Enhanced Flame Retardancy of Silica Fume-Based Geopolymer Composite Coatings Through In Situ-Formed Boron Phosphate from Doped Zinc Phytate and Boric Acid

Silica fume-based geopolymer composite coatings, an approach to using metallurgical solid waste, exert flame retardancy with ecological, halogen-free, and environmentally friendly advantages, but their fire resistance needs to be improved further. Herein, a silica fume-based geopolymer composite flame-retardant coating was designed by doping boric acid (BA), zinc phytate (ZnPA), and melamine (MEL). The results of a cone calorimeter demonstrated that appropriate ZnPA and BA significantly enhanced its flame retardancy, evidenced by the peak heat release rate (p-HRR) decreasing from 268.78 to 118.72 kW·m−2, the fire performance index (FPI) increasing from 0.59 to 2.83 s·m2·kW−1, and the flame retardancy index increasing from 1.00 to 8.48, respectively. Meanwhile, the in situ-formed boron phosphate (BPO4) facilitated the residual resilience of the fire-barrier layer. Furthermore, the pyrolysis kinetics indicated that the three-level chemical reactions governed the pyrolysis of the coatings. BPO4 made the pyrolysis Eα climb from 94.28 (P5) to 127.08 (B3) kJ·mol−1 with temperatures of 731–940 °C, corresponding to improved thermal stability. Consequently, this study explored the synergistic flame-retardant mechanism of silica fume-based geopolymer coatings doped with ZnPA, BA, and MEL, providing an efficient strategy for the high-value-added recycling utilization of silica fume.