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Article

Combined Functionalization of Carbon Nanotubes (CNT) Fibers with H2SO4/HNO3 and Ca(OH)2 for Addition in Cementitious Matrix

by
Eduardo Batiston
1,2,
Paulo Ricardo de Matos
2,*,
Philippe Jean Paul Gleize
2,
Roman Fediuk
3,*,
Sergey Klyuev
4,
Nikolai Vatin
5 and
Maria Karelina
6
1
Department of Civil Engineering, Community University of the Chapecó Regions, Chapeco 89809-900, Brazil
2
Department of Civil Engineering, Federal University of Santa Catarina, Florianópolis 88040-900, Brazil
3
Polytechnic Institute, Far Eastern Federal University, 690922 Vladivostok, Russia
4
Department of Theoretical Mechanics and Strength of Materials, Belgorod State Technological University Named after V.G. Shukhov, 308012 Belgorod, Russia
5
Institute of Civil Engineering, Peter the Great St. Petersburg Polytechnic University, 195251 St. Petersburg, Russia
6
Department of Road-Building Materials, Moscow Automobile and Road Construction University, 125319 Moscow, Russia
*
Authors to whom correspondence should be addressed.
Fibers 2021, 9(3), 14; https://0-doi-org.brum.beds.ac.uk/10.3390/fib9030014
Submission received: 8 January 2021 / Revised: 28 January 2021 / Accepted: 1 February 2021 / Published: 1 March 2021
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Abstract

:
Acid treatment is commonly used to improve the dispersion of carbon nanotubes (CNT) in a cementitious matrix, but it causes undesired delay on cement hydration kinetics. This work reports a combined CNT functionalization method with H2SO4/HNO3 and Ca(OH)2 for addition in a cementitious matrix. Results showed that the Ca(OH)2 exposure neutralized the active sites generated by acid exposure, compensating the delay in hydration. As a result, CNT exposed to H2SO4/HNO3 for 9 h and further Ca(OH)2 treatment led to equivalent hydration kinetics than un-treated CNT did with improved stability.

1. Introduction

Carbon nanotubes (CNT) incorporation in cement-based materials can improve their mechanical performance by acting as bridges across cracks and voids, therefore increasing the strength and toughness of the composite [1,2,3,4]. In addition, the very high specific surface area (SSA) of CNT tends to accelerate the early hydration kinetics of cement, resulting in mechanical strength gains even at early ages [5,6,7,8]. However, properly dispersing CNT is a challenge due to its high SSA, clustering formation, and Van der Waals attractive forces [9,10,11,12].
The main methods for dispersing CNT are mechanical dispersion (e.g., high-speed ball milling) [13,14,15,16], ultrasonic dispersion [17,18,19,20], electric field induction [21,22,23], surface modification (also called functionalization) [1,24,25], or a combination of these methods. Among the functionalization methods, the most common is by exposing the CNT to a highly acidic environment (e.g., solution of sulfuric and nitric acid) [26,27,28]. In this condition, oxygen atoms from the acid can react with the carbon atoms of the defect sites and/or ends of CNT. As a consequence, some polar groups may be formed (e.g., –COH and –COOH), improving the wettability of CNT in water [29,30,31].
When CNTs are added to fresh cement mix, the polar groups formed by the functionalization process can bind with ions from the pore solution, delaying the cement hydration kinetics. This delay can slow down the formation of the microstructure of the cementitious matrix [4,32,33], impairing the mechanical strength of the composite at early ages [34,35,36,37]. In this context, Mendoza Reales et al. [38] evaluated the effect of a commercial dispersion of CNT on the hydration of a class G cement paste, finding that CNT addition delayed the induction period by up to 15 h, and this delay was proportional to the CNT content added. Torabian Isfahani et al. [39] produced cement pastes with 0 and 0.1 wt. % of functionalized CNT, finding that the incorporation of functionalized CNT reduced the 24-h cumulative heat by 3% compared with plain cement paste. Indirectly, Ahmed et al. [40] observed lower strength increases when functionalized CNT were added compared with un-treated CNT.
In order to prevent such undesired delay on cement hydration, this work investigated a combined functionalization of CNT with acid and calcium hydroxide for addition in cementitious matrix.

2. Materials and Methods

2.1. Materials

CNT manufactured by Cheap Tubes Inc. was used, with density of 1.7 g/cm3, external diameter of 8–15 nm, length of 10–50 nm, and 95% purity. Scanning electron microscopy (SEM) images of CNT are shown later in Section 3.1. A Portland cement type CP I-S [41] was used for paste preparation, the characteristics of which were provided by the manufacturer, as presented in Table 1. A polycarboxylate-based superplasticizer was used to achieve proper flowability, with pH of about 6, density of 1.09 g/cm3, and solid content of 30.5%.
The pastes were prepared with distilled water, using a water/cement ratio of 0.30 by weight, with the addition of both superplasticizer and CNT in 0.1 wt. % over the cement weight. Firstly, CNTs, water, and superplasticizer were mixed and sonicated for 20 min at 55 kHz in a low energy water bath sonicator (to prevent damaging the CNTs). Then, this suspension was mechanically mixed with the cement for 3 min.

2.2. CNT Treatment

Functionalization was conducted by immersing the CNT in a solution containing one part of HNO3 (65%) and three parts of H2SO4 (92%) in volume, for three different exposure times: three, six, and nine hours. The mixtures contained 100 mL of the acid solution and 10 mg of nanotubes. During the exposure, samples were kept in agitation using an ultrasonic bath. After the exposure to acid, samples were diluted in distilled water in the ratio of 100 mL of water for every 1 mL of acid + CNT solution. The solution was kept resting for 1 h, then filtered. The material retained on the filtration membrane was washed with distilled water and filtered until the residue reached a pH of 6 ± 1. In addition, a sample of CNT exposed to 9 h of acid treatment was further immersed in an aqueous solution of calcium hydroxide (Ca(OH)2) with a concentration of 1 g/L for 24 h, subsequently being washed and air-dried.
The samples were named according to the CNT treatment: CNT-0, -3, -6, and -9 respectively for zero, three, six, and nine hours of acid exposure, in addition to CNT-9-CH for the sample with coupled acid and Ca(OH)2 treatment.

2.3. Testing Methods

Zeta potential was conducted using a Zetasizer Nano ZS (Malvern) equipment in pH of 2–12. Raman spectrometry was conducted in a Ínvia (Renishaw) equipment with argon laser and wavelength of 514.5 nm within the green range of the visible light spectrum. SEM was conducted using a JSM 6701F (JEOL) microscope operating at 15 kV. Isothermal calorimetry was conducted on a TAM Air (TA Instruments) calorimeter at 21 °C for up to 50 h in paste samples of about 10 g.

3. Results and Discussion

3.1. CNT Treatment

Figure 1 shows the zeta potential and sedimentation of the CNT with different treatments. Un-treated CNT had an isoelectric point at a pH of 9.5 while increasing the H2SO4/HNO3 exposure time progressively decreased the pH of the isoelectric point. Consequently, CNT-9 had the highest stability among the samples (as visually confirmed in Figure 1b), with no isoelectric point and zeta potential from about −20 to −36 mV for pH of 8.5–12.0. The increase in exposure time to acid treatment progressively increases the formation of defects, i.e., active sites, as previously reported by [26,42]. In turn, subsequent Ca(OH)2 neutralization (CNT-9-CH) reduced the absolute zeta potential values compared with the non-neutralized suspension (CNT-9), indicating that the rapidly ionizable active sites created by the functionalization—which generate negative charge in solution—have been reduced.
Furthermore, the pH of cement pore solution is around 12, at 25 °C [43]. For this pH, all the CNT submitted to acid treatment had zeta potential from −33 to −36 mV, while un-treated CNT had −22.8 mV, confirming that H2SO4/HNO3 functionalization increased CNT stability in the pH of cement paste. The additional Ca(OH)2 neutralization led to a zeta potential of −26.3 mV, partially reducing the dispersibility of CNT when compared to acid-traded CNTs, but still 15% higher than un-treated CNT.
Figure 2 shows the Raman spectra of the CNT with different exposure times to H2SO4/HNO3 solution, where two bands are identified: D-bands (ID) at around 1340 cm−1, which corresponds to the disorder-induced band associated with structural defects, and G-bands (IG) around 1570 cm−1, associated with carbon-carbon stretching from graphite [44]. An increase in the ID/IG ratio was observed when CNTs were treated regardless of the exposure time, confirming the increase in the number of structural defects on the surface of the nanotubes [45]. Despite the increase in the number of defects with the functionalization, SEM images in Figure 3 showed that no significant changes in the overall morphology of CNT occurred, indicating that the acid treatment was not harmful to the integrity of the CNT.

3.2. Cement Hydration Kinetics

Figure 4 shows the isothermal calorimetry curves of the cement pastes, and Table 2 summarizes the testing results. The “Control” mix corresponds to a plain cement paste, i.e., without CNT addition. The incorporation of un-treated CNT (CNT-0) enhanced the early-age hydration of the cement compared with the Control mix, anticipating the occurrence of the main peak of heat release from 21.5 to 20.2 h and increasing the heat flow peak value from 2.03 to 2.18 mW/g of cement while reducing the induction period. The latter corresponds to the period of low reaction rate before the main cement reaction peak and is pointed out in Figure 4a. This behavior can be attributed to the high SSA of CNT: the incorporation of small particles provides extra surface to the nucleation and growth of the hydrated products resulting from the cement reaction, thus enhancing the early-age hydration kinetics [46,47,48,49], as reported by [5] for CNT incorporation in cement composite. In contrast, CNT functionalization delayed the cement hydration kinetics, and such delay increased as the acid exposure time increased: the main heat flow peak reduced down to 1.98 mW/g of cement and occurred up to 25.6 h for CNT-9. The polar groups created by the acid functionalization (e.g., –COH and –COOH) can bind with the Ca(OH)2 formed by cement hydration reactions [29,30]. Since the saturation of Ca2+ and consequent precipitation of Ca(OH)2 is recognized as the trigger for the end of the induction period [35], the capture of Ca2+ ions by those groups can delay that saturation and consequently the overall cement reactions.
Figure 4b shows the results for the pastes containing CNT with (CNT-9-CH) and without (CNT-0 and -9) subsequent Ca(OH)2 neutralization. The neutralization compensated the hydration delay caused by the acid treatment even for the highest exposure time investigated, resulting in a heat flow peak value of 2.11 mW/g of cement and occurrence time of 20.5 h for CNT-9-CH, similar to those of the CNT-0 mix (2.18 mW/g of cement and 20.2 h, respectively). This indicates that the polar groups formed in CNT bound with Ca2+ ions from the Ca(OH)2 neutralization solution before its addition in the paste, preventing the undesired delay in the cement hydration kinetics.

4. Conclusions

This work investigated the combined functionalization of CNT with H2SO4/HNO3 and Ca(OH)2 for addition in a cementitious matrix. Zeta potential and visual sedimentation indicated that the acid functionalization improved the dispersion and stability of CNT in water. Despite the increase in ID/IG ratio in Raman, SEM showed that H2SO4/HNO3 exposure did not lead to evident damage in the CNT’s morphology. However, the increase in the acid exposure time progressively delayed the early-age hydration kinetics of cement paste with the CNT. Further Ca(OH)2 neutralization limited the dispersion improvement promoted by acid treatment but compensated the undesired hydration delay, leading to equivalent cement hydration kinetics to un-treated CNT with improved stability.

Author Contributions

Conceptualization, E.B.; Data curation, P.R.d.M.; Formal analysis, R.F.; Funding acquisition, P.J.P.G. and M.K.; Investigation, E.B. and S.K.; Resources, P.J.P.G. and N.V.; Software, S.K.; Supervision, P.J.P.G.; Writing—original draft, E.B., P.R.d.M., P.J.P.G., R.F., S.K., N.V. and M.K. All authors have read and agreed to the published version of the manuscript.

Funding

The research is partially funded by the Ministry of Science and Higher Education of the Russian Federation as part of World-class Research Center program: Advanced Digital Technologies (contract No. 075-15-2020-934 dated 17 November 2020).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors acknowledge CAPES, CNPq and FAPESC from Brazil for the financial support, and LCME-UFSC for the SEM analysis.

Conflicts of Interest

The authors declare no conflict of interest.

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Fibers 09 00014 g001 550
Figure 1. Zeta potential (a) and sedimentation (b) of the carbon nanotubes (CNT) with different exposure times to H2SO4/HNO3 solution.
Figure 1. Zeta potential (a) and sedimentation (b) of the carbon nanotubes (CNT) with different exposure times to H2SO4/HNO3 solution.
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Figure 2. Raman spectra of the CNT with different exposure times to H2SO4/HNO3 solution.
Figure 2. Raman spectra of the CNT with different exposure times to H2SO4/HNO3 solution.
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Figure 3. SEM images of the CNT: before H2SO4/HNO3 exposure [×70,000 (a); ×20,000 (b)]; after H2SO4/HNO3 exposure [×80,000 (c); ×22,000 (d)].
Figure 3. SEM images of the CNT: before H2SO4/HNO3 exposure [×70,000 (a); ×20,000 (b)]; after H2SO4/HNO3 exposure [×80,000 (c); ×22,000 (d)].
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Figure 4. Isothermal calorimetry curves of the cement pates. (a) CNT with different exposure times Table 2. SO4/HNO3. (b) CNT with and without subsequent Ca(OH)2 neutralization.
Figure 4. Isothermal calorimetry curves of the cement pates. (a) CNT with different exposure times Table 2. SO4/HNO3. (b) CNT with and without subsequent Ca(OH)2 neutralization.
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Table
Table 1. Chemical and physical characteristics of the Portland cement used.
Table 1. Chemical and physical characteristics of the Portland cement used.
PropertyValue
Chemical composition (wt. %)
SiO220.17
Al2O34.06
Fe2O33.33
CaO60.96
MgO3.54
SiO33.43
K2O1.1
Na2O0.09
Loss on ignition3.05
Insoluble residue0.27
Physical property
Fc 1 day (MPa)23.8
Fc 3 day (MPa)29.2
Fc 28 day (MPa)40.2
Density (g/cm3)3.11
Blaine fineness (cm2/g)3460
Fc: nominal compressive strength.
Table
Table 2. Summary of the isothermal calorimetry.
Table 2. Summary of the isothermal calorimetry.
MixHeat Flow Peak
Value (mW/g Cement)Occurrence Time (h)
Control2.0321.5
CNT-02.1820.2
CNT-32.0523.2
CNT-62.0324.8
CNT-91.9825.6
CNT-9-CH2.1120.5
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Batiston, E.; de Matos, P.R.; Gleize, P.J.P.; Fediuk, R.; Klyuev, S.; Vatin, N.; Karelina, M. Combined Functionalization of Carbon Nanotubes (CNT) Fibers with H2SO4/HNO3 and Ca(OH)2 for Addition in Cementitious Matrix. Fibers 2021, 9, 14. https://0-doi-org.brum.beds.ac.uk/10.3390/fib9030014

AMA Style

Batiston E, de Matos PR, Gleize PJP, Fediuk R, Klyuev S, Vatin N, Karelina M. Combined Functionalization of Carbon Nanotubes (CNT) Fibers with H2SO4/HNO3 and Ca(OH)2 for Addition in Cementitious Matrix. Fibers. 2021; 9(3):14. https://0-doi-org.brum.beds.ac.uk/10.3390/fib9030014

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Batiston, Eduardo, Paulo Ricardo de Matos, Philippe Jean Paul Gleize, Roman Fediuk, Sergey Klyuev, Nikolai Vatin, and Maria Karelina. 2021. "Combined Functionalization of Carbon Nanotubes (CNT) Fibers with H2SO4/HNO3 and Ca(OH)2 for Addition in Cementitious Matrix" Fibers 9, no. 3: 14. https://0-doi-org.brum.beds.ac.uk/10.3390/fib9030014

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