Towards bio-sourced solutions for soil treatment: experimental study on seashell powder as precursor for alkali-activated binders

An experimental investigation on the use of mussel shell powder (MSP) as a precursor for Alkali Activated Binders (AAB) is presented in this study. The results of this study opens up new perspectives on the production of biowaste-based binders (mussel shells) for the mechanical improvement of soils. Two alkaline solutions of different chemical composition were considered for this study, namely 12 molar sodium hydroxide solution (12 M NaOH) and a mixture of NaOH and sodium silicate (Na2SiO3) solution. Mineralogical and microstructural changes of AAB were monitored over time by means of X-ray Diffraction (XRD), Thermogravimetric analysis (TGA), Scanning Electron Microscopy (SEM) and Mercury Intrusion Porosimetry (MIP). The alkaline environment induced by 12 M NaOH promotes the dissolution of calcium carbonate and the precipitation of metastable gels, then transformed into crystalline carbonate hydrates (i.e., pirssonite and gaylussite). The availability of silica in the system, induced by the use of NaOH+Na2SiO3, favors also the development of a second class of reactions (i.e. pozzolanic reactions) responsible for the precipitation of silicate hydrated gels. As a consequence, the binder activated by 12 M NaOH solution showed a reduction of Unconfined Compressive Strength (UCS) over time due to the transformation of metastable carbonate gel into crystals, whereas the binder activated with NaOH+Na2SiO3 showed a higher improvement of UCS for the precipitation of stable cementitious compounds. The carbon footprint of alkali-activated mussel shell binder was evaluated and compared with ordinary Portland cement. An insight into the mechanical effects induced by the Alkali Activated Mussel Shells Binders (AAMSB) on a dredged marine sediment was provided, highlighting the role of silicon availability in the alkaline activator on the effectiveness of the treatment.