Enhancing Sulfate Resistance of Spray-Based 3D Printed Recycled Tunnel Slag Concrete Through Polypropylene Fiber Optimization (2025-11)¶

This study investigates the influence of polypropylene (PP) fibers on the enhancement of sulfate resistance in spray-based 3D (S-3D) printed recycled tunnel slag concrete. The mechanical performance is evaluated with varying volume contents of PP fibers (0.2%, 0.4% and 0.6%), fibers lengths (3 mm and 6 mm), and mixing ratios (3 mm : 6 mm = 1:1, 2:1 and 1:2). Furthermore, the sulfate resistance of the specimens with optimal mechanical performance (F21-0.4) is analyzed based on the evolution of the density, mechanical properties and microstructure under different dry-wet cycles (30, 45, 60, 75, and 90). The results demonstrate that PP fibers can effectively enhance the mechanical performance and sulfate resistance of S-3D printed recycled tunnel slag concrete. F21-0.4 exhibits optimal comprehensive performance, achieving 28 d compressive and flexural strengths of 84.5 MPa and 16.1 MPa, representing significant improvements of 44.7% and 27.8%, respectively, compared with the control group (F-0). Following exposure to sulfate attack by dry-wet cycling, F21-0.4 demonstrated excellent strength stability. Its compressive strength decreases to 81.2 MPa after 90 cycles, yielding a corrosion resistance coefficient of 96.1%, which meets the 100-year service life requirements for Y1-class environments in railway tunnel applications. The synergistic optimization of the micro-pore structure, achieved through the mixed fiber length reinforcement and the densification effect of the S-3D printing process, significantly enhances sulfate resistance performance. The sulfate-attacked F21-0.4 shows optimal microstructural compactness with a porosity of 1.4%, representing a 40.0% increase from its pre-exposure value of 1.0%. It decreases by 61.1%, 33.3%, 26.3% and 44.0% compared with the control groups F-0, SF-0.4, LF-0.4 and F21-0.4-C, respectively. The optimized sulfate resistance facilitates the broader application of S-3D printed PP fiber-reinforced recycled tunnel slag concrete in tunnel engineering.