Sustainable blue economy: strategies for the valorization of mussel shell waste through circular economy and marine nature-based solutions

In recent years, the ecological and circular transition has highlighted the need to develop innovative strategies for waste management and recovery, including in the Blue Economy sectors. In this context, this thesis investigates the potential reuse of shell waste, proposing an integrated framework that combines circular economy principles with nature-based solutions. Due to the intrinsic characteristics of mussel farming, the supply chain generates a considerable volume of waste shells, destined almost exclusively for landfill disposal. These residues, composed of more than 90% calcium carbonate (CaCO₃), have chemical and physical characteristics that allow them to be valorized as secondary raw materials, reducing the pressure on the extraction of natural resources. The thesis develops an integrated methodological approach that combines environmental analysis, material characterization and application testing. A Life Cycle Assessment (LCA) of mussel production was conducted with the aim of quantifying the environmental impacts along the entire production chain and identifying the most critical stages towards more sustainable production. Subsequently, the work explores the technical valorisation of mussel shells as a secondary raw material, verifying its feasibility in various areas of application. On the one hand, it analyses the use of ground shells as bio-fillers for low environmental impact drainage flooring; on the other, it examines the use of shells as aggregate in cement mortars for 3D printing. The final part of the work proposes an innovative application of biogenic materials in the field of Marine Nature-Based Solutions (MNBS): the design and construction, using 3D printing, of ecological artificial reefs for the regeneration of marine habitats. In particular, a prototype circular reef has been developed, consisting of a cement mixture containing mussel shells, designed to promote the biological colonisation of flat oysters and other benthic organisms in the Gulf of La Spezia.