Effects of nanosilica additions on cement pastes

Some interesting experimental results derived from the addition of nanosilica particles into cement pastes are reported. The addition of nanosilica to cement pastes has been carried out, either in powder form or as a colloidal suspension (stabilised by a dispersant), and has been compared to the addition of microsilica. It has been found that nanosilica additions improve the mechanical properties (compressive strength) of the resulting cement paste with respect to additions of microsilica. Furthermore, it has been seen that this effect cannot stem from a pozzolanic reaction. In fact, XR-Diffraction studies have confirmed that, contrary to the case of microsilica or silica fume, in the case of nanosilica, the consumption of Ca(OH)₂ is completely negligible. Consequently, the improvement of the mechanical properties must be attributed to a new and yet unidentified process. Some ²⁹Si NMR experiments have been carried out, as it is expected that this NMR study could lead to an understanding of this intriguing result. It is noteworthy that these experiments suggest that nanosilica promotes the growth of Si chains, and thereby the formation of denser structures.