Effects of nanosilica on the calcium silicate hydrates in Portland cement–fly ash systems

Cementitious materials have a complex chemistry and naturally form nanostructures in the hydration process, a network of calcium silicate hydrates. It is considered that nanoparticles such as nanosilica could act as a pozzolanic material as well as a seeding agent for nucleation and acceleration of the formation of the calcium silicate hydrate network. This work evaluates the effect of nanosilica on the calcium silicate hydrate network and microstructure of hardened ternary, quaternary and quinary system Portland cement based pastes. The quinary system, containing Portland cement, limestone, fly ash, microsilica and nanosilica, and ternary combinations (Portland cement, limestone and fly ash) showed mean calcium/silicon atomic ratios of the calcium silicate hydrate gel in the 28 d old hardened paste of 1·2 and 2·3 respectively. Fourier transform infrared spectroscopy results show the presence of the bridging silicate tetrahedra (Q2) characteristic of a peak at around 980 cm−1 and a shoulder at around 1060 cm−1 in the calcium silicate hydrate gel network of the ternary, quaternary and quinary combinations; these bands are thus more pronounced for the nanosilica-enhanced formulations. The sample obtained in the presence of microsilica and nanosilica (quinary combination) showed evidence of a more intricate calcium silicate hydrate gel network (bridging tetrahedra) characteristic of a honeycomb-like structure opposed to the ternary combination (control sample).