Rheological Enhancement of Bitumen Through Valorisation of Organic Waste Additives for Enhanced Railway Track Performance at Medium and High Temperatures

In railway engineering, waterproofing layers (or blankets), composed of bitumen and mineral aggregates, are a viable and promising solution, but further innovation is needed to achieve higher mechanical characteristics and a better balance between virgin and recycled materials. Based on the above, this study investigates the potential improvements in rheological properties of a railway-oriented hard bitumen, HD, modified with a ternary system consisting of an organic compound (OC), a multifunctional additive (MA), and a modified fibrous additive (MFA) obtained from the utilisation of organic waste. In this context, comprehensive empirical, rheological, and chemical tests were performed on reference and modified bituminous blends. Results confirmed that a synergistic blend of 0.5% MA, 2% OC, and 3% MFA achieved a high R% (93.99) and G*, as well as minimal Jnr, which resulted in the best rank. This ternary combination increased viscosity by 124.97% at 150 °C, improved rutting resistance by 60.69%, and improved binder-level fatigue indicator (G*sinδ) by 45.40% under the unaged DSR sweep conditions. A mechanistic interpretation was provided. These findings support the use of OC, MA, and MFA as sustainable modifiers to enhance the high-temperature rheological performance of HD binders for railway waterproofing layers and sub-ballast applications. Future research should evaluate (1) long-term field performance and compatibility with ballast and subgrade materials, and (2) moisture damage resistance. Indeed, this cannot be inferred from binder rheology alone and should be verified through mixture-level moisture susceptibility and binder–aggregate adhesion tests.