Mechanical Anisotropy, Water Absorption, and Alkali Leaching of Two Regional 3D-Printed Concrete Formulations for Artificial Coral Reef Applications

Three-dimensional concrete printing (3DPC) offers geometric versatility for fabricating complex artificial coral reef structures; however, how regional differences in binder composition affect mechanical anisotropy, water absorption, and alkali leaching remains insufficiently characterized. This study compares two regional formulations: Mix_Vn (Vietnam, 40% fly ash) and Mix_Cn (China, 30% fly ash + 20% ground granulated blast-furnace slag, GGBS). Compressive strength was evaluated in three orthogonal orientations, water absorption was measured in tap and seawater, and pH evolution was monitored under four immersion regimes. Mix_Cn exhibited higher and more isotropic strength, reduced water absorption, and slower alkali release compared with Mix_Vn. In particular, GGBS substitution improved pore connectivity and stabilized pH near 9.0 under renewed seawater, whereas Mix_Vn exceeded pH 10.5 within two days in static tap water. These findings demonstrate that the GGBS-containing regional formulation (Mix_Cn) exhibited more favorable short-term performance across all evaluated metrics under the investigated laboratory conditions, contributing to low-carbon concrete construction through substantial Portland cement replacement by industrial by-products.