Evaluating ecological concrete cubes with recycled oyster shell aggregates for oyster reef restoration

Oyster reef restoration faces increasing demand for cost-effective and ecologically functional substrates, particularly in regions with abundant oyster shell waste. This study proposes a modular ecological concrete cube composed of recycled oyster shells (as coarse aggregate) and Portland cement, excluding fine aggregates, to evaluate whether this material can deliver sufficient mechanical strength and ecological performance for reef restoration, while reducing cost and promoting sustainability. We suspended ecological concrete cubes (100 × 100 × 100 mm) beneath oyster-culture rafts at 1.5 m depth in Maowei Bay, China, for one year, testing five oyster shell grain size groups (<1 cm to >4 cm) and a cement-only reference. Results showed that compressive strength declined with increasing shell grain size, ranging from 32.45 ± 3.17 MPa (<1 cm) to 0.58 ± 0.27 MPa (>4 cm), while porosity increased from 4.76 ± 0.73% to 54.99 ± 1.49%, indicating a strength-porosity trade-off. All cubes remained intact after one year of field deployment. Ecologically, pure cement cubes surpassed shell-containing concretes in oyster biomass (47.89 ± 5.43 g) and surface coverage (33.89 ± 8.08%) by the end of 12 months of deployment, likely due to surface roughening from biogenic deposition. In contrast, cubes with >2 cm shell aggregates showed the highest Shannon-Wiener biodiversity index (up to 0.55), attributed to enhanced microhabitat complexity and porosity. These findings suggest that crushed oyster shells are a feasible material for producing ecological concrete for oyster reef restoration and biodiversity enhancement. Smaller shell sizes (<2 cm) support rapid oyster colonization, while larger sizes (>2 cm) enhance biodiversity. This approach offers a scalable strategy for resource recycling and functional habitat creation in coastal environments.