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Abstract: Coral reef degradation—driven by climate change, pollution, and destructive human activities—continues to threaten marine biodiversity, prompting widespread use of artificial reefs (ARs), with concrete ARs being the most common intervention. Traditional concrete ARs, while widely used, are often heavy, less porous, and challenging to transport and deploy, limiting their restoration effectiveness. In response, this study introduces an innovative coral reef restoration strategy through the development of 3D-printed and precast ARs made from a waste-derived lightweight foamed concrete mix with apparent porosity of 60–80% (vs. 5–20% in conventional concrete), densities above 1030 - to 1200 kg/m3 (50% lighter than standard concrete), and compressive strengths exceeding 10 MPa—suitable for underwater use. The formulation partially replaces ordinary Portland cement (OPC) with fly ash and limestone, and incorporates a protein-based foaming agent derived from fish waste, stabilized by nanosilica extracted from rice hull ash. This circular approach transforms agro-industrial and fishery by-products into high-value, marine-safe materials. Initial runs focused on printability, buildability, and strength considering foam to paste ratio (30-40%) and water to cementitious ratios showed excellent layer stability. Printed specimens achieved a 28-day compressive strength of 15.79 MPa at 1329 kg/m³, while cast samples ranged from 1150 to 1360 kg/m³. Microstructural analysis confirmed an open pore network and favorable water absorption properties that support biofilm formation and marine colonization. These findings affirm the material's ecological compatibility and structural resilience, making it a viable substrate for long-term reef restoration. The project demonstrates how circular economy principles and eco-engineered materials can drive sustainable innovation in marine conservation and scalable solutions to biodiversity loss.
Key Words: coral reef degradation, artificial coral reefs (AR), 3D-Printed and Precast ARs, fish-waste foaming agent, lightweight foamed concrete.
