Silica Fume in ConcreteSilica 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• 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 concreteSilica Fume SummarySilica Fume (AKA)• Condensed silica fume• Microsilica• “Micropoz” (trademark)• Silica dust• Volatilized silicaSilica Fume is NOT:• Precipitated silica• Fumed silica• Gel silica• Colloidal silica• Silica flourSilica Fume Health IssuesThe committee is not aware of any reported health-related problems associated with the use of silica fume in concrete.--ACI 234R• 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 measuresSilica Fume Health IssuesSilica Fume ProductionCoal and quartzWood chipsQuartz gravelMetals That ProduceSilica Fume• Silicon metal - typically greater than 97% silicon• Ferrosilicon alloys -ranging from 40 to 90% silicon alloyed with ironSilica Fume Product Forms• As-produced powder• Water-based slurry• Densified • Blended silica-fume cement• PelletizedProduct 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 tankersProduct 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)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 applicationsProduct 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 marketedProduct 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!Premium -- WhiteStandard -- GreySilica Fume ColorsSilica Fume --Chemical Propertiesz Amorphousz Silicon dioxide > 85%z Trace elements depending upon type of fumeParticle 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/ft3Specific gravity 2.2Surface area (BET) 60,000 to 150,000 ft2/lbSilica Fume -- Physical PropertiesParticle size (typical) < 1 µ mBulk density(as-produced) 130 to 430 kg/m3(slurry) 1320 to 1440 kg/m3(densified) 480 to 720 kg/m3Specific gravity 2.2Surface area (BET) 13,000 to 30,000 m2/kgSilica Fume -- Physical Properties0.020.040.060.080.0100.00.1 0.3 0.5 0.7 0.9Diameter, micrometersPercent passingTypical particle size distributionComparison of Chemical and Physical Characteristics -- Silica Fume, Fly Ash and CementComparison of Chemical and Physical Characteristics -- Silica Fume, Fly Ash and CementSilica FumeSilica FumeFly AshFly AshCementCementSiO2Content8585--979735 35 --484820 20 --2525Surface Area m2/kg17,000 17,000 --30,00030,000400 400 --700700300 300 --500500Pozzolanic Activity(with cement, %)120 120 --21021085 85 --110110n/an/aPozzolanic 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)How Does Silica Fume Work in Concrete?zPhysical effectzChemical effectSilica Fume: Physical EffectThe presence of any type of very small particles will improve concrete properties. This effect is termed either “particle packing” or “micro filling”.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 JournalPhysical EffectSilica Fume: Chemical EffectSilica fume is simply a very effective pozzolanic materialA 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 116RWhat is a Pozzolan?portland cement + water=calcium silicate hydrate+calcium hydroxidepozzolan + calcium hydroxide+water=calcium silicate hydrateThe 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