# Adhesion Promoters in Bituminous Road Materials: A Review
## 2 The Structure of Bitumen and Its Interaction with Mineral Aggregates  
Bitumen is a dark, oily, and viscous residue of the vacuum distillation of petroleum, which is widely used in road paving, roofing membranes, and other waterproofing materials. Bitumen is also known to be a colloid in which asphaltenes (n-pentane insoluble) [14] form complex micelles dispersed into an oil phase of saturated paraffins, aromatics, and resins [1; 15; 16] with weak chemical affinity for aggregate, whereas the aggregate is characterized by a strong affinity for water. The result is that bitumen can be easily replaced by water. Several studies showed that bitumen polar functionalities, such as polynuclear aromatics, phenolics, pyrrolics, pyridinics, 2-quinolones, sulfoxides, sulfides, ketones, anhydrides, and carboxylics have different adsorption powers on aggregate active sites (see [1] and references therein). Adhesion is promoted by bitumen polar components, which are able to lower the interfacial energy between bitumen and mineral aggregates. The adhesive strength depends on the relative capacity of the functional groups to adsorb at the aggregate surface and the relative desorption by water. As a matter of fact, due to the complex and variable composition of the materials involved, a few investigations have been reported on the identification of suitable physico-chemical factors able to describe the wetting behavior of the bitumen-aggregate interface [17; 18]. Very recently, [PERSON] et al. identified bitumen viscosity and rock heterogeneities as two indicators to consider for a good wetting quality [19]. For example, asphaltenes are mostly subjected to oxidation reactions owing to their aromatic structure, and may form heavier and more complex molecules whose result is an increased stiffness and decreased flexibility of the binder. Therefore, any physico-chemical phenomenon that leads to an asphalt composition modification may determine a change in bitumen viscosity. These consequences may also weaken the aggregate/asphalt bond because the binder would not satisfactorily wet the mineral substrate, hence the bond strength could not be optimal. This last effect may also partially depend on the chemical composition of mineral aggregates. In the simplest case stripping, a type of dewetting, the bitumen film is detached from the surface of the aggregate. However, it can also show itself as a retraction of the bitumen film from the stone caused by water mobility within the film or coming out of the stone itself (see Figure 1). In addition, regarding the WMA technology, if the moisture included in the aggregate does not completely evaporate during mixing due to the relatively low mix temperatures, residual water may still be on the aggregate surface, which in turn could lead to an increased susceptibility to moisture damage. The stripping phenomenon has been firmly attributed to the presence of residual water as a principal consequence of adhesion failure of bitumen onto aggregates [20; 21; 22; 23]. It has also been ascertained that mineralogy (rock chemical composition), surface texture, absorption, surface ageing, surface coatings, and particle shape can affect the efficiency of an adhesive bond in an asphalt-aggregate mixture [24; 25]. Aggregates are characterized by chemical heterogeneity resulting from several factors such as oxidation, coatings, adsorbed films, ledges, kinks, dislocations, and crystallographic anisotropy [26; 27]. The charged aggregate surface attracts and orients bitumen polar molecules. Indeed, mineral rocks used for asphalt concrete can be of an acidic type, whose surfaces have a tendency to become negatively charged, or of a basic type, whose surfaces are prone to become positively charged. Acidic aggregates include those with high silica contents (quartz-based granites), while basic aggregates include mostly carbonates (calcite-based limestones). An irregular aggregate surface on the microscale level could hamper the spreading phenomenon.