# A Review of Nanomaterials' Effect on Mechanical Performance and Aging of Asphal Mixtures
## Nanomaterials  
### Type of Nanomaterials  
Theoretically, any material can be synthetized in the nanometric scale, generically, by processing macroparticles of the respective material. The nanomaterials more studied for asphalt binder and asphalt mixture modification are types of nanosilica and of nanoclay. Nanosilica is the term used to designate nanoparticles of silicon dioxide (SiO\({}_{2}\)). Silicon dioxide is an inorganic material produced mainly from silica precursors, e.g. synthetized from silica fume or chemically processed from rice husk ash [10; 11; 12; 13; 14; 15; 16]. It has a molecular mass of 60.08 g/mol and the appearance of a white powder. Figure 1 presents a comparison of the volume taken by a sample of 2.50 g of nanosilica and 2.50 g of limestone filler (in the case of the filler, it is only the fraction under 63 \(\upmu\)m). One can see that the nanoparticles occupy considerably more volume. Concerning the asphalt binder modification, the good dispersion ability and large surface area are its most interesting characteristics. Table 1 presents the properties of the nanosilica used by several researchers. The clays are materials that can be found abundantly in nature. Although presenting some natural variability in their constitution, such ease of access made them known materials with many applications. Currently, there are few processes to extract nanoclay from a layered clay [26; 27]. Montmorillonite, is a smectite clay material derived from bentonite ore [28], is the most common natural nanomineral used by industry [29]. Majority of the clays present a layered structure, which consists of a Silica tetrahedron connected to an alumina octahedron, coordinated by oxygen atoms or hydroxyl groups, with the overall thickness of a single layer approaching one nanometer [30]. The complete separation (exfoliation) of the nanoclay layers will result in a large surface area, up to 800 m\({}^{2}\)/g [9], as well as, very high aspect ratio, typically 100 to 1500 [31]. Generally, the natural nanoclays have hydrophilic properties. The hydrophilic behavior may cause difficulties to disperse the nanoclay homogeneously in the asphalt binder, which has organophilic properties [32]. To mitigate such a problem, the raw nanoclays can be modified by replacing the interlayer cations with quaternized ammonium or phosphonium cations, preferably with long alkyl chains, originating an organically modified or organophilic nanoclay [5], e.g. cloisite is an organically modified nanoclay which base is montmorillonite. Table 2 presents the properties of the nanoclay used by several researchers, where, in all the cases, the base of the studied nanoclays was montmorillonite. The dispersion of nanoclay in the asphalt matrix can create immiscible, intercalated, or exfoliated nanostructures [33]. In an intercalated structure, there is an expansion of the nanoclay interlayer spacing that is occupied by asphalt molecules. In an exfoliated structure, the layers of the nanoclay are exfoliated (completely separated) and the individual layers are distributed throughout the polymer matrix. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Particle Size** & **Specific Surface** & **True Density** & **Bulk Density** & \multirow{2}{*}{**SiO\({}_{2}\) (\%)**} & \multirow{2}{*}{**Reference**} \\ (nm) & **Area (m\({}^{2}\)/g)** & **(g/cm\({}^{3}\))** & & & \\ \hline
12 & 175–225 & 2.6 & – & \(\geq\)99.8 & [17] \\
20–30 & 130–600 & 2.1 & – & \(\geq\)99 & [18] \\ \(<\)10 & 600 & 2.4 & 0.10 & \(\geq\)99 & [19; 20] \\
15 \(\pm\) 3 & 160 \(\pm\) 12 & – & 0.14 & \(\geq\)99.9 & [21] \\
30 & 440 & – & 0.063 & – & [22] \\
20–30 & 180–600 & 2.4 & – & 99 & [23] \\
30 & 200 \(\pm\) 35 & – & 0.03-0.06 & 99.8 & [24] \\
70 & 64 & 2.2–2.6 & – & – & [25] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Properties of nanosilica used by several authors. Figure 1: Mass of 2.50 g of limestone filler under 0.063 mm (**left**) and nanosilica (**right**).