Soft Calcite from mussel shells: a bio-derived additive for the geotechnical enhancement of marine clays

Marine dredged sediments, mostly rich in clay minerals, pose significant challenges in geotechnical engineering due to their complex properties and variability. This study shows preliminary experimental results on the treatment of dredged sediments using a peculiar needle-like form of calcite, called Soft Calcite (SC), obtained by a controlled dissolution of the aragonitic layer from Mytilus Galloprovincialis sp. shells [1]. When used as a partial replacement for cement, this material can play an active role in the chemo-mechanical stabilization of sediment mixtures. Recent research [2] highlights that biogenic calcium carbonate in mussel shells is more liable to decomposition than lithogenic sources, making it more reactive in hydration reactions with sediment silica and cement minerals. Builds on these findings, here we discuss the results of three different treatments of mussel shells to obtain the SC in a more sustainable and scalable way. These treatments were designed to enhance the reactivity of SC and its effectiveness in the mechanical improvement processes of soft marine sediments. To assess the role of different treatments, SC was subjected to thermogravimetric analysis (TGA), X-ray Diffraction (XRD) and total organic carbon (TOC) measurement. The efficacy of SC can be attributed to three main factors: (i) the higher specific surface area compared to traditional Portland cement, promotes hydration product precipitation and nucleation; (ii) the unique morphology of the SC prismatic needles, which are integrated into the mixtures forming a 3D scaffolding microstructure, as revealed by SEM images and synchrotron nanocomputed X-ray tomography (Syn nCT), facilitates the formation of a reticulated network that better hosts CSH gel during cement hydration; and (iii) the impact of different treatment time on the consistency index, water content, and strength of the sediment-cement mixtures prove the persistence of SC after 28-days of curing, which further strengthens the mixture by filling the matrix. The innovative properties of this material, including its ability to capture and retain harmful pollutants [3], suggest its potential for use as an effective barrier in port areas, addressing both the stabilization of dredged sediments and the remediation of polluted environments. [1] Murphy J. et al. (2020) Matter 3:2029-2041 [2] Petti R et al. (2024) Géotechnique 0:1-20 [3] Topić Popović N. et al. (2023) Appl Sci 13(1):623