Structure of Calcium Silicate Hydrate (C‐S‐H): Near‐, Mid‐, and Far‐Infrared Spectroscopy

The mid‐, near‐, and far‐infrared (IR) spectra of synthetic, single‐phase calcium silicate hydrates (C‐S‐H) with Ca/Si ratios (C/S) of 0.41–1.85, 1.4 nm tobermorite, 1.1 nm tobermorite, and jennite confirm the similarity of the structure of these phases and provide important new insight into their H 2 O and OH environments. The main mid‐IR bands occur at 950–1100, 810–830, 660–670, and 440–450 cm −1 , consistent with single silicate chain structures. For the C‐S‐H samples, the mid‐IR bands change systematically with increasing C/S ratio, consistent with decreasing silicate polymerization and with an increasing content of jennite‐like structural environments of C/S ratios >1.2. The 950–1100 cm −1 group of bands due to Si‐O stretching shifts first to lower wave number due to decreasing polymerization and then to higher wave numbers, possibly reflecting an increase in jennite‐like structural environments. Because IR spectroscopy is a local structural probe, the spatial distribution of the jennite‐like domains cannot be determined from these data. A shoulder at ∼1200 cm −1 due to Si‐O stretching vibrations in Q 3 sites occurs only at C/S lessthan equal to 0.7. The 660–670 cm −1 band due to Si‐O‐Si bending broadens and decreases in intensity for samples with C/S > 0.88, consistent with depolymerization and decreased structural order. In the near‐IR region, the combination band at 4567 cm −1 due to Si‐OH stretching plus O‐H stretching decreases in intensity and is absent at C/S greater than ∼1.2, indicating the absence of Si‐OH linkages at C/S ratios greater than this. The primary Si‐OH band at 3740 cm ‐1 decreases in a similar way. In the far‐IR region, C‐S‐H samples with C/S ratio greater than ∼1.3 have increased absorption intensity at ∼300 cm −1 , indicating the presence of CaOH environments, even though portlandite cannot be detected by X‐ray diffraction for C/S ratios <1.5. These results, in combination with our previous NMR and Raman spectroscopic studies of the same samples, provide the basis for a more complete structural model for this type of C‐S‐H, which is described.