Type and characteristics of silica fume
There are several types of silica fume. All product forms have advantages and disadvantages that may affect concrete performance, material handling, efficiency, and product addition rates. The treatment of materials has a deep relationship with the type of production, Material handling methods have been developed to use silica fume in its as-produced form, densified or compacted form, and as water-based slurry.
As-produced silica fume
Silica fume, as collected, is an extremely fine powder. This material is referred to as “as-produced silica fume.” Due to its extreme fineness and low bulk density, there may be serious processing problems. The flow of some as-product silica fume particles is very difficult, causing blockages in pneumatic conveying equipment, sticky storage silos, etc., or other problems associated with production. Partial problems can be overcome by properly designing the loading, transport, storage and dosing systems.
The silica fume is directly discharged into the truck mixer and added to the concrete. However, this method is not popular because of the dust generated and high labor costs. Since as-product silica fume is more difficult to handle than other forms of silica fume and transportation costs are higher, silica fume is not widely used in ready-mixed concrete.
To overcome the difficulties associated with transporting and handling the as-produced silica fume, some suppliers have concentrated on supplying silica fume as a water-based slurry. Silica fume slurry usually contains 42 – 60% silica fume. The 50% solids slurry will contain approximately 700 kg/m3 of material, while the as-produced silica fume is 130 to 430 kg/m3. Despite the high water content, the slurry is usually transported more than as-produced silica fume economic.
Densified (compacted) silica fume
Dry, densified(or compacted) silica-fume products are also available. These products are dense enough to be transported economically. They may be handled like portland cement or fly ash at a concrete plant. The densification process greatly reduces the dust associated with the as-produced silica fume.
One method of producing densified silica fume is to place the as-produced silica fume into a silo. Compressed air is blown in from the bottom of the silo, causing the particles to roll. When the particles roll, they will collect. The heavier agglomerates fall to the bottom of the silo and are collected periodically. Because the agglomerates are held relatively weakly together, they decompose with mixing and agitation during concrete production. Most of the published information and the densified silica fume used in the field are derived from air densification.
Another method of producing densified silica fume is mechanically compressed material. Although mechanically densified silica fume has been available in the US market, we do not currently know of any mechanically densified silica fume in the world.
Pelletized silica fume
As-produced silica fume may also be pelletized by mixing the silica fume with a small amount of water and often a little cement, typically on a disk pelletizer. The method can be made into pellets of various sizes and can be processed in landfills. In Norway, granules have been used as fillers for retaining walls in private and public areas. Granulation is an irreversible process – particles are too difficult to decompose during concrete production.
Pelletized silica fume is not used as a mixture of concrete; however, a Canadian cement producer mixes granules with Portland cement clinker to form a mixed cement. Based on the published data, the mixed cement containing granular silica fume and the mixed cement directly added with silica fume were compared. They concluded that the mechanical properties and durability characteristics of silica fume concrete are different regardless of the product form of silica fume. It is similar.