# Oil Sludge as a Rejuvenator for Aged Bitumen: Structural and Thermal Effect
## 1 Introduction  
Highways and related infrastructure are critical to national economic development, and asphalt pavements--due to their flexibility and durability--remain a key component of this network [1]. However, over time, exposure to oxidative aging, UV radiation, and environmental stressors such as moisture and traffic loading leads to severe degradation in the physicochemical properties of bitumen, the primary binder in asphalt concrete [2, 3]. The aging process involves the transformation of light fractions into heavier ones--aromatics into resins and then into asphaltenes, eventually forming carbenes and carboids [4, 5]. These changes increase binder stiffness, reduce flexibility, and impair pavement performance [6, 7, 8].  
Bitumen aging is typically divided into short-term (technological) aging, which occurs during production and mixing with aggregates, and long-term (operational) aging, which takes place over time under environmental influences such as UV radiation, oxygen, and traffic loads [9]. To counteract these effects, rejuvenators are employed to reverse aging-induced alterations. An effective rejuvenator should not only soften aged bitumen by reducing viscosity and increasing elasticity, but also rebalance its molecular composition, particularly the ratio of maltenes to asphaltenes [10; 11; 12]. The restored binder should regain its flexibility and resistance to thermal and mechanical stresses, thereby prolonging pavement service life.  
A wide range of rejuvenators has been explored, including petroleum-based oils, waste engine oils, and bio-based alternatives such as castor, soybean, and palm oils [13; 14; 15; 16]. These additives have shown promise in improving the flexibility of aged asphalt, restoring functional group balance, and reducing chemical aging markers such as carbonyl and sulfoxide indices. More recent studies have focused on renewable sources like agricultural residues, wood chips, and waste cooking oils, aligning with global trends toward sustainability [17; 18; 19; 20; 21].  
Other industrial by-products, including aromatic extracts, vacuum residues, and used lubricants, have also demonstrated rejuvenating potential [22; 23; 24; 25; 26]. Among them, oil sludge--a hazardous semi-solid residue from petroleum refining--has drawn increasing attention due to its similar hydrocarbon structure to bitumen [27]. Globally, petroleum operations generate substantial volumes of oil sludge: each U.S. refinery produces up to 30,000 tons annually, while Chinese petrochemical industries generate approximately 3 million tons per year [28]. In Kazakhstan, the annual production reaches around 120,000 tons. This growing volume of hazardous waste poses both environmental risks and economic losses, reinforcing the need for sustainable reuse solutions. While traditionally considered waste, oil sludge contains valuable light fractions and aromatic compounds that may enable its application as a low-cost, circular economy rejuvenator.  
Building upon these findings, the present study seeks to deepen the understanding of oil sludge's role as a rejuvenator for aged bitumen. The primary objective is to evaluate its effectiveness in restoring the physicochemical and structural properties of aged binders and to explore the potential of utilizing this hazardous waste material within a sustainable circular economy framework. The novelty of this work lies in its focus on a regionally sourced oil sludge and its potential dual benefit of enhancing asphalt performance while mitigating environmental burden.