# Oil Sludge as a Rejuvenator for Aged Bitumen: Structural and Thermal Effect
## 4 Discussion  
While previous studies have explored oil sludge in asphalt mixtures, this research is the first to systematically examine its rejuvenating effect on aged bitumen at the molecular and microstructural levels. By combining rheological, spectroscopic, morphological, and thermogravimetric analyses, the study demonstrates that oil sludge not only enhances flexibility and thermal stability but also alters the colloidal balance of the binder. This multiscale insight into its rejuvenating mechanism underscores the potential of oil sludge as a sustainable alternative to conventional rejuvenators. The rejuvenating mechanism of oil sludge in aged bitumen systems is primarily attributed to its content of maltenic fractions, including light aromatic and resinous components, which facilitate the redistribution of the colloidal structure of bitumen. These components enhance the solubility and dispersion of asphaltenes and restore the balance between solid and liquid phases within the binder. According to the asphaltene colloidal theory, bitumen is viewed as a micellar system where asphaltenes are stabilized by surrounding aromatics and resins [48, 49]. Aging disrupts this equilibrium by depleting stabilizing aromatics, which leads to the formation of dense, fractal-like asphaltene aggregates, as confirmed by SAXS and SARA studies [50]. Rejuvenation with oil sludge reintroduces these key fractions, promoting the fragmentation and redispersion of aged asphaltene clusters. This trend is quantitatively confirmed by SARA fractionation results. In our previous study [31], the asphaltene-to-maltene (A/M) ratio in virgin bitumen was 0.355. Upon treatment with 1%, 3%, and 5% oil sludge, this ratio progressively decreased to 0.320, 0.280, and 0.257, respectively. Such a shift suggests a partial reversal of oxidative aging and restoration of the colloidal balance. Similar observations were reported by [PERSON] et al. [50], where rejuvenation narrowed the size distribution of asphaltene aggregates and increased the aromatic/resin content, improving colloidal stability. Figure 5: TGA results for aged bitumen (1) and bitumen rejuvenated with oil sludge at 1% (2), 3% (3), and 5% (4) dosages. FTIR spectra revealed a reduction in sulfoxide group intensities with increasing oil sludge content, suggesting partial reversal of oxidation processes. This aligns with the concept that maltenes from oil sludge dilute oxidized polar components, thus increasing the maltene-to-asphaltene ratio. SEM micrographs confirmed the structural homogenization of the binder, indicating uniform dispersion and effective interaction between oil sludge and the aged matrix. Asphaltenes are known for their rigid, polar, and aggregation-prone molecular structure, which significantly contributes to increased stiffness and loss of flexibility in aged bitumen. Therefore, the observed reduction in their relative content provides compelling evidence of colloidal rebalancing and structural rejuvenation of the binder [46]. This mechanism is schematically illustrated in Figure 6, which visually demonstrates the transition from an aged, asphaltene-rich structure to a rejuvenated and more homogeneous bituminous matrix with improved dispersion, elasticity, and colloidal stability. In the figure, dark grey clusters represent aggregated asphaltenes; blue circles correspond to maltenes; black curly lines indicate oxidative fragments; green elements show light aromatics and resins derived from oil sludge. Thermogravimetric analysis (TGA) further supported the mechanism by showing a decrease in residual mass with the addition of oil sludge, especially at a 5% concentration. This may imply a disaggregation or partial dissolution of thermally stable asphaltenes and other high-molecular-weight structures, improving the thermal responsiveness and plasticity of the bitumen. Collectively, these findings demonstrate that oil sludge functions as more than a mere softening agent.