How Nanotechnology Can Change theConcrete World Part Two of a Two-Port Series

Successfully mimicking nature's boffom-up constructionprocesses is one of the most promising directions.

Konstantin SoboievFacultad de Ingenieria CivilUniversidad Autonoma de Nuevo LeonSan Nicolas de los Garza, Mexico

Miguel Ferrada GutierrezCognoscible TechnologiesSantiago, Ct)ile

general overview of nano-technology was present-ed in part one of this two-part series {see pi 4 in theOctober issue). In this

part, potential applications of nanotech-nology to concrete are presented.

How Nanotechnoloay Can ChangeConstruction Materials

The majority of recent nanotechnologyresearch in construction has focused onthe structure of cement-based materialsand their fracture mechanisms.''' ' ' ' ^ Newadvanced equipment makes it possible toobserve a structure at its atomic level.Moreover, the strength, hardness andother basic properties of microscopic and

nanoscopic phases of materials can bemeasured.^''

Atomic force microscopy (AFM) has beenapplied to the investigation of the amor-phous C-S-H gel structure.This has led tothe discovery that this product has a highlyordered structure at the nanoscale (Fig. 6). ^

Understanding of nanoscale structurehelps to influence important processesrelated to production and use of construc-tion materials, including strength develop-ment, fracture, corrosion and tailoring ofdesired properties. For instance, the devel-opment of paints and finishing materialsthat are self-cleaning, discoloration resist-ant, antigraffiti protected, high scratchresistant and wear resistant is extremelyimportant for facade and interior applica-tions. Self-cleaning Hydrotect tile, windov^/glass, and water-based paint have beendeveloped by TOTO based on photocata-lyst technology.'^''''The self-cleaning effectrelated to decomposition of organic pollu-tants and gases is achieved when titania

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GSH crystallized2 X 2 pm

Atomic resolution20 X 20 nm

Higbly ordered structure of C-S-H gel (Ca/Si = 0.9} observed at a nonoscale.^^Courtesy Eric Lesniewska, Nanosciences Group, Physics lab. University of Bourgogne, France.

photocatalyst thin film is set on asurface to emit active oxygen underultraviolet light.

Another aspect of self-cleaning isprovided by the hydrophilicity of thesurface.This helps to rinse away dustand dirt. Other examples are relatedto nanometer-thin coatings of con-ducting polymers that protect car-bon steel against corrosion"*' ' ^ orwindow glass covered with an invisi-ble silver nanolayer to enhance thethermal insulation

Nanochemistry with Its bottom-uppossibilities offers new products thatcan be effectively applied in concretetechnology. One example is relatedto the development of new super-plasticizers for concrete, such as poly-carboxylic ether (PCE) polymer-basedSky.This product has been developedrecently by Degussa.A nanodesignapproach helped to realize theextended slump retention ofconcrete mixtures.^''

It has been proposed that nanopar-ticles can be incorporated into con-ventional building materials.Theythen possess advanced or smartproperties required for the construc-tion of high-rise, long-span or intelli-gent civil and infrastructuresystems.'*' ''' ' ' ^ For example, silicananoparticles {nanosilica (Fig. 7)) can

be used as an additive for high-per-formance and self-compacting con-crete that has improved workabilityand strength.^^"" Nanosilica hasbeen proved to be an effective addi-tive to polymers to improvestrength, flexibility and

Kang et al. ^ investigated epoxycomposites filled with functionalizednanosilica particles obtained usingthe sol-gel process.The synthesizeduniform silica particles were eithermodified by substituting the surfacesilanol groups by epoxide ring, amineand isocyanate groups or by calcinat-ing the nanosilica to remove surfacesilanol groups. It was found that mod-ified particles could chemically bondto epoxy matrix, which resulted in adecrease in the coefficient of thermalexpansion (CTE) of the composites.With increased nanosilica content, thecomposites exhibited an additionaldecrease in CTE, an increase in glasstransition temperature and adecrease in damping.

The combination of carbon nano-tubes and conventional polymer-based fibers and films is anotherchallenge. For example, the incorpo-ration of 10% SWNTs into thestrongest artificial fiber, Zylon, resultsin a new material with 50% greaterstrength.'*" It is expected that betterexfoliation (separation of the bun-dles to release the individual nano-

Ultrafine fumed silica particles observedusing TEM.^^ Courtesy Andri Vital, EMPAMat'ls Testing & Research. Labaratary farHigh-Performance Ceramics, Duebendoif,Switzerland.

tubes) and improved dispersion andalignment of the individual nano-tubes can increase the performanceof composite fibers or decrease thevolume of the nanotubes used.''"

Based on research conducted at theUniversity of Texas-Dallas, Dalton etal.^^'*' introduced a further break-through related to SWNT-reinforcedfibers.The composite fiber compris-ing 60% of nanotubes and 40% ofpoly{vinyl alcohol) (PVA) producedby continuous spinning with a modi-fied coagulation method achievedstrengths of >1.8 GPa.These fibersmatched the energy-absorbingcapacity of spider silk up to thebreaking point of silk at 30% (165J/g) and continued to absorb energyuntil reaching a toughness of 570J/g, as compared with 50 and 33 J/gfor Spectra and Kevlar fibers, respec-tively.^^ Application of these newsuper fibers in composite materials ispromising.

Concrete with NanoparticlesMechanical properties of cementmortars with nano-iron-oxide andnanosilica were studied by Li et al. ^Experimental results demonstratedan increase in compressive and flex-ural strengths of mortars that con-tained nanoparticles. It was foundthat increased nanosilica contentimproved the strength of the mortars.

American Ceramic Society Bulletin, Vol. 84, No, 11

CONCRETE NANDTECHNOLaGY

Table 1. Effect of Gaia on Performance ofConcrete Mixtures^

Mixture parameters

Ceinen1tvp6|EN-197)

>fnenl content (kg/m^)

Admixture dosage (%)

Air content (%)

Slump (mm)

Alter 5 min

Alter 30 min

After 60 min

After 90 min

Ambient temperature [°C)

lompressive strength (MPo)

At I dA t 7 d

At2Ed

Plain

Il/A-P 42.SR

460

2.7

60

25

15

20

11.1

32.7

45.2

With Goia

tl/AP42.5R

460

1.3

1.1

200

210

160

140

20

6S.277.3

91.7

iReferente 35.

Moreover, the strength of thecement mortars with nanoparticleswas even higher than the strength ofmortars with silica fume. SEM studyproved that the nano-iron-oxide andnanosilica particles filled the poresand decreased the content of calciumhydroxide within the hydration prod-ucts. These effects resulted in theimprovement of the mechanicalproperties of the cement mortarswith nanopartictes.

A laboratory study of high-volumefly-ash high-strength concrete thatincorporated nanosilica was per-formed by Li.'' investigation of thehydration process confirmed that thepozzolanic activity of fly-ash can besignificantly improved by the applica-tion of a nanosilica. It was concludedthat the use of nanosilica led toincreased early age and ultimatestrength of high-volume fly-ash con-crete.The developed concrete withnanosilica had a 3d strength 81%higher compared with plain concrete.The 2-year strength of the developedconcrete was n5.9MPa (higher thanthe strength of reference portlandcement concrete of ~103.7 MPa).

Collepardi et ai.^ ' '' investigatedlow-heat self-compacting concreteswith mineral additives (ground lime-stone, fly-ash and ground fly-ash).Nanosilica (5-50 nm) at a dosage of

l -2%of cementitious materials wasused as a viscosity-modifying agent.A constant water/cement ratio {W/Qof 0.58 was used for all mixtures, anda slump flow of 780-800 mm wasmaintained by adjusting the dosageof an acrylic-polymer-based super-plasticizer.To maintain the specifiedslump flow, the superplasticizerdosage was increased by-0 .21% pereach percent of nanosilica used.Theaddition of natiosiiica made the con-crete mixture more cohesive andreduced bleeding and segregation.At the same time, the nanosilica hadlittle effect on the slump loss meas-ured within the first 30 min.

Although the nanosilica did notaffect the compressive strength ofthe concrete that contained variousforms of fly-ash, the compressivestrength of the concrete that con-tained ground limestone was slightlydecreased.The best performancewas demonstrated by concrete withground fly-ash, 2% nanosilica and1.5% superplasticizer.This concretehad the highest compressivestrength of 55 MPa at an age of 28 dwith the desired behavior in thefresh state, i.e., low bleeding, perfectcohesiveness, better slump flow andlittle slump loss.The investigationalso showed that nanosilica did notaffect concrete

One of the first commercialnanoadmixtures for concrete, Gaia,was developed by Ulmen S.A. andCognoscible Technologies to substi-tute for silica fume at ready-mixedconcrete facilities certified by ISO14001,"Environmental ManagementSystems."This product is available inliquid form that facilitates the satis-factory distribution of nanosilica par-ticles in concrete.Gaia combines theeffects of water reduction and slumpincrease.^^ According to Ferrada etal.,^^ the concrete mixtures with Gaiaexhibit perfect workability withoutsegregation or bleeding.This makesthe design of self-compacting con-crete an extremely easy task. Aslump loss of 30% in 1.5 h at anambient temperature of 20°C hasbeen reported for concrete mixturesthat include Gaia (Table 1).

The application of Gaia at a dosageof 1.3% (by weight, as a dry content)provides almost a twofold increasein concrete compressive strength atages of 7 and 28 d.'^ The earlystrength of the concrete with Gaia is68.2 MPa, which is ~3 times higherthan that of reference concrete(Table 1 and Fig. 8).The 28 d com-pressive strength of the concretemade with Gaia demonstrates a clas-sical dependence on W/C (Fig. 9)and, based on the available test data,a single formula is proposed to pre-dict the 28 d strength of the investi-gated concretes: f g - 208.38e-3°s8iiv/c(at R^ = 97%).

Based on the available data, thepositive action of the nanoparticleson the microstructure and propertiesof cement-based materials can beexplained by the following

•Well-dispersed nanoparticlesincrease the viscosity of the liquidphase, which helps to suspend thecement grains and aggregates,which, in turn, improves the segrega-tion resistance and workability of thesystem.

• Nanoparticles fill the voidsbetween cement grains, whichresults in the immobilization of freewater (filler effect).

• Well-dispersed nanoparticles act ascenters of crystallization of cementhydrates, which accelerates thehydration.

• Nanoparticles favor the formationof small-sized crystals (such as calci-um hydroxide and AF^) and small-sized uniform clusters of C-S-H.

• Nanosilica participates in the poz-zolanic reactions, which results in theconsumption of calcium hydroxideand formation of an additional C-SH.

• Nanoparticles improve the struc-ture of the aggregate contact zone,which results in a better bondbetween aggregates and cementpaste.

• Crack arrest and interlocking effectsbetween the slip planes provided bynanoparticles improve the toughness,shear, tensile strength andflexuralstrength of cement-based materials.

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Future DevelopmentsMuch progress in concrete science isto be expected in coming years bythe adaptation of new knowledgegenerated by a quickly growing fieldof nanotechnology.The develop-ment of the following concrete-relat-ed nanoproducts can be anticipated:• Catalysts for the low-temperaturesynthesis of clinker and acceleratedhydration of conventional cements;• Grinding aids for superfine grindingand mechanochemical activation ofcements;• Binders reinforced with nanoparti-cles, nanorods, nanotubes (includingSWNTs), nanodampers, nanonets ornanosprings;• Binders with enhanced/nanoengi-neered internal bond between thehydration products;• Binders modified by nanosizedpolymer particles, their emulsions orpolymeric nanofilms;• Biomaterials (including those imi-tating the structure and behavior ofmollusk shells);• Cement-based composites rein-forced with new fibers that containnanotubes as well as with fibers cov-ered by nanolayers (to enhance thebond and corrosion resistance or tointroduce new properties, such aselectrical conductivity);• Next-generation superplastlcizersfor total workability control andsupreme water reduction;• Cement-based materials withsupreme strength, ductility andtoughness;• Binders with controlled internalmoisture supply to avoid/decreasemicrocracking;• Cement-based materials with engi-neered nanostructures andmicrostructures that exhibit supremedurability;• Ecobinders modified by nanoparti-cles and produced with substantiallydecreased volume of portlandcement component (down to10-15%) or binders based on thealternative systems (magnesia, phos-phate, geopolymers and gypsum);

• Self-healing materials and repairtechnologies that use nanotubes

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28

Strength development of concrete with Gaia.

and chemical admixtures;• Materials with self-cleaning/air-purifying features based on photo-catalyst technology;• Materials with controlled electricalconductivity, deformative properties,nonshrinking and low thermalexpansion; and• Smart materials, such as tempera-ture-, moisture- and stress-sensing orresponding materials.

Mechanochemistry and nanocata-lysts could change the face of themodern cement industry by thegreat decrease of clinkering temper-ature and even realizing the possibil-ity of cold sintering of clinker miner-als in mechanochemical reactors.Nanobinder can be proposed as alogical extension of two concepts:densified system with ultrafine parti-cles (DSP)'' and modified multicom-ponent binder (MMCB)""' ^ down tothe nanolevel (Fig. 10).

In these systems, the densificationof binder is achieved with the helpof ultrafine particles: silica fume (SF)dispersed with superplasticizer (SP)in DSP and finely ground mineraladditives (FGMA) and SF modified bySP in MMCB.These particles fill thegaps between cement grains. In

these systems, portland cementcomponent is used at its standarddispersion to provide the integrity ofcomposition. In contrast to DSP andMMCB, the nanobinder can bedesigned with a nanodispersedcement component applied to fillthe gaps between the particles ofmineral additives (including FGMA).

In the nanobinder, the mineral addi-tives (optionally, finely ground),which act as the main component,provide the structural stability of thesystem.The microsized or nanosizedcementitious component (which alsocan contain the nanosized particlesother than portland cement) acts asa glue to bind less-reactive particlesof mineral additives together. Suchnanosized cementitous componentcan be obtained by the colloidalmilling of a conventional (or speciallysintered/high C S) portland cementclinker (the top-down approach) orby self-assembly, which usesmechanochemically inducedtopochemical reactions (the bottom-up approach).

Editor's Note

The references as well as Figs. 9 and 10can be found as part of the onlineversion of the article atwww.ceramicbulletin.org.

American Ceramic Society Bulletin, Vol. 84, No. 11