Calcined Crab Shell as a Sustainable Supplementary Cementitious Material in Cement Pastes: Chemical Interaction, Microstructural Evolution, and Mechanical Performance

The growing demand for sustainable construction materials has stimulated interest in alternative binders derived from waste resources. This study investigates the use of calcined crab shell (CCS), a calcium-rich marine biowaste, as a partial replacement for Portland limestone cement. Cement pastes containing 0%, 5%, 10%, and 15% CCS were prepared and evaluated through compressive strength, water absorption, open porosity, bulk density, SEM, XRD, FTIR, and TGA analyses. The results showed that incorporating 10% CCS produced the most favorable performance, increasing compressive strength from 17.6 MPa to 33.6 MPa after 28 days of curing. This improvement was accompanied by reduced porosity, increased bulk density, and the development of a denser and more homogeneous microstructure. Physicochemical analyses suggest that CCS acts both as a filler and as a source of reactive calcium species. The CaO generated during calcination may participate in hydration processes and influence the formation of hydration products, contributing to matrix densification. In contrast, the incorporation of 15% CCS resulted in increased porosity, a less homogeneous microstructure, and lower mechanical performance. These findings indicate that replacing Portland limestone cement with up to 10% CCS can improve the properties of cement pastes while promoting the valorization of marine shell waste and reducing cement consumption, thereby supporting the development of more sustainable construction materials.