Silica Fume Concrete
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23-09-2008, 12:31 AM

Silica fume is also referred to as micro silica or condensed silica fume but the term silica fume has been generally accepted. It is a by-product of manufacture of silica and ferrosilicon alloys from high purity quartz and coal in a submerged arc electric furnace. It is reported that the addition of ultra fine particle in HPC improves the strength of concrete. The optimum silica fume proportions in between 20% and 25% by weight of concrete. In it self silica fume does not have any binding properties, but it reacts with calcium hydroxide on hydration of cement and produces the gel i.e. calcium silicatehydrate which has good binding properties. Silica fume has been used in manufacture of high strength concrete.. .
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03-08-2012, 04:00 PM

Silica fume in concrete

.pdf   1Silica fume.pdf (Size: 595.17 KB / Downloads: 195)


Silica fume (SF)* is a by-product of the manufacture of
silicon metal and ferro-silicon alloys (Figure 1). The
process involves the reduction of high purity quartz (SiO2)
in electric arc furnaces at temperatures in excess of
2000°C. SF is a very fine powder consisting mainly of
spherical particles or microspheres of mean diameter
about 0.15 microns, with a very high specific surface area
(15,000–25,000 m2/kg). Each microsphere is on average
100 times smaller than an average cement grain. At a
typical dosage of 10% by mass of cement, there will be
50,000–100,000 SF particles per cement grain (Figure 2).
Class A SF (in accordance with an amendment to BS EN
13263[1]), contains at least 85% by mass of non-crystalline
SiO2. Alkalis are present only in small amounts (< 1%).
Class B fume, which has an SiO2 limit of 75%, is beyond
the scope of this paper.


The appropriate use of SF in concrete can give a range of
benefits in design, construction and performance of many
types of concrete structure – including high-rise buildings,
industrial floors, civil engineering and marine structures
(see Figures 3 and 4). It can be used for precast and insitu
concrete, but this Information Paper concentrates on
the latter.
SF has specific benefits during construction, including:
increased cohesiveness of the fresh concrete, which
can lead to improved handling characteristics
curing can start earlier as there is no need to wait for
bleed water to dissipate. (Bleeding is a form of
segregation where the solid components of the
concrete settle downwards, leaving water on the top
surface. It continues until the cement paste has
stiffened enough to end the settling process)
high early strength (in excess of 25 N/mm2 at
24 hours).

Track record of SF in concrete

Until the mid-1970s, nearly all SF was discharged into the
atmosphere during ferrosilicon manufacture. Once
environmental concerns necessitated the collection and
landfilling of SF, it became economically justified to use it
in various applications, in particular in high performance
concrete[8]. SF was first sampled and characterised in the
1950s and formally adopted into design codes for concrete
during the 1970s. Thereafter its use around the globe.

Batching and mixing

How SF is added to the fresh concrete depends on
whether it is being used in powder or slurry form. For
powder, correct timing of addition is crucial and should
be in accordance with the manufacturer’s instructions or
agreed method statements. Failure to do this could result
in SF forming ‘balls’, inadequate dispersion, and build‑up
of cementitious material in the ready-mixed truck.
Addition of slurry is more straightforward. In a wet batch
plant, SF slurry can be added directly into the pan mixer,
preferably at the same time as the mixing water. One of
the best addition methods involves adding SF through a
water weigher/loading sock. The time at which the
addition is made can depend on the target consistence
or slump of the concrete (for example, half way through
the mixing cycle for lower slump classes such as S2
consistence concrete)*.

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