This study investigates the reuse of mussel shell waste as a secondary raw material in bio-cement mortars designed for the additive manufacturing of artificial reefs for marine habitat restoration. The novelty of the research lies in combining a high recycled shell content (60 wt.%), low-clinker cement, and two 3D-printing techniques: Extruded Material Systems (EMS) and Powder-Based Systems (PBS). Mechanical performance was evaluated through flexural and compressive tests after 7, 28, and 91 days under both air and freshwater curing conditions, while environmental impacts were assessed through Life Cycle Assessment (LCA). The LCA evaluated both the environmental performance of shell-based mixtures compared with conventional materials and the impacts associated with the investigated fabrication techniques. The best-performing bio-mixtures achieved compressive strengths up to 46.01 MPa and flexural strengths up to 9.91 MPa after freshwater curing, demonstrating the suitability of shell-based mortars for submerged applications. LCA results showed reduced impacts in land use and mineral resource depletion compared with conventional mixtures, despite slightly higher energy and water demands associated with shell pre-treatment. The results demonstrate the technical and environmental feasibility of integrating aquaculture waste into sustainable 3D-printed marine restoration solutions.

High-performance concrete for 3D printing has recently attracted significant attention due to its potential to create structural elements without the need for traditional reinforcement. While various formulations have been proposed by researchers, evaluations are often limited to mechanical performance and printability, while cost and environmental impact are generally overlooked. This study expands the analysis by also considering cost and environmental impact, aiming to identify the optimal mix using a multi-criteria decision-making analysis (MCDMA). In the first phase, several high-strength mortar formulations were developed and assessed based on mechanical strength, printability, environmental impact, and cost. In the second phase, the most promising mix from the initial evaluation was further modified by incorporating different types of fibers, including aramid, carbon, glass, cellulose, and polypropylene. Comprehensive testing—covering mechanical properties and printability—together with cost and a life cycle assessment were conducted to determine the most effective mortar formulations. One of the main findings is that adding 0.05% of 20 mm length cellulose fibers in weight to a mortar containing Cem I 42.5R can increase the compressive strength by more than 9% without affecting the cost or environmental impact, also allowing the obtainment of a mortar apt for 3D printing. This increase in the compression strength is presumably related to a lateral restriction in movements of the mortar, which makes it increase the maximal principal stresses, and thus, its strength.