Assignment of SiO vibrational modes in the IR spectra of C-S-H phases based on single-crystal polarized IR and Raman spectra of 14 Å tobermorite, jennite, and jaffeite

This article explains the vibrational modes in the SiO stretching range in the IR spectra of synthetic C-S-H phases with varying C/S ratios. These are compared with selected in situ spectra of hydrates of OPC, and those of synthetic crystalline hydrates. The assignments were supported by ²⁹Si NMR and trimethylsilylation (TMS) data. IR and Raman polarized spectra of oriented crystals of 14 Å tobermorite, jennite and jaffeite enabled direct observation of the SiO vibrational modes. They were successfully resolved based on the involvement of specific silicon (paired, bridging) and oxygen (bridging, non-bridging) atoms, and were compared with existing theoretical data. The resemblance between the IR spectra of synthetic C-S-H and those formed upon hydration of OPC, proves the suitability of model C-S-H phases for understanding hydration processes. Some uncertainties in the assignment of the C-S-H bands observed in existing in situ IR experiments are discussed, and potential sources of error identified.