Hydrated borates at non ambient conditions: pivotal experiments in the production of neutron shielding concretes.

Hydrated borates (e.g., colemanite, kernite, ulexite, borax, tincalconite) are the most common ore minerals of boron, an important geochemical marker, in pegmatitic and granitic systems, for petrogenetic processes and a strategic element in a series of technological applications. Hydrated borates have been listed as critical raw materials by the EU [1], and they could be used as aggregate in neutron-shielding Sorel or Portland concretes, enhancing the adsorption of concrete towards thermal neutrons. The main structural units in hydrated borates are Bφx units (fundamental building blocks, i.e., tetrahedra and planar trigonal group where φ is an anion, O2- or OH- ), connected in such a way to form clusters of polyions connected to alkaline/Earth alkaline (mainly Na+ , K+ , Ca2+, Mg2+) polyhedra. In these structures, H2O molecules and OHform a complex and pervasive hydrogen-bond network, which reinforce the connection between the polyions clusters and the cationspolyhedrons, playing a paramount role in the stability of the crystalline edifice [2, 3]. In the last 4 years, a number of studies have been performed at high temperature and pressure unveiling phase transition driving deformation mechanisms’ that lead to the formation of their high-pressure polymorphs. Critically, the pressure at which hydrated borates undergo a phase transition is related to the water content of the mineral itself. The aim of this contribution is to provide insides on the high-pressure behavior and structure evolution of selected hydrate borate minerals. These studies at non ambient conditions are pivotal to produce neutron shielding tiles of Sorel concretes.