Combining Raman Spectroscopy, DFT Calculations, and Atomic Force Microscopy in the Study of Clinker Materials

Raman spectroscopy and Raman mapping analysis, combined with density functional theory calculations were applied to the problem of differentiating similar clinker materials such as alite and belite. The Portland cement clinker 217 (further: clinker) was analysed using colocalised Raman mapping and atomic force microscopy mapping, which provided both spatial and chemical information simultaneously. The main constituents found in the clinker were alite, belite, portlandite, amorphous calcium carbonate, and gypsum. Since phonon bands of alite and belite greatly overlap, and their distinction is important for the hydration process during cement setting, we provided the calculated phonon density of states for alite Ca₃SiO₅ (<M>Pc structure) and belite Ca₂SiO₄ (β P2₁/n structure) here for the first time. Both calculated phonon densities have similar distribution of phonon modes, with a gap between 560 and 810 cm^-1. A comparison of the calculated phonon frequencies for Ca₃SiO₅ and Ca₂SiO₄ shows that the lowest calculated phonon frequency of β-Ca₂SiO₄ lies at 102 cm^-1, while for <M>Pc alite the lowest phonon frequency is predicted at 27 cm^-1. Low frequency Raman spectroscopy could therefore be used for a clearer distinction of these two species in a clinker material.