Research on the Flame Retardancy Properties and Mechanism of Modified Asphalt with Halloysite Nanotubes and Conventional Flame Retardant

The inflammability of asphalt road will promote fire spread in the tunnel and produce lots of toxic smoke. To improve the fire resistance of asphalt pavement, mineral powder flame retardants are generally replaced by flame retardants in equal amounts. In this study, the effects of the synergistic flame retardancy system of halloysite nanotubes (HNTs) and conventional flame retardants (CFR) on the flame retardancy performance and mechanism of asphalt were investigated. Firstly, the flame retardancy properties of the HNTs and CFR composite modified asphalt were investigated based on the Cleveland open cup method (COC), Limiting oxygen index meter (LOI), and Cone calorimeter tests (CCTs). Then, the flame retardancy mechanism of the modified asphalt was studied based on Thermogravimetric analyzer (TGA), Fourier-transform infrared (FTIR), and Scanning electron microscopy (SEM). The results show that adding HNTs could improve the flame retardancy of the CFR modified asphalt binder. When 1 wt % HNTs and 8 wt % CFR were used, the limiting oxygen index of asphalt increased by 40.1%, the ignition temperature increased by 40 degrees C, while the heat release rate, total heat release, the smoke production rate, total smoke release, and other parameters decreased with varying degrees. Based on TG, FTIR, and SEM, the targeted flame retardancy mechanism and synergistic effect of HNTs/CFR flame retardancy system were revealed and summarized as three stages: (1) Stage 1, aluminum hydroxide (ATH) absorbs heat through thermal decomposition and inhibits the decomposition of lightweight components in asphalt; (2) Stage 2, aluminum diethyl phosphate (ADP) decomposes and produces organic phosphoric acid, which catalyzes crosslinking and ring thickening of asphalt and the quenching effect of phosphorus free radicals to block the combustion; and (3) Stage 3, HNTs plays an important role in increasing the integrity and density of the barrier layer. In addition, the Al2O3 produced by the decomposition of ATH, the carbon layer formed by the ADP catalyzed pitch, and HNTs play a significant synergistic effect in the formation of the barrier layer. Thus, the combination of HNTs and CFR has been proved to be a prospective flame retardancy system for asphalt.