Effects, and Precautions for Use of Silica Fume - ACC

About silica fume

Silica fume is amorphous, powdered byproduct obtained from the smelting process of ferrosilicon, industrial silicon by a baghouse.

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The average particle size of silica fume is 0.1-0.15 μm, which is a few percent of the average particle size of cement. And it has an extremely strong surface activity with a specific surface area of 15-27 m2/g. The unique properties of silica fume make it widely used in applications such as marine, harbor terminals, subways, tunnels, bridges, high-rise buildings, chemical industry. It mainly improves the flowability and denseness of the product and significantly increases the high-temperature strength and thermal shock properties.

A large amount of silica fume is used in cement or as a concrete admixture to improve their performance, formulate concrete with ultra-high strength, wear resistance, scour resistance, corrosion resistance, and anti-penetration, anti-freeze, early strength characteristics, for dams, large reservoirs, hydropower, seaport terminals, railroad bridges, highways, runways of airports, tunnels and skyscrapers and other projects.

Silica fume can also be used in the production of refractory materials and ceramic products to improve the strength and durability of products, and used in paints, coatings, resins, rubber and other polymer materials fillers, it can play a role in improving the overall performance of materials.

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Effects and advantages of silica fume

For concrete

• Significantly improve the strength and flowability of concrete.
• To manufacture low-permeability, waterproof concrete for subways, tunnels, basements of high-rise buildings.
• In the manufacture of concrete for environments like marine, silica fume’s high dense properties can effectively prevent the penetration and erosion of sulfate and chloride ions to concrete, avoiding corrosion of reinforcing steel in concrete, thus extending the service life of the concrete.
• In water conservancy and bridge projects, concrete not only needs the aforementioned properties, but also has very demanding requirements on its wear resistance and scour resistance, which makes silica fume an essential ingredient.
• In the production of high early strength concrete for rapid construction.
• In the production of high-strength shotcrete used for support during the construction of tunnels, subways and large foundation structures.
• In the production of concrete for underwater construction projects (e.g. bridge piers, dams, drilling platforms).
• Pavement concrete for highways and large bridges.

For refractory materials

Silica fume applied to refractory materials has the following characteristics.

• Improve the flowability of refractory castables, reduce water addition, making them easy to shape and greatly improving production efficiency.
• Substantial improvement in the denseness and strength of the refractory materials due to the void filling effect of silica fume’s ultra-microstructure.
• Silica fume has high activity, and it is easier to form mullite with alumina trioxide (Al2O3), so that the high temperature strength and thermal shock resistance of refractories are significantly improved. In addition to the common use in refractory castables, it is also being used in a large number of applications in fused cast refractories and sintered refractories.

Silica fume – uses and precautions

For refractory

According to the different properties of refractory raw materials, orthogonal design is carried out and the best formulation is selected through comparative tests. Refractory materials usually use undensified silica fume.

For high performance concrete

• Compatibility: silica fume is well compatible with water reducer, fly ash, mineral powder and fiber.
• Slump: due to its gap-filling effect, the viscosity of concrete is high, so the slump design requirement of concrete mixed with silica fume is 2 to 3 cm higher than that of conventional concrete.
• Dosage: for the manufacture of high-strength concrete and shotcrete, the dosage of silica fume is 6-12% of the amount of cement; for underwater concrete, the dosage is slightly higher; and for the use of silica fume as a pumping aid, the dosage is reduced to about 2%. The final dosage is determined through the mix ratio test.

Adding and mixing

• Densified silica fume is usually used in concrete industries. It is possible to choose manual feeding or automatic feeding.
• Due to the unique ultra-fine characteristics of silica fume, it is required to be added into the aggregate immediately after feeding, and no silica fume powder should be added to the concrete that has already been mixed properly.
• The mixing time of concrete with silica fume is 30-40s longer than that of conventional concrete to obtain good uniformity.
• There are no special requirements for the transportation of concrete mixed with silica fume, which is the same as conventional concrete.

Pouring and curing

• Normally silica fume concrete should be properly vibrated 10-30s.
• Time between successive concrete pours should not exceed 100 min.
• Silica fume concrete curing process to prevent premature evaporation of water is very important. Therefore, in maintenance process, cover the concrete with film, lay the sack, and spray it with water.

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