Influence of Thermal Treatment and Particle Size on the Physicochemical Properties and Filler Performance of Oyster Shell-Derived CaCO3 in Mortar

The cement industry contributes approximately 7–8% of global CO2 emissions, motivating the development of sustainable supplementary materials. This study evaluates the partial replacement (10 wt.%) of Portland cement with calcium carbonate (CaCO3) derived from oyster shells, both untreated and thermally treated at 600 °C, in non-structural mortar blocks. Structural and physicochemical characterization was performed using XRD, SEM, EDS, BET, and TGA to assess phase composition, morphology, and surface properties. Thermal treatment modified the textural characteristics of CaCO3, reducing the crystallite size and increasing the specific surface area (from 5.8 to 25.6 m2/g), without phase transformation. Compressive strength results, relative to a reference mortar (13.6 MPa), showed comparable performance, with variations generally within ±10%, although slightly larger deviations were observed for specific particle sizes. Finer calcined particles yielded the highest strength (15.0 MPa), reinforcing the combined influence of particle size and thermal treatment. These results suggest that CaCO3 acts primarily through a filler effect, improving particle packing and matrix interaction. Both untreated and heat-treated CaCO3 satisfied strength requirements for non-structural applications, supporting the valorization of oyster shell waste as a sustainable material in cement-based systems.