## 4 A Cheap Solution: The Use of Inorganic Nanoparticles to Improve Mechanical Properties  
Nanosilica is probably one of the cheapest materials among all the nano-sized ones, and this renders its use quite affordable, due to its high stability, ability to disperse, filler effect and its pozzolanic reactivity [44; 45]. Besides, the rutting resistance, fatigue cracking and anti-stripping properties are enhanced [46]. Several authors have added nano-SiO\({}_{2}\) to a bare bitumen to demonstrate enhancement of performance (softening point increases according to the nanosilica content, while penetration decreases) as well as a clear improvement in storage stability [47]. [PERSON] et al. [48] self-consistently found that the penetration value and the ductility decrease with the amount of nanosilica whereas the softening temperature and rotational viscosity increase. The effect of silica is clear also in the increase of the Marshall stability and of resistance against rutting at loading conditions, as well as the fatigue life of warm mix asphalts as found by [PERSON] et al. [49]. They conclude that increasing mixing times would positively affect the properties of the binder, because, obviously, longer times can guarantee a better dispersion. However, as the authors correctly point out, this would increase the energy consumption and consequently the costs. As [PERSON] et al. themselves admit [50], nanosilica are the most studied materials as asphalt binder modifiers. This is unavoidable, in our opinion, due to the low cost and the ease of supply. Despite the wide use of nanosilica, however, comparative studies among different kinds of nanoparticles are also \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Materials** & **Unit Price (f/g)** \\ \hline  & Nano SiO\({}_{2}\) & 0.08–0.15 (b) \\  & & 1.6–6.5 (c) \\ \hline  & Nano Iron & 4.8–17 \\  & & 0.1–2.6 (b) \\ \hline  & & 3.4 \\ Inorganic nanoparticles & Fe\({}_{2}\)O\({}_{3}\) & 0.1–0.3 (b) \\  & & 4.42–11.5 (c) \\ \hline  & Nano ZnO & 1.48 \\  & & 1.23–1.94 (c) \\ \hline  & Nano CuO & 2–7.2 \\  & & 1.24–9.8 (c) \\ \hline  & Nano Al\({}_{2}\)O\({}_{3}\) & 1.8–9 \\  & & 1.3–6.9 (c) \\ \hline  & Nanoclay (800 nm) & 1.16 \\ \hline  & Single Walled CNT (SWCNT) & 65–195 \\  & & 54.7–263 (c) \\ \hline  & Double Walled CNT (DWCNT) & 43 \\  & & 51.21 (c) \\ \hline  & Multiwalled CNT (MWCNT) & 1.5–33 \\  & & 30.9–34.4 (c) \\ \hline  & Graphene Nanoplatelets (GNP) & 6–11 \\  & & 15–114 (c) \\ \hline  & Graphene Oxide (GO) & 68–165 \\  & & 172 (c) \\ \hline  & Reduced Graphene Oxide (rGO) & 48 \\ \hline  & Carbon Nanofibers (CNF) & 14 \\  & & 65 (c) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Costs of inorganic and organic nanoparticles used as additive in bitumens and asphalt concrets; (a) Laser Synthesized; (b) from [42]; (c) from [43]. important: thanks to comparative studies, in fact, it was found that nano-TiO\({}_{2}\) has more problems than nano-SiO\({}_{2}\) in chemical compatibility with the apolar bitumen [51] while hydrophilic bentonite and Zero-valent nano-iron have better affinity with bitumens [50]. Specific influences of nano- TiO\({}_{2}\), ZnO, Al\({}_{2}\)O\({}_{3}\) and Fe\({}_{2}\)O\({}_{3}\) in the creep rate and strain modulus of hot mix asphalt have been also highlighted [52]. Nano-CuO was, instead, found to enhance resistance to rutting deformations of bitumens and aged bitumens but only at temperatures lower than 64 \({}^{\circ}\)C [53]. The authors also found that the addition of nano-CuO to bitumen enhances its fatigue cracking, while other authors have found that nano-CuO can be used as phase change material to increase thermal properties (glass transition temperature, melting peak temperature, specific heat capacity) [54]. [PERSON] and other co-workers [55] explored the potential of CaCO\({}_{3}\) nanoparticles, besides those of SiO\({}_{2}\) and TiO\({}_{2}\). Confirming that silica are the particles giving greatest fatigue resistance, the authors detected a possible antioxidant effect of nano-TiO\({}_{2}\) and nano-CaCO\({}_{3}\).