Thermal and compressional behaviour of natural borates: a potentially aggregates in radiation-shielding concretes

Natural borates represent the first worldwide source of boron. Boron is a key constituent in different industrial sectors, including glass, ceramics, agricultural, metallurgical, electronics, textile, cosmetics, and chemistry. Recent technological developments have further expanded the use of borates, as underlined by the doubling of global production in the last decade. The recent addition (on 2014) of borates to the European Union list of Critical Raw Materials is a further evidence of the large global demand for this commodity. Due to the ability of 10B (ca. 20% of the natural boron) to absorb thermal neutrons, related to its high cross-section for the 10B(n,α)7Li reaction (~3840 barns), several recent studies investigated the utilization of natural borates as light aggregates in radiation-shielding materials, such as concretes. In this light, the use of natural borates would provide also an economic advantage: synthetic B4C, for example, proved to be efficiently adopted in this field, but its use is hindered by the high costs of synthesis. In order to characterize the phase stability field, the thermo-elastic properties, and the mechanisms of thermal-induced dehydration, we have investigated the behavior at non-ambient T and P of some of the most common hydrous borates, i.e. kernite, colemanite, kurnakovite, ulexite, and meyerhofferite, by means of in situ single-crystal synchrotron Xray diffraction. In situ non-ambient conditions were obtained using diamond anvil cells (for high-P), nitrogen cryostats (low-T), and gas blowers (high-T). High-pressure experiments show that all the analyzed borates remain stable at pressures exceeding those to which radiation-shielding materials may be subjected. Among them, colemanite shows the lowest bulk compressibility (KV0=67(4) GPa and KV0’=5.5(7), where = ∂KV0/∂P; βV0 = 1/KV0 = 0.0149(9) GPa-1). The refined isothermal bulk moduli for ulexite and kurnakovite are ~ 37 GPa (βV0 ~ 0.0270 GPa-1), which lie between those of other minerals commonly used as aggregates in concretes, while kernite and meyerhofferite are slightly softer (KV0 ~ 30 GPa; βV0 ~ 0.0333 GPa-1). The high-temperature experiments on kurnakovite and colemanite show that the presence of structural H2O leads to dehydration processes that result in a structural collapse. In the light of a potential application as aggregates, this phenomenon is more critical in kurnakovite (~48% H2O), where the crystal structure is no longer stable above 120 °C, while in colemanite (~22% H2O) significant dehydration starts at T > 240°C. The structural collapse of kurnakovite at relatively low temperatures implies severe questions on its potential applicability in radiationshielding concretes, while the thermal-induced dehydration of colemanite should not represent an issue for several applications of radiation-shielding materials.