<p>This study examined calcined oyster shell powder (COSP) as a lime substitute in earthen building materials using a controlled substitution design and a simplified cradle-to-gate carbon assessment. Five mixtures with increasing COSP replacement levels were prepared, and their fresh-state behavior, 28-day compressive strength, EDS-derived elemental ratio, SEM morphology, and calculated binder-related carbon emissions were evaluated together. The mini-slump results showed that COSP substitution caused only limited changes in spread diameter, while the post-vibration morphology varied across the substitution series. The 28-day compressive-strength results indicated that the COSP-containing mixtures remained within a comparable strength range for stabilized earthen materials, although they did not show a clear strength-enhancing effect compared with the lime-based reference mixture. The elemental ratio R = Ca/(Si + Al) varied non-monotonically, and SEM observations suggested differences in local matrix compactness and heterogeneity among mixtures. Under both baseline and low-carbon electricity scenarios, the calculated mass-based binder-related carbon emissions decreased with increasing COSP replacement. When normalized by compressive strength, the carbon intensity results became more sensitive to the measured mechanical response, indicating that COSP substitution should be evaluated as a strength–carbon trade-off rather than as a purely mechanical optimization problem. Overall, the results suggest that COSP can serve as a potential low-carbon lime substitute in earthen building materials under the tested conditions. The main contribution of this study is to provide a coupled comparison of mechanical response, elemental composition, microstructural observation, and carbon-accounting results for shell-derived calcium substitution in earthen materials. The findings should be interpreted within the selected experimental design and carbon-accounting assumptions.</p>