An althernative method for the immobilization of a Pb/(Bi) waste stream in an alkali-activated material based on vitrified bauxite residue

This study focuses on identifying a new immobilization matrix for Pb and Pb/Bi alloys that are used as coolants in lead-cooled small-modular and fast reactors respectively. The base material for those matrices is Fe-rich glasses (Vitrified Bauxite Residue), which is used as a precursor for an Alkali-activated material. After having described the chemical-physical characteristics of the matrix, the study attempts to find an appropriate formulation for the realization of the Alkali-activated material, which is able to comply with the waste acceptance criteria (e.g. strength) and possesses good fresh properties. Specifically, a key aspect of this research concerns the mineralogical and microstructural characterization of the waste form, in order to highlight the binding mechanism of Pb and Bi within the alkaliactivate matrix. Results show that good compatibility can be obtained with Pb but not with Pb/Bi, since Bi introduces variability in terms of polarizability, reactivity and compatibility with geopolymer network. The alloy can form intermetallic compounds that interact weakly with the matrix, causing instability, component migration or the formation of unwanted phases that compromise the strength of the material. The presence of Bi also favors the formation of crystalline phases or microcracks, reducing their overall compatibility with the system. Good mechanical and fresh properties of the Alkali-activated material itself are expected, since the Alkali-activated material, like cement, can reach a good mechanical strength (compatible with the Italian criteria, requiring at last 10 MPa compressive strength). This research demonstrate that Alkali-activated materials are well suited to replace a large part of traditional cement.