Composition
Condensed silica
fume consists primarily of pure silica in non-crystalline form and contains
more than 90% silicon dioxide (Detwiler et al., 1996). Small amounts of
magnesium, iron, and alkali oxides are also present. Most particles are
smaller than 1 mm with an average diameter of
0.1 mm. ASTM C 1240 limits the percent retained
on the 45-mm sieve to 10%, the moisture content
to 3%, and the loss on ignition (LOI) to 6%.
Replacement
Condensed silica fume is used as a replacement for portland cement. Silica fume can replace portland cement in the range of 9-15% by mass of cement.
Advantages
Using silica fume concrete creates the following advantages:
Since it has a high surface area, condensed silica fume increases the amount of water needed in the mixture, so it is recommended to use it along with a high-range water-reducing admixture. The condensed silica fume/portland cement mixture must be protected for the first 7 days from drying shrinkage. To optimize the benefits resulting from the use of condensed silica fume, it must be thoroughly dispersed in the cement mixture. A disadvantage to utilizing silica fume in cement is the cost. Condensed silica fume costs from $400-$1,000/ton, which is significantly higher than portland cement ($90/ton). When using this or any other alternative cementing material with portland cement, it is necessary to create trial mixtures to ensure proper proportioning for the desired properties.