The effect of the addition of nanoparticles of silica on the strength and microstructure of blended Portland cement pastes

The complex behaviour of concrete depends on the properties and response of the main cement hydration products; calcium silicate hydrates (C-S-H), calcium silicate aluminates and calcium hydroxide. C-S-H is nanosized; therefore, the addition of nanoparticles can induce modifications affecting larger scale phenomena. In this research the addition of silica nanoparticles (nS) to blended cement formulations was investigated with the aim of enhancing durability and lowering environmental impact. The hydration products, microstructure and compressive strength of the early and later ages hardened cement paste were compared in two series of cement pastes. The first one contained Portland limestone cement, limestone and nanosilica at 0.1%, 0.5%, 1% and 1.5% by mass of solids and the second series, Portland limestone cement, limestone, fly ash and silica nanoparticles at 0.1%, 0.2%, 0.5% and 1% by mass of solids. The water to binder ratio was kept constant at 0.3 and specimens were tested up to 90 days of age. The size of nanosilica was confirmed by transmission electron microscopy. Investigating the performance of the first series of pastes, it was concluded that nS exhibits a pozzolanic behaviour. In the second series of pastes, the results presented do not suggest depletion of the Ca(OH)2 in the cement paste. The delayed pozzolanic reaction of the fly ash particles was attributed to the addition of silica nanoparticles, which created denser hydration products, acting as ion penetration barriers around fly ash particles. Thermogravimetric analyses and scanning electron microscopy provided a further justification of the hypothesis. Transmission electron microscopy was also used to determine the size of the nanoparticles. The research reported was part of a much broader research project supported by the EU, and involving industrial and academic partners throughout Europe, to investigate nanotechnology enhanced cements.