Strength Anisotropy and Alkali Leaching of 3D-Printed Concrete with Fly Ash, GGBS, and Regional Sands for Artificial Coral Reefs

Three-dimensional concrete printing (3DPC) enables the fabrication of complex artificial coral reef structures, yet the role of binder composition in controlling mechanical anisotropy, water absorption, and alkali leaching remains insufficiently understood. 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 provide the first comparative evidence that GGBS incorporation simultaneously mitigates anisotropy and durability concerns in 3DPC, making Mix_Cn the recommended formulation for marine ecological applications and contributing to low-carbon concrete construction through substantial Portland cement replacement by industrial by-products.