Examining the Multi-Scale Toughening Mechanisms and Mechanical Anisotropic Behavior of 3D Printed Concrete Reinforced with Calcium Sulfate Whiskers and Mixed Fibers (2025-11)¶

This study systematically evaluated the effects of calcium sulfate whiskers (CSW), polyoxymethylene fiber (POMF), and polypropylene fiber (PPF) on the printability, mechanical properties, and fracture behavior of 3D printed concrete (3DPC). Performance variations among mono-fiber and hybrid-fiber designs were examined through compressive strength, flexural strength, and toughness tests under different loading directions (X, Y, and Z). The results showed that incorporating CSW alone effectively enhanced the strength of 3DPC through pore refinement, matrix densification, and bridging among hydration products, with maximum increases of 47.6 % in compressive strength and 68.1 % in flexural strength. However, its contribution to post-cracking toughness was limited. Incorporating POMF or PPF to form hybrid systems significantly enhanced the energy dissipation capacity during crack propagation, with the effect being particularly pronounced under Y-axis loading (perpendicular to the printing path). Comprehensive analyses revealed that the CSW–PPF system achieved an optimal balance between strength and toughness. Its superior performance was attributed to high fiber distribution density, strong interfacial anchorage, and the coexistence of pull-out and rupture failure mechanisms. The findings verified that a multi-scale bridging and toughening strategy effectively harmonized strength and toughness in 3DPC, based on the composite mechanism of whiskers and conventional fibers. Therefore, the CSW–PPF system is recommended as the appropriate composite design.