## 3 The Main Drawback: the Costs  
This is a crucial point: at what extent a given technological field can bear the costs of reducing the size? Each size regime has specific properties and can give application-specific added-value, so it is not true that smaller means better. There is a delicate equilibrium among properties, costs and added value to be taken into account.  
At current prices, for the nanomodifications to be cost effective, a significant improvement in durability has to be attained. It is expected that the production of nanoparticles in bulk quantity and the use of alternative sources will significantly reduce the costs. For example, rice husk, an abundant waste biomass with high content of silica was identified as a potential low-cost resource for the production of nanosilica particles [39]. Therefore, it is necessary to carry out economic, ecological and environmental evaluations of nanomodified asphalt before a large-scale application in practical engineering.  
The use of nanoparticles implies costs which make nanoparticles-additivated bitumens and asphalts much more expensive. The costs involved in bitumen and asphalt productions depend on different factors as depicted by [PERSON] et al. [40]. However, they are of the order of 3-350 (\(\xi\)/ton) for common bitumens/asphalts as listed in Table 1  
In comparison to this, the cost of nanomaterials to be used as asphalt additives is much higher. This is due to the use of expensive equipment and technology and the highly qualified personal involved and so on.  
Products with high purity and narrow size range and high specific surface may demand higher processing efforts, higher complexity of the apparata involved and higher skills of the people involved, thus implying higher final costs. For this reason, it is not possible to define a cost for a given nanomaterial, but only a rough range. Mind that the prices for nanoparticles are not given in \(\xi\)/ton like for bitumens, but in \(\xi\)/kg or even in \(\xi\)/g! Let us examine Table 2, which reports some indicative ranges for nanoparticle costs, for some comments. From a perusal of Table 2 it can be seen that the cheapest material is nanosilica, which can cost down to 80 \(\xi\)/kg (see 0.08 \(\xi\)/g, first entry in the table). However, its surface modification to increase its compatibility with the apolar nature of the bitumen increases the cost a lot, so a silane-modified nanosilica cost between 180 \(\xi\)/kg to 450 \(\xi\)/kg. If organic particles are used the cost increases severely: the most expensive family of such nanomaterials is that of single walled nanotubes, whose cost can span from 65 \(\xi\)/g to about 200 \(\xi\)/g. Multiwalled carbon nanotubes (MWCNTs) are cheaper, ranging from about 2 \(\xi\)/g up to 40 \(\xi\)/g. Among the organic carbon nanostructures, however, carbon nanofibers and nanoplatelets are the most affordable solutions, since their cost hardly exceeds 15 \(\xi\)/g [41].  
Of course, specialized technology development and the increasing market of carbon and graphene families will cause a decrease in their price, probably making such kind of materials more appealing for bitumen and asphalt modification. However, at the moment, the use of inorganic nanoparticles is still the most affordable solution.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Materials** & **Average Unit Prices (\(\xi\)/ton)** \\ \hline Bitumen 35/50 & 350 \\ Natural aggregates & 10 \\ Electric arc furnace steel slag aggregates & 3 \\ Reclaimed asphalt pavement aggregates & 5 \\ Recycled concrete aggregate & 5 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Cost evaluation of typical asphalt mixtures.