Toward alleviating anisotropy of 3D printed cementitious composites through rheological regulation and interfacial modification

The inherent anisotropy generated by interlayer defects and weak interfaces between printed filaments remains a critical challenge for the structural application of 3D printed concrete (3DPC). This study presents a physicochemical strategy to alleviate anisotropy in 3D printed white cementitious materials (3DP-WCMs) by leveraging the combined effects of needle-shaped wollastonite (WL) and spherical silica fume (SF). Oscillatory shear tests indicated that the addition of WL and SF significantly broadened the linear viscoelastic region (LVR) and delayed the onset of structural flow, suggesting the formation of a rigid physical network in the fresh state. Considering both fresh viscoelastic behavior and hardened mechanical properties, C80W12S8 was identified as the optimal mixture. At 28 days (28 d), the compressive strength of C80W12S8 reached 43.10 MPa in the A direction, while its flexural strength increased by 46.4% relative to C100 in the weakest C direction. Importantly, the proposed strategy substantially reduced mechanical anisotropy while simultaneously lowering the embodied carbon through partial replacement of WPC by WL and SF. The embodied carbon was reduced by 15.6% for C80W12S8 and 45.9% for C50W30S20 compared with C100. Meanwhile, the flexural anisotropy coefficient ( I f ) of C80W12S8 at 28 d decreased from 0.29 to 0.13, while its compressive anisotropy coefficient ( I c ) dropped from 0.52 to 0.40 relative to C100. In addition, separate-hydration analyses supported a possible mechanism in which Ca 2+ -mediated electrostatic interactions may promote the association of SF with WL surfaces, while SF provided nucleation sites for the formation of a denser and more continuous C-S-H network.