Effects of aggregate mineral surface anisotropy on asphalt-aggregate interfacial bonding using molecular dynamics (MD) simulation

Abstract

The effects of aggregate mineral surface anisotropy on asphalt-aggregate interfacial bonding have not been studied so far. This paper presents a research that applied molecular dynamics (MD) simulation to investigate these effects. The analysis is presented by studying the interfacial bonding characteristics between asphalt and two aggregate minerals, namely a-quartz and calcite. The two aggregate minerals were selected to represent acidic and alkaline aggregates respectively. Asphalt was represented by a four-component model. To obtain a stable conformation for the asphalt model, its molecular structure was optimized using the simulated annealing (SA) method based on geometry and energy considerations. The physical properties including density and glass transition temperature for the four-component asphalt model was considered to justify the model chosen. For the aggregates, six commonly exposed surfaces {0 0 1}, {1 0 0}, {1 0 1} of a-quartz and {1 0 4}, {2 1 4}, {0 1 8} of calcite were cleaved respectively to build asphalt-aggregate interface models with the four-component asphalt model. It was found that different aggregate mineral surfaces have significant influences on the adhesion properties between asphalt and minerals. For α-quartz-asphalt models, the adhesion interaction is dominated by van der Waals energy while van der Waals interaction and electrostatic interaction are both important for calcite-asphalt models. In addition, from the atomic scale, the adhesion mechanism between asphalt and different mineral surfaces was also found strongly related to the mineral surface atomic density and dangling bonds of surface active ions. The analysis shows that the calcite (an alkaline aggregate) has a stronger bonding strength with asphalt than the α-quartz (an acidic aggregate). The findings of MD simulations agree well with experimental findings of past researches concerning the relative asphalt-aggregate bonding strength of alkaline limestone and acidic silica aggregates. The study demonstrates that surface anisotropy of aggregate minerals is a significant factor to be considered in the study of bonding characteristics of asphalt-aggregate interface.