Ericaria amentacea, a habitat-forming macroalga of high ecological importance in the Mediterranean, has been the focus of restoration project involving the deployment of 3D-printed concrete units along the Ligurian coast of Italy. Previous studies demonstrated that these prototypes supported effective colonization under laboratory and field conditions, but revealed insufficient strength to withstand marine loading, which restricted their long-term applicability. To address this limitation, the present study evaluates conventionally cast mortars in which crushed seashells were introduced as a partial replacement for natural fine aggregates. The use of shells, an abundant by-product of fisheries, is aligned with sustainability objectives and circular economy strategies. Mechanical and durability tests showed that shell-based mortars preserved flexural strength while exhibiting lower air permeability and capillary absorption, properties associated with improved performance in marine environments. The results confirm the feasibility of seashell aggregates in restoration substrates, providing a durable and scalable alternative for E. amentacea habitats recovery in the Mediterranean.