Calcined oyster shell powder as a lime substitute in earthen building materials

This study examined calcined oyster shell powder (COSP) as a substitute for lime in earthen building materials using a controlled substitution design and a simplified cradle-to-gate carbon assessment. Fresh-state behavior, 28-day compressive strength, EDS-derived elemental ratio, SEM observations, and calculated binder-related carbon emissions were considered together. Five mixtures with increasing COSP replacement levels were prepared. The fresh-state results showed only limited changes in mini-slump spread diameter, although the post-vibration morphology differed across the substitution series. The 28-day compressive strength showed a non-monotonic trend, decreasing from A1 to A3, recovering in A4 to a value close to that of the reference group, and decreasing again in A5. The elemental ratio \(\:R=Ca/(Si+Al)\)also varied non-monotonically, and mixtures with intermediate \(\:R\)values showed denser-looking SEM morphology and comparatively higher strength retention. Under both baseline and low-carbon electricity scenarios, the calculated binder-related carbon emissions decreased with increasing COSP replacement, and this trend remained after normalization by compressive strength. When the mechanical and carbon results were considered together, A4 provided the closest compressive strength to the reference group while also showing lower calculated carbon intensity. These results indicate that COSP can function as a lime substitute in earthen building materials while reducing calculated binder-related carbon emissions at selected replacement levels. However, the carbon trend was more regular than the mechanical trend, and the findings should therefore be interpreted within the experimental conditions and carbon-accounting assumptions used in this study.