Pressure-induced anomalous behavior of thaumasite crystal

This work investigated the structural responses of thaumasite crystal, an important phase to understand the structural integrity of concrete-based structures, using synchrotron-based X-ray diffraction and first-principles calculations. The 100 peak was immediately diffused upon the contact of pressure-transmitting medium, but regenerated under subsequent pressurization. Under high pressure, it showed complex nonlinear responses; lattice parameters a and b became stiffer first (between 1.06 and 2.32 GPa) then lattice parameter c became significantly incompressible (beyond 2.32 GPa). The densification of hydrogen bond network in the channels surrounded by calcium silicate columns and the interaction between the network and the medium caused the first nonlinear response and completely weakened the periodicity of lattice parameters a and b (beyond 5.37 GPa). However, this amorphization phenomenon did not leave a permanent damage on the crystal, leading to the reshaping of the weakened crystallinity upon the release of pressure. Simulation results further elucidated the compressive mechanism of thaumasite crystal. It confirmed that deformation due to pressure mainly took place in the channel space, thus strengthening the hydrogen bonds. It also suggested a potential symmetry breaking of hexagonal structure that makes the stiffness characteristics of the crystal highly anisotropic under pressure.