Sulfur-driven phase stabilization of SBS-modified bitumen: comparative effects of polymeric and rhombic sulfur

Abstract This study investigates the effect of sulfur allotropy on the phase stabilization of SBS-modified bitumen by directly comparing polymeric (Sp) and rhombic (Sr) sulfur at an identical low dosage of 0.3 wt.%. A 70/100 paving-grade bitumen modified with 3 wt.% SBS-L was used as the reference polymer-modified binder. The addition of sulfur markedly improved storage stability while maintaining enhanced high-temperature performance. The SBS-L binder exhibited pronounced phase separation during hot storage, with a top-to-bottom temperature difference (ΔT) of 2.7 °C. The incorporation of Sp reduced ΔT to 0.4 °C, whereas Sr reduced it to only 0.1 °C, corresponding to reductions of approximately 85% and 96%, respectively. Sulfur-containing SBS binders also showed improved compositional stability after short-term aging, with Sr providing the lowest susceptibility to component redistribution and rheological aging. Both sulfur-containing systems exhibited high resistance to permanent deformation after RTFOT aging. The Sp-containing binder provided the lowest non-recoverable creep compliance and the highest elastic recovery, whereas Sr showed the highest phase uniformity and compositional stability. Overall, sulfur allotropy significantly influences the stabilization mechanism and performance balance of SBS-modified bitumen, with rhombic sulfur being particularly effective in suppressing phase separation.