Strength and microstructure of colloidal nanosilica enhanced cement pastes.

The macrolevel properties of concrete; strength and durability are dependent on the hydration of cement. The main products of the chemical reactions of cement clinker with water, are calcium silicate hydrates (C-S-H), calcium aluminate hydrates and calcium hydroxide. C-S-H, the major binding and strengthening factor within the hydrating cement paste, is nanosized. Therefore, it is believed that the addition of nanoparticles can modify the characteristics of the C-S-H in nano enhanced pastes, inducing subsequent alterationsat larger scale phenomena. In this research the addition of nanoparticles of silica, referred to as nanosilica(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 (from 1 day until half a year old) hardened cement paste were compared in a series of cement pastes. The reference paste contained Portland limestone cement and additional limestone (summing up to 60% Portland cement and 40% limestone by mass) and the nS enhanced pastes contained nanosilica at 0.1%, 0.5%, 1.0% and 1.5% by mass of solids. The water to binder ratio was kept constant at 0.3.The size of nanosilica was confirmed by transmission electron microscopy to be in the order of 8 nm to 50 nm, diameter. Performance evaluation of the pastes, indicated that nS exhibits a pozzolanic behaviour consuming calcium hydroxide to form additional C-S-H. Additionally, for pastes with 40% substitution of cement by limestone, low nS content (0.1% to 0.5%) is favouring strength gain even at later ages. Thermogravimetric analyses and scanning electron microscopy provided a further justification of the above hypothesis.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.

Author: S. Papatzani, K. Paine, J. Calabria-Holley

Section: Science and production

Keywords: blended cements, nanosilica, cement hydration