AbstractFillers are known as dust fractions in asphalt mixes having a predominant particle size less than 75 micrometer (μm). Typical filler concentration in an asphalt mix varies from 4% to 12% (by aggregate weight). The effects of filler intrinsic properties on mastic behavior can get magnified with the concentration of filler. Thus, in filler-rich mixes like stone matrix asphalt (SMA), the filler characteristics and concentration can be sensitive factors, playing significant roles in the performance of asphalt mastic and mix. The study considered six different types of fillers to capture the effect of filler characteristics on mastic. The mastics were prepared with a constant filler concentration by volume (F/Bvol=0.49), which was determined by a typical SMA mix design. The fillers were characterized based on their surface area, size distribution, shape, Rigden voids (RV), clay content, and chemical composition. The filler effects on mastic were evaluated based on the stiffness (complex modulus) and stiffness ratio. The weak correlation observed between the mastic stiffness and the fixed and free binder reveals that the RV and filler fixing factor (FFF) can mislead regarding the stiffening potential of filler material. The surface areas by Brunauer–Emmett–Teller (BET) and Blaine’s methods were insufficient to represent the stiffening potential while the stiffness showed a better correlation with the concentration of finer gradations in the filler evinced by a higher coefficient of P10 (percentage passing 10 μ sieve size) and fineness modulus (FM) parameters. Further, the micromechanics based understanding of mastic behavior explains the mastic stiffening phenomenon as the combined effects of adsorbed asphalt layer thickness and interparticle interaction. The linear relation observed between stiffness ratio and newly derived micromechanism stiffening factor (MMSF) indicates that in filler-rich mastics, the stiffness may be improved with a finer sized filler (lower FM) and higher volume concentration. However, the adsorbed layer thickness didn’t show correlation with any of the filler properties considered. The study recommends a limiting value for mastic stiffness in terms of stiffness ratio which is established based on MMSF factor. A limiting value for stiffness of mastic could be significant to avoid dry mixes and subsequent poor fatigue performance.