From ore to shield: The structural behaviour of critical borates in neutron-shielding concretes

Hydrated borates, a group that includes minerals such as colemanite, ulexite, and borax, serve as the primary ore sources for boron. This element is not only a vital geochemical marker used to track and interpret petrogenetic processes, but it also represents a strategic resource essential for high-end technological applications. Highlighting their economic and industrial significance, these minerals have been recently classified as critical raw materials by the European Union (EU Commission, 2017). Consequently, they are being increasingly investigated as specialized aggregates in the development of neutron-shielding concretes. This interest is driven by their exceptional thermal neutron adsorption capacity combined with a characteristically low density, typically measured between 2.3 and 2.5 g/cm3. From a structural perspective, hydrated borates are characterized by an intricate architecture composed of Bφx units, such as tetrahedra and trigonal planes, which are organized into complex polyion clusters. These clusters are interconnected by alkali or alkaline-earth polyhedra, most commonly involving cations such as sodium (Na+), calcium (Ca2+), and magnesium (Mg2+). Within these crystalline frameworks, H2O molecules and OH groups establish an extensive and sophisticated hydrogen-bond network. This network is not a secondary feature; rather, it is fundamental to the overall stability and structural integrity of the entire crystalline edifice (Comboni et al., 2022; 2024). A comprehensive characterization of the crystal-chemistry, elastic properties, and long-term stability of these natural borates is essential for modern engineering. Specifically, understanding their structural behaviors at varying temperature (T) and pressure (P) conditions is a fundamental requirement for the accurate modeling and prediction of their performance when adopted as aggregates in technical concretes. The aim of this contribution is to explore the behavior of these minerals at non-ambient conditions, providing a detailed description of their deformation mechanisms at the atomic scale