Q. As a producer, I would like to learn more about adding silica fume to my mix. What are its effects and where can I get more information?
A. Silica fume is a by-product of producing silicon metal or ferrosilicon alloys in smelters using electric arc furnaces. These metals are used in many industrial applications to include aluminum and steel production, computer chip fabrication, and production of silicones, which are widely used in lubricants and sealants.

Silica fume significantly improves the properties of fresh and hardened concrete. The material’s potential use was known as early as the late 1940s. But it wasn’t widely used until the development of powerful dispersants, or high-range water-reducing admixtures, or superplasticizers. Once these admixtures became available, using silica fume in concrete became possible.

Fresh concrete made with silica fume is more cohesive and therefore less prone to segregation than concrete without silica fume. Too offset this increased cohesion when placing, silica fume concrete is typically placed at 40 to 55 mm greater slump than concrete without silica fume in the same placement.

Also, because of the very high surface area of the silica fume and the usually very low water content of silica fume concrete, there will be very little, if any, bleeding. Once silica fume content of about 5% is reached, there will be no bleeding in most concretes.

In addition to improved durability, the lack of bleeding allows a more efficient finishing process to be used with silica fume concrete flatwork.

For more information on silica fume in concrete, the Silica Fume Association has published the Silica Fume User’s Manual. The manual is a comprehensive guide for ready-mix and precast concrete producers, specifiers, and contractors that describes the best practices for the successful use of silica fume in the production of high-strength concrete.

The manual was produced under a cooperative agreement between the Federal Highway Administration and the association and will be of great value to the HPC bridge community.

Copies of the 183-page, soft-cover book are available at no charge by filing out the on-line order form at www.silicafume.org. An electronic version can be downloaded as a searchable PDF file.