Low rigidity despite high asphaltene content in bitumen: The role of resins

This research presents a laboratory study on three different bitumens, chosen as representative, coming from three different oils with different origins (Saudi Arabia, Venezuela, Russia) for assessing their structural differences due their different origins. The three bitumens have penetration indexes of 160/220, 70/100, 70/100 respectively, whereas their S.A.R.A. compositions showed marked differences particularly in the asphaltene and resin content, being the bitumen from Saudi Arabia particularly rich in such fractions, and that from Venezuela possessing the lower content of these. Rheological characterization carried out as a function of frequency and temperature suggests that: (i) the modulus at an angular frequency of 1 rad/s takes the lowest value for sample from Saudi Arabia, in accordance with the penetration index trend of the samples (ii) from a microscopic point of view, the average number of first neighbors involved in the flowing process is around one. Further analysis involved the extraction of the asphaltenic fraction according to the standard ASTM D4124 procedure: the melting point of the asphaltene fraction is 175 and 195 °C for the samples from Venezuela and Russia, respectively, while it can hardly be detected for the sample from Saudi Arabia. This anomalous behavior has been interpreted with the aid of X-Ray scattering data, which highlighted a lower order in asphaltene assembly at higher level of aggregation, which means that asphaltene molecules are arranged to form local assemblies with structural well-defined order only at short range, being higher levels of aggregations more disordered. All the data self-consistently show that the bitumen from Saudi Arabia, thanks to a higher content of resins, can stabilize more efficiently aphaltenes and therefore the latter can form ordered stacks only ad short range (only few Angstroms), leaving a more amorphous structure at longer ranges with consequently higher penetration index and lower modulus.