Sodium Sites and Hydration State in C–S–H Phases Synthesized under Alkaline Conditions from ¹H and ²³Na NMR Experiments

Alkali ions play a crucial role in cement hydration as they are the first species to dissolve, resulting in a pH increase, which promotes the hydration reactions. The location of the alkalis after hydration is less understood; however, it is known that significant amounts are associated with the principal binding component, the calcium-silicate-hydrate (C-S-H) phase. The present work investigates the potential of 23Na NMR to study adsorption, incorporation, and hydration states of sodium in pure C-S-H phases with Ca/Si ratios of 0.8 and 1.4, synthesized from Ca(OH)2 and silica in 0.5 M NaOH solutions. The 23Na NMR spectra (14.1 T) of the synthesized samples are dominated by a featureless resonance from sodium coordinated to water in [Na(H2O)6]+ complex ions. Dehydration of the samples in an oven, by long-term MAS spinning, or by storage at different humidities results in a systematic shift of the main resonance toward lower frequency, reflecting sequential removal of H2O from the sodium hydration shell. Lineshape analyses reveal that the low-frequency shifts result from increasing quadrupole couplings for constant isotropic chemical shifts. Rehydration of the samples shows that water can be incorporated into the hydration shell again. 23Na MAS and MQMAS NMR spectra at different magnetic fields (14.1 and 22.3 T) allow identification of three distinct sodium environments in the C-S-H samples, for which 23Na isotropic chemical shifts and quadrupolar product parameters have been determined with good accuracy. The three resonances are assigned to a surface Na site and two interlayer Na sites in the C-S-H structure.