Effects of nanosilica addition on workability and compressive strength of Portland cement pastes

The effect of nano-size particles of amorphous silica (nanosilica) on the rheological behaviour and mechanical strength development of cementitious mixes is addressed. Mini-slump and rheometric tests were carried out on cement pastes made with three dose levels of nanosilica at different water/binder ratios. Cement paste workability resulted to be significantly lower than expected for the adopted water/binder ratios, as a consequence of instantaneous interactions between nanosilica sol and the liquid phase of cement pastes, which evidenced the formation of gels characterised by a significant water retention capacity. The resulting reduction of the mix workability was avoided by suitable addition procedures of superplasticizers. No appreciable improvement in the compressive strength development of cementitious mixes by nanosilica addition was observed, in contrast with some results from literature. This confirms conflicting experience on the problem, but some parameters affecting the strength development were identified and discussed.