The rate of pozzolanic reactivity of pyrogenic nanosilicas as compared with nanosilica sols

Pyrogenic nanosilicas and nanosilica sols are two types of nanosilicas, which have been investigated in numerous studies of cement-based materials. Different production process of these materials leads to different characteristics of them. For instance, nanosilica sols are monodispersed particles in water; while, in the production process of pyrogenic nanosilicas, particles fuse together and form primary aggregates with sizes up to 100 nanometers. The primary aggregates also bind together and form agglomerates with sizes from a few micrometers up to a few hundred micrometers. This article presents the results of an investigation on the rate of lime consumption of the product Aerosil 200 (a pyrogenic nanosilica with surface area of 200 m2/g) and Levasil 200/30 (a nanosilica sol with surface area of 200 m2/g and concentration of 30%). The results show that despite the agglomeration state of the pyrgenic nanosilica, this material has faster pozzolanic reactivity than the nanosilica sol in lime and cement pastes. The results also indicate that the nanosilicas have accelerating influence on the hydration degree of cement at early ages. However, by progress of hydration and from 7 days lower hydration degree of cement compared to the plain paste was observed. It appears that considerable water absorption of nanosilicas is responsible for the lower available water for hydration of cement and consequently lower hydration degree of cement.