Journal of Materials Research and Technology Journal of Materials Research and Technology
Original Article
Effect of multi-walled carbon nanotubes (MWCNTs) on the strength development of cementitious materials
Ali Naqia,, , Naseem Abbasb,, , Nida Zahrac, Abasal Hussaind, Syed Qasim Shabbire
a School of Civil & Environmental Engineering, Chung-Ang University, Seoul 06974, Republic of Korea
b School of Mechanical Engineering, Chung-Ang University, Seoul 06974, Republic of Korea
c Department of Physics, Government College University, Faisalabad 38000, Pakistan
d School of Civil Engineering, Dalian University of Technology, Dalian 116-024, China
e School of Civil & Environmental Engineering, Yonsei University, Seoul, Republic of Korea
Received 11 April 2018, Accepted 05 September 2018

The proposed study aimed to investigate the potential use of pristine multi-walled carbon nanotubes (MWCNTs) as nano reinforcement in enhancing mechanical properties of hybrid MWCNT/silica fume cement composites. Dispersion of MWCNTs was facilitated utilizing very fine particles of silica fume which also helped in an improved interfacial bond between MWCNTs and the cement matrix. The MWCNTs dispersion within the hardened cement matrix was qualitatively assessed by Field Emission Scanning Electron Microscope (FESEM) analysis. It was also observed that addition of MWCNTs accelerated the hydration process. The test results showed an increment in compressive strength by 12.4% and reduction in autogenous shrinkage by 8.5% for hybrid MWCNT/silica fume cement composites containing 0.01% MWCNTs (by wt. of binder). However, higher additions (greater than 0.03%) of MWCNTs appeared to have adverse effects on specimens. It was found that properly dispersed MWCNTs filled the fine pores in the cement matrix by providing an additional nucleation site for the formation of calcium silicate hydrate (C-S-H) that results in a denser microstructure, which in turn enhanced the strengths and limited the autogenous shrinkage.

Multi-walled carbon nanotubes, Silica fume, Dispersion of MWCNTs, Compressive strength, Autogenous shrinkage
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Concrete is the most widely used construction material for buildings, bridges, roads and dams all over the world. It is the second most used material (after water) on earth. However, cement paste phase of concrete is apparently very brittle material and possesses very low tensile strength and low strain capacity. Volume changes in concrete are generally attributed to the drying of concrete but there is an additional problem to drying shrinkage in early stages that occur without any moisture transfer to the surrounding environment. This volume decrease is termed as autogenous shrinkage and is related to the chemistry and changes in the internal structure [1,2]. Autogenous shrinkage should be controlled as it leads to cracking [3]. The phenomenon of autogenous shrinkage was identified a long time ago, but its practical importance in concrete technology was highlighted in recent years. In high-strength concrete, low water-binder ratio and inclusion of natural admixture, e.g. silica fume, result in a substantial drop in internal relative humidity value of the paste specimen during sealed hydration [4]. This autogenous change in relative humidity is closely related to the autogenous shrinkage, which results in internal tensile stresses due to restraint from the aggregates. To control the aforementioned problem researchers started testing macrofibres and microfibers as a mean of reinforcement in cementitious composites at micro level [5,6]. However, cementitious composites show flaws at nanoscale where fiber reinforcement is not effective [7].

Carbon nanotubes (CNTs) are of particular interest because of their high mechanical, electrical and thermal properties and their ability to prevent cracks growth at the nanoscale [8,9]. The nanotubes get their high stiffness and strength from the carbon–carbon (C–C) covalent sp2 bonding between individual carbon atoms. Carbon nanotubes are considered as the strongest and stiffest nanomaterials discovered yet in terms of elastic modulus and tensile strength, with a reported Young's modulus of nearly 1 tera-pascal (TPa), a tensile strength of up to 100giga-pascal and yield strain of 12% [10,11].

The two main challenges to encounter in order to achieve an effective CNTs/cement composite are, well dispersed CNTs within cement paste matrix and the bonding between nanotubes and the cement paste [12]. High aspect ratio and strong surface attraction between CNT particles make it extremely difficult to ensure uniform dispersion within the cement paste [13]. Poor dispersion results in the formation of defect sites in the cement paste matrix that leads to the formation of CNT bundles and limit the effectiveness of CNTs [14]. Researchers used a combination of physical (ultrasonication) and chemical (use of surfactant) techniques to better disperse CNTs in cement-based materials [15]. However, using a surfactant as a dispersant can cause issues of connectivity of carbon nanomaterials within cement matrix [16]. A strong bond between CNTs surfaces and cement paste is necessary for crack bridging that can provide considerable mechanical reinforcement [17]. Sanchez and Ince [18] utilize very fine particle sizes of silica fume, size ranging from 100 to 500nm, to overcome dispersion and bonding obstacles of nanofibers within cement matrix. Silica fume particles can mechanically separate the agglomerated MWCNTs within the cement paste matrix allowing the elimination of ultra-sonication process.

Cwirzen [19] used functionalized carbon nanotubes for the reduction of autogenous shrinkage. His initial test results showed an almost 50% decrease for 1.4wt.% of CNTs. Experimental results from Konsta-Gdotous et al. [20] showed a decrease in autogenous shrinkage with an increase amount of long MWCNTs compared to the reference plain cement paste. Specimens reinforced with 0.048wt.% of MWCNTs showed an increase of 30–40% higher flexural strength over plain paste specimen. Cwirzen et al. [21] achieved 50% higher compressive strength for the cement paste specimen doped by MWCNTs (0.045wt.%), which were dispersed by sonication prior to mixing in matrix. Collins et al. [22] demonstrated that with the use of polycarboxylate admixture in combination with ultrasonication exerted positive influence on compressive strength of cement paste. Shah et al. [20,23] have investigated the effect of various dosages of CNTs, varying between 0.02wt.% and 0.1wt.% by weight of cement, in cement paste specimens and reported that low concentrations of CNTs leads to more enhancement in mechanical properties of nanocomposites. Researchers have also studied the effects of CNTs on frost durability [24] and sensing properties of pastes and concretes. Li et al. [25] first developed piezo-resistive MWCNTs/cement composites using functionalized CNTs and measured the piezo-resistivity under uniaxial compression. Saafi [26] developed the piezo-resistive SWCNT/cement sensor for damage detection in concrete structures. Han et al. [27] integrated self-sensing CNTs concrete into concrete pavement and applied this payment to a real time detection of vehicles passing.

The present study experimentally investigates the effects of MWCNTs as nano reinforcement to enhance the compressive strength and minimize the autogenous shrinkage of MWCNTs/silica fume cement composites. The study involves a real time internal relative humidity [IRH] sensor embedded in fresh MWCNTs/silica fume cement composites and connected to a CR1000 data logger to measure the fluctuations in relative humidity values as autogenous shrinkage is closely related to the relative humidity. Previous studies did not undertake the relative humidity values for measuring autogenous shrinkage in cement composites. Cement composites with eight different amounts of MWCNTs addition: 0; 0.01; 0.02; 0.03; 0.05; 0.10; 0.20; and 0.30wt.% of binder were made. For each mixture the amount of silica fume applied was fixed at 10% by weight of binder. The role of silica fume in facilitating the dispersion of nanotubes and improving the interfacial bond between the cement phase and nanotubes is also studied. A polycarboxylate based superplasticizer (SP) was applied to improve the flow of MWCNTs/cement composites and to enable proper dispersion of nanotubes [18,28]. The MWCNTs dispersion within the hardened cement matrix was qualitatively assessed by FESEM analysis.

2Experiment details2.1Materials

Type 1 Ordinary Portland cement (OPC) (Sungshin Cement Co., Ltd.) and silica fume (Gansu Sanyuan Silicon Materials Co., Ltd.) were used in this study. Table 1 shows chemical composition of OPC and silica fume. Pristine MWCNTs that are a proprietary product of KUMHO Petro-chemicals were used in the specimens. The MWCNTs were produced with purity greater than 90% by weight. Table 2 shows the general specifications of MWCNTs used in this work. The polycarboxylate based superplasticizer was collected in liquid packing from Sure Chemical Co., Ltd.). The specifications of superplasticizer are listed in Table 3.

Table 1.

Chemical composition of Portland cement and silica fume.

Constituent  OPC (mass %)  Silica fume (mass %) 
SiO2  21.9  93.0 
Al2O3  4.9  0.4 
Fe2O3  3.7  0.5 
CaO  62.3  0.7 
MgO  2.0  0.5 
K20.5  0.8 
Na20.3  – 
SO3  2.2  – 
LOI  1.78  1.5 
Sum  99.5  97.4 
Table 2.

Specifications of MWCNTs (K-Nanos 100P).

Property  Unit  Value 
CNT type  –  Bundle 
Bundle diameter  μm  3–15 
Bundle length  μm  10–50 
Outer diameter  nm  10–15 
Bundle density  g/ml  0.02–0.04 
Purity  >90  0.45 
Crystallinity  IG/ID  0.6–0.8 
Table 3.

Specifications of polycarboxylate based superplasticizer.

Property  Specification 
Visual appearance  Pale brown viscous liquid 
Density (23°C)  1.12±0.2kg/l 
pH (23°C)  9.0±0.5 
Solid content (%)  40±0.5 
Stability (0°C, 24h)  No crystallization 
Cl (%)  ≤0.10 
Na2SO4 (%)
Na2O+0.658K2O (%) 
2.2Mixes preparation

A total of eight mixes were prepared and examined. Eight different quantities of MWCNTs, 0; 0.01; 0.02; 0.03; 0.05; 0.10; 0.20; and 0.30wt.% of binder were added. The concentrations are denoted by C0.00; C0.01; C0.02; C0.03; C0.05; C0.10; C0.20; and C0.30, respectively. Silica fume was added in an amount of 10% by weight of binder (S10) in each mixture to evaluate its effect on MWCNT/cement composite. The amount of water added to all the mixtures was kept constant at 20% by weight of binder. The idea of utilizing less water-binder ratio is to enhance CNTs dispersion by very fine particles of silica fume. When the water quantity in the cement matrix is reduced, the possibility of collision between the agglomerated MWCNTs and silica fume particles is increased [28–30]. Another reason of avoiding high water-binder ratio is the formation of agglomerates as black streaks of nanotubes within the cement paste samples following mixing [31]. The amount of superplasticizer (SP) varied between 0.05 and 0.8wt% of the binder. Table 4 shows mix design of test specimen. In order to prepare hybrid MWCNT/cement composites, mixing of material was carried out using standard Hobart mixer, specified in American Society for Testing Materials (ASTM) C 305. All the dried materials including cement, silica fume and MWCNTs were mixed for 5min. Water was added followed by superplasticizer after 2min and mixed again for an additional 5min at low speed. Mixing was finished with 5min additional mixing at high speed.

Table 4.

Mix design of test specimen. Unit [g].

Cement  Silica fume 
900  100  0.00  200  0.50 
900  100  0.10  200  0.70 
900  100  0.20  200  1.10 
900  100  0.30  200  1.50 
900  100  0.50  200  2.00 
900  100  1.00  200  2.50 
900  100  2.00  200  5.50 
900  100  3.00  200  7.80 
2.3Experimental procedure

All the specimens were demolded after 24h of casting and cured in water saturated with calcium hydroxide till the time of testing. The hardened specimens after 7-days curing were crushed and pre-conditioned for the microstructure and fracture surfaces analysis using FESEM-6330F, JEOL Ltd., Tokyo, Japan. Moreover, energy dispersive spectroscopy (EDS) technique was used to confirm the presence of MWCNTs and distinguish them from needle-like products which have similar structure like MWCNTs, by verifying chemical composition.

Cube specimen of 50×50×50mm size were prepared with MWCNTs concentration of 0; 0.01; 0.02; 0.03; 0.05; 0.10; 0.20; and 0.30wt.% of binder, cured at a constant temperature of 23±1°C and relative humidity of 100% according to the ASTM C 192. Compressive strength testing was done according to ASTM C 109 at 1–3-7 day's age. The internal relative humidity (IRH) in specimens was continuously monitored using SHT75 temperature and humidity sensors. In order to use this sensor, a data logger is attached to collect data and uses CR1000 software manufactured by US company. Fig. 1a shows an experimental view of the IRH measurement of the SHT75 temperature and humidity sensor and hybrid MWCNT/silica fume cement composite prepared for use in this study. Fig. 1b shows vicat needle test setup to check the setting time of the specimens. Fig. 1c shows specimens wrapped in aluminum foil after final set while Fig. 1d shows apparatus utilized to measure the length change of the specimen.

Fig. 1.

(a) RH sensors embedded in the specimen. (b) Measurement of final setting time of paste using vicat needle. (c) Specimens wrapped in aluminum foil after final set. (d) Apparatus used to measure length change of prism specimen.

The autogenous shrinkage testing of hybrid MWCNT/silica fume cement composites was done in accordance with ASTM C 490. The paste was cast into a 25×25×254mm prism. The setting time of paste was determined by using vicat apparatus according to ASTM C 191. Immediately after final set, specimens were demolded and wrapped using aluminum foil. A length comparator was used to measure the distance between the studs from the final setting time up to 6 days.

3Results and discussion3.1Compressive strength

The addition of pristine multi-walled carbon nanotubes has affected the compressive strengths of cement/silica fume paste specimens. The values were measured after 7 days of water curing. Fig. 2 shows compressive strengths of hybrid MWCNT/silica fume cement composites reinforced with varying amounts of MWCNTs (by wt. of binder) at different ages. The maximum compressive strength was achieved in samples with 0.01% MWCNTs. The specimens had 4.4%, 9.7%, and 12.4% higher compressive strengths than that of reference specimens at 1, 3 and 7 days respectively. Also, other specimens with low dosages showed higher compressive strengths as compared to the reference specimens. The 1, 3, and 7-day compressive strengths of specimens with 0.02% MWCNTs were about 0.2%, 7.9%, and 6.8% greater, while specimens with 0.03% MWCNTs produced 5.2%, 3%, and 4.1% higher compressive strength than that of reference specimens, respectively. Morsy et al. [32] showed similar results by improving compressive strength by 11% on addition of CNT (0.02wt.%) and nano metakaolin (6wt.%) while Cwirzen et al. [21] improved compressive strength by 50% for specimen with an addition of 0.045% MWCNTs compared to reference samples.

Fig. 2.

Compressive strength of hybrid MWCNT/silica fume cement composites reinforced with varying amounts of MWCNTs at different ages.

An amount of maximum of 1% nanotubes (by weight of total cement) addition to the mixes enhanced the compressive strength of the cement composites [7,33]. The plausible reason for this effect is that the smaller concentrations of MWCNTs agglomerates were easier to be dispersed by the silica fume into individual fibers that might act as pores and filled the pore space between the cement grains with hydration phases and reduced the capillary porosity [34,35]. The reduction in porosity leads to a denser microstructure than that of reference mix. Work by Nochaiya [36] showed that MWCNTs additions results in fewer mesopores than that of control mix. Due to their very small size, MWCNTs provide very large reactive surface areas that can provide additional nucleation site. The interfacial bond between the hydration product and MWCNTs depends on the surface energies of MWCNTs and hydrated product, while surface energies of CNTs ranged from 27 to 45 mj/m2[37]. The interface between MWCNTs and the hydrated matrix is improved by bridge effect and well dispersed MWCNTs would act as reinforcement fibers [22]. Fig. 3a and b shows carbon nanotubes bridging hydrates and nano-cracks respectively. Fig. 4a and b shows individually dispersed fibers embedded in the hydrated matrix. The FESEM images confirmed that these individually dispersed MWCNTs fibers could act as additional nucleation sites for the calcium silicate hydrate formation. Fig. 4c shows MWCNTs covered with hydration products. Makar and Chan [38] also obtained same images where single-walled carbon nanotubes (SWCNTs) were covered with C–S–H layer. Fig. 4d shows MWCNTs entangled in clumped silica fume. Chaipanich et al. [39] and Torkittikul et al. [40] showed an increase in compressive strength by 10% for specimens reinforced with 1% CNTs by weight of cement at 7 days testing age. While the test results of Musso et al. [41] showed an increase of 11% in compressive strength utilizing 0.5% pristine CNTs by weight of cement. However, all these test results include sonication method to disperse CNTs prior to mixing with other aggregates. On the other hand, in this study, additions of MWCNTs, 0.05; 0.10; 0.20; and 0.30wt.% obtained lower compressive strengths as compared to the reference specimens. The compressive strengths of specimens with 0.05wt.% MWCNTs were 0.1, 3.2 and 1.0% lower. Specimens with 0.10wt.% MWCNTs showed 0.7, 4.2 and 4.1% lower compressive strengths, while specimens with 0.20 and 0.30wt.% MWCNTs showed 10.4, 12.9, 12.4, 17.7, 21 and 20% less compressive strengths compared to the reference specimen at 1, 3 and 7 days respectively. Similar results are revealed by Morsy et al. [32] that compressive strength increased at low dosage of MWCNTs until it reaches an optimal amount of 0.02wt.% and then values started to drop. Kowald [42] incorporated MWCNTs in range of 0.5 and 5wt.% into the cement paste and found that low concentrations have enhanced the compressive strength while higher dosages have reduced the strength values compared to the reference specimen. This decrease in compressive strength for the higher amount of MWCNTs can be related to the inadequate dispersion as well as presence of agglomerates and bundles of MWCNTs around cement grains hindering the formation of hydration product resulting in weak bond. Also, nanotubes may not be wetted adequately thus causing pullout resulting in the formation of cracks [43].

Fig. 3.

SEM image of (a) MWCNTs bridging hydrates, (b) evidence showing MWCNTs bridging a nano-crack.

Fig. 4.

SEM image of (a, b) individually dispersed MWCNTs fibers embedded in hydration product (c) MWCNTs covered with hydration product (d) MWCNTs entangled in clumped silica fume(d).

3.2Internal relative humidity (IRH)

Fig. 5 shows internal relative humidity values of all specimens with varying amounts of MWCNTs. Specimen with the dosage of 0.01wt.% MWCNTs showed least drop in relative humidity among all other specimens that is approximately 5.5% less drop in IRH as compared to the reference specimen (silica fume composites). Specimen with 0.02wt.% MWCNTs dosage showed 2.5% less drop in IRH compared to reference specimen while specimen with 0.03wt.% dosage of MWCNTs showed nearly equal drop in IRH compare to the reference one. Fewer drops in relative humidity compared to the reference specimen indicate that smaller additions results in lower hydration rate of cement. In the case of low dosages, the individually dispersed MWCNTs had decreased the number of fine pores in the specimen, which results in a reduction of capillary stresses and ultimately lower autogenous shrinkage. However, specimens with 0.05; 0.10; 0.20; and 0.30wt.% MWCNTs produced quick drops of internal relative humidity with 8.5 and 8% more drop in IRH than that of the reference specimen. The reason for these higher drops in relative humidity can possibly be the smaller internal pore structure in the paste and inability of MWCNTs to fill the fine pores due to agglomerates and bundles of MWCNTs. Indeed, according to the Kelvin equation, small pore sizes gives reduce radii of water meniscus resulting in quick decrease of the internal relative humidity, which ultimately induce stress and enlarge shrinkage.

Fig. 5.

Internal relative humidity of hybrid MWCNT/silica fume cement composites reinforced with varying amounts of MWCNTs.

3.3Autogenous shrinkage

Fig. 6 shows the measured autogenous shrinkage as a function of time for varying dosages of MWCNTs. Figs. 7 and 8 exhibit a graphical presentation of shrinkage values at 3 and 6 days’ age. The shrinkage values were zeroed at the final setting time, which varies with varying amounts of MWCNTs incorporated in the specimen. According to the measured autogenous shrinkage values, specimen with an addition of 0.01wt.% MWCNTs showed highest reduction to 1413.385μmm/mm followed by specimen with 1503.937μmm/mm reduction for an addition of 0.02wt.% MWCNTs in comparison to the reference specimen that showed reduction to 1543.307μmm/mm at 6 days age. These measured values are in accordance with the findings of Blandine et al. [44]. Their experiment showed highest autogenous reduction at an additional level of 0.01wt% which is 50% less shrinkage value than the reference paste specimen. Experimental results of Metaxa et al. [45] also showed decreased autogenous shrinkage values for the cement paste reinforced with 0.048wt.% MWCNTs compared to the plain cement paste. The influence of MWCNTs to the autogenous shrinkage is related to the hydration of cement. Vicat needle test was utilized to analyze the effect of MWCNTs on the cement hydration. It was observed that MWCNTs have an accelerating effect on hydration of cement as varying dosages of MWCNTs showed differing final setting time. Table 5 shows final setting time of specimens with and without an addition of MWCNTs. It is evident from the table that the final setting time of hybrid MWCNT/silica fume cement composites was always less than the reference specimen that confirms shortening of setting time of hybrid MWCNT/silica fume cement composites by incorporation of MWCNTs. These results endorse the findings of Maker and Margeson who found a similar trend of final setting time for CNTs [46]. Fig. 9 shows the position of peak energy dispersive (keV) from spectrum. Table 6 describes atomic % of different elements in elemental analysis of energy dispersive spectroscopy that confirms the presence of dispersed multi-walled carbon nanotubes in the mixture. Owing to their very small size the well-dispersed MWCNTs (diameter 8–15nm) can fill the fine pores between the cement grains and their very large reactive surface areas can act as an additional nucleation site for the formation of calcium silicate hydrate (C-S-H). This leads to reduced capillary stresses that result in lower shrinkage. The mechanism for MWCNTs effect on cement hydration resembles to Jeffrey J. Thomas model that shows impact of well-dispersed nanoparticles on cement hydration [34]. Fig. 10 shows hydration of cement [A] and with nanoparticle [B] at different times after mixing. FESEM images confirmed the presence of individually dispersed MWCNTs fibers within the hydration product that possibly provided additional nucleation site for C-S-H formation. However, higher additions of MWCNTs 0.03; 0.05; 0.10; 0.20; and 0.30wt.% were not found to be effective in autogenous shrinkage reduction of the hybrid MWCNT/silica fume cement composites as these additions showed increment in measured shrinkage compared to the reference specimen. Although, these results contradict with the findings of Cwirzen [19] and Konsta-Gdotous et al. [20] which showed a decrease in autogenous shrinkage with an even higher percentage of MWCNTs (1.4 and 0.048wt.% long MWCNTs respectively) compared to the reference plain cement paste. The reason for increased autogenous shrinkage in this study is related to the quick drops in internal relative humidity, possibly due to the smaller internal pore structure in the cement paste and inability of MWCNTs to fill the fine pores due to agglomerates and bundles of MWCNTs. In fact, according to the Kelvin equation, small pore sizes gives reduce radii of water meniscus resulting in quick decrease of the internal relative humidity, which ultimately induce stress and enlarge shrinkage.

Fig. 6.

Autogenous shrinkage of hybrid MWCNT/silica fume cement composites reinforced with varying dosages of MWCNTs as a function of time.

Fig. 7.

Autogenous shrinkage of specimens with varying dosages of MWCNTs at 3-days age.

Fig. 8.

Autogenous shrinkage of specimens with varying dosages of MWCNTs at 6-days age.

Table 5.

Influence of MWCNTs on cement paste final setting time.

Specimen ID  Setting time
S10-C0.00  5:55 
S10-C0.01  5:45 
S10-C0.02  5:45 
S10-C0.03  5:35 
S10-C0.05  5:25 
S10-C0.10  5:15 
S10-C0.20  5:00 
S10-C0.30  4:40 
Fig. 9.

EDS analysis of 0.3wt.% hybrid MWCNTs/silica fume paste.

Table 6.

Elemental analysis of energy dispersive spectroscopy.

Element  Weight (%)  Atomic (%) 
4.75  8.55 
48.99  66.19 
Mg  0.44  0.39 
Al  0.34  0.27 
Si  1.09  0.84 
0.19  0.13 
Ca  42.87  23.12 
Fe  1.33  0.51 
Fig. 10.

Hydration of cement (A) and with nanoparticle (B) at different times after mixing (1–3) (Ref. Jo B.).

4Summary and conclusion

In this study, mechanical properties of hybrid MWCNT/silica fume cement composites have been investigated at concentrations of 0.01; 0.02; 0.03; 0.05; 0.10; 0.20 and 0.30% pristine multi-walled carbon nanotubes (by weight of binder). The specimens were made with 0.20% water-binder ratio and tested through the compressive strength test and autogenous shrinkage test. In this study, the dispersion of the MWCNTs was carried out using very fine particles of silica fume and dry mixed with cement in the Hobart mixer. The MWCNTs dispersion within the hardened cement matrix was qualitatively assessed by FESEM analysis. The effects of MWCNTs incorporation on the compressive strengths, internal relative humidity, and autogenous shrinkage were systematically investigated. The main results of this investigation are summarized as follow:

  • i)

    Silica fume particles were found to be effective to mechanically break the agglomerates of low dosages of MWCNT to smaller sizes.

  • ii)

    Individually dispersed MWCNTs filled the fine pores between the cement particles and accelerated the cement hydration by utilizing their very large reactive surface that acted as a nucleation site for the formation of C-S-H resulting in a denser microstructure, higher compressive strengths and reduced autogenous shrinkage.

  • iii)

    Optimum dosages of MWCNTs were found to be ranging between 0.01 and 0.02wt.% of binder for maximum strength development.

  • iv)

    FESEM images confirmed the multi-wall carbon nanotubes bridging hydrates and nano-cracks respectively.

  • v)

    Higher dosages (greater than 0.03wt.%) of MWCNTs appeared to have adverse effects on MWCNT/cement composites due to the formation of smaller internal pore structure within the paste and inability of the entangled and clumped MWCNTs to fill the fine pores.

Conflicts of interest

The authors declare no conflicts of interest.


This research was supported by Chung Ang University under Chung-Ang University Young Scientist Scholarship (CAYSS) program. Furthermore, the support from Korean Conformity Laboratories staff in testing of specimens is greatly appreciated.

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Corresponding authors. (Naseem Abbas
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