Optimization and evaluation of pond ash–bottom ash geopolymer binders: Taguchi-based mix design, microstructure, and economic analysis

Coal-fired power plants generate large volumes of pond ash PA and bottom ash BA, which are often landfilled. This study develops geopolymer binders from PA–BA blends using Taguchi L9 design, varying PA:BA ratios 100:0, 50:50, 0:100, NaOH molarity 8, 10, 12 M, silicate ratio 1.5, 2.0, 2.5, and curing temperatures 30, 60, 90°C. All nine mixes exhibited adequate workability slump flow: 172–190 mm. The optimal mix, R1M12S2.5T90 100% PA, 12 M NaOH, silicate ratio 2.5, cured at 90°C, achieved compressive strengths of 32 MPa at 24 h and 45 MPa at 28 days, with 2320 kg/m3 density, 5.5% water absorption, 11.4% porosity, 4.4 km/s UPV, and 3.98 mm/√h sorptivity – comparable to M30 OPC concrete. Even 100% BA mixes reached 30–32 MPa at 28 days under strong activation. ANOVA revealed NaOH molarity as the dominant factor for strength 47.3%, absorption 49.2%, and sorptivity 46.5%, followed by curing temperature 20–23%. Estimated production cost ranged ₹3450–3750/m3, similar to M30 concrete. The findings demonstrate that durable, structural-grade geopolymer binders can be produced entirely from PA and BA without pretreatment, offering a sustainable alternative to traditional cement.