29Si MAS NMR Spectroscopy of Poorly-Crystalline Calcium Silicate Hydrates (C-S-H)

Carbonation of C–S–H and C–A–S–H samples studied by 13 C, 27 Al and 29 Si MAS NMR spectroscopy

“…At high Ca/Si ratios, the Q b 2 peak is not clearly resolved, reflecting that the silicate chains become shorter and that the C-S-H is mainly composed of silicate dimers. Again, this is in accord with earlier 29 Si MAS NMR [23][24][25][26][27][28][29][30] and Raman spectroscopic [31] studies of synthesized C-S-H samples and it reflects that the high-Ca/Si ratio C-S-H samples (Ca/Si~1.5) have a defect tobermorite-like structure, with silicon absent from almost all bridging tetrahedral sites, rather than a jennite-like structure [21], which ideally contains Q 2 sites only (infinite silicate chains). The 29 Si MAS NMR spectra in Fig.…”

“…Jennite (Ca 9 Si 6 O 18 (OH) 6 ·8H 2 O [21]) exhibits Ca/Si = 1.5 and Ca/Si = 1.75 corresponds to the ratio typically found in mature Portland cement pastes [22]. For the low-Ca/Si ratio samples, three distinct peaks are resolved, corresponding to end groups and dimers (Q 1 , −79.1 ppm), the bridging sites (Q b 2 , −83.2 ppm) and paired sites (Q p 2 , −84.9 ppm) of the silicate chains, in agreement with earlier 29 Si MAS NMR studies of synthesized C-S-H phases [23][24][25][26][27][28][29][30]. Moreover, the observation of these resonances agrees with the basic dreierketten silicate-chain structure of 14-Å tobermorite [19], although infinite chains in the ideal structures are not observed for the synthesized samples.…”

“…The 29 Si MAS NMR spectra were acquired on a Varian INOVA-400 MHz (9.4 T) spectrometer using a home-built CP/MAS probe for 5 mm o.d. PSZ rotors, ν R = 12.0 kHz, γB 1 /2π = 40 kHz, τ p = 2.5 μs, and a relaxation delay of 30 s. The 27 Al MAS NMR spectra were recorded at 14.1 T on a Varian Direct Drive VNMR-600 spectrometer, employing a homebuilt CP/MAS probe for 4 mm o.d. PSZ rotors, ν R = 13.0 kHz, γB 1 /2π = 60 kHz, τ p = 0.5 μs, 1 H decoupling (γB 2 /2π = 50 kHz), and a relaxation delay of 2 s. 13 C and 29 Si chemical shifts are referenced to an external sample of tetramethylsilane (TMS), while the 27 Al chemical shifts are relative to an external sample of a 1.0 M aqueous solution of AlCl 3 ·6H 2 O.…”

“…13 C and 13 C{ 1 H} CP/MAS NMR are used to detect the amount of carbonate ions and the carbonate ions in a hydrous environment, respectively, using the same approach as in our recent 13 C MAS NMR study of the hydration of Portland-limestone cements [17]. Finally, 27 Al MAS NMR is used to follow the structural changes that occur for Al in C-A-S-H phases upon decomposition caused by carbonation.…”

Solubility and structure of calcium silicate hydrate

“…This shoulder has been observed by a number of investigators [3,4,19,31,[64][65][66][67], but its assignment is still uncertain. It is generally assumed that the shoulder is produced by a subset of the Q 2 Si, and it is likely to represent at least some of those present as bridging tetrahedra [4,67]. The current results are consistent with (but not proof of) this last hypothesis, as the shoulder becomes increasingly prominent as the Ca/Si ratio decreases.…”