The suitability of alkali activated slag as a substrate for sessile epibenthos in Reef Cubes

Abstract Reef Cubes® are novel artificial reef units intended to enhance habitat complexity and provide hard substrate around marine man-made infrastructure. If made with Portland Cement, Reef Cubes® could create numerous negative environmental impacts, including a high carbon footprint. Alkali-activated materials (AAMs) are a collection of alternative binders associated with lower embodied emissions but alterations to concrete chemistry can affect the development of marine fouling communities. The objectives of this study were to evaluate the effect of replacing Portland Cement with an AAM binder on the development of macrofouling communities on Reef Cubes®. 25 cm sided Reef Cubes® were manufactured using three different concrete material types and deployed in the subtidal zone of Torbay, Devon, UK. The material types were an alkali activated slag concrete (Type: AAM), a cement-limestone blend (CEM-II) concrete (Type: C) and a cement-limestone blend (CEM-II) concrete with an additional micro silica pozzolan and an exposed aggregate texture (Type: CP). After 1 year of immersion the Reef Cubes® were retrieved, and fouling communities were analysed visually or scraped and weighed to gauge biomass. Univariate metrics of species richness, species diversity, total live cover, total biomass, calcareous mass and live biomass; and multivariate community compositions were compared. There were no significant differences in species richness, species diversity, total biomass, calcareous mass and live biomass between the material types. Total live cover was significantly different; with C appearing the highest, followed by AAM and CP, however pairwise comparisons were not significant. The community compositions on the AAM and C Reef Cubes® were not significantly different. Both were significantly different to the CP material type, which had a higher abundance of erect Bryozoans. The results suggest that compared to cement-based concretes, AAMs are a satisfactory substrate for the development of epibenthic communities on Reef Cubes®. There was also evidence contrary to common artificial reef practice that exposing the aggregate and incorporating a pozzolan improves biodiversity.