## 3 Traditional Use of Nanotechnology Products in Road Pavement Engineering
### Chemistry of Organofunctional Nano-Silanes for Application in Pavement Engineering  
Most of the nanotechnology products currently used in the built environment to protect, preserve and/or strengthen building materials are based on the silane (SiH\({}_{4}\)) derivative of silicon (the same derivative, which has found application as the basis for the development of stone consolidants for more than a 150 years [4]). Each hydrogen (H) atom in the silane (SiH\({}_{4}\)) can be replaced by any other element or group(s) to form a large variety of nano-products with numerous fields of application. The most common reactive groups are hydrogen (H), chloride (Cl), fluorine (F) and a RO group, where R is the general symbol for the alkyl group, also known as the organo-functional group in silane-chemistry (also referred to as the hydrocarbon group discussed and explained in the composition of the emulsifying agents demonstrated in Figures 2 and 3), which may, inter alia, include chemical compounds such as CH\({}_{3}\) (methyl) or CH\({}_{3}\)CH\({}_{2}\) (ethyl). The CH\({}_{3}\)-Si bond is a very stable (non-reactive) bond (known as organo-silane), with low surface energy with hydrophobic (water-repellent) or oleophilic (oil-loving) characteristics. The reactive group RO is referred to as an alkoxy (alkyl + oxygen) group [4]. Silicon (Si) forms the centre of the RO group together with the second functional group(s) (X) (e.g., methoxy, ethoxy, etc.) to form the chemical molecule RO-Si-X. These secondary functional (pendant) groups may be highly reactive (in cases where the element of carbon (C) is excluded from the formulation) and define the functionality of a specific nano-silane [4]. The functional group (R) will attach to organic material (such as bitumen or an alternative material compatible polymer with an oil basis), while the functional group(s) (X) will attach to inorganic material (granular material such as rock/gravel/soil). Hence, an organo-functional nano-silane combines the functionality of a reactive group with a non-reactive group in a single molecule. It should be noted that the organofunctional nano-silane is not replacing the stabilising agent in the stabilisation of granular road building materials. The molecule is too small to effectively bridge the gaps between the material fractions. However, in practice, the nano-silane can very effectively fulfil the role of a bonding agent (adhesive agent, a well-known concept in the asphalt industry), which is designed to permanently bind the stabilising agent (the bitumen or equivalent organic substitute with similar characteristics) to the granular materials (stone/aggregate/soil). Si-O chemical bonds are some of the strongest found in nature (refer to Figure 3). The relatively small size of the nano-silane particle (depending on the type of nano-silane and the quality of the product, the size of nano-silanes may vary from about 5 nm to less than 1 nm) results in a considerably high surface area per volume ratio [7]. In other words, a relatively small amount of the nano-silane is required to totally encapsulate all particles of the granular material (stone/gravel/soil) that is being treated/stabilised (for example, 1 litre of nano-silane could easily have the same coverage area of about a 1000 litres of bitumen, with a bitumen particle being in the order of 1000 to 5000 nm in size) [7; 17]). As mentioned, the organofunctional nano-silane particle will, in effect, have the properties of a \"coupling agent\" commonly referred to in the asphalt industry as an aggregate adhesive. Aggregate adhesives or aggregate promoters are well-known terminologies in pavement engineering, and such nanotechnologies have been used for at least three decades [31] as modifiers to bitumen emulsions. However, the use of silicon-based nanotechnology products in pavement engineering is relatively new and has been developed mainly over the last 15 years.