Bio-colonisation, durability, and microstructural analysis of concrete incorporating magnetite and oyster shell waste aggregates in marine environment

Abstract

This study evaluates the impact of bio-colonisation and seawater attack on the durability, microstructure and mineralogy of two distinct concrete formulations. These formulations are designed to meet the specifications of marine infrastructure applications, particularly those intended as biomimetic solutions for infrastructures typically used for boat anchoring, which are commonly responsible for the displacement and destruction of marine habitats. The first formulation incorporates magnetite aggregates, resulting in a heavy concrete capable of stabilising the base structure of the biomimetic concrete mooring. In the second formulation, natural gravel is partially substituted by oyster shell waste, to reduce the carbon footprint of these marine infrastructures. Experimental tests were conducted to evaluate the bio-colonisation and durability of the two concrete formulations under marine exposure. After 24 months of immersion, the surface biomass on both formulations exhibited similar kinetic bio-receptivity, primarily attributed to the binder and surface roughness rather than the type of aggregates used. The porosity accessible to water decreases in marine conditions, suggesting that the biofilm contributes to this decrease. Durability results indicate that while both concrete types deteriorate under seawater exposure, the oyster shell aggregates demonstrated better resilience to natural seawater aggressiveness compared to magnetite aggregates over long-term exposure. Elemental mapping showed no obvious zonation of elements. However, a slight increase in surface roughness was observed, with no macroscopic damage detected. This research enhances our understanding of how magnetite and oyster shell waste aggregates respond to bio-colonisation and seawater attack, which are critical factors in the development of biomimetic marine infrastructure.