Innovative solutions for the geo-chemo-mechanical stabilisation of marine sediments

The work displays the results of an intense experimental research into the geo-chemo-hydromechanical stabilisation of sediments dredged from a fill-in basin in the Port of Taranto (Ionian Sea, Italy). This research was conducted in collaboration with ETH-Zurich and Italcementi HeidelbergGroup and founded by a MIUR- PON RI DRSATE PhD grant. Taranto, as many port areas worldwide, is facing the problems of disposing two materials currently considered as waste: dredged sediments and mussel shells. Starting from this premise and on the basis of the extensive literature concerning the stabilisation of marine sediments with the use of hydraulic binders and the related environmental issues, this study proposes the treatment of Taranto sediments through innovative and eco-sustainable solutions. In detail, the stabilisation solutions in which commercial cements were replaced by mussel shell powder (CemShells) were widely illustrated based on the experimental tests conducted on three types of commercial cement: Type III Termocem Green Blast furnace cement, Type I traditional Portland Cement 52.5R and Sulfoaluminate cement. In order to include the case of the treatment of contaminated sediments in the stabilisation solution, this study also focuses on the artificial contamination of sediments with 5000 ppm Lead to obtain heavy metal contaminated sediment and with 8% kerosene to obtain an organic contaminated sediment. In the case of kerosene-contaminated sediment, Biochar was also tested as an extra additive of commercial cements used in the case of treatment of uncontaminated sediment. Moreover, in order to stabiles kerosene- polluted sediment, Biochar was also added to CemShells to form another Bio-Binders named CemShellBio. In order to analyse the efficacy of the stabilisation solutions in terms of effectiveness and environmental sustainability, this study focused on multiscale and multidisciplinary investigations entailing the dredged unpolluted and polluted (artificially) sediments as well as the same materials when treated with the commercial cements and the original binders i.e., the CemShells and the CemShellBios. The goal of the investigation was to obtain a complete information on the sediment behaviour at both micro- and macro- scale in the natural and treated state. The investigation focused on: i) Chemical, geochemical and thermal, ii) microstructural and iii) geotechnical characteristics of the natural and treated geomaterials. Microstructural tests were carried out on more than 60 samples. The microstructural testing routine included: pore size distribution with mercury intrusion porosimetry analysis (MIP), microstructure morphology at high magnification imaging with scanning electron microscope (SEM) and mineralogical composition with X-Ray diffraction (XRD). Chemical, geochemical, and thermal tests were carried out on more than 150 samples. The chemical and geochemical testing programme included measurements of pH, electrical conductivity (EC), cation exchange capacity (CEC), carbon content (TIC/TOC) on matrix solid. Moreover, measurement of Chlorides and Sulphate, pH, conductivity, and salinity were also conducted on curing water (and/or pore water in natural sediments). All geochemical tests aimed at studying the chemical interaction between the clay particles of sediments and the minerals of cements. In addition, they targeted the interaction of sediments and cements with mussel shells and Biochar. Thermal tests included the thermogravimetry on treated sediment samples to quantify heat and hydration time occurring in the stabilisation process. Finally, chemical tests were carried out on eluate of both untreated and treated sediments to study the chemical efficiency, in term of leachability, of each treatment. Geotechnical tests were carried out on more than 80 samples. Standard procedures were properly adapted to consider the influence of salt in the pore fluid and organic compounds of the solid matrix. The geotechnical testing programme included tests the analysis of the physical properties, composition, and state of sediments. Moreover, oedometer tests were carried out to determine the 1D compression and consolidation properties of the soil and CIU triaxial tests were performed to measure isotropic compressibility and effective strength parameters. Furthermore, unconfined strength was also measured by means of unconfined compression test (UCT) and originally used texturometer tests. The integration of the microstructural, geotechnical, and geochemical/chemical data produced a global picture on the effects of the chemo-mechanical interaction processes on the geotechnical behaviour of both untreated and treated sediments. This study can provide a useful guide towards an original, effective, and more sustainable stabilisation solution for unpolluted and polluted sediments.