11-04-2012, 10:57 AM
Silica Fume in Concrete
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Silica Fume
Very fine noncrystalline silica
produced in electric arc furnaces as
a byproduct of the production of
elemental silicon or alloys containing
silicon; also known as condensed
silica fume or microsilica.
Silica Fume Summary
• Smoke by-product from furnaces
used in the production of ferrosilicon
and silicon metals
• Amorphous silica with high SiO2
content, extremely small particle size,
and large surface area
Silica Fume is NOT:
• Precipitated silica
• Fumed silica
• Gel silica
• Colloidal silica
Silica Fume Health Issues
The committee is not aware of any
reported health-related problems
associated with the use of silica fume
in concrete.
• Silica flour
• Highly reactive pozzolan used to
improve mortar and concrete
Product Characteristics --As-Produced Silica Fume
• As produced directly from bag house
• Extremely fine and dusty
• Difficult to handle pneumatically -- sticky
• Self agglomerating with a tendency to
create small weak lumps
• Low density yields small loads (8 - 10 tons)
(7 - 9 Mg) in bulk tankers
Physical Effect
The carbon black and plain cement
mixes showed comparable strengths
at both 7 and 28 days, even though
the carbon black mixes contained 10
percent less cement (by mass) ...
physical mechanisms do play a
significant role, particularly at early
ages.
The Transition Zone
The transition zone is a thin layer
between the bulk hydrated cement
paste and the aggregate particles in
concrete. This zone is the weakest
component in concrete, and it is also
the most permeable area. Silica fume
plays a significant role in the transition
zone through both its physical and
chemical effects.