Optimization of sustainable ternary low-carbon geopolymer binders using the best–worst multi-criteria decision-making method

Introduction This study focuses on optimizing the performance and sustainability of ternary blended geopolymer mortars (TBMs) incorporating Ground Granulated Blast Furnace Slag (GGBS), metakaolin (MK), and paper sludge ash (PSA). The need for environmentally friendly alternatives to ordinary Portland cement has driven the development of multi-criteria decision-making approaches for sustainable material design. Methods Sixteen mix designs with varying binder proportions and alkali molarities were experimentally evaluated for mechanical, durability, environmental, and economic performance. The Best–Worst Method (BWM) was employed to determine the relative importance of six criteria: workability, compressive strength, water absorption, energy consumption, CO 2 emissions, and cost. Results Results indicate that increasing MK and PSA contents reduced workability, while balanced GGBS–MK proportions and moderate alkali molarity improved strength. The optimal mix (70% GGBS, 15% MK, 7.5% PSA, 4M NaOH) achieved a compressive strength of 63.67 N/mm 2 and water absorption of 1.43%, indicating dense microstructure development. Energy consumption (5.06 MJ/kg) and CO 2 emissions (0.54 kg CO 2 -eq/kg) were significantly lower than conventional OPC mortars. BWM analysis identified compressive strength as the most critical criterion and CO 2 emissions as the least significant. The ranking results indicated Mix M15 as the optimal composition. Discussion The proposed BWM-based decision-making framework effectively integrates performance, environmental, and economic aspects, providing a practical approach for designing sustainable and high-performance geopolymer mortars.