussion
### Light Microscopy to Determine Asphaltene Melting Point  
This is a microscopy technique that also combines the use of a light microscope with the concept of basic calorimetry. It involves monitoring the response of asphaltene particles to a gradual increase in temperature, and this response can be observed in real time under the microscope. Even though this technique is a qualitative method, its validity and accuracy has been verified using differential scanning calorimetry, as shown in previous studies [2; 12; 17]. This microscopy technique is a form of real-time observation of the transition from one state to another, a parameter that differential scanning calorimetry usually measures. Bitumen, after aging, exhibits an increase in rigidity in its solid form and viscosity in its liquid form. Since the asphaltenes are the fraction of bitumen susceptible to oxidative aging [22; 27], it is only logical to investigate the response of asphaltenes to an increase in temperature. Aged asphaltenes become more heat resistant, and melt at higher temperatures compared to unaged virgin bitumen [23; 29]. The melting point of an asphaltene is the temperature at which it transitions from a crystalline to an amorphous state. The images in Figure 5 show the asphaltenes of each sample and the temperatures at which they transition from crystalline to amorphous states. Figure 5: Light microscopy images indicating asphaltene melting points of virgin and aged 70/100 bitumen. In Figure 5, it can be observed that with each cycle of aging, the melting point of the asphaltenes of the virgin bitumen continues to increase. The highest melting point was observed in the sample that underwent three PAV aging cycles. Upon the addition of the R1 recycling agent to the bitumen that was aged once, it can be observed from Figure 6 that the melting point of the asphaltenes of the recycled bitumen reduced from 170 \({}^{\circ}\)C to 155 \({}^{\circ}\)C, which is 5 \({}^{\circ}\)C lower than the virgin bitumen. This shows the capacity of the R1 recycling agent to restore the thermal properties of the asphaltenes of aged bitumen. Upon the aging of this recycled sample, the melting point of the asphaltenes increased, this time to 165 \({}^{\circ}\)C, which is 5 \({}^{\circ}\)C higher than the melting point of the virgin bitumen asphaltene sample. This aged bitumen was modified again with the R1 recycling agent, and the melting point decreased to 160 \({}^{\circ}\)C (the same melting point as the virgin bitumen asphaltene sample). A final aging cycle was carried out on the twice-recycled bitumen, and it was observed that the asphaltene melting point increased once again to 165 \({}^{\circ}\)C. This could mean that the R1 recycling agent is capable of interacting chemically with the asphaltenes of the aged bitumen, thereby restoring their properties that were lost due to oxidative aging. On the other hand, in the case of the R2 recycling agent, when it was added to the bitumen sample that was aged once, the melting point of the asphaltenes reduced from 170 \({}^{\circ}\)C to 160 \({}^{\circ}\)C, as can be seen in Figure 7. This recycled sample was aged again, and the melting point of the asphaltenes increased to 165 \({}^{\circ}\)C. This twice-aged sample was modified again with R2, and the melting point of the asphaltenes was even higher (180 \({}^{\circ}\)C, which is the same melting point as the neat bitumen aged twice). This means that the second modification of the bitumen with the R2 recycling agent was not effective, and even worsened the effects of aging on the asphaltenes thermal properties. This twice-recycled bitumen was aged for the third time, and the asphaltene melting point remained the same at 180 \({}^{\circ}\)C. It can thus be said that the R2 recycling agent, after the first recycling process, did not in any way improve the bitumen sample. This could be because R2 is a mere fluxing agent that softens the bitumen and reduces its viscosity. It does not truly interact chemically with the aged bitumen to restore its virgin properties.