## 2 Materials and Sample Preparation
### Nano Hydrophobic Silane Silica  
Nano-silica is an important asphalt modifier with superior adhesion, tear resistance and heat aging resistance. However, the nano-silica exhibits hydrophilicity due to the polysilane inside the nano-silica and the reactive silanol groups present on the outer surface. The hydroxyl groups on the surface can be classified into three types: isolated hydroxyl group, continuous hydroxyl group, and twin hydroxyl group. Isolated hydroxyl group is a free hydroxyl group that does not participate in the reaction; the continuous hydroxyl group is formed by two hydrogen groups that generate hydrogen bonds and associate with each other; two hydroxyl groups attached to the same Si are called twin hydroxyl groups. The isolated hydroxyl group and continuous hydroxyl group have no hydrogen bond. The reinforcing effect of nanosilica as a modifier mainly comes from the active structure (-Si-OH) on the surface of the particles, and it is easy to bond the surrounding ions. The surface of nanosilica is uniformly distributed with a layer of silanol and siloxane group with strong water absorption. The water molecules can be physically covered on the surface of the particles or chemically bonded to the hydroxyl groups on the Si atoms. Therefore, nanosilica shows a strong affinity for water. In order to reduce the number of surface silanol structures and change the surface functional groups of the primary nanosilica particles, the surface modifier is used to modify the nanosilica to realize the transition from hydrophilic to hydrophobic. There are many surface treatment agents for nanosilica, and silane coupling agent surface treatment is the most common application method. The nano hydrophobic silane silica is obtained by grafting silane coupling agent onto the surface of nanosilica to carry out surface modification. The surface modification method of nano hydrophobic silane silica is detailed as follows: the X group in the silane coupling agent first undergoes hydrolysis by contacting with moisture, and then forms a temporary oligomer by dehydration condensation. The hydroxyl groups on the surface of nanosilica can react with the oligomeric structure to generate hydrogen bonds, then the nanosilica and the oligomer continue to undergo condensation and dehydration reaction by heating, drying, etc., and finally the silane coupling agent is successfully grafted onto the surface of the nanosilica by a covalent bond, as is shown in Figure 1. After surface modification, the number of silanol structures on the surface of the nanosilica is reduced, the structure of the surface functional groups and the atomic layer of the nanosilica particles is changed. Thereby, the surface properties such as physicochemical adsorption of nanosilica are changed, the surface free energy of the nanoparticle and the phenomenon of agglomeration between particles is reduced. The initial decomposition temperature of nano hydrophobic silane silica obtained by grafting the silane coupling agent on the surface of the \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Technical**} & \multicolumn{2}{c}{**Penetation (0.1 mm)**} & \multicolumn{2}{c}{**25 \({}^{\circ}\)C**} & \multicolumn{1}{c}{**Softening**} & \multicolumn{1}{c}{**Flash Point**} & \multicolumn{1}{c}{**Solubility**} & \multicolumn{1}{c}{**Density**} \\
**Parameters** & **15 \({}^{\circ}\)C** & **25 \({}^{\circ}\)C** & **30 \({}^{\circ}\)C** & **Ductility (cm)** & **Point \({}^{\circ}\)C** & **(c\({}^{\circ}\)C)** & **(\%)** & **(g-cm-3)** \\ \hline Test results & 25.3 & 86.9 & 142.4 & \textgreater{}130 & 44.6 & 340 & 99.9 & 1.003 \\ Test procedure & GB/T0606-2011 & GB/T0605-2011 & GB/T0606-2011 & GB/T0611-2011 & GB/T0607-2011 & GB/T0603-2011 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Technical parameters of base asphalt. silica is about 250 \({}^{\circ}\)C, and the loss on ignition (1000 \({}^{\circ}\)C, 2 h) is 1.5-2.0, which ensure the workability and effectiveness of nano hydrophobic silane silica in the asphalt pavement construction.