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 coating, an approach for utilization of metallurgical solid waste, exerts flame retardancy with ecology, halogen-free and environmentally friendly advantage, but its fire resistance needs to be improved further. Herein, the silica fume-based geopolymer composite flame-retardant coating is benignly designed by doping boric acid (BA), zinc phytate (ZnPA), and melamine (MEL). The results of cone calorimeter demonstrate that appropriate ZnPA and BA significantly enhance its flame retardancy, evidenced by that the peak heat release rate (p-HRR) decreases from 268.78 to 118.72 kW·m-2, the flame performance index (FPI) increases from 0.59 to 2.83 s·m2·kW-1, and the fire resistance index increases from 1.00 to 8.48, respectively. Meanwhile, the in-situ formed boron phosphate (BPO4) facilitates the residual resilience of the fire-barrier layer. Furthermore, the pyrolysis kinetics indicate that the three-level chemical reaction model governs the pyrolysis of coatings. BPO4 makes the pyrolysis Eα climb from 94.28 (P5) to 127.08 (B3) kJ·mol-1 during 731-940°C, corresponding to the improved thermal stability. Consequently, it explores the synergistic flame-retardant mechanism of silica fume-based geopolymer coatings doped with ZnPA, BA, and MEL, providing an efficient strategy for high value-added recycling utilization of silica fume.