Mechanical and rheological properties evaluation for lignin-modified asphalt mixtures

Lignin, a renewable and low-cost by-product of the paper industry, has emerged as a promising modifier for sustainable asphalt production. This study investigates the effects of kraft lignin and black liquor on the mechanical, rheological, and aging properties of bitumen and asphalt mixtures. Adding 20% black liquor and 9% kraft lignin reduced the optimum binder content by 6.63% and 2.69%, respectively, while enhancing Marshall stability by 19.7% and 17.9%. These improvements are attributed to the rich organic composition of black liquor, promoting stronger binder interactions. Aging tests confirmed better oxidative stability, and storage stability tests showed minimal temperature differences (≤2°C), indicating good uniformity. The use of lignin led to a 12% reduction in penetration, higher softening points (59.2°C for black liquor, 61.0°C for kraft lignin), and improved viscosity and fire point–enhancing workability and safety. Although ductility decreased, it stayed within acceptable limits. Machine learning models–Gradient Boosting, Random Forest, Decision Tree, and Linear Regression–were applied to predict Marshall stability using lignin content, bitumen percentage, and temperature. Among them, Gradient Boosting showed the highest accuracy with a strong correlation (R = 0.9030). Lignin-modified binders offer environmental benefits and enhance asphalt durability, promoting greener pavement solutions.