Revealing aging behavior retarding mechanism of zinc oxide/expanded vermiculite composite modified bituminous mixture

Zinc oxide/expanded vermiculite composite (ZnO/EVMT), a novel modifier, can synergistically improve the thermal oxidative and ultraviolet (UV) aging resistance of bituminous mixture. However, the existing characteristics of ZnO/EVMT and the recovered binder in mixture is unclear, preventing the understanding of its aging behavior retarding mechanism. In this research, the modified bituminous mixture was prepared and treated with four aging methods, including short-term and long-term oven aging, long-term continuous and holoclimate UV aging. The ZnO/EVMT and bituminous binder in the aged mixtures were recovered. Then, the microscopic and thermodynamic characteristics of the recovered ZnO/EVMT and the retarded aging behavior of the recovered binder were characterized. Finally, the aging behavior retarding mechanism of ZnO/EVMT modified bituminous mixture was revealed. Results show that the recovered ZnO/EVMT can maintain the integrity of its original structure in both bituminous binder and mixture, and is distributed on the bitumen film surface of the loose mixture. ZnO/EVMT modified bitumen forms an exfoliation phase structure, enhancing bitumen heat barrier effect. The incorporation of ZnO/EVMT into bituminous mixture can reduce the production of carbonyl or sulfoxide group contents and the increment of large-size molecule content under different aging methods. The aging behavior retarding mechanism lies in the two-dimensional layered structure of its EVMT component and the semiconductor characteristics of its ZnO particle component, as well as the synergistic effect between the two components. Therefore, it can be expected that the mechanism by which ZnO/EVMT retards mixture aging behavior is beneficial not only for regulating its own performance and promoting its application but also for extending the expected service life and enhancing the durability of bituminous pavements.