Rheological, adhesive, and chemical characterization of asphalt mastic modified with spent coffee grounds: A comparative study with limestone filler

Abstract

Spent coffee grounds (SCG) were evaluated as an organic filler in asphalt mastic at 10 %, 15 %, and 20 % by volume, benchmarked against limestone (LM). The experimental program combined DSR temperature sweeps, pull-off adhesion tests on basalt, granite, and marble substrates, SARA fractionation, and SEM, performed under fresh, RTFOT-aged, and PAV-aged conditions. At 20 wt% replacement, SCG reduced the complex shear modulus G* from 3637 to 2885 kPa at 16 °C (−21 %) and lowered the isostiffness temperature from 31.22 °C to 25.43 °C after PAV aging (−5.79 °C), while limestone at 20 wt% raised G* to 6461 kPa (+78 %). The rutting factor G*/sinδ decreased by 19–23 % with SCG and increased by 71–80 % with LM at 64 °C, whereas the fatigue factor G*sinδ at 25 °C dropped by 18–22 % with SCG, indicating improved fatigue resistance potential. Limestone mastics exhibited 25–83 % higher pull-off force than SCG ( p < 0.05, all 18 comparisons, Benjamini–Hochberg-corrected), yet SCG mastics maintained moisture retention of 83–97 % on basalt, comparable to LM (75–97 %). Failure mode analysis showed all mastics cohesive under dry conditions, with SCG transitioning to adhesive failure under moisture exposure (notably at 15–20 % SCG on marble and granite after PAV aging). SARA analysis confirmed aromatics rose from 60.6 % to 65.7 % and asphaltenes fell from 10.7 % to 8.2 % with increasing SCG, yielding an Instability Colloidal Index decrease from 0.187 to 0.154. These results establish SCG not as a direct limestone substitute but as a complementary modifier suited to fatigue-prone warm-climate binder courses and hybrid SCG–LM filler blends, offering environmental benefits through agricultural waste valorization.