Elkem Microsilica®

for Superior Concrete

How to improve strength and durability of concrete

ELKEM MATERIALS GIVES YOU ADVICE ON THE USE

OF ELKEM MICROSILICA.Elkem Microsilica is a mineral additive that improves theproperties of both fresh and hardened concrete.

Elkem Materials has long experience in the use of ElkemMicrosilica in concrete, and can offer a total package to theuser. This package includes advice and guidance in themanufacture of concrete for various purposes and in thenecessary equipment, as well as training in the use of ElkemMicrosilica and technical assistance in the development ofnew applications.

The purpose of this publication is to introduce benefits ofElkem Microsilica and to give general guidance for its properuse as an additive.

BACKGROUND.Elkem Microsilica is a pozzolan. Pozzolans have been usedfor ages. Classical Greeks and Romans used cementsmade from volcanic ash that had pozzolanic properties.

The Coliseum is just one example of the durability and last-ing strength of such ancient concrete structures.

Elkem Microsilica, because of its fineness and high reactiv-ity, is a particularly valuable pozzolanic material. Thus it findswidespread use in the production of quality concrete.

THE PRODUCTION OF ELKEM MICROSILICA.

Elkem Microsilica is manufactured in silicon and ferro-siliconsmelting plants.

Elkem Microsilica has a bulk density of 200-300 kg/m3 fromthe furnace. In this form the material is difficult to handle,store and transport. Therefore it is usually processed furtherby densification or by slurrification.

In the densification process, the bulk density of the materialincreases to more than double the original value.

The other main form of delivery is as a water-based slurry,where, at 50% dry content, one cubic meter of slurry will con-tain about 700 kg Elkem Microsilica.

ELKEM MICROSILICA

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THIS IS ELKEM MICROSILICA.Elkem Microsilica contains 85-98% amorphous silicon diox-ide (SiO2) in the form of microscopic spherical particles. Theaverage diameter of these particles is 0.10 – 0.15 micron;this is about the same as particles in cigarette smoke. Aconsequence of the diminutive particle size is that ElkemMicrosilica has a specific surface area which is about 100times that of cement.

The large surface area and the high content of amorphoussilicon dioxide gives Elkem Microsilica superior pozzolanicproperties.

Since the silicon dioxide in Elkem Microsilica is present inan amorphous, not a crystalline state, Elkem Microsilica isnot considered to be hazardous to health.

WHAT IS CONCRETE?Concrete is the most widely used construction material to-day. About 1 m3/year is used for every man, woman andchild on the planet. This universal acceptance follows fromits basic properties: it is easy to shape, it is readily availablelocally, and it has strength and durability. Concrete containscement, water, aggregates (sand and gravel), chemicaladmixtures and mineral additives. Elkem Microsilica is amineral additive that gives superior properties to concrete.

THE HYDRATION PROCESS WITHOUTELKEM MICROSILICA.

Hydration of Portland cement is the reaction between cementand water which makes calcium silicate hydrates (CSH).CSH is the “gel” or binder that gives concrete its strength.

Calcium hydroxide forms as a by-product in the hydrationprocess. Calcium hydroxide (15-25% of hydration products)is a weak and soluble material that does not contribute to thestrength or the durability of the concrete.

A-Portland cementB-WaterC-CSHD-Calcium hydroxide

CEMENT GRAIN

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ELKEM MICROSILICA IN THE HYDRATION PROCESS.

Elkem Microsilica reacts with the calcium hydroxide thatforms during the hydration of cement. This pozzolanic reac-tion creates an increased amount of CSH binder.

Thus the content of weak calcium hydroxide decreases,while the content of CSH, which provides strength and dura-bility, increases. This is one of the main effects of ElkemMicrosilica.

ELKEM MICROSILICA FILLS THE GAPS INCONCRETE.

One of the basic principles of concrete technology is theneed for proper packing of the particles. The purpose is toobtain the smallest possible volume of empty or water-filledspace in the material. The tiny particles of Elkem Microsilicahelp the particle distribution and the packing of the system.

Very strong surface forces act between the particles ofElkem Microsilica. These forces may prevent proper disper-sion of the particles in the fresh concrete. Accordingly, dis-persing admixtures are value components of microsilica-concrete.

WATER REDUCING ADMIXTURES.Good workability of a concrete often requires more waterthan needed for hydration of the cement. The excess watercreates voids in the concrete.

Water reducing admixtures make it possible to reduce theamount of excess water. These chemicals reduce the inter-particle forces and make the particles more mobile and moredensely packed.

Such admixtures are essential to the manufacture of highperformance concrete, including concrete with ElkemMicrosilica.

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ORDEMs PROGRESSO

AGGREGATES.Concrete contains up to 70% aggregates by volume.Consequently, the quality and properties of the aggregatesare of major importance.

Aggregates are divided into fine and coarse fractions.Occasionally filler materials are also used. This is materialwith a maximum grain size of 0.125 mm.

The small particle size of Elkem Microsilica reduces theneed for fine aggregate and filler.

ELKEM MATERIALS FOR FREEDOM OF CHOICEAND SECURE SUPPLY.

Elkem Materials has production facilities at many sitesaround the world. Distribution facilities for Elkem Microsilicahave been established in several locations.

Our experience shows that the slurry forms of ElkemMicrosilica have several benefits in transport, dispensingand effect in concrete. For some users, other forms of theproduct may be more appropriate. Accordingly, ElkemMicrosilica is also available as powder: densified or undensi-fied.

All products may be shipped in a variety of packaging,adapted to the need of the customer. Several productionand distribution facilities offer the user the freedom of choiceand secure supply.

ELKEM MICROSILICA: STORAGE AND HANDLING.

Elkem Materials can provide help in the design of practicalsolutions for the storage and handling of Elkem Microsilica.

COARSEAGGREGATE

COARSEAGGREGATE

FINEAGGREGATE

FINEAGGREGATE

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CEMENTCEMENT

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MIXING.Elkem Microsilica is available as a slurry or as a dry powder.When using slurry, the mixing time required is roughly thatused for ordinary concrete without microsilica. For dry,densified powder the mixing time should be increased, butthe need for additional mixing will vary with mix design andthe actual mixer used.

CLEANING OF EQUIPMENT.The use of Elkem Microsilica can make concrete morecohesive and “sticky”. This can make the concrete more difficultto remove during cleaning of mixing and other handlingequipment.

Good housekeeping in the production plant will assure lesswear on the equipment and effective blending of the ingredi-ents of the concrete.

The mixer needs careful cleaning frequently, during breaksas well as at the end of the shift. The use of form releaseagents will reduce the labor required to maintain a cleanoperation.

WORKABILITY.The workability of concrete often improves when the slumpincreases. Since addition of microsilica will give fresh con-crete higher shear strength, microsilica concrete can appearstiffer and more difficult to move at a given slump. However,microsilica also decreases viscosity, so that at given slump,microsilica concrete will flow more easily.

If concrete with Elkem Microsilica is to behave as “normal”concrete, one should increase the slump slightly (with ad-mixtures). This can be done because Elkem Microsilica pro-vides a stable fresh concrete with much reduced tendency toseparation and segregation.

In self-compacting concrete and concrete for pumping, theuse of Elkem Microsilica gives improved robustness andstability; therefore improved performance.

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ELKEM MICROSILICA PROVIDESREDUCED SETTLING AND

MORE HOMOGENOUS CONCRETE.A layer of water will normally form on the horizontal surfaceof concrete without Elkem Microsilica. This is the conse-quence of settlement in the concrete and water rising to thesurface. This phenomenon is commonly referred to asbleeding.

The bleed-water will often transport fine particles such ascement and filler, and form a slurry on the surface of theconcrete. After the slurry has dried, this represents a weaklayer on the surface of the concrete.

Too wet or excessively vibrated concrete can segregate.The aggregates sink so that the distribution of particles in theconcrete is irregular. (Bleeding also increases.)

Elkem Microsilica prevents both bleeding and segregation.This is mainly due to the strong internal forces caused by thelarge surface of the particles in Elkem Microsilica, and partlydue to improved particle packing.

ELKEM MICROSILICA MINIMIZESBLEEDING UNDER LARGEAGGREGATE PARTICLES.

Bleeding and settling under large grains of aggregate arecommon in concrete.

The bleeding is reduced or eliminated by the use of ElkemMicrosilica. This gives improved bond between paste andaggregate as well as a less permeable concrete.

ELKEM MICROSILICA PREVENTSBLEEDING UNDER REINFORCEMENT.

Settlement cracks can sometimes be observed over thereinforcing steel in reinforced concrete due to bleeding andsettling (as with large grains of aggregate).

Elkem Microsilica reduces bleeding. This gives improved bondand better protection against corrosion of the reinforcement.

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PROVIDE PROTECTION AGAINSTPLASTIC SHRINKAGE.

When water evaporates from the concrete surface at ahigher rate than it can be renewed from the concrete under-neath, the surface dries. The suction of water creates strongcapillary forces that act on the material and attempt to pullthe particles closer together. If the tensile strength of theconcrete is unable to resist these forces (as will normally bethe case in fresh concrete) the consequence will be cracksthat run from the surface to the interior of the concrete.Since this shrinkage and cracking process takes place whilethe concrete is in a plastic state, the phenomenon is calledplastic shrinkage.

A layer of bleed-water on the surface of fresh concrete mayprotect the concrete against harmful drying.

Since Elkem Microsilica prevents bleeding, concrete withmicrosilica may be subject to plastic shrinkage. The practicewhich eliminates plastic shrinkage is the same for allconcretes: proper curing, such as curing compound, waterspray, cover with plastic sheeting and hessian or ponding.

DEVELOPMENT OF STRENGTH.Curing of concrete begins after the initial set. The rate ofstrength gain is initially large and decreases after the first fewdays. As a rule of thumb, the concrete reaches 80% of its finalstrength after 28 days curing in water at 20 degrees C.

For a given 28-day strength, concrete with Elkem Microsilicamay display a somewhat reduced rate of strength developmentjust after pouring. However, after less than 2 weeks, the rate ofstrength gain will be higher than in concrete without microsilica.The relations between the rates of strength developmentdepend on several factors. These factors include concretequality, type and amount of cement and curing temperature.

HEAT OF HYDRATION.The hydration of cement (its reaction with water) is a processthat releases heat. The pozzolanic reaction between ElkemMicrosilica, calcium hydroxide and water also generatesheat in such a fashion that the hydration of 1 kg. of microsilicausually gives about the same heat as the hydration of 1 kg ofcement. Thus, for the same characteristic strength (cementreplacement at 1 kg microsilica to 3 kg of cement), concretewith Elkem Microsilica will develop less total heat than con-crete without microsilica.

This effect will reduce temperature loads in massive struc-tures, but it also means that concrete with microsilica needsmore attention if poured in winter.

CONCRETESETS

1 hour 5 h 1 day 28 d

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CURING

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EFFECT OF CURING TEMPERATURE.The rate of strength gain depends on the curing temperature.

For a given 28-day strength (i.e. cement replacement by 1part Elkem Microsilica for 3 parts cement), microsilicaconcrete will be more influenced by the curing temperaturethan “ordinary” concrete.

Also, concrete with Elkem Microsilica responds well tocuring at elevated temperatures, but more care must betaken in winter to ensure adequate strength of the concretebefore removal of the formwork.

CONCRETING AT LOW OR ELEVATED TEMPERATURES.

For a given target strength (cement replacement, seeabove), concrete with Elkem Microsilica will develop lessinternal heat than concrete without microsilica. The rate atwhich the properties of concrete develop is also more sensitiveto temperature if the concrete contains Elkem Microsilica.Therefore concrete construction at low temperatures usingElkem Microsilica as cement replacement should include:

- Heating of the microsilica-concrete at the batching plant;and/or

- Added insulation to protect the fresh microsilica-concreteagainst heat loss.

Fresh concrete always needs protection in the form of in-sulation during winter; this is particularly important forconcrete that contains Elkem Microsilica.

At high curing temperatures, the effect is the opposite;concrete with silica fume will gain strength at a higher ratethan comparable conventional concrete.

ELKEM MICROSILICA INCREASESSTRENGTH AND REDUCES PERMEABILITY.

Elkem Microsilica improves particle packing in the freshconcrete, and the pozzolanic reaction leads to a replace-ment of weak calcium hydroxide by strong CSH-gel. Thesetwo effects give concrete with Elkem Microsilica improvedstrength and permeability.

Elkem Microsilica also improves the interface betweenpaste and aggregate; a region where in conventionalconcrete channeling often occurs and calcium hydroxidetends to accumulate.

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ELKEM MICROSILICA MEANS INCREASEDCOMPRESSIVE STRENGTH.

If the cement and water contents of a given concrete areconstant, the addition of Elkem Microsilica means a signifi-cant increase in compressive strength. (In a cementreplacement application, Elkem Microsilica may replacesome of the cement for a given target 28-day strength)

TENSILE STRENGTH.The tensile strength of concrete will be influenced by thepresence of weak zones, such as bleed-pockets undergrains of aggregates. In concrete with Elkem Microsilica thenumber of such areas will be reduced.

However, as a general rule of thumb, microsilica concretehas the same relation between compressive strength andother mechanical properties as does conventional concrete.

ELKEM MICROSILICA PROTECTS AGAINSTCORROSION OF REINFORCEMENT.

The corrosion process has two distinct stages: initiation andpropagation (growth). Initiation means that the protectivepassive layer on the steel surface is destroyed; the two com-mon causes are chloride penetration and carbonation of theconcrete.

Elkem Microsilica in concrete gives a more closed porestructure and the concrete is much less permeable thanconcrete without microsilica.

For the process of corrosion initiation, this means thatmicrosilica concrete is more resistant to the ingress of chlor-ides that can promote corrosion. Elkem Microsilica has noparticular influence on the rate of carbonation.

Regardless of the quality of the concrete, corrosion initia-tion will often take place. One example is reinforcement inthe bottom of cracks.

In the propagation stage, the rate of corrosion is normallygoverned by oxygen transport or internal electrical resist-ance in the concrete.

Concrete with Elkem Microsilica has an internal electricalresistance that is several times that of conventional concrete.The increase in electrical resistance means a correspondingreduction in the possible rate of corrosion.

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ELKEM MICROSILICA FOR REDUCED PERMEABILITY.

A concrete with low permeability is more durable than a morepermeable one. The use of Elkem Microsilica gives very lowpermeability. Concrete with Elkem Microsilica is thereforeparticularly suited to aggressive environments. These includesewage conduits and treatment plants, agricultural applica-tions, industrial environments, marine or coastal structuresand structures exposed to chlorides, such as bridges orparking structures.

ELKEM MICROSILICA IMPROVES THE FROST RESISTANCE OF CONCRETE.

Freezing water in the interior of concrete can expand andcause disruption of the concrete. In the presence of salt onthe concrete surface, the danger of frost damage is particu-larly severe. Frost damage is countered by the use of en-trained air. The air voids provide relief for water escapingfrom water-filled pores where the water is freezing.

There are two reasons for good frost resistance in proper-ly proportioned concrete with Elkem Microsilica:— The stability of the fresh concrete promotes good distribu-tion of the entrained air and prevents the disruption of theair-void system during transportation and handling of theconcrete.— The low permeability of the concrete can prevent waterfrom entering the concrete.

! Note that structural detailing is of outstanding importancein preventing frost attack. Proper drainage prevents pondingon the concrete surface. Ponding means that the concretewill become saturated sooner or later. In saturated concretefrost damage is likely.

ELKEM MICROSILICA PROMOTES RESISTANCE AGAINST CHEMICAL ATTACK.

Most forms of chemical attack on concrete are typified eitherby leaching of calcium hydroxide or by ingress of harmful,disruptive substances, such as sulfates or nitrates.

Since Elkem Microsilica reduces the amount of solublecalcium hydroxide, the risk of leaching is reduced.

The addition of Elkem Microsilica reduces the permeabilityof concrete; this retards the arrival of deleterious substancesto the interior of the concrete.

Consequently, concrete with Elkem microsilica will possesssuperior resistance to chemical attack.

no Microsilicaw Microsilica

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ELKEM MICROSILICA AS A MEASUREAGAINST ALKALI-SILICA REACTION (ASR).

Alkalis in solution and as solids are normal constituents ofhardened concrete. If the aggregates contain reactive silica,these alkalis can, in the presence of water, form a gel byreaction with the silica. The gel can cause internal expansionwhich is deleterious to the concrete.

Elkem Microsilica promotes reduced permeability and ef-fectively binds the alkalis in the hardened concrete. ThusElkem Microsilica eliminates the risk of alkali-silica attack.

In Iceland, ASR was an enormous problem, however allconcrete since 1979 has contained microsilica, and sincethen, ASR has not been a problem.

DRYING SHRINKAGE.Concrete is a porous material that adjusts to the ambient hu-midity. Changes in moisture content influence internalforces of attraction in the material. Cured concrete will there-fore shrink and swell as it exchanges moisture with the en-vironment.

Drying shrinkage is the reduction in volume that is the con-sequence of drying of the concrete. It is usually dis-tinguished between reversible and irreversible shrinkage.The extent of drying shrinkage depends very much on thewater content of the concrete, and there will not be any sig-nificant difference in behavior between concretes with or with-out Elkem Microsilica.

ELKEM MICROSILICA IMPROVES BOND.Elkem Microsilica reduces settling and bleeding. In addition,the packing density of particles improves, including the walleffect on the interface between small and large particles.These factors mean improved bond to embedded materialsand substrate, such as old concrete or rock.

ALKALIS WATER

reactive aggregates

alkali-silica gel

expansion, cracking andconcrete distress

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REMOVAL OF FORMWORK.If Elkem Microsilica is added to an existing mix design, thestrength necessary for removal of formwork will be gainedearlier than for the original concrete without microsilica.

If Elkem Microsilica replaces some of the cement, the usershould be aware that concrete with microsilica has somewhatreduced very early strength (24 hrs). This is particularly thecase when the fresh concrete is cold when poured.

The rate of strength development can be increased by theuse of rapid-hardening cement, by heat treatment and by theuse of insulation to prevent heat loss from the concrete.

ELKEM MICROSILICA MEANSLESS PATCHING.

By using Elkem Microsilica, the slump of concrete can be in-creased significantly without increasing the risk of segregationand bleeding. This means improved compaction of the freshconcrete and accordingly less repair after removal of theformwork.

Joints between pours: Bleeding will often cause the forma-tion of a weak layer on the surface of a horizontal joint ofconcrete. This layer must be removed before the next pourcan be made.

Elkem Microsilica reduces the bleeding to such a degree asto prevent the formation of this weak layer.

VACUUM TREATMENT.Vacuum treatment of a concrete surface removes water fromthe concrete surface and the nearby interior of the concrete.This treatment provides locally improved compaction and re-duction of the water/cement ratio at and near the surface.

Since bleeding is negligible in concrete with Elkem Microsilica,such concrete does not need and should not be exposed tovacuum treatment.

Additionally, since fines can cause clogging and otherproblems in the vacuum process, concrete that is to bevacuum-treated should not have a high content of fines. Thisis another reason why concrete with microsilica is not suitedto vacuum treatment.

Removal of formwork

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ELKEM MICROSILICA FOR BETTER SHOTCRETE.

In Norway Elkem Microsilica has been used in shotcrete formany years. The reason is that Elkem Microsilica enhancesimportant qualities of the shotcrete, such as:

Improved internal cohesion so that the concrete becomesmore sticky and gives improved adhesion, both to base and tounderlying shotcrete.

This means:-Reduced rebound by a factor of 3-5.-The thickness of the layer in each pass can be increased,up to 200 mm or more.-The use of fibers is easier.Since thicker layers can be established and still adhere,

the amount of accelerating admixtures can be reduced. Thismeans that for a given strength, the content of cement need notbe increased to compensate for the effect of the acceleratingadmixture.

The compressive strength is improved.The concrete is easier to pump.The enhanced properties that Elkem Microsilica providesare particularly well utilized in shotcrete technology. These benefits apply in both the wet and the dry processes.

ELKEM MICROSILICA FORUNDERWATER CONCRETING.

For concreting jobs under water, the internal cohesion of aconcrete with Elkem Microsilica means that it has significantreduction in tendency to wash-out.

Since the workability and slump can be increased, the tremiepipe can be sunk further into the in-place concrete thanwhen placing conventional underwater concrete. This furthersimproved filling of the form and limited risk of water intrusion.The displacement of water along the sides of the formworkimproves and the top of the pour will be more level.

QUALITY CONTROL OF CONCRETEWITH ELKEM MICROSILICA.

The quality of concrete with Elkem Microsilica is maintainedwith the same tools and procedures as the quality of con-ventional concrete.

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Elkem – A World Class Producer

Elkem is one of Norway’s largest industrial companies.

Many of today’s most essential technical devices and consumer goodshave components which contain Elkem products. These productsmaintain the highest quality standards and are in demand in a steadilygrowing market.

Elkem’s main products are ferroalloys, silicon metal, aluminium andmicrosilica. Elkem’s production facilities around the world provide theworld market with a wide range of metals and materials through morethan 15 sales offices in 12 countries. Elkem is one of the largestproducers of ferroalloys in the world, particularly with respect toferrosilicon. Elkem is a leading supplier of ferrochrome to the Europeanproducers of stainless steel, and is the western world’s largestmanufacturer of silicon metal to the electronics and chemical industries.

Elkem’s aluminium production plays a major role in a number ofspecialised markets in Europe. The company’s operations are based onclean, replenishable hydroelectric power, and Elkem itself produces asubstantial amount of energy.

Elkem Materials – The World’s LeadingMicrosilica Supplier

Elkem Materials markets Elkem Microsilica®, an ultrafine powder createdduring the production of ferrosilicon and silicon metal.Microsilica is today used in prestigious projects throughout the worldwherever concrete durability is the prime concern.

Elkem Materials is part of Elkem ASA, and was set up in 1982 to marketand promote microsilica into a variety of industries on a world-wide basis.One priority was that the products and services promoted shouldcontribute to raising environmental standards within Elkem and thecountries it operates in. This emphasis continues to be maintained asElkem continuously focuses on:

With its advanced research and development facilities, Elkem Materials iscommitted to further improvements in new product technology and indeveloping new solutions and product improvements for our customers.This commitment is consistent with Elkem Materials’ role in protectingour environment, which will continue to remain a main focus of the com-pany.

ExcellenceInnovationQuality

Elkem Materials – at your service

NS-EN ISO 9001 CERTIFICATED FIRM

NORWEGIANACCREDITATION

No. S03

Disclaimer:

The information given is based on many years of research and field experience and is accurate to the best knowledge of Elkem Materials.

However, due to the numerous factors that can affect theperformance of a concrete, with or without microsilica, ElkemMaterials offers this information without guarantee and accepts noliability for any direct or indirect damage from its use.

If further information or assistance is required, please contact yourlocal representative or the office number provided below.

Elkem Microsilica® is a registered trademark owned by Elkem Materials

Elkem ASA, Materials, P.O.Box 8126, Vaagsbygd, N-4675 Kristiansand – Norway

www.concrete.elkem.com

Representative:

The world’s most comprehensive internet site dedicated to microsilica concrete

Switchboard: +47 38 01 75 00Telefax: +47 38 01 49 70E-mail:microsilica.materials@elkem.no

Elkem Materials is the world’s largest supplier ofmicrosilica and we are the leader in marketing andimplementing microsilica technology worldwide.We also actively participate in internationaldevelopment of technical and quality standards.

Elkem Materials offers a total package to ourcustomers.This includes advice and guidance withrespect to materials handling and technicalassistance in the manufacture of high performanceconcrete for a variety of applications.

To our customers Elkem Materials stands for areliable supply of quality-assured Elkem Microsilica,either through our sales offices around the worldor through local representatives.

50 years of research and practical use have led toElkem Microsilica becoming a key component forthe enhancement of concrete properties such as:

• improved pumpability

• less segregation

• increased strength

• reduced permeability to water and chlorides

• high sulfate resistance

• better protection of steel reinforcement.

Elkem Microsilica is now used in more and moreprojects wherever durability is the prime concern.

Tel.: +965 1820044 - Fax: +965 24582548www.spec-kw.com - sales@spec-kw.com