Silica Fume Final

8/6/2019 Silica Fume Final

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SILICA FUME CONCRETESILICA FUME CONCRETE

&&HIGH VOLUMEHIGH VOLUMEFLY ASH CONCRETEFLY ASH CONCRETE


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SILICASILICA FUMEFUME CONCRETECONCRETE


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SILICA FUME also referred to as Microsilica or Condensed Silica Fume.

It is another material that is used as an artificial pozzolanic admixture.

Reduction of high purity quartz (SiO2) in electric arc furnaces attemperatures in excess of 2000C.

By-product of the manufacture of silicon metal and ferro-silicon alloys.

Silica fume rises as an oxidised vapour.

It cools, condenses and is collected in cloth bags.


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Very fine powder consisting mainly of spherical particles or microspheresof mean diameter about 0.15 microns, with a very high specific surfacearea (15,00025,000 m2/kg).

Each microsphere is on average 100 times smaller than an average cementgrain.

At a typical dosage of 10% by mass of cement, there will be 50,000100,000 SF particles per cement grain.


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TYPESTYPES OFOF SFSF

Class A SF

y Contains at least 85% by mass of non-crystalline SiO2.

Alkalis are present only in small amounts (< 1%).

Class B SF

y Has a SiO2 limit of 75%.


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FUNCTIONFUNCTION OFOF SFSF ININ CONCRETECONCRETE

y SF functions in a concrete as a highly efficient pozzolan

y Reacts chemically with the calcium hydroxide producedby the hydration of the Portland cement (PC) to formcalcium silicate hydrates (C-S-H) which bind theconcrete together.

y Highly reactive due to the high proportion of non-crystalline SiO2 and the large surface area.


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CHARACTERISTICSCHARACTERISTICS

y In bulk, SF is generally dark grey to black or off-white incolour

y Can be supplied as a densified powder or slurrydepending on the application and the available handlingfacilities.

y SF is available globally.

y There is no health hazard associated with the use of SFas non-crystalline silica is non-hazardous.


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USESUSES ANDAND BENEFITSBENEFITS OFOF SFSF ININCONCRETECONCRETE

y The appropriate use of SF in concrete can give arange of benefits in design, construction and

performance of many types of concrete structureincluding high-rise buildings, industrial floors,civil engineering and marine structures.

y It can be used for precast and insitu concrete.


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BENEFITSBENEFITS DURINGDURING CONSTRUCTIONCONSTRUCTION

yIncreased cohesiveness of the fresh concrete, which can lead

to improved handling characteristics

yCuring can start earlier as there is no need to wait for bleedwater to dissipate.

yHigh early strength (in excess of 25 N/mm2 at 24 hours).

The dense microstructure of concrete containing SF leads tomajor improvements in mechanical performance andresistance to chemicals (such as acids, fuel oil, chlorides andsulphates).


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CharacteristicsCharacteristics ofof hardenedhardened SFSFconcreteconcrete

y Lower permeability and improved durability (due to the fineparticle size and reactivity of SF)

y Greater resistance to abrasion and impact thanconventional concretes of similar strength grade

y Compressive strengths in excess of 60 N/mm2 are easilyachieved. Higher flexural strength and modulusof elasticitythan conventional concretes of equal compressive strength

y SF can be used as an ingredient in high performanceconcretes containing micro-fibres to combat explosivespalling during exposure to fire. A properly designed SFhigh performance concrete containing micro-fibres with alow water/cement ratio will outperform conventionalconcretes in terms of resistance to spalling during fire.


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Silica fume for use in concrete is available in wet or drySilica fume for use in concrete is available in wet or dryforms. It is usually added during concrete productionforms. It is usually added during concrete productionat a concrete plant as shown in the photoat a concrete plant as shown in the photo


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The greatest cause of concrete deteriorationThe greatest cause of concrete deterioration

in the US today is corrosion induced byin the US today is corrosion induced bydeicing or marine salts. Silicadeicing or marine salts. Silica--fume concretefume concretewith a low water content is highly resistant towith a low water content is highly resistant topenetration by chloride ions.penetration by chloride ions.


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The photo shows the "oneThe photo shows the "one--pass" finishingpass" finishingprocess in which the silicaprocess in which the silica--fume concrete isfume concrete is

placed, consolidated, and textured with littleplaced, consolidated, and textured with littleor no waiting time between operations. Toor no waiting time between operations. Togain the most benefits from using silica fume,gain the most benefits from using silica fume,the concrete must be cured effectively.the concrete must be cured effectively.


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y Environmental benefits (due to reducedcement contents and improved service life).

y ideally suited to the most demandingapplications such as concrete slipways, damspillways and

y hardstandings where chloride, chemical or

abrasion resistance are required.y SF concretes have performed well under

these circumstances, as they are chemicallystable and have very low permeability.


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TrackTrack recordrecord ofof SFSF inin concreteconcrete

y Until the mid-1970s, nearly all SF was discharged into theatmosphere during ferrosilicon manufacture. Once

environmental concerns necessitated the collection andland filling of SF, it became economically justified to use itin various applications, in particular in high performanceconcrete.

y Following the introduction of standards for the material,global consumption now exceeds 300,000 Mt per annum.

y SF was first used in the UK in the early 1980s when majorconstruction projects in the scrap, waste and marineindustries commenced and concretes able to better resistthese aggressive environments were required.


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StandardsStandards andand specificationsspecifications

y The compliance of SF to relevant standards is critical toensure that its performance in concrete meetsexpectations.

y There are a large number of sources of fume material,most of which may not comply with the standards.

y The standards covering SF in mainland Europe, the UK andthe US are:

BS EN 13263 Silica fumes for concrete.x Part 1: 2005 Definitions, requirements and conformity

criteriax Part 2: 2005 Conformity evaluation

ASTM C1240-97b Standard specification for silica fumesfor use as a mineral admixture in hydraulic cement

concrete, mortar, and grout.


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StandardsStandards andand specificationsspecifications

y These require the material to be tested for

compliance at the plant of origin.y Sources of SF that do not meet the

requirements of the standards may causeproblems in concrete relating to rheology,

admixture dosages, strength performanceand permeability.


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PerformancePerformance andand durabilitydurability datadata

y The chemical resistance and durability of SF concrete arethe result of its very low permeability. Impermeable

materials resist the ingress of water, gases, saltsolutions and aggressive liquids.

y Drying shrinkage is also typically reduced.

y However, as with all concretes, proper attention to

design, quality control and curing are paramount toensure that these benefits are achieved.

y SF concretes can achieve specific requirements for 50and 100 year design lives.

y

Permeability data for high performance SF concretes inshows lower water permeability (by at least a factor of10) compared with a typical RC32/40 mix.


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ECONOMICSECONOMICS

y Savings associated with speed of build (due to finishingcharacteristics)

y Use of more slender structural elements can lead tomore rent-generating space

y Potential materials cost savings (through using slenderelements)

y Concrete strength can be tailored to the requiredstrength through the height of a building, which can leadto standardised column formwork throughout thebuilding, simplifying the design and construction process

y SF can bring significant cost and process benefits but itshould be recognised that SF is typically 34 times asexpensive as PC (on a w/w basis).


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KEYKEY FEATURESFEATURES OFOF SFSF ININ CONCRETECONCRETE

Fresh concretey Increased cohesiony Reduced bleedingy Can increase speed of build. Light vehicle

traffic often possible by 24 hours,formwork can be stripped earlier

y Appropriate temperature control to ensurethat the benefits of rapid strength gainare realised.


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KEYKEY FEATURESFEATURES OFOF SFSF ININ CONCRETECONCRETE

Hardened concrete

y Enhanced mechanical properties (including early age

strength)y Increased flexural strength and modulus of elasticity

y Reduced permeability/enhanced durability

y High impact and abrasion resistance

y Exposure classes for 50 and 100 year design life

achievable (dependant on cover to reinforcement)y Increased chemical resistancey Enables production of high performance concrete.


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GeneralGeneral

y Environmental benefits

y

Potential for cost saving, though thematerial costs more than PC, weight forweight.


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HighHigh--strength silica fume concretestrength silica fume concretewas used to stiffen the Key Bankwas used to stiffen the Key BankTower in Cleveland, Ohio.Tower in Cleveland, Ohio.

Silica Fume Final

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