Study on waste eggshell-derived calcium oxide-modified bitumen using response surface methodology

Waste eggshells, composed predominantly of calcium carbonate (> 94%), represent an abundant resource that can be converted into calcium oxide (CaO) for sustainable bitumen modification. This study explores the performance enhancement of bituminous binders modified with CaO produced from calcined waste eggshells. Eggshells were thermally treated at 900 °C to obtain high-purity CaO, and its formation was verified through XRD, FTIR, and SEM-EDX. The modifier was incorporated into 60/70 grade bitumen at dosages ranging from 1% to 4%, and its effects were assessed using a dynamic shear rheometer. The CaO-modified binder exhibited remarkable improvements in high-temperature behavior, with the complex modulus (G*) rising by 540%, phase angle decreasing by 14%, and rutting resistance (G*/sin δ) increasing by 574% as compared to the control binder. Moreover, a response surface methodology (RSM) framework was employed to assess the combined influence of CaO content and temperature, yielding statistically reliable cubic models (R² > 0.95). Optimization results identified 2.3% CaO at 58 °C as the most effective combination, which was experimentally validated with less than 5% deviation from predicted values. Mixtures prepared using the optimized binder demonstrated enhanced mechanical performance, reflected by increases of 13.76% in Marshall stability, 16% in indirect tensile strength, and 5% in tensile strength ratio, confirming improved stiffness and moisture resistance. Overall, the findings establish waste eggshell-derived CaO as an eco-friendly and technically viable modifier capable of improving both rheological properties and structural performance of bituminous binders and mixes.