Enhanced structural performance of combustion bottom ash concrete via compression casting technique: Mechanical, microstructural, and sustainability insights

Global concerns over industrial waste management have prompted growing interest in reusing combustion bottom ash (CBA) as a sustainable fine aggregate in structural concrete. However, CBA tends to increase mixture porosity and weaken the interfacial transition zone, limiting its structural potential. This study develops and validates a high-performance concrete incorporating CBA through a compression casting technique (CCT), designed to enhance matrix densification and interface stability. Experimental programs evaluated mixtures with 0%, 50%, and 100% sand replacement at a 45 MPa design strength, assessing mechanical behavior through axial stress–strain response, peak strain, elastic modulus, and microstructural analysis via scanning electron microscopy (SEM). The CCT approach significantly improved mechanical performance, achieving compressive strengths of 71 MPa, 58 MPa, and 50 MPa for 0%, 50%, and 100% CBA, corresponding to 55–43% gains over traditionally cast specimens. Strength enhancement correlated with reduced porosity and improved aggregate–paste bonding, confirmed by SEM. Analytical and machine-learning models accurately predicted key mechanical properties, demonstrating strong validation (error indices of 7–13%). Sustainability analysis further indicated 20% lower embodied energy, CO₂ emissions, and cost per strength unit. The results establish CCT as a viable method for producing structurally efficient, low-carbon concretes using 100% CBA, offering both scientific insight into microstructural mechanisms and practical pathways for sustainable structural engineering.