Structural and spectroscopic characterization of ettringite mineral –combined DFT and experimental study

• Ettringite mineral studied by experimental and Density Functional Theory methods.
• Calculations refined positions of the H atoms and the hydrogen bonds were analyzed.
• All calculated vibrational modes were distinguished and assigned.
• The vibrations of the SO42− anions (C3v) in the ettringite structure were analyzed.
• A comparison of the vibrational spectra of ettringite and thaumasite is presented.

The structure of the ettringite mineral was studied by means of FTIR spectroscopy and single crystal X-ray diffraction method. The experimental study was combined with the first principle calculations based on density functional theory (DFT) method. Predicted structural parameters (unit cell vectors and positions of heavy atoms) are in a very good agreement with the experimental data. Moreover, calculations also enabled to refine the positions of the hydrogen atoms not determined precisely by the single crystal X-ray measurement. The detailed analysis of the hydrogen bonds in the ettringite structure was performed and several groups of the hydrogen bonds were classified. It was found that the water molecules from the coordination sphere of Ca2+ cations act as proton donors in moderate O–H···O hydrogen bonds with SO32− anions. Further, multiple O–H···O hydrogen bonds were identified among water molecules themselves. In addition, also hydroxyl groups from the [Al(OH)6]3− octahedral units are involved in the weak O–H···O hydrogen bonding with the water molecules. The calculated vibrational spectrum showed all typical features observed in the experimental FTIR spectrum. Moreover, performing the analysis of the calculated spectrum, all vibrational modes were distinguished and assigned. Such a complete analysis of the measured IR and/or Raman spectra is not fully possible, specifically for the region below 1500 cm−1, which is characterized by a complex curve with many overlapped bands. A comparison of the vibrational spectra of ettringite and thaumasite (mineral structurally similar to ettringite) revealed the origin of the most important differences between them.

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