Soft X-ray absorption near-edge investigations of Mg-containing mineral phases relevant for cementitious materials

This study shows that soft X-ray absorption spectroscopy coupled with ab initio calculations performed by FDMNES is a powerful tool to investigate Mg-rich mineral phases. Such phases play an important role in low-pH cements and for the interaction of cementitious materials with clay minerals. FDMNES can simulate spectral features occurring at the near and medium range distance around an absorbing atom by using two modelling approaches: firstly, the multiple scattering theory (MST) approach and, secondly the finite define method (FDM). It was observed that the modelling of different cluster sizes of the investigated phase is important for the interpretation of the spectra. Furthermore, the FDMNES code is sensitive to different crystallographic sites as well as symmetries. Here we present, as example, calculations for the Mg K-edge of the talc structure which are compared to the experimental spectrum. The results show that FDM computations with a cluster size of 8 angstrom have to be conducted and, the sum of the M1 and M2 octahedral sites occupied by Mg have to be considered in order to model the talc spectrum. Such methodological approaches are crucial for investigating amorphous phases present in cementitious materials and, they can provide a better understanding of the mechanisms responsible for the formation of new phases.