# Char from Pyrolysis of Waste Tires to Increase Bitumen Performances
## 3 Results and Discussion
### Bitumen Characterization
However, the effect of char addition on the increase in G\({}^{\prime}\)@50 \({}^{\circ}\)C being quite high is noteworthy. Interesting information can also be derived from the simulated aging experiments. It turns out that G\({}^{\prime}\)@50 \({}^{\circ}\)C and T* in each aged bitumen are higher than in the corresponding unaged sample. This is due to the increase in the number of polar functional groups of the molecules in bitumens [38] with their ultimate aggregation and constrained dynamics typically caused by the aging process. Therefore, the chemical reason for this change will not be discussed deeply here, and the reader can see paper [39] for further reading. However, a quantitative analysis is insightful: to highlight the effect of char addition, an estimation of the change in G\({}^{\prime}\)@50 \({}^{\circ}\)C and T* with the aging time (i.e., the slopes of the plots G\({}^{\prime}\)@50 \({}^{\circ}\)C and T* vs. aging time) as a function of char addition was evaluated. Results are reported in Figure 5. It can be seen that the increase in T* with the aging time is almost constant for all the amounts of chars added to the bitumen and it does not differ from the values for the reference (undoped) bitumen. In addition, no significant effect is seen if the flowing gas is changed. In synthesis, it can be concluded that the addition of char does not affect the increase in T* upon aging, but it almost halves the aging effect as reflected by the increase in G*@50 \({}^{\circ}\)C upon aging. This highlights a special effect exerted by the chars on the rigidity at working conditions of the bitumen, rather than on its transition temperature. This is also in line with the indications of the data in Figure 4, where the comparison with the inert CaCO\({}_{3}\) highlighted a more pronounced effect on G*@50 \({}^{\circ}\)C than on T*. The anti-aging effect could be ascribed to the effect of char particles, reasonably slowing down the kinetic processes of the molecules in bitumens by presenting to them a solid obstacle (the interfacial barrier). In this way, some of the bituminous molecule degrees of freedom are hindered. Since char can be classified among substances under the definition of _activated carbon_, the char carbon surface reactivity can also give a further chemical effect, in our opinion. For example, the presence of small amounts of zinc sulfide can give rise to further interactions between the metal and the asphalthene or resins present in the bitumen, as already suggested in recent works [37]. After all, the peculiar physico-chemical properties of activated carbon have been of added value in many applications, filtration, cleaning and adsorption of gases, liquids and contaminants [40,41], so that some anti-oxidation effects [42], and some increased thermal storage stability [43], have also been recently found in bituminous materials. Rheological data allowed the derivation of the rutting parameter. It is defined as G*/sin \(\delta\). It was found that under usage conditions [44] (50 \({}^{\circ}\)C), the observed values are always higher than 1 kPa for unaged samples, meeting the limits imposed by the Superior Performing Asphalt Pavements method (Superpave SHRP, Strategic Highway Research Program) [45]. It must be underlined that a recent study [46] highlighted that pyrolysis of materials rich in carbon, like waste tires, can give carbon-rich char, which can be well accommodated within the bituminous matrix. In this regard, the indications of the present work corroborate that study well, by suggesting the use of CO\({}_{2}\) as the flowing gas to be used during pyrolysis for even more marked effects. The chemical explanation for this effect, i.e., the higher reactivity of CO\({}_{2}\) with respect to N\({}_{2}\), allows for suggesting that other reactive gases could be explored for future experiments in the piloted design of char for bitumen modification. Figure 5: Variation of the transition temperature T* with aging time as a function of char wt%.