Effect of borax and magnesia on setting, strength, and microstructure of acid-activated fly ash geopolymers

Flash setting and narrow processing windows hinder the practical use of phosphoric acid-activated fly ash geopolymer (PAAFG). Although borax (retarder) and magnesia (accelerator) are individually known to affect phosphate systems, their combined action, optimal proportions, and mechanistic roles in PAAFG remain unclear. This study examined the effects of phosphoric acid concentration (PAC), liquid-to-solid ratio (L/S), and systematically varied borax-to-magnesia mass ratio (B/M) and total dosage (TBM) on setting, strength, and phase/microstructure evolution. Lower PAC, higher L/S, and borax prolonged setting, whereas magnesia accelerated hardening; B/M = 2 maximized setting time, enabling a wide workability window. Strength increased with PAC up to 6 mol/L and L/S = 0.40 and peaked at 11.52 MPa for B/M = 1 and TBM = 8 wt%, decoupling workability control from strength optimization. XRD/FTIR/SEM show that balanced co-addition suppresses excessive crystalline phosphates and promotes dense, phosphate-rich amorphous gels; overdosing either additive disrupts gel continuity. The work delivers a composition-processing control (B/M-TBM) and mechanistic basis for tuning PAAFG from “no-flash, castable” to “early-strength” regimes, advancing practical mixture design of acid-activated FA binders. • PAAFGs with varying PAC, L/S ratios, borax, and magnesia were developed. • The synergistic use of borax and magnesia promoted gel structures such as Si-O-B-O-P and Al-O-P. • The optimal B/M ratio and total dosage improved matrix density and enhanced compressive strength. • Excessive borax or magnesia hindered geopolymerization and disrupted crystalline phase formation.