The benefits of using silica sol of 3-150 nm in diameter in self-compacting concrete are discussed. The addition of polycarboxylate superplasticisers to self-compacting concrete to reduce the water requirements increases segregation and bleed, requiring additions of chemicals such as cellulose ethers, welen, guar and xanthan gum for control. The resulting concrete is sticky, heavy, slow-flowing, with a high yield value and difficult to place and clean. Silica sol may be used as an alternative to these organic compounds, the rapid formation of a calcium-silicate-hydrate gel giving a concrete which is more stable in the fluid state and of enhanced strength when hardened. Sticky, slow-flowing mixes are avoided. Its use enables a reduction in both cement content and fines whilst still achieving a specification of 30 MPa at 28 days. 5 refs.