| Asphalt pavement is applied in large scale road tunnels widely in recent years, and the fire safety of asphalt material has drawn wide attention at home and abroad. Using flame retardants to improve the flame retardancy and smoke suppression of asphalt has emerged as the mainstream technology to solve this issue. And the trend of flame retardants is be non-toxic, smoke suppressed, efficient and compounded. Therefore, researching the combustion and pyrolysis characteristics of asphalt and developing a cost-effective inorganic composite flame retardant system with improved flame retardant/pavement performance following the trend of flame retardants shows important significance.Thermogravimetry-differential scanning calorimetry-fourier transform infrared spectrum (TG-DSC-FTIR) analyzer, cone calorimeter and thermal analysis kinetics method were used to study the combustion characteristics and kinetics of the multi-components in asphalt, and analyze the influence of oxygen concentration on the pyrolysis/combustion characteristics and the smoke release of asphalt. Then, the mechanism of flame retardancy and smoke suppression of hydrated lime (HL) and montmorillonite (MMT) was investigated, and the synergistic effect of flame retardant with aluminum hydroxide (ATH) was also analyzed. At last, an inorganic composite flame retardant system (HAM) with HL, ATH and MMT was proposed, and its flame retardant and smoke suppress performance and pavement performance was studied. The main conclusions are as follows:1. The pyrolysis process of asphalt shows a single stage, and the pyrolysis of the four-components have little mutual effect. Wherein saturate, aromatic and resin are accordant with diffusion mechanism, and saturate has the lowest pyrolysis temperature, largest mass loss rate and least residual ratio, so suppressing pyrolysis of saturate can effectively reduce the flammability of asphalt. The combustion process of asphalt shows two main stages. Saturate and asphaltene react in stage Ⅰ and Ⅱ respectively, while aromatic and resin account for the largest proportion in both two stages of asphalt combustion, and suppressing the combustion of aromatic and resin is critical for the inhibition of the combustion process and smoke release.2. The combustion of asphalt is under the oxygen-poor environment during tunnel fire, with the decreasing of the oxygen concentration, heat release rate of asphalt combustion decreases significantly; the activation energy of the first stage increases, the reaction exhibits hysteresis, the amount of organic volatiles (methane, aldehydes, olefins, aromatic compounds and etc.) increases. Combustion intensity of the second stage significantly decreases with the decreasing of the oxygen concentration, then the amount of combustion products (CO2, CO, and etc.) decreases, and the amount of residue increases. Furthermore, as the heating rate increases, the combustion rate of asphalt increases synchronously, the combustion temperature occurs a hysteresis, the mass loss percentage of the first stage and the degree of incomplete combustion also increase.3. HL has a flame retardancy, in index such as limit oxygen index (LOI) and heat release rate, closed to that of magnesium hydroxide (MH), and shows better smoke and CO suppression than that of MH. To be different from the gas-phase mechanism of MH, HL works mainly through a condensed-phase mechanism, on the one hand HL suppresses the release of flammable volatiles contents of saturate, on the other hand parallel reactions of dehydration and carbonation of HL will occur during asphalt combustion, the CaCO3 from the carbonation can promote the asphalt surface to form a dense inert layer which can promote the crosslink and aromatization of aromatic and resin, thus inhibiting the release of heat and flue gas. Meanwhile, HL and aluminum hydroxide (ATH) have different flame retardant mechanism and reaction temperature ranges, and a good a synergistic flame-retardant exists between HL and ATH.4. Adding a small amount of montmorillonite (MMT) can promote the formation of barrier layer during asphalt combustion, and reduce the heat release rate and postpone the burnout time. However, MMT cannot significantly increase the LOI of asphalt. The addition of MMT and ATH has a synergistic effect on flame retardancy, it can further reduces the heat release rate and burn-out rate of asphalt.5. An inorganic composite flame retardant system (HAM) was formed by using MMT modified asphalt and flame retardant fillers with part of HL and ATH. When the dosage of HL, ATH and MMT is 12.5%,12.5% and 3% respectively, the LOI of asphalt can reach 25.0%. Comparing with the base asphalt mixtures, the ignition time HAM is prolonged by 73 s, the average heat release rate and total heat release is decreased by 35.1% and 45.3% respectively, and the total smoke release and CO yield is decreased by 48.6% and 55.1% respectively. Furthermore HAM can effectively improve the high and low temperature performance and the water stability of asphalt mixtures. HAM shows both good flame retardant/pavement performance and high cost efficiency. |
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