The detection of bridging tetrahedra, or Q2(L), in the silica chains constituting C–S–H gels is controversial. While some authors maintain that 29Si MAS-NMR can be used to distinguish between Q1 and Q2 units but not between the silicon environments associated with Q2 (intermediate position) and Q2(L) (intermediate and bridging position) units, others claim the contrary. The present article addresses this issue with a report on 29Si MAS-NMR studies of a number of calcium silicate hydrate gels. The findings showed that nuclear magnetic resonance can differentiate between Q2 and Q2(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 Q1 units. Under these conditions, in which it was practically impossible to distinguish between Q2 and Q2(L), the presence of a single signal for all Q2 units was assumed.

Supported by the Ministry of Science and Innovation under project BIA2010-17530.

Peer reviewed