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 H2O and OH environments. The main mid-IR bands occur at 950-1100, 810-830, 660-670, and 440-350 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 similar to 1200 cm(-1) due to Si-O stretching vibrations in Q(3) sites occurs only at C/S less than or equal to 0.7, The 660-670 cm(-1) band due to Si-O-Si bending broadens and decreases in intensity for samples with CIS > 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 similar to 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-B samples with C/S ratio greater than similar to 1.3 have increased absorption intensity at similar to 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.