Micro- and macro-structural performance of hydraulic lime mortars with oyster shell incorporation and bio-based surface healing

The effect of incorporating oyster shell waste as a partial replacement of lime (24%) and sand (30%) was studied in natural hydraulic lime mortars treated with a bacterial surface self-healing technique based on microbiologically induced calcite precipitation (MICP). These incorporation levels were selected from a previous experimental campaign because they combine waste replacement with limitations on water-ingress-related performance. The main objective was to assess whether the bio-based surface treatment could mitigate the performance limitation loss associated with high oyster shell content. Control mortar and mortars incorporating oyster shell powder (OSP) and oyster shell aggregate (OSA) were surface-treated with the bacterial solution, while samples of these same mortars were exposed to distilled water under identical conditions. Bulk density, open porosity, capillary water absorption, flexural strength, and microstructural characteristics were evaluated. At the specimen level, treatment-related changes in bulk density (up to 2%), open porosity (up to 3.2%), and capillary coefficient (up to 5.7% in the 24% OSP mix) were consistent in direction but not statistically significant. However, a paired within-specimen comparison of treated and untreated faces revealed a statistically significant reduction in near-surface capillary uptake (≈9.7%), most notably in shell mortars. Flexural strength was higher after treatment, but this reflects the combined effect of treatment and additional curing. Microstructural analyses indicated relative calcite enrichment in bio-treated shell mortars, providing convergent but indirect evidence of treatment-associated precipitation. Therefore, the combined use of oyster shell waste and bacterial MICP represents a promising route toward more resource-efficient lime-based mortars, pending confirmation under matched curing conditions and service exposure.