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

Abstract

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 ≤ 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.

Chemical acceleration of a neutral granulated blast-furnace slag activated by sodium carbonate

“…ATR-FTIR results (Fig. 5) also supports the view that only part of Al in these pastes is in C-S-H gel structure; the peak about 960 cm -1 suggests that all AAS pastes have close Al/Si ratios (higher Al uptake would shift the peak towards lower wave number [37,38,41]). Table 4 Mean atomic ratios of AAS pastes at 1 day.…”

“…A well-developed band at 957 cm -1 and a shoulder at 1117 cm -1 can be seen in the spectra of the paste accelerated with a combination of silica fume and slaked lime. The band at 957 cm -1 is assigned to the Si-O stretching vibrations generated by Q 2 tetrahedra that comprise C-S-H and more specifically C-(A)-S-H gel [37][38][39]. The shoulder at 1117 cm -1 can be associated with the formation of a silicon-rich gel with some sodium in its composition and a higher degree of polymerization than the C-(A)-S-H gel [38,40].…”

“…In the case of pastes accelerated with OPC and the combination of silica fume and slaked lime, the band also shifts towards higher wave numbers of 963 cm -1 . The shift indicates a higher degree of polymerization, a decrease in the Ca/Si and an increase in the Si/Al ratio in the C-(A)-S-H gel structure [37,38,41]. …”

Hydration water and microstructure in calcium silicate and aluminate hydrates

“…During hydration, the total intensity of these two bands decreases, because of the conversion of the water into hydrated phases. Contextually the intensity of the sharp peak at 7083 cm −1 , due to the first O-H stretching overtone of portlandite [52], increases.…”

“…Five Gaussian functions (α, β, γ , δ and ε) were employed to fit the experimental data. In particular, the fifth Gaussian, ε, was used to account for the very sharp peak at 7083 cm −1 coming from the overtone of the O-H stretching in Ca(OH) 2 [52]. The β and γ Gaussians, respectively at 6600 and 6900 cm −1 , are attributed to a continuum of strongly hydrogen bonded water molecules rather than to specific molecular classes [53], while α (around 7070 cm −1 ), is attributed to the oscillation of weak O-H bonds (i.e.…”