Multi-scale synergistic toughening of steel slag-based cementitious materials: Mechanisms and design via polypropylene fiber, rubber powder and GGBS

Conventional single-component modification often fails to adequately mitigate the poor toughness and high brittleness of steel slag (SS)-based cementitious materials. To address this limitation, a novel multi-scale collaborative design strategy is proposed. The synergistic toughening mechanisms of polypropylene fibers (PPF), rubber powder (RP), and ground granulated blast furnace slag (GGBS) were systematically investigated. At the microscale, GGBS (GGBS:SS ratio = 1:1) promoted the formation of C-A-S-H gel (increasing its content by 14 %), and molecular dynamics simulations indicated that Al 3 + from GGBS entered the interlayer of C-S-H to form bridging structures, increasing the tensile strength of the gel from 0.98 GPa to 1.54 GPa. At the mesoscale, 5 % RP, acting as an elastic filler, effectively filled the matrix pores (reducing the porosity by 10 %), improved the interface structure, and enhanced the material’s deformation capacity. At the macroscale, 0.5 % PPF bridged cracks, significantly inhibiting crack propagation. Based on the response surface methodology, the optimal composition was determined as 4.61 % RP, 0.47 % PPF, and GGBS:SS ratio = 1.39, and achieved a 28-day peak flexural load of 7.39 kN, which was 35 % higher than that of the control group. This study revealed a multi-scale collaborative toughening mechanism of “microscopic gel strengthening - mesoscopic pore optimization - macroscopic fiber bridging”, providing a theoretical and experimental basis for design of high-toughness SS-based cementitious materials. • Optimal composition (4.61 % RP, 0.47 % PPF, GGBS:SS=1.39) yields 7.39 kN flexural load at 28 d, 35 % over control. • Synergistic "fiber bridging - elastic filler - gel strengthening" mechanism across scales enhances strength and toughness. • GGBS raises C-A-S-H content 14 % and its tensile strength to 1.54 GPa via Al 3+ bridging, per molecular dynamics.