Development of a new method to quantify filler dispersion in bituminous mastics by transmission microscopy and image analysis

This study introduces a novel method to analyze how fillers disperse in bituminous binders. A custom-designed laboratory setup enables precise temperature control (140 °C) and mixing speeds (200, 800, 1500 rpm) for small-scale mixtures (5% filler) over 5, 15 and 30 min. To assess dispersion, microscopy-based visualization approach using optical microscopy (10× objective) and image analysis (ImageJ). Each particle is segmented and its projected area converted into an equivalent diameter, yielding a surface-fraction distribution (in logarithmic size bins). We evaluate the D 50 (Area50) position and the 50–100% range to detect coarse aggregates or over-shearing. The interest of the method is illustrated by comparing how limestone (CaCO 2 ) and quartz (SiO 2 ) fillers disperse in two different bitumens: naphthenic and paraffinic. Results indicate that quartz disperses more readily, quickly approaching a near-homogeneous state, whereas limestone requires higher shear and longer mixing times to approach the reference filler. Paraffinic bitumen shows a higher initial fine‑particle fraction; nonetheless, the naphthenic binder de‑agglomerates faster and ultimately matches, or slightly surpasses, the paraffinic dispersion. Mixing at 1500 rpm can fracture particles, skewing the size profile. The protocol delivers ± 1% repeatability across replicates, providing a robust tool for optimising mastic formulation and enhancing pavement durability.