what is silica fumes or micro silica? and whr is it used?
wat r its advantages?
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Answer Posted / microsilica
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 percement grain (Figure
2).Class A SF (in accordance with an amendment to BS EN13263
[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
beyondthe scope of this paper.
SF functions in a concrete as a highly efficient pozzolan,
that is, it reacts chemically with the calcium hydroxide
produced by the hydration of the Portland cement (PC) to
form calcium silicate hydrates (C-S-H) which bind the
concrete together. SF is highly reactive due to the high
proportion of non-crystalline SiO2 and the large surface
area.In bulk, SF is generally dark grey to black or off-
white in colour and can be supplied as a densified powder
or slurry depending on the application and the available
handling facilities. SF is available
globally.Specifications for factory-produced cements
containing SF (referred to as CEM II/A-D) are included in
BS EN197-1[3]. Specifications are also given for its use as
a Type II (ie pozzolanic or latently hydraulic) addition to
concrete in BS EN 206-1[4] and BS 8500[5].There is no
health hazard associated with the use of SF as non-
crystalline silica is non-hazardous.
The dense microstructure of concrete containing SF leads to
major improvements in mechanical performance and resistance
to chemicals (such as acids, fuel oil, chlorides and
sulfates). Characteristics of hardened SF concrete
include:Lower permeability and improved durability (due to
the fine particle size and reactivity of SF)Greater
resistance to abrasion and impact than conventional
concretes of similar strength grade Compressive strengths
in excess of 60 N/mm2 are easily achieved. Higher flexural
strength and modulus of elasticity than conventional
concretes of equal compressive strength SF can be used as
an ingredient in high performance concretes containing
micro-fibres to combat explosive spalling during exposure
to fire. A properly designed SF high performance concrete
containing micro-fibres with a low water/cement ratio will
outperform conventional
concretes in terms of resistance to spalling during fire
(ACI 216)[7]Environmental benefits (due to reduced cement
contents and improved service life).
SF is ideally suited to the most demanding
applications,such as concrete slipways, dam spillways and
hardstandings, where chloride, chemical or abrasion
resistance are required. SF concretes have performed well
under these circumstances, as they are chemically stable
and have very low permeability.
Note：http://www.chinasilicafume.com
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