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The use of silicafume incombination withfly ash

On Fly Ash

Silica fume from theferro-silicon industryand fly ash from coalfired power plants areboth important inmodern concretetechnology. Used intogether with portlandcement, theycontribute to concretewith selectedproperties.

There are significantdifferences betweenthe products; morethan 600 million tons offly ash are producedannually, while the totalavailabilityof silica

fume is about onetenth of a percent ofthis volume. Also,standards for silica

fume are relativelytight, while variations infly ash performanceparameters reflect the

great variations infeedstock for theburners, in burnertechnology etc.

Fly ash has standardsin many countries. Asignificant problem isthat two ashes, bothmeeting a specificstandard, can give verydifferent performancein concrete. Therefore,strict procedures forevaluation andspecification of ash forconcrete are required.

Fly ash usually isbeneficial in providinglong-tem strength and

Triple Blend With Fly Ash

Elkem Microsilica®

CONCRETE

C5-03Applications

Tsing Ma, Hong Kong

Bandra Worli, MumbaiBandra Worli, Mumbai

Low bridgesHigh bridge

Sub-sea tunnel

Low bridgesHigh bridge

Sub-sea tunnel

Low bridgesHigh bridge

Sub-sea tunnel

Great Belt, Denmarkimpermeability.However, fly ash has alow rate of hydrationwhich means that bothshort term strength andchloride resistance aretypically detrimentallyinfluenced. This slowdevelopment ofproperties is criticalwhen the structure inquestion will beexposed to a chlorideenvironment after 2-3days or less.Fly ash hasdocumented goodperformanceconcerning theresistance to chloridepenetration. Atvolumes of 20 % andup there is a very goodeffect -- after hydrationis complete. Lesservolumes, less effect.The cause ofbeneficiation from flyash is believed to be toa minor part frombetter particle sizedistribution, for themajor part from bindingof chlorides by thealuminium in the fly-ash.

Silica fume and itseffects on chlorideresistance is welldocumented. Alsothere is a lot ofexperience in usingsilica fume to providehigh strength, toreduce heat ofhydration and improvea number of otherproperties of concrete.

There are three mainreasons for using silicafume:In fresh concrete,

silica fume give astable, non-segregating concretean also acts as apumping aid, reducingviscosity in theconcrete and the riskof blocking duringpumping.In hardened concrete,

silica fume can providehigher and morereliable strength thanany other method.In hardened concrete,

the effect of microsilicais to provide highresistance to mostaggressivemechanisms, fromASR through sulphateattack to reinforcementcorrosion due tochlorides

Particle packing is onereason. In the case offly ash, the particle is

often finer that thecement, this meansthat the small silicafume particles canperform better inparticle packing sincethe intermediateparticle space, slightlysmaller than cement, isfilled by the fly ash.The chemical bindingof chlorides by fly ashdue to its content ofaluminium workstogether with the porerefinement due to silicafume to give excellentperformance in achloride environment,

Due to low reactionrate, fly ash has oftenbeen used in HPC toreduce the heat ofhydration and will alsogive good flow in freshconcrete. However,this gives a problem infly ash concrete is theearly age, what to do

until the fly ash hashydrated sufficientlyto have strength and toprotect againstaggressives.

In a triple blend, thesilica fume takes careof properties in theearly age, while fly ashadds its contribution atlater ages

The combination ofsilica fume and fly ashhas been used inpractice andextensively studied.The major projectslisted at the end speakfor themselves, someinformation fromlaboratory studies areimportant.

Bouzoubaâ &al(2002)did a largestudy to study all

Microsilica (SilicaFume)

Triple (ternary)blends - why?

Experience

Laboratory studies

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Coastal viaduct, HK

Triple blends have characterised several major projects done with microsilica.Great Belt, DenmarkØresund, Denmark/SwedenConfederation Bridge, CanadaTsing Ma, Hong Kong

aspects of triple blend.For durability the mainconclusion is that thetriple blendoutperforms the binaryblends, also theaddition of silica fumereduced the sensitivityof fly ash concrete tothe effects ofinsufficient curing.Several otherproperties eitherimproved or remainedthe same.

Lynsdale and Khanstudied chloride andoxygen permeability oftriple blends. Theirmain conclusion is theternary blends enablednegligible chloridetransport even at earlyages, both fly ash andsilica fumecontributing. At low w/bwith 10% silica fume,15-20% fly ash gavethe lowest chloridetransport of the tests.

Nassim andSuksawang (2003) intheir verycomprehensive studyhas a main conclusion:“Combining silica fumeand fly ash enhancesthe durability andmechanical propertiesof HPC. In fact, it ishighly recommendedthat a minimum of 5percent silica fume be

added to fly ashconcrete to improve itsdurability. Moreover,the ductility of concreteincreases whencomparing to ACIrecommendation. “

Ramakrishnan and Sigl(2001) is anothercomprehensive studythat also includesstructures built usingthe triple blendtechnology. In theirsummary they state:The addition of fly ashand silica fumereduced the chloridepermeability ofconcrete significantlywhile increasing thecompressive strength.Based on the analysisof results obtained,one mix was chosen,as the best mix havingall the propertiesrequired for a highperformance bridgedeck. Another highstrength HPC mix wasselected for the girdersto satisfy the strengthrequirements for theearly release ofprestress strands andat 28 days. The totalcost of the HPCbridges and thestandard SDDOT(South DakotaDepartment ofTransport) presentdesign bridges is

almost the same.However the life-cyclecost may be cheaperbecause of theanticipated longer lifeand reducedmaintenance costs forthe HPC bridges.

Elkem ASA, Materials

P.O.Box 8126, Vaagsbygd

N-4675 Kristiansand

Norway

Telephone: +47 38 01 75 00

Telefax: +47 38 01 49 70

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e-mail: microsilica.materials@elkem.no

Elkem Microsilica is a registered trademark and belongs to Elkem Materials®

CONCRETE REFERENCE PROJECT MARCH 2002 C4-28

ElkemMaterials

E

Bandra Worli in MumbaiAs an example, these are performance data from pile-caps in the Bandra Worli project

Chloride Ion penetration- ASTM C 1202: 600 CoulombsWater Permeability (DIN 1048): NilMaximum temperature at the core: 68 C.Max. temperature difference < 20 C.

°°

Bandra Worli Pile Caps

Compressive strength

20

40

60

80

0 10 20 30 40 50 60

DaysM

Pa

Concrete specificationkg/m

Cement (53 Grade) 300Micro silica 40Flyash 196Coarse aggregate 20mm 577Coarse aggregate 10mm 500Natural Sand 423Crushed Sand 327Free water (litres) 134Water Binder ratio 0.25Admixture (litres) 13.4

3

References

N. Bouzoubaâ, A. Bilodeau, V. Sivasundaram, and B.Fournier Development of Ternary Blends for High-Performance Concrete MATERIALS TECHNOLOGYLABORATORY MTL 2002-5 (TR-R) July 2002

Moncef Nehdi. Ternary and Quaternary CementsSustainable Development. Concrete international / APRIL2001

PKshemendra Nath. P, MICT High Performance MicrosilicaConcrete For Bandra-Worli Sealink, Mumbai; India. TheInstitute Of Concrete Technology. Annual TechnicalSymposium Concrete For A New World 5 April 2005

B J Magee, M R Jones R K Dhir Performance Of ConcreteContaining Ternary Binders In Chloride-Laden EnvironmentsProc. Dundee conference

S.P. Pandeya,*, A.K. Singha, R.L. Sharmab, A.K. TiwariaStudies on high-performance blended/multiblendedcements and their durability characteristics Cement andConcrete Research 33 (2003) 14331436

G. J. Lynsdale and M. I. Khan Chloride and OxygenPermeability of Concrete Incorporating Fly Ash and SilicaFume in Ternary Systems SP 192-45

M.D.A. Thomas, M.H. Shehata, S.G. Shashiprakash, D.S.Hopkins, K. Cail Use of ternary cementitious systemscontaining silica fume and fly ash in concrete Cementand Concrete Research 29 (1999) 1207-1214

P. F. McGrath and R. D. Hooton Influence of BinderComposition on Chloride Penetration Resistance ofConcrete SP 170-16

M.R. Jones, R.K. Dhir and B.J. Magee Concrete ContainingTernary Blended Binders: Resistance To Chloride IngressAnd Carbonation Cement and Concrete Research, Vol.27, No. 6, pp. 825-831, 1997

Shondeep L. Sarkar, Moussa Baalbaki, and Pierre-ClaudeAitcin. Microstructural Development in a High-StrengthConcrete Containing a Ternary Cementitious System. 1991by the American Society for Testing and Materials (CementConcrete and Aggregates)

Hani Nassif , Nakin Suksawang, Anthony Chmiel.Development of High-Performance Concrete forTransportation Structures in New Jersey.FINAL REPORTAugust 2003. FHWA-NJ-2003-016

Dr. V. Ramakrishnan, Dr. Arden Sigl Evaluation of HighPerformance Concrete in Four Bridge Decks as well asPrestressed Girders for Two Bridges Final Report Study SD98-