Biogenic vs. geogenic calcium carbonate: multiscale analysis in the mechanical improvement of dredged sediments

The aim of this study has been to evaluate the effectiveness of biogenic calcium carbonate derived from uncalcined mussel shells as a partial cement substitute for the mechanical improvement of fine marine sediments. If mechanical properties of the resulting shell-cement-sediment mixtures were not significantly inferior compared to those with pure cement, this could open a promising path towards simultaneous valorisation of two high-impact marine wastes: dredged sediments and discarded mollusc shells. Within this context two related research questions have been addressed. The first, a more fundamental one, focused on quantifying and understanding potential differences between biogenic and commonly used geogenic sources of calcium carbonate as partial cement replacement. Within this context a systematic comparison was conducted between mussel shell powder and limestone. The effects of two particle sizes (fine and coarse) and two replacement ratios (25% and 50%) were assessed. The results indicate that replacing 25% of cement with fine mussel shell powder yields the best mechanical performance, with an average increase of 7% in consistency index (CI) and 15% in extended shear strength from texturometer apparatus (STEXT) compared to mixtures with limestone. The enhanced performance is primarily attributed to the biogenic nature of the carbonate (aragonite), its elongated morphology, and its higher specific surface area (3.43 m²/g), which together promote pozzolanic reactions and the nucleation of hydration products. The second, more practical research question explored the actual effectiveness of shell powder as a cement substitute. A multidisciplinary and multiscale characterization was conducted on selected mixtures composed of a highly plastic clayey silt sediment from the Port of Taranto, treated with three different types of cement (Portland, blast furnace slag, and calcium sulphoaluminate), and with cement partially replaced by fine mussel shell powder at two replacement ratios (25% and 50%). The analyses performed (CI, STEXT, pH_w, XRD, MIP, SEM) confirm that Portland and blast furnace slag cement-based mixtures incorporating mussel shell powder at a 25% replacement level achieve mechanical and structural performance comparable to or even slightly higher than that of the corresponding cement-only mixtures. Specifically, average increases in CI (about 9%) and S_TEXT (about 3%) are recorded in shell-containing mixtures respect to the traditional ones. Furthermore, shell-containing mixtures exhibit increased microporosity and reduced macroporosity, associated with a denser microstructure. In contrast, the sediment treated with calcium sulphoaluminate cement did not exhibit clear signs of cementation. This result may be attributed to the nature of the hydration products of this type of cement and the presence of high amounts of soluble sulphates. Overall, the findings demonstrate that mussel shell powder is a technically effective and environmentally sustainable solution for the stabilization of marine sediments, yielding performance that matches or even moderately exceeds that of conventional cement-based treatments, potentially contributing to reduced binder consumption, a lower carbon footprint, and the advancement of circular economy practices in the sustainable management of port materials.