# A Review on Bitumen Rejuvenation: Mechanisms, Materials, Methods and Perspectives
## 4 The State-Of-The-Art
### Images Techniques as Useful Complementary Tools
In fact, the rheology of the BM1 rejuvenator shows a lower viscosity compared to bitumen, while CM1 rejuvenator has different rheological characteristics. Indeed, it displays a much lower shear modulus at low frequencies. There is even a behavior of dilatant fluid or shear thickening at a frequency of about 3-5 Hz. The Authors thought that this was due to the presence of suspended particles like structures in rejuvenator CM1 or the formation of such structures at higher shear rates. However, the addition of rejuvenator BM1 into the aged bitumen causes a decreasing of the complex shear modulus, while the phase angle increases to the value of the virgin bitumen. CM1 rejuvenator on the aged bitumen leads to a lower complex shear modulus than pristine bitumen for both the concentrations tested. On the contrary, as expected, the phase angle is almost equal to (10% _w/w_ CM1) or higher than the pristine bitumen (25% _w/w_ CM1). It is worth noting that in a blend with aged bitumen any signature of the dilatant nature of the pure rejuvenator CM1 is completely lost. Figure 19: (**a**) Complex shear modulus of pristine, aged, doped aged and rejuvenator; (**b**) phase angle of pristine, aged and doped aged bitumen (reprinted from Reference [63] with the permission of SAGE Publications). In our opinion, the AFM analysis conducted by [PERSON] et al. is commendable. To our eyes, it seems to be almost the most complete job among all the AFM-based works we have examined. In fact, the authors were not limited to the identification of domains and bee structures, having identified new types of structures (e.g., tertiary and quaternary) providing an accurate description. Figure 20(ai) shows the AFM phase image for the pristine bitumen. It is possible to estimate the length of the domains long axes which falls into the range 2-6 \(\upmu\)m. The topography images show \"wrinkling\" in the middle of the domain. Finally, the domains result to be buried about 2-5 nm with respect to the average height of the continuous phase. On the contrary, the aged bitumen is very different from the pristine one. Figure 21 shows the same elliptical domain (i) and matrix (ii) of pristine bitumen. Moreover, it is possible to observe the tertiary phase, which consists of fine dark arcs and spots dispersed throughout the matrix (iii). This tertiary phase displays the lowest phase shift, and it is the softest phase. From the topography (shown in Figure 20 (ii)), it turns out that the elliptical domains are 5-8 nm lower (buried) the matrix surface, on average, while the tertiary phase protrudes above the matrix phase by 3-5 nm. Figure 14:0 AFM (**a**) phase images, (**b**) topography images of pristine, aged and doped aged bitumen (reprinted from Reference [63] with the permission of SAGE Publications). From Figure 20(aiii), it is possible to notice that the addition of 20% of BM1 to aged bitumen restores the microstructure. However, now it is possible to distinguish a new phase at the boundary of the domains and at the interstitial spaces of consecutive domains. A new quaternary phase is found, see Figure 21(biv). This new phase has the highest phase shift (highest stiffness), and it appears in almost circular shapes with sizes in the range 15.2-4 um. As it can be seen from Figure 20 (iv) and (v), the addition of the CM1 additive at both concentrations causes a change in morphology: needle-like particles, 20-90 nm wide and 50-250 nm long, appear. The network becomes the main phase at a microstructural level at the increasing of CM1 concentration. In order to understand if the microstructure evolves with time, the Authors analyzed the samples, again, after seven days. The aged bitumen and the aged doped with 20% BM1 did not show any change over time. On the contrary, the aged bitumen doped with 10 and 25% of CM1 evolve over time (see Figure 22). The biggest change can be observed for the blend with the lower (10%) concentration of CM1 rejuvenator (see Figure 22(aiii)).