Abstract In the pursuit of greener and sustainable materials for developimg cementitious composites, several agricultural and undustrial wastes are continually explored for use in the process. The current study focused on the use of 20%, 25% and 30% Oyster shell powder (OSP) as a partial substitute for Ordinary Portland cement, and 100% Oyster shell aggregate (OSA) and 100% recycled concrete aggregate (RCA) (OPC) as total replacement for fine aggregate in mortar production. The mechanical properties of the mortars such as compressive strength, flexural stremntgh and water abosprion were determined. Also, microscale analysis using SEM-EDX was conducted on selected mortars. The results showed that the control mortar demonstrated the maximum compressive strength, however, OSP is adequate as binder at upto 25% replacement level, producing strength somewhat close to that of the control mortar. Results obviously showed that the replacement amounts of OSP, RCA and OSA greatly influence the hydration process of the mortar matrix, and the overall performance.