Green low-carbon SBR composites incorporating steel slag and desulfurization ash: fabrication, mechanical and flammability performance

Steel slag (SS) and desulfurization ash (DA), two high-volume industrial solid wastes, create increasing environmental burdens and land occupation due to their limited utilization. To achieve high-value utilization of SS and DA while reducing carbon black (CB) usage, SS was modified by ultrafine ball milling combined with a silane coupling agent (KH550) to prepare modified ultrafine SS powder (MSS). DA was converted into calcium sulfate whiskers (CSW) via hydrothermal heterogeneous cryogenic rapid oxidation. CSW reinforced MSS-based styrene-butadiene rubber composites (MSS/CSW–SBR) were then prepared with partial replacement of CB. The effects of MSS and CSW on curing behavior, mechanical properties, and flame retardancy were systematically investigated, and the underlying mechanisms were discussed. The results show that the scorch time (t 10 ) increases, whereas the optimum cure time (t 90 ) decreases. The cure rate index (CRI) increases by 18.4-22.5% relative to SBR-0. MC-5/1 delivers the highest tensile strength (19.72 MPa) and tear strength (100.56 kN·m -1 ), representing increases of 11.54% and 5.96% compared with SBR-0, respectively. The MSS/CSW system exhibits a higher LOI (18.3-18.5%) than SBR-0 (17.2%). The residual carbon increases from 2.9% to 4.8-8.6%. KH550 modification reduces MSS surface polarity and improves its compatibility with SBR, while dispersed MSS act as rigid reinforcing domains within the rubber matrix. CSW retards crack propagation through bridging, pull-out, and fracture. During pyrolysis, CSW undergoes endothermic dehydration and releases water vapor, and the residual CaSO 4 integrates with char to form a thermal barrier, producing a denser char layer. An MSS:CSW mass ratio of 5:1 (MC-5/1) is identified as the optimal formulation with the best overall performance. This study provides theoretical insights and practical guidance for the synergistic resource utilization of SS and DA as rubber fillers.