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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.
-- ACI 116R
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•Smoke by-product from furnaces used in the production of ferrosilicon and silicon metals
•Amorphous silica with high SiO2content, extremely small particle size, and large surface area
•Highly reactive pozzolan used to improve mortar and concrete
Silica Fume Summary
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Silica Fume (AKA)
•Condensed silica fume•Microsilica
• “Micropoz” (trademark)•Silica dust
•Volatilized silica
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Silica Fume is NOT:
•Precipitated silica•Fumed silica•Gel silica
•Colloidal silica•Silica flour
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Silica Fume Health Issues
The committee is not aware of any reported health-related problems
associated with the use of silica fume in concrete.
--ACI 234R
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•Silica fume may contain trace amounts of crystalline quartz --requires warnings on bags
•Treat as respirable dust•Refer to materials safety data sheets
(MSDS) for safety measures
Silica Fume Health Issues
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Silica Fume Production
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Coal and quartz
Wood chips
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Quartz gravel
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Metals That ProduceSilica Fume
•Silicon metal - typically greater than 97% silicon
•Ferrosilicon alloys -ranging from 40 to 90% silicon alloyed with iron
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Silica Fume Product Forms
•As-produced powder•Water-based slurry•Densified •Blended silica-fume cement•Pelletized
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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
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Product Characteristics --Silica-Fume Slurry
• 50 - 52% solids (as-produced silica fume dispersed in water)
• Storage tanks require agitation and protection from freezing
• Transported in bulk tankers 4,000 gallons (12 tons of silica fume) (15 kL, 10 Mg)
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Product Characteristics --Densified Silica-Fume
•Reversible agglomeration process• Flows well pneumatically• Bulk transportation is economical, 22
tons (20 Mg) on a bulk tanker• Product density can be controlled for
handling conditions and applications
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Product Characteristics --Blended Silica-Fume Cement
• Primarily used in Northeastern Canada, limited availability in US
• Fixed silica fume content of 7.5% to 8.5%• Produced from as-produced, densified, or
pelletized silica fume• Portland cement-silica fume blend is primary
product. One blend of silica fume, fly ash, and portland cement now being marketed
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Product Characteristics --Pelletized Silica Fume
•Dustless•Non-reversible agglomeration• Small pellets, typically 3/8 to 1 inch (10 to
25 mm) diameter•Utilized in interground silica fume blended
cement•Not suitable for direct use in concrete!
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Premium -- White Standard -- Grey
Silica Fume Colors
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Silica Fume --Chemical Properties
AmorphousSilicon dioxide > 85%
Trace elements depending upon type of fume
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Particle size (typical) < 4 x 10 -6 in.Bulk density
(as-produced) 8 to 27 lb/ft3
(slurry) 11 to 12 lb/gal(densified) 30 to 45 lb/ft3
Specific gravity 2.2Surface area (BET) 60,000 to
150,000 ft2/lb
Silica Fume -- Physical Properties
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Particle size (typical) < 1 µ mBulk density
(as-produced) 130 to 430 kg/m3
(slurry) 1320 to 1440 kg/m3
(densified) 480 to 720 kg/m3
Specific gravity 2.2Surface area (BET) 13,000 to
30,000 m2/kg
Silica Fume -- Physical Properties
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0.020.040.060.080.0
100.0
0.1 0.3 0.5 0.7 0.9Diameter, micrometers
Per
cent
pas
sing
Typical particle size distribution
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Comparison of Chemical and Physical Characteristics -- Silica
Fume, Fly Ash and Cement
Comparison of Chemical and Physical Characteristics -- Silica
Fume, Fly Ash and CementSilica FumeSilica Fume Fly AshFly Ash CementCement
SiO2 Content 8585-- 9797 35 35 -- 4848 20 20 --2525
Surface Area m2/kg 17,000 17,000 -- 30,00030,000 400 400 -- 700700 300 300 -- 500500
Pozzolanic Activity(with cement, %) 120 120 -- 210210 85 85 -- 110110 n/an/a
Pozzolanic Activity(with lime, psi) (MPa) 1,200 1,200 -- 1,660 1,660 800 800 -- 1,000 1,000 n/a n/a
(8.3 (8.3 -- 11.4)11.4) (5.5 (5.5 -- 6.9)6.9)
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How Does Silica Fume Work in Concrete?
Physical effectChemical effect
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Silica Fume: Physical Effect
The presence of any type of very small particles will improve
concrete properties. This effect is termed either “particle packing” or
“micro filling”.
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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.
-- Detwiler and MehtaACI Materials Journal
Physical Effect
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Silica Fume: Chemical Effect
Silica fume is simply a very effective pozzolanic material
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A siliceous or siliceous and aluminousmaterial, which in itself possess little or no cementitious value but will, in finely divided form and in the presence of
moisture , chemically react with calcium hydroxide at ordinary temperatures to
form compounds possessing cementitious properties.
-- ACI 116R
What is a Pozzolan?
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portland cement + water=
calcium silicate hydrate+
calcium hydroxide
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pozzolan + calcium hydroxide+
water=
calcium silicate hydrate
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The Transition ZoneThe 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.
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