Activation of retarded seawater-mixed cementitious materials for 2K 3D concrete printing

Abstract

Seawater-mixed concrete for 3D printing offers a promising solution for marine infrastructure construction in coastal and island regions facing shortages of freshwater and labor. However, the high chloride concentration in seawater accelerates the initial setting of concrete, thereby hindering pumping and extrusion in one-component (1 K) printing. The two-component (2 K) printing approach, which combines retarded cementitious mixtures with aluminate-based accelerators, enables set-on-demand behavior and provides a potential solution for seawater-based printing. Nevertheless, the early-age behavior of seawater-mixed cementitious materials incorporating aluminate-based accelerators, such as aluminum sulfate (A$) and calcium aluminate cement with anhydrite (CAC-C$), remains largely unexplored, particularly regarding their interactions with seawater ions and Portland cement. This study systematically investigates the structural build-up and early-age hydration of seawater-mixed cementitious materials incorporating A$- and CAC-C$-based acceleration slurries. Structural build-up was quantified using constant shear rate rheometry, while water state evolution, hydration kinetics, and phase assemblages were characterized using low-field 1H NMR, isothermal calorimetry, thermogravimetric analysis, and X-ray diffraction. The results demonstrate that ettringite (AFt) precipitation governs structural build-up in both systems due to its high water-binding capacity and voluminous needle-like morphology. Seawater significantly enhances AFt formation in the A$-based system, leading to rapid stiffening but a narrowed printability window. In contrast, seawater initially suppresses AFt formation in the CAC-C$ system due to delayed sulfate dissolution and the preferential formation of Friedel’s salt; this effect is transient, and AFt formation resumes at later ages. Consequently, the CAC-C$ system exhibits superior robustness and greater suitability for seawater-mixed concrete 2 K printing.