## 1 Introduction  
Carbon reduction commitments after the UN Climate Change Conference (COP26) 2021 and Paris summit have sensitized governments on the environmental concerns more than ever. As one of the main carbon-affiliated sectors, within the road industry, several technologies have been proposed to reduce the carbon equivalent of asphalt pavement construction and maintenance interventions [1, 2]. None of these technologies and initiatives is more practical and generally accepted in the road industry than the recycling of reclaimed asphalt (RA). Although, in terms of material functionality, recycling RA has been improved over the past few decades, several limitations still remain. For instance, it has been shown that when RA is incorporated in a high ratio in an asphalt mix, producing quality recycledasphalt is dependent on the correct choice of the type, dosage, and application method of the recycling agent [3]. To date, many different design flowcharts or blending charts have been proposed to optimize the dosage of the recycling agents, mostly considering only the physical properties determined by conventional binder tests, i.e., viscosity, penetration, softening point, etc. However, recent research shows that the mechanism of a complete rejuvenation is not a unilateral but a multilateral action [4]. Different recycling agents have unique and varying potential to address the deficiencies of aged binders to different extents [5; 6]. In other words, it can be said that all of the additives that are being used for recycling purposes could be considered as recycling agents; however, not all of them are rejuvenating agents. To investigate the multilateral mechanism of recycling, in addition to physical and mechanical properties characterization, which is usually the primary step, several techniques have been introduced in research and practice. These can be divided into microscopic, physico-chemical, and chemical techniques, and so on. Among these techniques, microscopic analysis, such as atomic force microscopy (AFM), that provides knowledge on the microstructural properties is a widely popular approach. Regardless of what the different features in AFM images correlate to, there is a consensus on the fact that while some recycling agents damage the chemical morphology of aged bitumen, there are some others that could restore it [2; 7]. This restoration could be achieved either by dispersing the so-called bee structures and reducing their size, or regenerating the dominant para phase [8; 9]. This may or may not be observable during conventional bitumen tests [10; 11]. Compared to AFM, light microscopy has been less applied in this context. This is due to the technical inadequacy of light microscopy for bitumen observation purposes. However, the authors have successfully used this technique in previous studies to investigate asphaltene thermal properties, including melting point. The technique is based on the fact that during the oxidation of bitumen, the asphaltene clusters tend to increase in size and texture, as explained by the Yen-Mullins model. Based on the results, while the addition of one of the recycling agents reduced the melting point of the asphaltenes to values close to the reference value, the others did not [12]. In the investigation of the chemical composition of virgin and aged bitumen, SARA analysis is a technique frequently used for investigating recycling agents and differentiating rejuvenators from those of flux oils (softeners). A real bitumen rejuvenation mechanism involves the rejuvenator influencing changes in the colloidal matrix of aged bitumen, which restores the asphaltene-maltene ratio to a state similar to that of virgin bitumen [13]. Recycling agents, especially fluxing oils, may reduce the viscosity and modulus of the overall bitumen through lowering the viscosity of the continuous solvent phase, but may have little effect on the intermolecular agglomeration and self-assembly of the polar micelles [14].