Experimental investigation and GEP-based design of sustainable concrete incorporating untreated-CBA

Coal bottom ash (CBA), a waste by-product from coal combustion, is typically landfilled, posing environmental and disposal challenges. This study aims to assess the feasibility of using untreated CBA as a partial replacement for cement and/or sand in concrete, with a focus on mechanical performance, predictive modeling, and sustainability impacts. An experimental program was conducted on mortar and concrete mixtures incorporating CBA at replacement levels up to 30% by weight. Compressive strength tests were performed to evaluate mechanical behavior under different substitution scenarios. Additionally, a Gene Expression Programming (GEP) model was developed to predict compressive strength for sand-replacement cases, and a material-level comparative analysis of cost and embodied carbon was carried out. The results indicate that replacing cement with untreated CBA led to a reduction in compressive strength due to its limited pozzolanic activity. In contrast, replacing sand with CBA improved strength, with a 4.7% increase observed at a 20% replacement level. Combined cement and sand replacement resulted in only minor strength reductions at lower substitution levels. The results confirm that partial replacement of sand or both cement and sand collectively with CBA not only retains structural integrity but also contributes significantly to cost and carbon footprint reduction, thereby supporting a more sustainable and environmentally responsible approach for CBA management. The GEP model successfully produced explicit symbolic equations for strength prediction that can be directly used for practical mix design optimization.