Reefcrete: Reducing the environmental footprint of concretes for eco-engineering marine structures

The ecological value of engineered marine structures can be enhanced by building-in additional habitat complexity. Pre-fabricated habitat units can be cheaply and easily cast from concrete into heterogeneous three-dimensional shapes and surface topographies, with proven ability to enhance biodiversity on artificial structures. The net ecological benefits of enhancement using concrete, however, may be compromised on account of its large environmental footprint and poor performance as substrate for many marine organisms. We carried out a pilot study to trial alternative cast-able “Reefcrete” concrete mixes, with reduced environmental footprints, for use in the marine environment. We used partial replacement of Portland cement with recycled ground granulated blast-furnace slag (GGBS), and partial replacement of coarse aggregate with hemp fibres and recycled shell material. We calculated the estimated carbon footprint of each concrete blend and deployed replicate tiles in the intertidal environment for 12 months to assess their performance as substrate for marine biodiversity. The hemp and shell concrete blends had reduced carbon footprints compared to both ordinary Portland cement based concrete and the GGBS based control concrete used in this study. At the end of the experiment, the hemp and shell blends supported significantly more live cover than the standard GGBS control blend. Taxon richness, particularly of mobile fauna, was also higher on the hemp concrete than either the shell or GGBS control. Furthermore, the overall species pool recorded on the hemp concrete was much larger. Community compositions differed significantly on the hemp tiles, compared to GGBS controls. This was largely explained by higher abundances of several taxa, including canopy-forming algae, which may have facilitated other taxa. Our findings indicate that the alternative materials trialled in this study provided substrate of equal or better habitat suitability compared to ordinary GGBS based concrete. Given the growing interest in ecological engineering of marine infrastructure, we propose there would be great benefit in further development of these alternative “Reefcrete” materials for wider application