Mechanical Properties of Crushed Oyster Shell Mortar and Conceptual Design of a Modular Wave-Dissipating Block

Construction of cement-based coastal components requires natural aggregates, while oyster farming and processing generate waste shells. This study compared mortar flow, strength, and chloride-migration screening responses across crushed waste oyster shell (CWOS) replacement levels. CWOS replaced 0%, 5%, 10%, or 15% of river sand by mass (Control = 0%), following reference sand sieve-fraction mass proportions with a fixed mixing-water amount and total fine-aggregate mass. Flow values were 280, 260, 230, and 180 mm. CWOS-10 reached 28 d flexural and compressive strengths of 9.20 ± 0.30 and 63.13 ± 0.53 MPa, or 94.8% and 95.0% of Control. Its 28 and 90 d coefficients from modified fixed-voltage rapid chloride-migration screening (30 V, 8 h) were (3.86 ± 0.04) × 10−12 and (3.19 ± 0.04) × 10−12 m2/s, higher than Control. CWOS-10 retained approximately 95% of both 28 d Control strengths at 10% river-sand replacement and was selected as a candidate for future cement-based prototypes. A four-panel modular wave-dissipating block concept incorporated local panel replacement, with a 3D-printed model illustrating connection and assembly features. These findings demonstrate the potential of waste oyster shells for partial sand replacement in mortar, enabling shell-waste reuse while reducing natural-sand consumption.