supplementary cementitious materials (scm) - national precast concrete association - usa - october,...

8/3/2019 Supplementary Cementitious Materials (SCM) - National Precast Concrete Association - USA - October, 2004 Issue

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(SCMs). Supplementary cementitiousmaterials are often incorporated in theconcrete mix to reduce cement con-tents, improve workability, increasestrength and enhance durability.

Background

The use of SCMs dates back to theancient Greeks who incorporated vol-canic ash with hydraulic lime to createa cementitious mortar. The Greekspassed this knowledge on to theRomans, who constructed such engi-neering marvels as the Roman aque-ducts and the Coliseum, which stillstand today. Early SCMs consisted ofnatural, readily available materials suchas volcanic ash or diatomaceous earth.

More recently, strict air-pollutioncontrols and regulations have producedan abundance of industrial byproductsthat can be used as supplementary

cementitious materials such as fly ash,silica fume and blast furnace slag. Theuse of such byproducts in concreteconstruction not only prevents theseproducts from being land-filled but alsoenhances the properties of concrete inthe fresh and hydrated states.

SCMs can be divided into two cate-gories based on their type of reaction:

hydraulic or pozzolanic. Hydraulicmaterials react directly with water toform cementitious compounds, whilepozzolanic materials chemically reactwith calcium hydroxide (CH), a soluble

reaction product, in the presence ofmoisture to form compounds possess-ing cementing properties. Part 1 of thisarticle focuses specifically on pozzolan-ic SCMs. Part 2 will address hydraulicSCMs, blended SCMs for enhanced per-formance and a summary of SCMs and

their influence on the fresed properties of concrete.

The word pozzolan waderived from a large depoVesuvius volcanic ash locatown of Pozzuoli, Italy. PoSCMs can be used either to the cement or as a replportion of the cement. Mo

SCM will be used to replaof the cement content for property-enhancement reaa brief overview of some ocommon pozzolans used ifactured concrete product

Silica fumeSilica fume is an industr

of high-purity quartz withand wood chips in an elecnace during the productiometal or ferrosilicon alloyremoved from exhaust gasand condenses into ultrafisilica glass. Silica fume hatent of amorphous silicon

percent to 94 percent SiOcal in shape and is extremhaving an average diameteone-tenth of a micron (0.1average silica particle is roone-hundredth the size ofgrain.

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Concrete is by far the most wide-ly used construction materialbecause of its low cost, avail-

ability of raw materials, strength, dura-bility and, most importantly, versatility.Worldwide, more than one ton of con-crete is produced every year for eachperson on the planet; looking at North

America alone, this number jumps toroughly 2.5 tons produced per personper year.

The key to concretes success is itsversatility and no other sector of theconstruction industry utilizes this attrib-ute more than the manufactured con-crete products industry. Concrete canbe designed to withstand the harshestenvironments while taking on the mostinspirational forms. Engineers are con-

tinually pushing the limits with the helpof innovative chemical admixtures andsupplementary cementitious materials

TECHN ICAL

SupplementaryCementitiousMaterialsB Y A D A M D . N E U W A L D

What are SCMs and how can you use them to your advantage?

Part I: Pozzolanic SCMs

This is the first of a two-partseries covering SupplementaryCementitious Materials. Webegin with a discussion onpozzolanic SCMs. Part 2, whichwill focus on hydraulic SCMs,will appear in the January/

February 2005 issue ofMC Magazine.

SCMsPOZZOLANIC VS. HYDRAULIC EFFECTS

Differences in How Various Classes ofSupplementary Materials Respond

Pozzolanic Hydraulic

Silica Fume

Low CaO

Medium CaO

High CaO

Slag

Class F Fly Ash

Class C Fly Ash


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Silica fume can be used as an addi-tion to cement, but is usually used as a

5 percent to 10 percent replacement bymass for cement. Silica fume is typical-ly more expensive than cement and isconsidered a property-enhancing mate-rial. Silica fume is regularly used inhigh-strength concrete applications orin concrete products that will be sub-jected to abrasive or corrosive environ-ments such as coastal applications,

bridge decks or water conveyancestructures.

Silica fume is available in a variety offorms. As-produced silica fume isextremely fine and can be delivered inbags or in bulk. The fineness of thismaterial and the ease with which itbecomes airborne may create handlingproblems that raise health concerns,which is why many precasters prefer touse densified or slurried silica fume.

Silica fume is densified by placing thematerial into a silo and blowing com-pressed air in from the bottom. Theparticles then begin to tumble and sticktogether. Densified silica is available inbags or bulk and mixing times mayneed to be increased to ensure that theparticles adequately break down duringthe mixing process.

Slurried silica is a water-based mate-rial containing roughly 42 percent to 60percent by mass of silica fume. Mostslurries contain a high-range water-reducing admixture (HRWRA) to offsetthe increased water demand associatedwith its use. Some silica fume is pel-letized for landfill purposes and shouldnot be used in concrete.

Silica fume improves the strength anddurability of concrete by creating adenser cement matrix when comparedto conventional concrete. Research hasfound that when silica fume is used ata 15 percent replacement level, thereare roughly 2 million silica fume parti-cles for each grain of cement present.

This ultimately reduces the porosity ofthe hydrated cement matrix throughimproved particle packing. Silica fumefills the voids between cement particlesjust as cement fills the voids betweensand and sand fills the voids betweencoarse aggregate.

Silica fume also modifies the pastestructure around aggregates and otherembedded items. This critical region isknow as the interfacial transition zone(ITZ) and in conventional concrete ischaracterized by a massive calciumhydroxide layer laden with voids creat-ing a weak link between the paste andaggregates. Due to their small size, sili-ca fume particles pack around the

aggregate more efficiently, reducingporosity, modifying the paste structureand preventing bleeding. Manyresearchers believe this mechanismgives silica fume concrete its increasedstrength gain when compared to con-ventional concrete.

Silica fume is a pozzolan and willconsume roughly 50 percent of the cal-

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Hydration is the result of a chemical reaction that occurs between water and the

chemical compounds present in portland cement. Portland cement is predominately com-

posed of two calcium silicates which account for 70 percent to 80 percent of the

cement. The two calcium silicates are dicalcium silicate (C2S) and tricalcium silicate (C3S).

The other compounds present in portland cement are tricalcium aluminate (C3A), tetra-

calcium aluminoferrite (C4AF) and gypsum. For the sake of simplicity this discussion

focuses only on the reaction between the calcium silicates and water.

The reaction of dicalcium silicate and tricalcium silicate with water (abbreviated as

H) produces calcium silicate hydrate (C-S-H) and calcium hydroxide (CH), as illustrated

in the following chemical equations.

2C2S + 9H (water) C3S2H8 + CH

2C3S + 11H (water) C3S2H8 + 3CH

C-S-H accounts for more than half the volume of the hydrated cement paste while CH

accounts for about 25% of the paste volume. The remainder of hydrated portland

cement is predominantly composed of Calcium Sulfoaluminates (ettringite) and capillary

pores.

C-S-H is a poorly crystalline material with a variable composition that forms extremely

small particles less then 1.0 m in size. C-S-H is the main cementitious compound, or

glue, that gives concrete its inherent strength. The structure of C-S-H becomes muchmore stable and resistant to subsequent environmental changes upon prolonged moist

curing or curing at elevated temperatures. Calcium hydroxide, on the other hand, is a

well-crystallized material with a fixed composition. CH contributes somewhat to con-

cretes inherent strength because it will form large crystals inside voids, thereby reducing

porosity. However, CH is a soluble compound, meaning it will move throughout the pore

system in the presence of water, making it extremely vulnerable to chemical attack.

All you really need to remember is that C-S-H is a superior reaction product because it

creates a denser microstructure that increases strength, reduces the permeability of the

concrete and improves its resistance to chemical attack. The formation of CH, on the

other hand, increases the concretes porosity and is susceptible to sulfate attack. The poz-

zolanic reaction converts the soluble CH to C-S-H, increasing the overall strength and

durability of the concrete.

Cement Hydration andPozzolans


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cium hydroxide present within the first 28 days when used ata 10 percent replacement level under normal curing condi-tions. The pozzolanic reaction is extremely sensitive to tem-perature and will be greatly accelerated by steam curing andother accelerated curing methods making it possible toachieve much higher early strengths. Due to its extremelysmall size and high surface area (20,000 m2/kg), silica fumeminimizes bleeding, which may lead to plastic shrinkagecracking. Moist curing should begin as soon as possible to

prevent this from occurring.At replacement levels of only 2 percent to 3 percent silicafume may actually reduce the water demand, otherwise thewater demand of silica fume concrete is substantially higherbecause of its extremely high surface area. It is best to use adispersing agent such as a HRWRA when using silica fume toovercome surface forces, ensuring adequate particle disper-sion. Silica fume concrete may be slightly darker in color andhas been reported as being sticky during finishing.

As previously mentioned, silica fume is a property-enhanc-ing material and can be used to meet durability requirementsin project specifications. Precasters often incorporate silicafume in parking garage products; marine structures such assea walls, docks and pilings, and coastal bridges; and high-

strength mix designs for such products as precast concretebank vaults. Silica fume also can be used to increase thecompressive strength of lightweight concrete as well as con-ventional concrete, making it possible to reduce the wallthickness or other dimensions of a product to overcometransportation limitations.

Fly Ash

Fly ash is by far the most widely used supplementarycementitious material in the manufactured concrete productsindustry because of its low cost (about half that of cement),availability and property-enhancing characteristics. Fly ash isa byproduct of the combustion of ground coal for use in elec-tric power plants. It is a fine residue of mineral impuritiesthat melt and recrystallize within the air stream movingthrough the combustion boiler. The material is then collectedfrom exhaust gases using electrostatic precipitators or filters.According to the American Coal Ash Association, roughly 12.5million tons of fly ash were used in the production of con-


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crete in the United States during 2002.Fly ash was first used in large-mass

concrete structures such as dams toreduce cost and minimize the heat ofhydration. Additional research revealedproperty-enhancing benefits of fly ash,including resistance to certain harmfulchemicals, sulfate attack and alkali silicareaction (ASR). The oil crisis during the

1970s also led to the construction ofadditional coal-burning power plantsthroughout the United States creating

an abundance of fly ash.Fly ash is a variable material and its

composition is determined by the chem-ical composition of the coal used by thepower plant. There are two basic formsof fly ash characterized by the percent-age of compounds present in the mate-rial. The four main constituents are sili-con dioxide (SiO2), aluminum oxide

(Al2O3), iron oxide (Fe2O3) and calciumoxide (CaO). ASTM C 618 classifies flyash based on the sum of the first threeconstituents (SiO2, Al2O3, Fe2O3). Whenthis sum exceeds 70 percent the Class Fdesignation is given to the materialwhile their sum must exceed only 50percent to be classified as a Class C flyash. Class C fly ashes also containhigher levels of calcium oxide usuallyexceeding 20 percent. Class F fly ashesare pozzolanic in nature while Class Cfly ashes react both pozzolanically andhydraulically and will be covered inPart II of this article.

Class F fly ashes have lower calciumcontents and are typically derived from

higher-ranked coals containing clayeymineral impurities. These coals are typi-cally found east of the MississippiRiver. The principal reaction product ofClass F fly ash is suggested to be moregel-like and denser than that from port-land cement hydration. Class F flyashes react more slowly than portlandcement, compromising the initial

strength gain of the fly ash concrete.Longer set times can be expected asthe quantity of fly ash increases, there-fore finishing operations may need tobe delayed. When using fly ash in themanufactured concrete products indus-try, accelerated curing methods andextended moisture curing should beused to initiate the pozzolanic reaction

and improve initial strength gain.Fly ash is spherical in shape andgreatly improves the workability offresh concrete by acting like small ballbearings during the mixing and placingprocess. Typically the amount of watercan be reduced by 2 percent to 3 per-cent for every 10 percent of fly ashused to replace cement. Depending onhow fine the fly ash and the content ofunburnt carbon, the water content canusually be reduced by about 6 percentto 10 percent with a 25 percent cementreplacement.

Unfortunately, fly ash has an adverseeffect on maintaining a stable air-voidsystem, especially for higher carbon

content Class F fly ashes. The carboncontent of fly ash is often given by theLoss on Ignition Value (LOI) shown onthe material certification report. Thisvalue is obtained by drying the sampleof ash and then massing it. The sampleis then ignited at 750 C in a muffle fur-nace. The loss in weight represents thequantity of unburnt carbon present inthe material and is often a good indica-tion of how it will affect the air contentof the concrete. Trial batches shouldalways be cast prior to using a newmaterial. The air content of the concreteshould be measured regularly whenusing a fly ash with a LOI value greaterthan 3 percent.

Either type of fly ash can be used asa cement replacement to reduce pro-duction costs. Class F fly ashes havealso been found to improve sulfateresistance better than Class C flyashes. However, some ashes with highalumina contents are not as effective inimproving sulfate resistance. The fol-lowing equation developed by the

Bureau of Reclamation can be used to assess the suitability ofa fly ash for improving sulfate resistance. Resistance factors(R) below 2.0 have been found to limit linear sulfate expan-sion to about 0.1 percent after three years of exposure.Typically the lower the resistance factor the better the sulfateresistance.

R = (CaO 5) / Fe2O3

Raw and processed natural pozzolansAs mentioned earlier, the ancient Greeks and Romans useda combination of lime and volcanic ash to make a cementi-tious mortar to construct many of the impressive monumentswhich still stand today. The earliest known use of a pozzolanactually dates back to about 4500 BC. It consisted of a mix-ture of lime and diatomaceous earth from the Persian Gulf.ACI defines natural pozzolans as either a raw or calcinednatural material that has pozzolanic properties. Calcining isthe process of altering the composition or physical state byheating a material below the temperature of fusion. Sourcesof natural pozzolans that do not require calcining to increasetheir reactivity are typically located west of the MississippiRiver. The price and availability of raw or processed naturalpozzolans is dependent on the location of such materials.

Research has indicated that most natural pozzolans produce

hardened concrete properties similar to industrial byproduct

pozzolans. Some investigators have even reported that naturalpozzolans are more effective in controlling alkali silica reac-tion than fly ash. More reactive pozzolans such as metakaolinand rice husk ash are often used in the same manner andproportions as silica fume.

Metakaolin is a calcined or thermally activated clay and isproduced with high purity kaolin-containing clay that is puri-fied by water processing prior to low temperature thermalactivation between 600 and 900 C. The material is then

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To learn more about SCMs, check out these standards.

ACI Documents

ACI 211.1, Standard Practice for Selecting Proportions for Normal, Heavyweight and

Mass Concrete

ACI 232.1, Use of Raw or Processed Natural Pozzolans in Concrete

ACI 232.2, Use of Fly Ash in Concrete

ACI 234, Guide for the Use of Silica Fume in Concrete

ASTM Documents

ASTM C 618, Standard Specification for Coal Fly Ash and Raw or Calcined Natural

Pozzolans for Use in Concrete

ASTM C 1240, Standard Specification for Silica Fume Used in Cementitious Mixtures

For Further Review...


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ground to a very high fineness (0.5 to 20 ?m) and marketed

as high-reactivity metakaolin. Research has shown that calci-um hydroxide produced during cement hydration will be com-pletely consumed when high-reactivity metakaolin is used at a20 percent replacement level. Metakaolin will increase theconcretes strength, reduce permeability and improve worka-bility when a water reducing agent is used. Metakaolin iswhite in color and ideal for use in architectural concrete.

Rice husk ash (RHA) is a natural byproduct from the pro-cessing of paddy rice. The husks, which are approximately 50percent cellulose, 30 percent lignin and 20 percent silica, areincinerated by controlled combustion leaving behind an ashthat predominantly consists of amorphous silica. Rice huskash is highly pozzolanic due to its extremely high surfacearea (50,000 to 100,000 m2/kg). Research has shown thathigher compressive strengths, decreased permeability, resist-ance to sulfate and acid attack, and resistance to chloridepenetration can all be expected when a high-quality RHA is

used in amounts of 5 percent to 15 percent by mass ofcement.

Incorporating pozzolans into a mix designThe choice to use natural and/or industrial byproduct poz-

zolans is based on availability and economics. If your plant islocated near a natural deposit or a source of industrial poz-zolans, then the material may be fairly inexpensive and canbe used to replace a portion of your c ement while improving

the performance of the concrete.Trial batches should always be utilized when changing a

mix design and proportions should be calculated in accor-dance with ACI 211.1, Standard Practice for SelectingProportions for Normal, Heavyweight and Mass Concrete.Trail batches also allow production personnel time to becomefamiliar with the new materials behavior. Highly reactive poz-zolans such as silica fume, metakaolin and rice husk ash mayactually increase initial strength gain, thereby reducing set

times. Conversely, less reactive pozzolans like fly ash maydelay the strength gain, possibly requiring accelerated curingmethods to achieve desired stripping strengths.

The pozzolans mentioned above are available in bulk quan-tities as well as smaller bags for limited-use applications. Youshould use the materials bulk density when ordering and besure your supplier has a sound quality control program inplace. Pozzolans may be delivered and stored in the samemanner as cement. Take appropriate precautions to preventcross-contamination should be taken, such as color codingsilos or feed lines. Cement should always be batched first fol-lowed by the SCM. Pozzolans tend to flow fairly easily, soconsider installing a positive shut-off value to prevent addi-tional material from flowing through the slide or screw oncethe device is stopped.

Generally speaking, most pozzolans will improve the worka-bility and cohesion of the mix due to improved particle pack-

ing. ACI even suggests that this may increase the life of dry-cast equipment. Because of differences in specific gravities, amix containing a pozzolan as a cement replacement by masswill typically produce a higher yield. The yield of the con-crete mix should be adjusted using the specific gravities ofthe actual materials used.

Pozzolans often have a variable chemical composition whichmay adversely react with different chemical admixtures. High-carbon content pozzolans typically require an increaseddosage rate of air-entraining admixtures to achieve a stableair-void system. Always check the c ompatibility of youradmixtures with the SCM.

One final precaution to consider is the safety of your pro-duction personnel. The fine nature of many of these materialsmay cause a considerable amount of them to become air-borne. Materials such as silica fume present a potential healthconcern which OSHA has recently taken an interest in.

Employees handling or working around airborne silica shouldalways wear proper personal protection equipment (PPE) suchas respirators and safety glasses.

Part II of this article will cover hydraulic supplementarycementitious materials, blended SCMs as well as a compre-hensive review of all SCMs and their property-enhancingcharacteristics. In the meantime, consult the literature listed inthe sidebar For Further Review to learn more aboutSCMs.
