Synergistic response surface optimization and composite acid activation for sustainable phosphoric acid-activated metakaolin geopolymers

To reduce the resource consumption, environmental impact, and high production cost associated with ordinary Portland cement (OPC) and metakaolin-based geopolymers, while evaluating the synergistic effects of different acid activators combined with phosphoric acid (PA) on geopolymer compressive strength, this study employed high-volume fly ash (FA) substitution to reduce metakaolin (MK) consumption and optimized the MK-to-FA ratio using response surface methodology (RSM). Composite acid activators were prepared by combining PA with aluminum dihydrogen phosphate (ADP), sodium polyphosphate (SP), and citric acid (CA), and their effects on the compressive strength of phosphoric acid-activated metakaolin-fly ash geopolymers (PMFG) were evaluated. The underlying mechanisms were investigated using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), and thermogravimetric analysis (TG), while the economic and environmental performance of the developed materials was also assessed. The optimized PMFG achieved a 28-day compressive strength of 40.9 MPa at an MK-to-FA ratio of 4.1:5.9 and a phosphoric acid concentration of 51.7 wt%. Among the three composite acid activators, the SP + PA system exhibited the strongest synergistic effect, increasing the 28-day compressive strength to 53.5 MPa, representing a 30.8% improvement over the reference PMFG. Microstructural characterization indicated that SP enhanced precursor reactivity and promoted the formation of a compact phosphate-based gel network. Compared with phosphoric acid-activated metakaolin geopolymers (PMKG) and OPC, the developed low-carbon composite acid-activated geopolymer exhibited lower production cost, energy consumption, and carbon emissions, demonstrating its potential as a sustainable and cost-effective alternative to conventional cementitious materials.