Development of Ambient-Cured Geopolymer Concrete: Investigating Bottom Ash as a Sustainable Alternative to Natural River Sand

Portland cement production is a major contributor to global CO2 emissions, and the scarcity of natural river sand limits conventional concrete production. Despite increasing interest in geopolymer concrete (GPC), studies on sustainable, low-temperature cured GPC using industrial by-products as fine aggregate replacements are limited. This study introduces a novel approach by utilizing coal bottom ash (BA) as a partial fine aggregate substitute in ambient-cured fly ash–based GPC, addressing both environmental concerns and resource scarcity. Concrete mixes containing 0%, 5%, 10%, and 15% BA were prepared and evaluated for workability, compressive and flexural strength, water absorption, microstructural characteristics, embodied energy, and CO2 emissions. The 28-day compressive (30.39 MPa) and flexural strength (3.18 MPa) were highest at 5% BA, attributed to microfiller effects and matrix densification, while water absorption remained within ASTM limits. Microstructural analysis confirmed additional N-A-S-H and C-(A)-S-H gel formation, and incorporation of BA reduced embodied energy and CO2 emissions by 57.7% and 93.2%, respectively, demonstrating significant mechanical and environmental advantages. These findings highlight the potential of BA as a sustainable fine aggregate in ambient-cured GPC, providing a novel, low-carbon solution for the construction industry.