An experimental investigation of flame retardant mechanism of hydrated lime in asphalt mastics

Highlights: • Hydrated lime can effectively suppress the heat and smoke release of asphalt mastic combustion. • The transformation of hydrated lime during asphalt mastic combustion was revealed. • Hydrated lime promotes the asphalt mastic surface to form a barrier layer by the carbonation reaction. In this study, the flame retardant mechanism of hydrated lime (HL) on asphalt mastics was investigated via a range of analytical techniques, including cone calorimeter test, and thermogravimetry and differential scanning calorimetry analysis. Experimental results indicated that the flame retardant mechanism of HL is the result of comprehensive effects including blocking, cooling and dilution. As compared with magnesium hydroxide (MH), the HL shows a stronger condensed phase flame retardant effect, which leads to a better performance in reducing total heat release rate, CO yield and total smoke release rate. Furthermore, it is noted that in the process of asphalt combustion, the parallel reactions of dehydration and carbonation of HL will occur, wherein the carbonation runs through the first and second stage of asphalt combustion. The generated calcium carbonate promotes the asphalt mastic surface to form a barrier layer, which limits the first and second stage of asphalt combustion and postpones the char combustion to an additional third stage. This leads to that the combustion of asphalt mastic with HL shows typical characteristics of char- or residue-forming materials.