Ageing impact of hydrated lime in bitumen-filler mastics using infrared spectroscopy

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This research study examines the chemical structural modifications induced by hydrated lime (HL) in bitumen mastics to advance the understanding of asphalt ageing mechanisms. Fourier Transform Infrared Spectroscopy (FTIR) was employed to characterise structural and compositional changes in mastics prepared with HL and with limestone filler (LF) as a control. The investigation considered filler type, content, and ageing conditions—unaged, short-term aged (Thin Film Oven Test, TFOT), and long-term aged (Pressure Ageing Vessel, PAV, 20 h and 40 h). A comprehensive structural chemical characterization derived from the IR spectra was conducted to assess various structural chemistry changes induced by HL in bitumen functional groups and the most important oxidative molecules. Particular attention was given to the oxidation of functional groups, especially carbonyl (C O) and sulfoxide (S O), which serve as indicators of oxidative ageing. The results demonstrate that HL induces marked alterations in FTIR spectra, including increased peak intensities and the formation of new peaks within the 600–2000 cm⁻¹ range. These chemical structural transformations, consistent across ageing conditions, reveal HL’s strong interaction with bitumen functional groups. Bond ratio analysis further highlights HL’s pronounced stabilizing effect on carbonyl and sulfoxide groups, confirming its superior anti-ageing performance compared with LF. Moreover, oxidation rate assessments show that HL reduces ageing kinetics, particularly under PAV conditions. Overall, the findings reinforce HL’s role as an active filler that enhances bitumen durability and provide evidence for FTIR as a rapid, reliable tool for evaluating bitumen–filler chemical interactions and ageing behaviour.