Examining the Implications of Wax-Based Additives on the Sustainability of Construction Practices: Multiscale Characterization of Wax-Doped Aged Asphalt Binder

Paraffin wax is a component of many additives and rejuvenators commonly used in the asphalt industry to promote sustainability by facilitating recycling and/or reducing mixing and compaction temperatures during pavement construction. However, the effect of wax-based additives on asphalt molecules and consequently on asphalt binder's performance characteristics has not been thoroughly understood. This paper uses a combination of a computational approach and an experimental approach to study the properties of aged asphalt binder specimens in the presence of paraffin wax. Differential scanning calorimetry showed that the introduction of paraffin wax to aged asphalt significantly reduces the glass transition temperature of the aged asphalt, with 10% wax-doped aged asphalt having a glass transition temperature 9 °C lower than the control asphalt. The above observations can be attributed to a plausible role of straight alkane chains of wax in disturbing the clusters of asphaltenes and increasing asphaltene mobility. The increase in asphaltene mobility is also reflected in the rheology of wax-doped aged asphalt; its percent recovery in the presence of 10% wax is reduced by 50% compared to the control. The results of molecular dynamics simulations revealed how paraffin wax molecules change the aggregation pattern of aged asphalt binder, as evidenced by the reduced formation of nanoaggregates in oxidized asphaltene in the presence of wax molecules. The reduced formation of nanoaggregates is described as a three-step mechanism: the attraction of wax molecules to nanoaggregates of oxidized asphaltene; the penetration of wax molecules to self-assembled asphaltene stacks; and disturbances in the formation of parallel oxidized asphaltene stacks. The results show the average aggregation number is reduced after adding paraffin wax molecules to an equilibrated system of oxidized asphaltene molecules. Furthermore, the radial distribution function confirms that, after the addition of paraffin wax molecules, the formation of parallel stacks of oxidized asphaltene is less likely. Thermogravimetric analysis shows that the addition of wax to aged asphalt binder increases the onset temperature of degradation; an increase of 25 °C is recorded when the wax dosage increases from 0% to 10%. The overall thermal stability of asphalt binder is reduced, as evidenced by lower residual mass in samples with higher wax content. In addition, the difference in critical cracking temperatures (determined based on the stiffness and the stress relaxation capability of the asphalt binder) increases as the wax content increases. This in turn indicates that the cracking properties of wax-doped binder are controlled mainly by the stiffness rather than by the stress relaxation capability; the control of cracking properties by stiffness could be attributed to the formation of a lamellar structure in wax-doped aged asphalt binder, causing a significant increase in stiffness. A molecular-level understanding of wax-asphaltene interaction helps explain how the performance of aged asphalt binder is affected by wax-based additives commonly added to asphalt binder to promote sustainability by facilitating the recycling of aged asphalt and/or reducing the mixing and compaction temperatures.