Temperature-dependent rheology and mechanical performance of underwater 3D printed high-strength mortar: Insights from microstructure

Abstract
High-strength mortar (HSM) derived from an ultra-high performance concrete (UHPC) formulation features a dense and cohesive matrix, making it a promising candidate for underwater 3D concrete printing in the automated construction and repair of marine infrastructure. However, the application of HSM in underwater concrete additive manufacturing remains largely unexplored, particularly regarding how aquatic thermal conditions govern material behavior. This research investigates the influence of low ambient underwater temperatures (20, 15, and 10 °C) on rheology, setting time, and early-age mechanical performance of 3D-printed HSM. Rheological and setting time evolution was characterized using modified test protocols designed for submerged conditions, while compressive strength and interfacial bonding were evaluated for specimens printed in both air and simulated underwater environments. The results indicate that lower underwater temperatures increased the stiffness of the fresh material, as evidenced by the higher plastic viscosity and shear stress, but delayed the initial setting time. This rheological stiffening critically restricted post-extrusion relaxation the viscoplastic spreading necessary for filaments to conform to adjacent layers. Consequently, lower temperatures limited self-adjustment at the interface, weakened inter-filament contact zones, and increased micro-entrapped porosity, as revealed in micro-computed tomography (micro-CT) results. X-ray diffraction (XRD) further indicated a higher content of unreacted silicate phases (Alite and Belite) at lower temperatures, reflecting a reduced degree of hydration. In addition, underwater-printed samples exhibited a more refined pore structure than their in-air counterparts. This was driven by a "controlled washout" mechanism, where fine particles settled into and self-leveled large process-induced voids during the manufacturing process. These findings establish a framework for balancing rheological stability with microstructural observations, providing essential guidelines for robust underwater additive manufacturing.