Construction and Building Materials 25 (2011) 145–149

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Construction and Building Materials

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Hybrid effect of carbon nanotube and nano-clay on physico-mechanicalproperties of cement mortar

M.S. Morsy *, S.H. Alsayed, M. AqelKing Saud University, College of Engineering, Specialty Units for Safety & Preservation of Structures P.O. Box 800, Saudi Arabia

a r t i c l e i n f o a b s t r a c t

Article history:Received 7 March 2010Received in revised form 5 May 2010Accepted 19 June 2010Available online 15 July 2010

Keywords:Carbon nanotubeNano-clayCement mortarThermal analysisCompressive strengthMicrostructure

0950-0618/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2010.06.046

* Corresponding author. Tel.: +966 14670631; fax:E-mail address: msmorsy@yahoo.com (M.S. Morsy

In this work, several nanomaterials have been used in cementitious matrices: multi wall carbon nano-tubes (MWCNTs) and nano-clays. The physico-mechanical behavior of these nanomaterials and ordinaryPortland cement (OPC) was studied. The nano-clay used in this investigation was nano-kaolin. Themetakaolin was prepared by thermal activation of nano-kaolin clay at 750 �C for 2 h. The organic ammo-nium chloride was used to aid in the exfoliation of the clay platelets. The blended cement used in thisinvestigation consists of ordinary Portland cement, carbon nanotubes and exfoliated nano metakaolin.The OPC was substituted by 6 wt.% of cement by nano metakaolin (NMK) and the carbon nanotubewas added by ratios of 0.005, 0.02, 0.05 and 0.1 wt.% of cement. The blended cement: sand ratio usedin this investigation was 1:2 wt.%. The blended cement mortar was prepared using water/binder ratioof 0.5 wt.% of cement. The fresh mortar pastes were first cured at 100% relative humidity for 24 h andthen cured in water for 28 days. Compressive strength, phase composition and microstructure of blendedcement were investigated. The results showed that, the replacement of OPC by 6 wt.% NMK increases thecompressive strength of blended mortar by 18% compared to control mix and the combination of 6 wt.%NMK and 0.02 wt.% CNTs increased the compressive strength by 29% than control.

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1. Introduction

The mechanical behavior of concrete materials depends onstructural elements and phenomena that occur in a micro and anano scale. As a result, nanotechnology can modify the molecularstructure of concrete which leads to improvement in the material’sbulk properties. Nanotechnology can also improve the mechanicalperformance, volume stability, durability, and sustainability ofconcrete. The revolutionary effects accompanying nanotechnologyallows the development of cost-effective, high-performance, andlong-lasting products of cement and concrete which can lead tounprecedented uses of concrete materials.

One of the most desired properties of nanomaterials in theconstruction sector is their capability to confer a mechanical rein-forcement to cement based structural materials. When usingnanomaterials three main advantages are considered. The firstadvantage is the production of high-strength concrete for specificapplication. The second advantage is to reduce the amount of ce-ment needed in concrete in order to obtain similar strengths anddecreasing the cost and the environmental impact of constructionmaterials. The third advantage is reducing the construction periods

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as nanomaterials can produces high-strength concrete with lesscuring time.

Carbon nanotubes (CNTs) are hollow tubular channels, formedeither by one single walls carbon nanotube (SWCNTs) or maltywalls carbon nanotube (MWCNTs) of rolled graphene sheets[1,2]. They have received an increasing scientific and industrialinterest due to their physical and chemical properties that is suit-able for different potential applications ranging from living matterstructure to nanometer-sized computer circuits and composites[3,4]. Since CNTs exhibit great mechanical properties along withextremely high aspect ratios (length-to-diameter ratio) rangingfrom 30 to more than many thousands, they are expected to pro-duce significantly stronger and tougher cement composites thantraditional reinforcing materials (e.g. glass fibers or carbon fibers).In fact, because of their size (ranging from 1 nm to 10 nm) and as-pect ratios, CNTs can be distributed in a much finer scale than com-mon fibers, giving as a result a more efficient crack bridging at thevery preliminary stage of crack propagation within composites.However, properties and dimensions of CNTs are strongly dependon the deposition parameters and the nature of the synthesismethod, i.e., arc discharge [5], laser ablation [6], or chemical vapordeposition (CVD) [6,7]. Carbon nanotubes used in this investigationwere produced by arc discharge technique with diameters from 3up to 8 nm and different length. The high specific strength, chem-ical resistance, electrical conductivity and thermal conductivity of

Table 2The dry mixes composition of blended cement (mass %).

Mixes OPC NMK CNTs

M0 94 6 0M1 94 6 0.005M2 94 6 0.02M3 94 6 0.05M4 94 6 0.1

146 M.S. Morsy et al. / Construction and Building Materials 25 (2011) 145–149

carbon nanotubes (CNTs) make them attractive for use as rein-forcement to develop superior cementitious composites [8,9].

Metakaolin has been recently introduced as a highly active andeffective pozzolan for the partial replacement of cement in con-crete. It is an ultrafine material produced by the dehydroxylationof a kaolin precursor upon heating in the temperature range of700–800 �C [10]. Metakaolin is a silica-based product that, on reac-tion with Ca(OH)2, produces CSH gel at ambient temperature.Metakaolin also contains alumina that reacts with CH to produceadditional alumina-containing phases, including C4AH13, C2ASH8,and C3AH6 [11,12]. Research results have shown that the incorpo-ration of metakaolin in concrete significantly enhances earlystrength [13]. Metakaolin increases resistance of concrete to alka-li-silica reaction [14], and its effect on sulfate resistance increasessystematically with increasing the replacement ratio of cement bymetakaolin [15]. Energy absorption or toughness of high-perfor-mance steel–fiber-reinforced concrete is increase with the intro-duction of high-reactivity metakaolin into the mix. Therefore, forapplications where both enhanced durability and high toughnessare required, the use of high-reactivity metakaolin concrete maybe advantageous [16]. However, other research has also shownthat increasing replacement levels of metakaolin produce increas-ing water demand, although this can be adjusted by adding a waterreducer to maintain the workability or flow properties [17].

Possessing nanoscale dimensions similar to calcium silicate hy-drates (C–S–H in cement nomenclature) which is the glue thatholds a cementitious matrix together, carbon nano fiber (CNFs)are expected to affect the nanoscale processes that control C–S–Hand to create hybrid CNF/cement composites with improved per-formance. However, considerable research efforts are focusing onCNFs in polymer composites [18,19] while limited efforts have fo-cus on their use in cement composites [20,21].

This research aimed to investigate the performance of hybridCNTs/nano-clay cement mortar composites in terms of microstruc-ture, physical, and mechanical properties.

2. Experimental work

2.1. Materials

The materials used in this study were nano-clay of Blaine surface area �48 m2/gand dimensions (200 � 100 � 20 nm), ordinary Portland cement (OPC), ASTM Type I[22]; supplied by Yamama Cement Company, Saudi Arabia .

The oxide composition of kaolin and ordinary Portland cement is shown inTable 1. The nano-clay used in this investigation was kaolin clay supplied by middleeast mining investments company (MEMCO), Cairo, Egypt. The nano-kaolin wasthermally treated at 750 �C for 2 h to produce active amorphous nano metakaolin.

The multi wall carbon nanotubes were synthesis in Egypt by arc discharge tech-nique with sizes diameter from 3 up to 8 nm with different length.

2.2. Mortar preparation and identification

The blended cement used in this investigation was ordinary Portland cement,nano metakaolin and CNTs. The OPC was substituted by 6% of nano metakaolin

Table 1The chemical composition of starting material.

Oxidecomposition

Ordinary Portlandcement (%)

Kaolin(%)

CaO 63.85 0.16SiO2 19.83 61.24Al2O3 5.29 20.89Fe2O3 3.53 6.38MgO 0.52 0.38SO3 2.43 0.17Na2O 0.21 1.61K2O 0.07 0.71TiO2 – 0.7P2O5 – 0.12Ignition loss 2.82 13.62

by weight [23]. Dispersant solution was prepared by adding organic ammoniumchloride solution containing 7 mg/g of clay. The nano metakaolin was added tothe dispersant solution and mixed to insure homogeneously. The solution is cov-ered and left for 24 h to insure that the clay plates had been exfoliated. The cement,exfoliated clay and CNTs were dry mixed for 5 min until homogeneity was achieved.The mortar was prepared using blended cement: sand ratio of 1:2 and water/binderratio of 0.5% as illustrated in Table 2.

The mortar pastes were molded into 5 cm cubes for compressive strength. Themolds were vibrated for 1 min to remove any air bubbles. The samples were kept inmolds at 100% relative humidity for 24 h, and then cured in water for 28 days. Thehardened cement mortar was removed from water before mechanical tests. Thecompressive strength was performed on wetted specimens. The crushed samplesresulted from compressive strength tests were grounded to be used for thermaland microstructure analysis. The evaporable water of the hydrated crushed sampleswas removed using the method described elsewhere [24].

2.3. Testing

2.3.1. Compressive strengthThe compressive strength tests were performed on a Toni Tech machine using

50 mm cube samples according to ASTM C-109 [25]. Three samples per batch weretested, and the average strength value was reported. The loading rate on the cubeswas 0.72 mm/min.

2.3.2. Differential scanning calorimeterDifferential thermal analysis conducted using a Shimadzu DSC 50 thermal ana-

lyzer at a heating rate of 20 �C/min. The samples chamber was purged with nitrogenat a flow rate of 30 cc/min.

2.3.3. Scanning electron microscopeThe scanning electron microscope (FEL-Spectra) was used for identification of

the changes occurred in the microstructure of the formed and/or decomposedphases. The SEM resolution was 4 nm.

3. Results and discussion

Fig. 1 shows the development of the mean values of compres-sive strength of blended mortar containing 6% exfoliated NMK ver-sus CNTs ratios at 28 days of hydration. Basically, the compressivestrength increased with the increase of CNTs until it reaches an

Fig. 1. Compressive strength of blended cement mortar containing exfoliated 6%NMK versus CNTs ratios at 28 days of hydration.

M.S. Morsy et al. / Construction and Building Materials 25 (2011) 145–149 147

optimal amount of 0.02% and then started to drop. Evidently, thereplacement of OPC by 6% NMK in blended mortar increases thecompressive strength by 18% compare to control mix. Theenhancement of compressive strengths of hardened cement mortardue to the addition of NMK can be explained by two mechanisms.The first strengthening mechanism was the packing effect of smallNMK acted as filler to fill into the interstitial spaces inside the skel-eton of hardened microstructure of cement mortar which leads toincrements in strength and density. The second strengtheningmechanism was the pozzolanic effect that combines glass-like sil-icon and alumina elements in NMK with the lime elements of cal-cium oxide and hydroxide in cement to add the bonding strengthand solid volume, resulting in higher compressive strength of hard-ened cement mortar.

Most pozzolanic reaction between the calcium hydroxide andamorphous NMK (silicon and alumina dioxide) normally reactsslowly during a prolonged period of moist curing. Since the plateletparticles of NMK have an average dimensions of (200 � 100 �20 nm) which is about 1000 times finer than average cement par-ticle of 20 lm resulting in an extremely large surface area, theNMK reacts very rapidly with the calcium hydroxide to form cal-cium silicate in an alkaline environment such as the pore solutionof fresh Portland cement paste. Additionally, the improvement ofcement mortar strength as CNTs loaded up to 0.02% was attributedto the crosslink of CNTs fiber with hydration product which lead toresist microcracks formation. Furthermore, at higher ratios of CNTsloaded the CNTs were agglomerated around cement grains leadingto partial hydration of cement grains and producing hydratedproduct having weak bond. Also, the fibers may not be wettedproperly thus causing fiber pullout resulting to formation andpropagation of microcracks.

Fig. 2 illustrates the variations of the differential scanning calo-rimetry (DSC thermograms of blended cement mortar containingexfoliated 6% NMK versus CNTs ratios at 28 days of hydration. Evi-dently, there were almost five endothermic peaks. The first endo-thermic peak located at 95 �C, which was mainly due to thedecomposition of calcium silicate hydrates (CSH).

The second endothermic peak observed at 174 �C represents thedecomposition of the gehlenite hydrate (C2ASH8). The third endo-thermic peak located at 380 �C, represents the decomposition ofhydrogarnet (C3ASH6). The fourth endothermic peak observed at470 �C represents the decomposition of CH. The fifth endothermicpeak appeared at 580 �C represents the decomposition of quartz.

Fig. 2. DSC thermograms of blended cement mortar containing exfoliated 6% NMKversus CNTs ratios at 28 days of hydration.

The mean features of the thermograms were characterized by aconsumption of the peak area of CH and an increase of the peakarea of CSH, C2ASH8 and C3ASH6 phases as the NMK loaded. The en-thalpy of formed CH during hydration decreases from 35.73 J/g to33.57 J/g for control and blended mortar containing 6% NMKrespectively.

Also the presence of NMK in mortar pastes leads to an increasein the enthalpy of CSH from 90.21 J/g to 113.91 J/g, whereas the en-thalpy of C2ASH8 increased from 0.421 J/g to 1.64 J/g and the en-thalpy of C3ASH6 increases from 0.078 J/g to 0.283 J/g. Moreover,the addition of NMK leads to the transformation of CH phases fromwell crystalline to ill-crystalline. Therefore, the increase of phase’senthalpy indicates the formation of well crystalline phases. Fur-ther, crystallizations produced by NMK and CH can fill up the poresand enhance microstructure and mechanical properties of cemen-titious materials. Furthermore, the loading of CNTs in cement athigher ratios decreases the enthalpy of CSH from 113.9 J/g at 0%to 92.69 J/g at 0.1% CNTs. At higher ratios of CNTs, the cementgrains were wrapped by CNTs particles which leads to a partialseparation of cement grains from hydration process. Basically,the partially hydrated cement grains decreases the formed hy-drates and bond strength of cement pastes. On the other hand atlower CNTs ratios, the formed hydrates were well crystalline andamorphous phases as well as the CNTs bridges the hydration prod-ucts and resist the formation of microcracks.

Fig. 3 shows the SEM micrographs of control and blended mor-tar containing NMK and CNTs hydrated for 28 days. Evidently, themicrostructure of the OPC mortar displayed the existence ofmicrocrystalline and nearly amorphous, mainly as calcium silicatehydrates (CSH). Furthermore, it can be seen calcium hydroxide(CH) crystals and air voids between the hydrated phases as shownin Fig. 3a. The SEM micrographs obtained for the NMK cementmortar indicated that the hydration products obtained were per-fectly dense structure. The calcium hydroxide was appeared asill-crystals as shown in Fig. 3b. Obviously, the pozzolanic reactionof NMK with calcium hydroxide liberated during hydration pro-duced additional CSH gel and ill-crystals CH which leads toimprovement in mechanical properties of blended mortar. TheSEM micrographs of mortar containing NMK and CNTs are illus-trated in Fig. 3c–f. It can be observed that the CNTs were disperseduniformly in the cement mortar and there was no obvious aggre-gation of CNTs. The microscopic observation also reveals that thesurface of CNTs was covered by CSH. The micrographs clearly dem-onstrated the dispersing potential of exfoliated NMK for CNTs incement mortar. The CNTs were found embedded as individual fi-bers in the paste and acting as bridges between hydrates andacross cracks (Fig. 3c and d). The NMK particles disrupted the fi-ber–fiber interactions (van der Waals forces) that held the CNTstogether as clumps. The small size of the NMK particles comparedto that of anhydrous cement particles (ca. 1000 times smaller) al-lowed them to work their way in-between the individual CNTsduring the dry mixing process, causing the CNTs to separate fromone another as mixing occurred, resulting in the separation offibers.

The presence of the small NMK particles, both in the clumpsthat remained after dry mixing and intermixed with the individual,dispersed fibers was thought to provide a ready source of siliconfor the generation of Ca–Si rich phases when combined with thehighly mobile Ca2+ ions. The CNTs may have provided potentialnucleation sites for the self-assembly of the Ca–Si rich phases. Basi-cally, as the CNTs loaded increases in cement mortar the SEMmicrograph indicated appearance of microcracks as presented inFig. 3e and f. Therefore, at higher concentration the CNTs can bere-agglomerate and slide on each other when the microcracksformed which will leads to weak bond in the microstructure andproduces lower strength.

Fig. 3. SME micrograph of ordinary Portland cement mortar; (a) control mortar, (b) mortar containing 6% NMK, (c) mortar containing 6% NMK and 0.005% CNTs, (d) mortarcontaining 6% NMK and 0.02% CNTs, (e) mortar containing 6% NMK and 0.05% CNTs and (f) mortar containing 6% NMK and 0.01% CNTs.

148 M.S. Morsy et al. / Construction and Building Materials 25 (2011) 145–149

4. Conclusions

The following conclusions may be drawn from the obtainedexperimental data.

� Replacement of OPC by 6% exfoliated NMK in cement mortarincreases compressive strength by 18% compare to control mix.� Due to its small particle size NMK facilitated CNTs dispersion

and improved the interfacial interaction between the CNTsand the cement phases. The CNTs were found as well anchoredin the hydration products throughout the paste.� The addition of CNTs (up to 0.02%) to NMK cement mortar

improves the compressive strength of the composites. Theimprovement was11% higher than blended mortar containing6% NMK but the addition of CNTs by 0.1% leads to decreasesthe compressive strength.

Acknowledgements

The authors gratefully acknowledge Dr. M. Etman for carbonnanotube synthesis, HBRC, Cairo, Egypt and technical support fromthe concrete Laboratory of KSU University.

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