96

to growing demand. At the same time, there is a need to create a strongconcrete by using composite materials based on cement. Utilizationof CNTs–CNFs occasionally has been investigated for applications inthe construction industry (6–9), although the expected improvementhas not been achieved (10–12). The biggest problem in potential useof the carbon nanomaterials for making strong concrete arises whenCNTs or CNFs are being introduced in the matrix material. Becauseof their high agglomeration and bundling tendency, carbon nano-materials cannot be easily and homogeneously dispersed in cement bya simple mixing procedure (12). Usually, multistep time-consumingprocesses are required. A bulk amount of CNTs–CNFs needs to bepurified and functionalized and only afterward mixed with matrices.However, even this procedure does not lead to significant enhance-ment of the concrete mechanical properties because of poor bondingbetween the CNTs–CNFs and cement (12).

A simple approach to growing CNTs–CNFs directly on the sur-face of cement particles is proposed (13). Under this approach, theproduced carbon nanomaterials are homogeneously dispersed in thematrix (or precursor matrix) and intermingled with the products. Anovel cement hybrid material (CHM) was synthesized in whichCNTs and CNFs are attached to cement particles by two differentmethods: screw feeder and fluidized bed reactors. CHM has beenshown to increase the compressive strength by two times and the elec-trical conductivity of the hardened paste—that is, concrete withoutsand—by 40 times.

EXPERIMENTAL DETAILS

CHM was synthesized by the chemical vapor deposition (CVD)method, which is considered to be the most viable and efficient processfor high-yield CNT production. Because cement particles naturallycontain both the catalyst and inert support substances required forCVD synthesis, this inexpensive basic building material has beenused for CNT and CNF fabrication, thereby making it possible toavoid many time-consuming steps of the catalyst-support prepara-tion. Portland sulfate-resistant (SR) cement (CEM I 42.5N) contain-ing about 4% Fe2O3 was examined (Table 1). The rest of the cementmaterials—SiO2, MgO, and Al2O3—are known to be good support-ing materials for the growth of CNTs (14–16). For the syntheses,acetylene was chosen as the main carbon source because of its lowdecomposition temperature and cost-effectiveness. CO and CO2 wereexamined as promoting additives to enhance the yield.

Direct Synthesis of Carbon Nanofibers on Cement Particles

Larisa I. Nasibulina, Ilya V. Anoshkin, Sergey D. Shandakov, Albert G. Nasibulin, Andrzej Cwirzen, Prasantha R. Mudimela, Karin Habermehl-Cwirzen, Jari E. M. Malm, Tatiana S. Koltsova, Ying Tian, Ekaterina S. Vasilieva, Vesa Penttala, Oleg V. Tolochko, Maarit J. Karppinen, and Esko I. Kauppinen

Carbon nanotubes (CNTs) and nanofibers (CNFs) are promising candi-dates for the next generation of high-performance structural and multi-functional composite materials. One of the largest obstacles to creatingstrong, electrically or thermally conductive CNT–CNF composites is thedifficulty of getting a good dispersion of the carbon nanomaterials in amatrix. Typically, time-consuming steps are required in purifying andfunctionalizing the carbon nanomaterial. A new approach under whichCNTs–CNFs are grown directly on the surface of matrix and matrixprecursor particles is proposed. Cement was selected as the precursormatrix, since it is the most important construction material. A novelcement hybrid material (CHM) was synthesized in which CNTs andCNFs are attached to the cement particles by two different methods:screw feeder and fluidized bed reactors. CHM has been proved to increasethe compressive strength by two times and the electrical conductivity ofthe hardened paste by 40 times.

Carbon nanotubes (CNTs) and nanofibers (CNFs) have recentlyreceived significant scientific attention owing to their extraordinaryand useful properties, such as exceptional tensile strength, elastic mod-ulus, and electrical and thermal conductivity (1, 2). These materialsare promising candidates for the next-generation high-performancestructural and multifunctional composite materials (3–5).

Cement is one of the most important building materials. The world’sproduction of cement has increased significantly in recent years due

L. I. Nasibulina, I. V. Anoshkin, A. G. Nasibulin, P. R. Mudimela, and Y. Tian,Department of Applied Physics and Center for New Materials, Helsinki Universityof Technology, Espoo, P.O. Box 5100, FIN-02150, Finland. S. D. Shandakov,Department of Applied Physics and Center for New Materials, Helsinki Universityof Technology, Espoo, P.O. Box 5100, FIN-02150, Finland; Laboratory of CarbonNanoMaterials, Department of Physics, Kemerovo State University, Kemerovo650043, Russia. A. Cwirzen, K. Habermehl-Cwirzen, and V. Penttala, Laboratoryof Building Materials Technology, Faculty of Engineering and Architecture, HelsinkiUniversity of Technology, Espoo, P.O. Box 5100, FIN-02150, Finland. J. E. M. Malmand M. J. Karppinen, Laboratory of Inorganic Chemistry, Department of Chemistry,Helsinki University of Technology, Espoo, P.O. Box 6100, FIN-02150, Finland. T. S.Koltsova, E. S. Vasilieva, and O. V. Tolochko, Material Science Faculty, State Poly-technical University, Polytechnicheskaya, 29, 195251, Saint Petersburg, Russia.E. I. Kauppinen, Department of Applied Physics and Center for New Materials, HelsinkiUniversity of Technology, Espoo, P.O. Box 5100, FIN-02150; VTT Biotechnology,Biologinkuja 7, 02044, Espoo, Finland. Corresponding author: A. G. Nasibulin,albert.nasibulin@hut.fi.

Transportation Research Record: Journal of the Transportation Research Board,No. 2142, Transportation Research Board of the National Academies, Washington,D.C., 2010, pp. 96–101.DOI: 10.3141/2142-14

Nasibulina et al. 97

A screw feeder reactor allowed for continuous feeding of the cat-alyst particles (Figure 1). This CVD reactor consisted of a quartz tube(with an internal diameter of 34 mm and a length of 100 cm) insertedin a resistively heated furnace (with a heated length of 60 cm), a pow-der feeder with an adjustable powder feeding rate, a copper screwfeeder, a powder collector, and a water cooling system to keep theends of the tube at room temperature. The residence time of thecement particles in the high-temperature reactor zone (about 30 cm)was regulated by motor rotation speed and varied from 1 to 6 perminute, which corresponds to 30 and 5 min, respectively. The exper-imental investigations were carried out with the powder feeding rateof 30 g/h in a temperature range of 400°C to 700°C.

The second experimental setup used for synthesis of CHM was afluidized bed reactor (Figure 2), which consists of a quartz tubeinserted in the vertical furnace (with a heated length of 60 cm). Thequartz tube was conically shaped with a junction from internal diam-eters of 6 to 34 mm with a cone zone length of 20 cm. For one exper-iment, 50 to 100 g of clinker (or cement) particles were filled up fromthe top and kept in a nitrogen atmosphere of 1,600 cm3/min to replaceoxygen and to heat the powder to the synthesis temperature for about5 min. Then CO at 530 cm3/min (for catalyst reduction purposes) andafter 5 min acetylene (carbon source) flows at 520 cm3/min wereintroduced. The nitrogen flow was gradually turned off during about5 min after fluidized conditions were achieved. Typical growth timewas 30 min. Acetylene and CO flows were then replaced by nitrogenand the reactor was cooled to room temperature outside the furnace.Typical yield of the final product was about 75 g/h. This method canbe easily scaled up.

Microscopic investigations were carried out with the help of high-resolution low-voltage field emission gun scanning electron micro-scopes (SEM, Leo DSM 982 Gemini and JEOL JSM-7500F) anda field emission gun transmission electron microscope (TEM, PhilipsCM200 FEG). X-ray powder diffraction (XRD) data were collectedon a D8 advance Bruker diffractometer by using CuKα (40 kV,40 mA) radiation. Raman spectra were recorded by using a frequencydoubled neodymium-doped yttrium aluminum garnet green laser(532.25 nm, 30 mW) and charge-coupled device detector. Ramanexperiments were performed at ambient atmosphere and temperature.Thermogravimetric investigations of pristine cement, activated car-bon, and CNT-modified cement particle samples were carried outwith a Netzsch STA 449 C thermobalance. The samples were slightlyground before the measurements. A sample of 9 to 18 mg was heatedin an Al2O3 crucible from room temperature to 750°C at a heating rateof 10°C/min in a dynamic air (40 mL/min) atmosphere.

To investigate the produced hybrid structures in the quality of abuilding component, cement pastes were prepared on the basis ofCHM, and their compressive and flexural strengths and electricalresistance were tested. The current investigations used beams withdimensions of 60 × 10 × 10 mm3 prepared with Teflon molds andcured in water at 20°C for 28 days. Each series consisted of threebeams. The cement paste specimens produced from a mixture ofpristine SR cement and SR CHM contained a polycarboxylate-basedsurfactant available under the commercial name Kolloment (GraceChemicals), whereas the cement paste specimens produced entirelyfrom SR CHM contained a mixture of two surfactants: Kollomentand Parmix (Finnsementti Oy). The electrical resistance R of thecement paste samples was measured by using two contacts pressedto opposite sides of the sample through soft graphite films withthe area of S separated by the distance of L. The resistivity wasdetermined as ρ = R � S/L.

EXPERIMENTAL RESULTS AND DISCUSSION

Systematic investigations of CNF growth were carried out in thescrew feeder reactor. The product composition could be varied fromno carbon precipitated on particles (below 450°C) to complete cov-erage of cement particles at higher temperatures. These structuresprovided a good dispersion of the CNTs and CNFs in cement, whichis essential for creation of very strong and electrically conductive

FIGURE 1 Schematic of experimental setup based on continuousfeeding of cement particles with a screw feeder.

FIGURE 2 Schematic ofexperimental setup based onfluidized bed conditions.

TABLE 1 Oxide ComponentContent of Portland CementsUsed for CNT–CNF Growth

Component Content (wt%)

CaO 63.1

SiO2 20.2

SO3 3.0

Fe2O3 4.0

Al2O3 2.2

MgO 2.0

K2O 0.3

Na2O 0.5

powdercollector

powderfeeder

GASEXHAUST

FURNACE

CO

2

COAr

cement

motor

watercooling

watercooling C

2H2

GASEXHAUST

FURNACETset=550 - 650°C

N2

CO

C2H

2

1600ccm

520ccm

530ccm

materials. Figure 3 shows SEM images of the product synthesized inthe screw feeder and fluidized bed reactor. The CNFs synthesized onthe surface of cement particles by the fluidized bed reactor are abouthalf as thick and less bundled. Typical lengths of the CNFs are severalmicrometers.

To determine quantitatively the carbon nanomaterial yield, thermo-gravimetric analyses (TGAs) were carried out. TGA revealed two fea-tures in the temperature ranges of 80°C to 280°C and 380°C to 560°Cfor SR CHM (Figure 4a). The first temperature mass decrease can beattributed to hydrocarbons, which were likely formed owing to acety-lene polymerization reactions; the second step can be explainedby burning out carbon from the CNTs and CNFs in an air atmo-sphere (17). The most reactive system resulting in the highest carbonyield of 1,660% corresponding to 15 g/h was found to be a mixture ofacetylene and CO. Interestingly, addition of CO2 (it is assumed thatCO2 present in the reactor can play the role of etching agent to removeamorphous carbon and therefore prevents catalyst particle encapsu-lation by amorphous carbon) to acetylene did not lead to increasedCNT–CNF yield as would have been expected from the literature(18, 19) but as shown later improved the properties of the cementpaste. CO alone, known to be a good carbon precursor for single-

walled CNT production (20–23), did not form any carbon productunder these conditions. Nevertheless, the important role of CO can beattributed to the reduction of iron oxide (24). Even though the absoluteyield of carbon nanomaterials was significantly higher for SR cement,the yield calculated per mass of available iron was found to be verysimilar for SR and white cements.

Raman experiments performed at ambient atmosphere and tem-perature showed two main features in the spectra: G band at about1,600 per cm and D band at 1,325 per cm (Figure 4b). These spectraare typical and can be obtained from multiwalled CNTs (25). Theresults of the Raman measurements did not significantly changewhen the type of cement or experimental conditions varied.

To examine possible changes in the cement particles under CHMsynthesis conditions XRD measurements were carried out. No majorchanges were found in the crystallinity, but a new peak correspond-ing to graphitized carbon appeared, and the gypsum phase vanished(Figure 4c). The latter can be explained by the changing water con-tent in the gypsum phase (26). The intensity of other peaks corre-sponding to different crystalline cement compounds such as Ca3SiO5,Ca2SiO4, and Ca3Al2O6 did not essentially change under the synthesisconditions.

98 Transportation Research Record 2142

FIGURE 3 SEM images of CHM structures synthesized in (a and c) screw feeder and (b and d ) fluidized bed reactors at 550�C in COand C2H2 atmosphere.

(a) (b)

(c) (d)

1 μμm 1 μm

200 nm 200 nm

Nasibulina et al. 99

The cement pastes made of CHM samples produced under differ-ent conditions revealed significant improvement in mechanical andelectrical properties after curing in water for 28 days. As shown inTable 2, the SR CHM can be used to prepare mechanically verystrong paste with compressive strength more than two times higherthan that of the paste prepared from the pristine cement. In addition,

up to 40 times better electrically conductive paste preserving itsmechanical properties can be produced on the basis of this material.To the best of the authors’ knowledge, these significant compressivestrength and electrical conductivity enhancements are the highestreported so far obtained with the help of CNTs. Until now, CNTsand CNFs added to cement resulted in a decrease or a rather small

100 300200 60050040060

65

70

75

80

85

90

95

1005

4

3

2

1

Mas

s ch

ange

[%]

Temperature [oC]

0 500 1000 1500 2000 2500 3000

21

GD

Inte

nsity

[au]

Raman shift [cm-1]

10 20 30 40 50 60

C

C

GG1

2

3

Inte

nsity

[au

]

2θ [ o ]

(a) (b)

(c)

FIGURE 4 (a) TGA curves of CHM produced at 600�C in C2H2 � CO (Curve 1) and at 550�Cin C2H2 � CO (Curve 2), at 525�C in C2H2 � CO (Curve 3), at 525�C in C2H2 � CO2 (Curve 4),and in pristine SR cement (Curve 5). (b) Raman spectra of pristine SR cement (Spectrum 1)and CHM produced from SR (Spectrum 2). (c) XRD spectra of pristine SR cement (Spectrum 1)and CHM produced at 500�C in C2H2 � CO2 (Spectrum 2) and at 575�C in C2H2 � CO2

(Spectrum 3) reveal gypsum (G) decomposition under thermal treatment and appearance ofgraphite (C) (au � atomic units).

TABLE 2 Mechanical (After 28 Days Curing in Water) and Electrical (1 Day AfterMechanical Testing) Properties of Cement Paste with Water–Cement or Water–CHMMass Ratio of 0.4

Synthesis Conditions

Fraction of TemperatureGas Flow Rate (cm3/min)

CompressiveCHM (%) (°C) C2H2 CO2 CO Strength (MPa) (MΩ-cm)

0 — — — — 25 9.7

100 550a 860 0 177 22 0.23

100 575b 660 660 0 55 1.3

100 500b 500 500 0 40 1.7

100 525 660 660 0 56 4.0

aScrew feeder rotation rate = 6/min.bScrew feeder rotation rate = 2/min.

ElectricalResistivity

100 Transportation Research Record 2142

FIGURE 5 SEM images of hardened cement paste (curing in water for 28 days) after mechanical test. Images reveal connectionformed after cement hydration process.

(a) (b)

(up to 20%) increase in compressive strength (27, 28). The mainproblem was in getting a good dispersion of carbon nanomaterialsin a matrix or its precursor. On the other side, the weakest compos-ite’s point, when carbon nanostructures are applied for advancedmaterials, is in the interfacial bonding between CNTs and the bindermatrix. To improve the situation, many researchers (27, 29–31) usedtreated CNTs–CNFs—for example, by using sulfuric and nitric acidsor ozone gas leading to the formation of oxygen-containing groupscapable of enhancing reinforcement efficiency between the hydratedcement and CNTs. From that point of view, for increasing the strengthof the hardened paste (or concrete) it is very important to produceCNTs–CNFs on the surface of cement particles—that is, on the sideof subsequent hydration cement products, which would provide betterstrength (and electrical contacts) between particles in the final product.Therefore, the carbon nanomaterials are easily and homogeneouslydispersed in the cement paste and are intermingled with the hydrationproducts during the hydration process.

The fractured parts of the hydrated (curing in water for 28 days)cement paste specimens were observed in SEM. The CNTs andCNFs originally attached to the cement particles appeared to benicely embedded in the hydration products of the calcium silicatehydrate phase (Figure 5). CNTs and CNFs bridged the neighboringcement particles surrounded by their hydration products, which canexplain the significant increase in mechanical strength and electri-cal conductivity on the hardened past compared with paste preparedfrom the pristine cement.

CONCLUSION

Cement particles were used as catalyst and support material, whichmade it possible to synthesize novel hybrid nanostructured materialin which CNTs and CNFs attached to cement particles and providedgood dispersion of the carbon nanomaterials in the cement. Thishybrid material was synthesized in two CVD reactors, which can be

easily combined into industrial cement production. The yield of theproduct in the fluidized bed reactor increased considerably.

The investigations based on TEM, SEM, XRD, TGA, and Ramanmeasurements showed high efficiency of the method for low-temperature and high-yield synthesis of CNTs and CNFs. Investi-gations of the physical properties of the paste made from the CHMrevealed up to a twofold increase in the compressive strength anda 40-fold increase in the electrical conductivity after 28 days ofcuring in water.

ACKNOWLEDGMENTS

This work was supported by the Academy of Finland and by theFederal Agency for Science and Innovation. S. D. Shandakov thanksthe European Commission for financial support through a Marie CurieIndividual Fellowship. The NanoMicroscopy Center of HelsinkiUniversity of Technology is acknowledged.

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The Nanotechnology-Based Concrete Materials Task Force peer-reviewed thispaper.