Enhancing Durability, Elasticity, and Resilience in Eco-Friendly Asphalt: Calcium-Induced Cross-Linking of Bio-Modified Binder

This study presents a novel approach to enhancing the elasticity and durability of bio-asphalt by inducing calcium crosslinking in bio-oil through the formation of calcium–carboxylate complexes. Experimental results show that the crosslinked binder exhibits up to an 18% increase in elastic recovery, as confirmed by Multiple Stress Creep Recovery (MSCR) tests. The improved performance is attributed to the formation of a robust internal network between CaCl₂ and fatty acid groups in the bio-oil, which enhances elasticity and reduces permanent deformation under repeated loading. Density Functional Theory (DFT) calculations reveal strong coordination between Ca²⁺ ions and carboxylate groups, with stepwise interaction energies of −339.4, −193.2, and −79.3 kcal mol⁻¹ for the first, second, and third oleate ligands, respectively. This coordination limits the mobility of hydrophilic acidic components, reducing their migration to siliceous surfaces—a key mechanism associated with moisture-induced damage. FTIR and contact angle measurements confirm improved resistance to dewetting and moisture exposure in CaCl₂-doped bio-bitumen. Additionally, the crosslinked structure inhibits alkane stacking, which contributes to resistance against low-temperature cracking. Beyond performance, this method supports sustainability by utilizing a low-energy curing process and renewable bio-based materials. The findings demonstrate the potential of calcium-induced crosslinking to address multiple limitations of bio-modified binders and provide a viable path toward more resilient and environmentally friendly asphalt pavements.