C – S – H Gels: Interpretation of ²⁹ Si MAS ‐ NMR Spectra

The detection of bridging tetrahedra, or Q 2 ( L ), in the silica chains constituting C – S – H gels is controversial. While some authors maintain that 29 Si MAS ‐ NMR can be used to distinguish between Q 1 and Q 2 units but not between the silicon environments associated with Q 2 (intermediate position) and Q 2 ( L ) (intermediate and bridging position) units, others claim the contrary. The present article addresses this issue with a report on 29 Si MAS ‐ NMR studies of a number of calcium silicate hydrate gels. The findings showed that nuclear magnetic resonance can differentiate between Q 2 and Q 2 ( L ) units in highly polymerized C – S – H gels with mean chain lengths ( MCL ) of over 12 and Ca / Si ratios of <1. At higher Ca / Si ratios, however, the MCL declined, with the concomitant rise in Q 1 units. Under these conditions, in which it was practically impossible to distinguish between Q 2 and Q 2 ( L ), the presence of a single signal for all Q 2 units was assumed.