Sustainability Implications of Regenerative Sulfur Blooms in Bituminous Composites

This paper investigates hydrophobic and self-regenerative characteristics of sulfur blooms at the surface of bituminous composites. It further examines the implication of sulfur blooms in the sustainability of outdoor construction elements containing bituminous composites. The occurrence of the abovementioned sulfur blooms is reported for the first time in this paper. The recent restriction on gaseous emissions from marine fuels has led refineries to remove sulfur from marine fuels resulting in a significant surplus of sulfur, some of which has been incorporated into bitumen such as the one studied here. Our study showed that over a period of several days sulfur blooms on the bitumen surface in millimeter-sized patches of microscale crystals. It was observed that blooms are regenerative and become more prevalent as bitumen goes through the oxidative aging process. Contact-angle measurements show that the bloom imparts roughness-enhanced hydrophobicity to the bitumen surface. We further studied the hydrophobicity of sulfur crystals using density functional theory (DFT). It was found that sulfur repels water molecules; water molecules were unable to make strong H-bonds with sulfur due to the lower electronegativity of sulfur atoms compared to oxygen atoms. It was also shown that sulfur has good interactions with polyaromatics such as those found in bitumen, which in turn deters sulfur crystallization. Reduced crystallization allows sulfur to easily migrate to the surface of bitumen and generate sulfur blooms patches. Due to the hydrophobic properties of sulfur blooms, the latter phenomenon can lead to a self-cleaning surface layer, which is continuously self-regenerated, while sulfur supply last in bitumen bulks provide that it is not crystallized. The study outcome promotes the sustainability of bituminous composites while increasing their durability and valorizing waste sulfur.