Mechanical properties and comprehensive evaluation of fly ash-metakaolin geopolymer mortar: combined effect of FA/MK ratio, NaOH concentration and curing condition

Geopolymers are widely regarded as the best low-carbon alternatives to ordinary Portland cement (OPC). However, most previous studies on precursor composition, alkali concentration, and curing conditions have been limited to single factors or partial combinations, failing to reflect the combined effects of all variables simultaneously. As a result, the optimal mixtures derived from isolated variables can easily fall into local optima and mask the true comprehensive potential of the material, significantly reducing their reference values. Therefore, this work designed a systematic experimental program covering 72 mix combinations, and a dynamic multi-objective evaluation model integrating the AHP-entropy weight method and grey clustering analysis was established to simultaneously assess mechanical performance, workability, and environmental impact (GWP). Comparative studies have shown that with the optimization of these factors, the comprehensive performance of Fly ash-metakaolin (FA-MK) geopolymer mortar has been significantly enhanced. The model identified an optimal mixture (FA/MK = 6:4, 12 mol/L NaOH, ambient curing) achieving a compressive strength of 65 MPa and a GWP of only 193.65 kg/t. Compared to higher-MK or heat-cured systems, this mixture avoids microstructural damage, reduces GWP by up to 66.36 kg/t, and improves workability, with a strength reduction of no more than 6.4 MPa. Compared to the pure FA system, compressive strength more than doubled at an acceptable environmental cost. More importantly, by integrating subjective AHP and objective entropy weighting, the model dynamically adjusts indicator weights to reflect specific engineering priorities, overcoming the limitations of traditional single-objective optimization. Beyond FA-MK geopolymers, it can be extended to other multi-source solid waste cementitious systems, serving as a flexible and practical tool for customized mixture design in sustainable construction.