Integrating waste materials in 3D-printed concrete for artificial coral: a pathway to low-carbon and smart coastal cities

This study investigates a low-carbon, waste-integrated concrete mixture for fabricating artificial coral units using three-dimensional concrete printing (3DCP). Fly ash from coal combustion and crushed sand from non-hazardous mining waste rock were incorporated at 25% and 50% replacement levels, respectively, to develop a printable, structurally stable mixture. A biomimetic coral geometry was designed and successfully fabricated using an extrusion-based 3DCP system. Mechanical testing was conducted on six flexural and twelve compressive specimens per printing orientation to evaluate anisotropic behavior. The results confirmed adequate mechanical capacity for handling and deployment of the hollow printed modules. A cradle-to-site life cycle assessment revealed that the 3D-printed unit has an embodied carbon footprint of 93.38 kg CO₂e per 300-kg block, representing a consistent 22% reduction compared to a conventional prefabricated counterpart, with formwork-related emissions, material waste, and transport distances standardized. This carbon advantage derives primarily from waste-material substitution and the elimination of temporary formwork. The findings demonstrate that waste-based 3DCP offers a technically feasible and environmentally advantageous solution for artificial reef production, contributing to sustainable, low-carbon, and ecologically responsive coastal infrastructure.