Utilizing the Potential of Solid Waste (Fly Ash and Bottom Ash) from Sumatera's Coal Power Plants for Innovative Geopolymer Concrete

Indonesia, with its vast coal reserves, relies heavily on coal-fired power plants, which supply around 50% of the nation's energy needs. These operations generate large quantities of fly ash and bottom ash (FABA) as residual byproducts, posing environmental and health risks if not properly managed. Geopolymer technology offers a sustainable pathway to convert these wastes into value-added construction materials. This study investigates the utilization of FABA from a Sumatera-based coal power plant as aluminosilicate precursors for geopolymer concrete synthesis. Sodium hydroxide and sodium silicate were employed as alkaline activators, and the effects of fly ash to bottom ash ratio, NaOH concentration, and curing temperature on compressive strength were systematically evaluated. The results demonstrate that FABA can be effectively applied in geopolymer production. The optimum formulation achieved a compressive strength of 29.4 MPa using 100% fly ash, 8 M NaOH, and curing at 90 °C. Increasing the NaOH concentration beyond 8 M did not improve compressive strength, while higher curing temperatures enhanced strength development but also induced microcrack formation. These findings confirm that FABA-based geopolymers provide a technically viable and environmentally sustainable alternative to conventional cementitious materials, supporting circular economy strategies for coal ash management in Indonesia.