I. Reasons why silica fume is not widely used in concrete
Silica fume is considered a "high-performance niche material." Although it can significantly improve concrete performance, its application in engineering projects is limited . The core reasons are its high cost, poor construction adaptability, and the higher cost-effectiveness of alternative solutions. Furthermore, application barriers and supply chain factors limit its use to high-grade/specialty concrete, and it is rarely used in ordinary projects. The specific reasons are as follows:
1. The economic cost is too high
The market price of silica fume is 10 to 20 times that of fly ash and 5 to 8 times that of mineral powder. Adding it to ordinary low/medium grade concrete will significantly increase the unit cost, resulting in extremely low cost-effectiveness in engineering projects . It will not be used unless there are special needs.
2. Difficulty in adapting to different job requirements
Silica fume has a large specific surface area and strong water absorption. When added, it will significantly increase the cohesiveness of concrete paste, which can easily lead to problems such as poor fluidity and high pumping resistance. It needs to be combined with high water-reducing polycarboxylate superplasticizer (which further increases costs). It also has high requirements for mix proportions and mixing processes. Ordinary commercial concrete plants are prone to fluctuations in construction quality.
3. The risk of contraction increases significantly
Adding Silica fume alone will significantly increase the drying shrinkage and autogenous shrinkage of concrete. If the mix proportion is not properly controlled or the curing is inadequate, cracks are very likely to occur, which will reduce the durability of the structure. Ordinary projects do not have the professional control conditions and dare not use it easily.
4. Standard engineering projects have no performance requirements
For mainstream projects such as civil buildings and municipal roads, C30 to C40 are the main grades. The combined use of fly ash and mineral powder can meet the requirements for strength, pumpability and foundation durability. There is no need for the ultra-high performance of silica fume , which is considered "excessive performance".
5. The alternative is superior and easier to implement
Fly ash and mineral powder are not only low in cost, but also improve the workability of concrete and reduce the heat of hydration. When combined, they can meet the performance requirements of more than 90% of projects. Moreover, the proportioning is mature and there are no barriers to operation in commercial concrete plants . They are a perfect and affordable alternative to silica fume.
In addition, the application of silica fume is also affected by standards and design guidelines: ordinary concrete design standards do not require the use of silica fume, and design institutes will not take the initiative to design it. Only in the design of high-grade concrete for special projects such as super high-rise buildings, nuclear power plants, and seaports will the addition requirements be clearly specified.
II. Types of concrete for which silica fume must be added/ mandatory
There are explicitly required or mandated by relevant standards or design specifications to contain silica fume in China, for reference by our international counterparts in the concrete industry.
1. Ultra-high strength concrete (the specifications clearly require the addition of active ultrafine admixtures, with silica fume being the preferred choice)
-Grade requirement: C80 and above ultra-high strength concrete (core tube of ultra-high-rise buildings, steel-concrete composite section of long-span bridges, precast ultra-high strength components)
-Standardized by: GB/T 51431, " Technical Specification for Application of Ultra-High Strength Concrete in China ", which requires the addition of active ultrafine admixtures such as silica fume and ultrafine mineral powder, with the silica fume content not less than 5% of the cementitious material. It is the core component for achieving low water-cement ratio and ultra-high strength, and there is no alternative.
2. Concrete for nuclear power plants (mandatory admixture in design to ensure nuclear-grade durability and radiation resistance)
-Applicable parts: Nuclear-grade concrete for nuclear power plant nuclear island foundations, containment structures, reactor buildings, etc.
-Key requirements: extreme resistance to chloride ion penetration, sulfate corrosion, low permeability and volume stability. The standard (" Technical Specification for Concrete Structures of Nuclear Power Plants in China " NB/T 20007) mandates the addition of silica fume at a dosage of 6% to 12%. It is an essential admixture for nuclear-grade concrete and must pass nuclear-grade material certification.
3. High-durability concrete for seaports/marine engineering (mandatory requirement in standards, resistant to harsh marine environments)
-Applicable parts: caissons for seaport terminals, abutments/ piers for cross-sea bridges, linings for subsea tunnels, and concrete structures for offshore platforms.
-Standard basis: JTJ 275, Technical Specification for Corrosion Protection of Concrete Structures in China's Seaport Engineering , mandates the addition of silica fume (or ultrafine mineral powder) to concrete in seawater splash zones , water level fluctuation zones, and underwater zones to enhance its resistance to chloride salt corrosion. In particular, silica fume is the preferred admixture in splash zones (the most severely corroded areas), with a dosage of 5% to 10%.
4. Chemical corrosion-resistant concrete (mandatory admixture in design to resist strong corrosive media)
-Applicable parts: Concrete structures in contact with strong acids, strong alkalis, and sulfates, such as foundations of chemical plant buildings, tank area floors, acid and alkali pools, and anti-corrosion pipe corridors.
-Core requirements: The dense microstructure formed by the filling of Silica fume and the reaction of volcanic ash can effectively block the intrusion of corrosive media. The design drawings will specify the addition of Silica fume , with a dosage of 6%~12%, and it should be used in conjunction with anti-corrosion additives.
-Applicable parts: Initial support of highway/railway tunnels, slope reinforcement dry/wet sprayed concrete.
-Standardized by: GB 50086, Technical Specification for Application of Shotcrete in China . It mandates the addition of Silica fume to early-strength, high-strength, and impermeable shotcrete (grade ≥ C40) at a dosage of 5% to 8%. This can significantly improve the bond strength, early strength, and impermeability of shotcrete, and reduce rebound.
6. Other special projects requiring mandatory admixture
-For super-frost-resistant concrete with a freeze-thaw resistance grade ≥ F300 (such as bridges and water conservancy projects in frigid northern regions), the specifications require the addition of Silica fume to improve density.
-Military-grade special concrete (civil defense projects, national defense works) is designed with mandatory addition of Silica fume to improve structural strength and penetration resistance.
-Prestressed high-strength concrete box girders (long-span bridges) are typically designed with mandatory addition of Silica fume to ensure low shrinkage and high density, thus avoiding prestress loss.
