## 1 Introduction  
Asphalt mixture is composed of bitumen, aggregate, and mineral filler [1]. The mixture is effectively a three-phase system consisting of aggregate, mastic, and air voids. The mastic consists of bitumen (dilute phase) and filler (particulates phase). The fillers are suspended in the bitumen matrix phase without particle-particle contact. The dispersion results in the formation of a colloidal suspension system. The mastic affects the rheology and mechanical performance of asphalt mixtures. The reinforcement role of filler is of volume-filling, physiochemical, and particle-interaction [1, 2]. In numerous studies, the positive role of filler has consistently been discussed in asphalt pavements [3, 4, 5, 6, 7, 8, 9, 10]. Some of the fillers, such as hydrated lime, are competitive with polymer additives [11]. [PERSON] showed that mineral filler affects the mastic by increasing its stiffness and moisture resistance [12]. In terms of complex modulus, the filler has an enhanced effect at high temperatures. Bitumen becomes softer at a higher temperature, while filler remains as a rigid entity. This combined effect is useful in reducing the tendency of rutting in hot weather [4]. The filler also acts as an extender, increases workability, and improves ageing characteristics [6]. [PERSON] et al. have discussed the effect of mineral fillers in asphalt mixtures with its historical development along with the effect of filler on temperature, moisture, ageing, and fatigue characteristics [13]. The increase in stiffness is quite a complex phenomenon, which has not been fully understood yet. Studies have tried to simplify this complex problem while attempting to reduce it to a few parameters. In 1998, [PERSON] and [PERSON] reported on the use of traditional as well as non-traditional approaches towards the stiffening potential [8]. One of the earliest references in 1915 is the work of [PERSON] on bitumen-filler suspension [14]. The role of the filler is not only void-filling, but it also includes some sort of physicochemical interaction between filler and bitumen. Limestone and Portland cement are considered to be better fillers as they adsorb a thicker layer of bitumen. The packing of filler plays an important role in the properties of mastics. The void content of the filler is the most important parameter to influence the properties of a mastic [10]. In 1947, [PERSON] showed that interaction between bitumen and filler is best explained by the theory of packing particles [15]. According to [PERSON] and [PERSON], the [PERSON] work on fillers provides a baseline for research on fillers. The Rigden voids incorporate the effect of the secondary properties of the filler in mastic suspension [15; 16]. These properties include bulk density, fractional voids, void diameter, and permeability of fillers, etc. The secondary properties are dependent on the intrinsic (primary) properties, including particle density, shape, texture, grain distribution, and surface area. The viscoelastic properties (viscosity, complex shear modulus, phase angle, and relaxation) of the mastic are dependent on the bitumen and dispersion of filler. A conceptual model was proposed by [PERSON] and [PERSON] to understand the mechanism of the filler stiffening effect in the diluted and concentrated regions of mastics. The Einstein coefficient (\(\mathrm{K_{E}}\)) and maximum packing fraction (\(\mathrm{\varphi_{m}}\)) are more useful as compared to Rigden voids. This is because Rigden voids ignore the effect of filler agglomeration on the stiffening effect [15; 17]. The high dispersion of filler in bitumen is indicated with a low value of maximum packing fraction, and \(\mathrm{K_{E}}\) shows a stiffening rate of mastics [13]. According to [PERSON] and [PERSON], \(\mathrm{K_{E}}\) and \(\mathrm{\varphi_{m}}\) are the important parameters to predict the stiffening effect, i.e., \(\mathrm{M/M_{1}=f(\mathrm{\varphi_{m}},\mathrm{K_{E}})_{\mathrm{r}}}\), where \(\mathrm{M}\) is any modulus (shear, elastic, etc.)