Anisotropic compressional behaviour of the Sorel cement F5-phase (Mg₃(OH)₅Cl·4H₂O) [Anisotropic compressional behaviour of the Sorel cement F5-phase (Mg3(OH)5Cl·4H2O)]

The crystal structure of the so-called F5-phase [Mg3(OH)5Cl·4H2O], the dominant crystalline component of the Sorel cement, has been reinvestigated by synchrotron powder X-ray diffraction data, with a deep description of the complex and pervasive H-bonding network. Its isothermal high-pressure behaviour (up to 5.10 GPa) has been studied by an in-situ compressional experiment, under hydrostatic conditions. The pressure vs volume pattern, modelled with an isothermal equation of state, revealed the occurrence of two anisotropic compressional regimes: the values of the isothermal bulk modulus (K0 = − V(∂P/∂V)) is 28.2(5) and 38.5(9) respectively up to 1.90 and at P > 2.10 GPa. The F5-phase shows a significant anisotropic compressional scheme, described as magnitude and orientation of the Eulerian finite unit-strain ellipsoid: ε1:ε2:ε3 ∼ 6.6:2.5:1.0 up to 1.9 GPa, and ε1:ε2:ε3 ∼ 4.7:1.8:1.0 at P > 2.1 GPa for the low- and high-P regimes, respectively. The structural causes, at the atomic scale, that govern the compressional anisotropy of the F5-phase are discussed.