Natural Abundance ⁴³ Ca NMR Spectroscopy of Tobermorite and Jennite: Model Compounds for C–S–H

There are few effective methods for characterizing the molecular scale structural environments of Ca 2+ in hydrated cements, which has limited our ability to understand the structure of, for example, Ca–silicate hydrate (C–S–H). 43 Ca nuclear magnetic resonance (NMR) spectroscopy has long been considered too insensitive to provide useful data in this regard, but 43 Ca magic angle spinning (MAS) NMR spectra reported here for synthetic tobermorite and jennite with naturally abundant levels of 43 Ca demonstrate that this is a viable approach. We show that spectra with useful signal/noise ratios can be obtained in a reasonable acquisition period (∼2 days) using an H 0 field strength of 21.1 T, 5 mm rotors spinning at a frequency of 5 kHz, and a double frequency sweep preparatory pulse sequence. Tobermorite and jennite produce relatively broad resonances due to their complex structures and structural disorder, however, the chemical shift differences between six‐coordinate 43 Ca in jennite and seven‐coordinate 43 Ca in 11 Å tobermorite are large enough that the signals are entirely resolved at this field. These data suggest that signal from ideal tobermorite‐like and jennite‐like sites in cement C–S–H can most likely be distinguished by 43 Ca NMR and that this method will be a powerful approach for studying cement‐based ceramic materials in the coming decade.