Phase transition and high-pressure behavior of ulexite, a potential aggregate in radiation-shielding concretes

The high-pressure behavior of ulexite [ideally NaCaB5O6(OH)6x5(H2O)], a B-bearing raw material (with B2O3 ca. 43 wt%) and a potential B-rich aggregate, has been studied by in-situ single-crystal synchrotron X-ray diffraction up to 9.5 GPa, in order to explore its stability field at high pressure, the anisotropy of its compressional behavior and the deformation mechanisms at the atomic scale. Ulexite undergoes a first-order iso-symmetric phase transition, reconstructive in character, between 6.44 and 7.13 GPa (to the ulexite-II polymorph); the crystal structure of ulexite-II was successfully solved and refined. For the low-P polymorph of ulexite (stable in all the working conditions as aggregate in concretes), the isothermal bulk modulus here obtained is KV0 = 36.9(5) GPa, with a strongly anisotropic compressional pattern (finite-Eulerian principal components of the unit-strain ellipsoid: e1:e2:e3 =7:3:1). Ulexite-II also shows a marked anisotropic compressional pattern and with a resulting bulk softening (KV0 = 26(4) GPa).