# Oil Sludge as a Rejuvenator for Aged Bitumen: Structural and Thermal Effect
## 3 Results
### Analysis of the Impact of Oil Sludge on the Chemical Composition and Structure of Bitumen
Notably, at 5% sludge (Figure 4d), the structure shows restored continuity and compactness, indicating improved compatibility and partial reconstruction \begin{table}
\begin{tabular}{l c} \hline \hline
**Sample** & **A\({}_{1030}\)/A\({}_{1456}\)** \\ \hline Aged bitumen & 0.395 \\
1\% OS & 0.352 \\
3\% OS & 0.265 \\
5\% OS & 0.259 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Sulfoxide index of aged and rejuvenated bitumen. Figure 3: FTIR spectra of aged bitumen samples after the introduction of oil sludge into the aged bitumen in the amount of 1% (a), 3% (b), and 5% (c). of the dispersed binder matrix. These observations confirm the microstructural healing effect of oil sludge, complementing the chemical evidence from FTIR and supporting the assumption that the sludge does not form a separate phase but is integrated at the colloidal or molecular level--similarly to the behavior observed by [PERSON] et al. [45], where phenol-cresol-formaldehyde resin demonstrated uniform morphology without distinct polymer-rich domains. Figure 5 presents the thermogravimetric curves of aged and rejuvenated bitumen samples. Notably, no significant mass loss was observed up to 200 \({}^{\circ}\)C, confirming the thermal stability of the samples under typical conditions of hot mixing and rejuvenation, which are usually conducted at temperatures below 160 \({}^{\circ}\)C. In the range of 200-400 \({}^{\circ}\)C, a gradual mass loss occurred, associated with the volatilization of maltenes and light aromatic compounds. The most intensive decomposition was observed between 450 and 500 \({}^{\circ}\)C, corresponding to the thermal degradation of heavier components such as resins and asphaltenes. Beyond 500 \({}^{\circ}\)C, the remaining mass mainly consisted of thermally stable substances, including both heavy organic residues and inorganic constituents introduced by the oil sludge, such as silicates, metal oxides, and sulfates. The sample containing 5% oil sludge exhibited more intensive thermal degradation, with a total mass loss of 90.84% and a significantly lower residual mass (9.16%) compared to the aged bitumen (19.25%). This considerable reduction suggests a decrease in thermally stable asphaltene structures due to the disaggregation and partial dissolution of asphaltene clusters by aromatic hydrocarbons present in the sludge. Figure 4: SEM images of aged bitumen (**a**) and after the introduction of oil sludge in the amount of 1% (**b**), 3% (**c**), and 5% (**d**). Images were acquired using accelerating voltages of 5 kV (**a**), 10 kV (**b**,**c**), and 2.0 kV (**d**), selected to optimize contrast and minimize surface charging based on sample composition and morphology. Standard post-processing (brightness/contrast adjustment) was applied uniformly across images without altering structural features. While a minor contribution of inorganic components to the final residue is acknowledged, the overall trend clearly indicates that the dominant mechanism is the suppression of coke-forming structures, resulting in a binder that is less rigid and more thermally responsive. These findings suggest that oil sludge contributes not only to softening but also to the molecular-level rejuvenation of aged bitumen, confirming its role as an active restorative agent rather than a simple fluxing additive [46, 47].