Comb-Shaped Polymers as Nanostructure Modifiers of Calcium Silicate Hydrate: A 29Si Solid-State NMR Investigation

Calcium silicate hydrate gel (C-S-H) is the complex phase mostly responsible for the binding properties and the mechanical resistance of Portland cement. The clarification of the C-S-H nanostructure and how the presence of organic additives affects it is still an intriguing and not trivial task, especially due to C-S-H scarce crystallinity and intrinsic complexity. In this work, we exploited Si-29 solid-state nuclear magnetic resonance (NMR) to investigate the effects of different comb-shaped superplasticizers on the silicate structure. The analysis of Si-29 solid-state NMR spectra shows that the additives increase the degree of polymerization and hence the average length of the silicate chains in C-S-H. This finding correlates well with the increase of the globule dimensions estimated by means of small angle scattering techniques showing that the comb-shaped polymers are able to tune the overall dimension of the C-S-H globule. This effect is dependent on the molecular architecture of the superplasticizer and allows a molecular imprinting to the globular structure of the C-S-H gel.