Performance evaluation of cement-stabilized coalmine overburden-furnace slag-fly ash mixes as economical and sustainable base course material for flexible pavement

The depletion of natural aggregates and the environmental challenges posed by waste disposal in landfills have become global concerns. This study addresses these issues by utilizing industrial waste materials, including coalmine overburden (OB) dump, basic oxygen furnace (BOF) slag, and fly ash, to develop a cement-treated base (CTB) material for flexible pavements. Mixture Design Response Surface Methodology was used to optimize the CTB material, focusing on 7-day unconfined compressive strength (UCS) and Poisson's ratio. The optimal blend, comprising 50% OB dump, 40% BOF slag, 5% fly ash, and 5% cement, achieved a UCS of 4.84 MPa and a Poisson's ratio of 0.25, meeting the Indian Road Congress (IRC) guidelines. The material also demonstrated a flexural strength of 2.06 MPa after 28 days, indicating strong fatigue resistance. Shrinkage studies showed a maximum dry shrinkage of 237 micro-strain and thermal shrinkage of 8.9 micro-strain/°C, both within permissible limits, ensuring minimal risk of shrinkage-induced cracking. X-ray diffraction and field emission scanning electron microscopy (FESEM) analysis confirmed the presence of strength-enhancing compounds, including ettringite, calcium silicate hydrate (C–S–H), and calcium aluminosilicate hydrate (C–A–S–H). Durability tests under wet-dry cycles revealed a weight loss of only 4.1%, and the UCS has increased to 6.1 MPa, highlighting the material's long-term durability under extreme weather conditions. An acid digestion test highlighted the enrichment of chromium, manganese, and iron in BOF slag, whereas fly ash exhibited high zinc content. Despite this, heavy metal concentrations leached from the CTB material remained well below the allowable limits stipulated by the Indian Standards, IS:10,500 (2012). Cost and carbon footprint analyses demonstrated that using waste materials instead of conventional aggregates reduced construction costs by 9.54% and CO₂ emissions by 9.3% for 1 km of highway construction. The results of this study confirm that the optimized CTB mix is an effective, sustainable, and eco-friendly solution for flexible pavement construction, promoting both cost-efficiency and environmental sustainability.