Mineralogical and Mechanical Characterization of Concrete Blocks for Artificial Reefs: A Comparative Study with Natural Coral Skeletons

Coral reefs are very important ecosystems for the planet, offering ecological and socio-economic benefits. However, they are under threat due to anthropogenic factors and environmental changes. This study assesses the feasibility of weathered Portland cement concrete as a material for marine artificial reefs by comparing its physicochemical and mechanical properties with those of natural coral skeletons from the coast of Paraíba, Brazil. Analyses included microstructural and physical characterization, compressive strength and ultrasonic pulse velocity tests, as well as pH monitoring. The results indicated that weathered concrete exhibits mineralogical similarity to corals, with the presence of carbonate phases and portlandite absent due to advanced carbonation. The compressive strength of the concrete (27.6 MPa) was significantly higher than that of the coral samples (1–6 MPa), while the porosity of the corals (34–41%) exceeded that of the concrete (14%). The alkaline nature of the concrete (pH 9.7) remained stable. Although differences in physical and mechanical properties are evident, the values are within the ranges reported for cementitious materials in marine applications. Mineralogical similarities between coral skeletons and concrete support its potential as a functional analog in artificial reefs, while adjustments in geometry and porosity are suggested to enhance ecological compatibility.