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Article

Studying the Incorporation of Multi-Walled Carbon Nanotubes in High-Performance Concrete

by
Esequiel Mesquita
1,
Ana Mafalda Matos
2,*,
Israel Sousa
1,
Mylene Vieira
1 and
Luís P. M. Santos
3,*
1
Laboratory of Rehabilitation and Building Durability (LAREB), Federal University of Ceará, Campus of Russas, Russas 62900-000, Brazil
2
CONSTRUCT-Labest, Faculty of Engineering, University of Porto, 4200-465 Porto, Portugal
3
Graduate Program in Materials Science and Engineering, Center of Technology, Federal University of Ceará, Campus of PICI, Fortaleza 60440-000, Brazil
*
Authors to whom correspondence should be addressed.
Sustainability 2023, 15(17), 12958; https://doi.org/10.3390/su151712958
Submission received: 20 July 2023 / Revised: 18 August 2023 / Accepted: 21 August 2023 / Published: 28 August 2023
(This article belongs to the Special Issue Sustainable Composite Materials)
Browse Figures
Review Reports Versions Notes

Abstract

:
The current work aimed to study nanomodified HPC with multi-walled carbon nanotubes (MWCNT). The effect of MWCNT concentration, from 0% to 0.6% of cement weight, was evaluated on HPC multi-level output properties, namely, the flowability, mechanical strength, electrical resistivity, and microstructure. In addition, a tentative, simplified, and more cost-effective method based on dispersion of a high-pH solution of hydroxide was also adapted to disperse the MWCNT before incorporation in fresh HPC mixtures. Adding 0.2–0.6% MWCNT reduced HPC workability even with a higher superplasticiser dosage. The electrical resistivity was 484.58 Ω m for the HPC without MWCNT at 28 days of curing, while the samples with 0.2%, 0.4%, and 0.6% MWCNT presented 341.41 Ω m, 363.44 Ω m, and 360.34 Ω m, respectively. The use of 0.2–0.6% MWCNT in HPC decreased the flexural and compressive strength by 20% and 30%, respectively. The HPC performance decrease with MWCNT seemed to be related to relatively significant agglomerations of the long MWCNTs, namely, in HPC-0.6% samples. New developments are needed to state a simple and cost-effective dispersion method for MWCNT incorporation in HPC. In addition, smaller dosages of MWCNT are suggested for future research works.

1. Introduction

Concrete is the most consumed building material in the world, with an annual overall flow estimated at 20–25 Gt [1]. Concrete is and will be the backbone for sustainable development since it fits the basic needs of society in housing and infrastructures. Thus, concrete is essential for achieving the Agenda 2030 sustainable development goals since it intersects with economic, social, and environmental spheres.
For a long time, researchers have made efforts to optimise concrete behavior, namely mechanical strength and workability. The development and implementation of technologically advanced concrete materials, such as high- and ultra-high-performance concrete (HPC and UHPC, respectively), may be seen as the beginning of a new era of what is genuinely the sustainable development of concrete as a high-tech bulk material at the forefront of science and engineering materials that can be adjusted not only to meet the classic requirements of mechanical properties and durability but also to contribute to reducing the environmental footprint of society’s consumption.
HPC and UHPC are advanced cementitious composites featuring self-compacting ability, superior mechanical properties, durability, and a polished aesthetic appearance. This material is characterised by its high mechanical strength, ductile behavior due to the incorporation of fibers, and high resistance to the penetration of aggressive agents. They can provide numerous opportunities for application, particularly in structural engineering [2]. HPC and UHPC have been applied in construction projects such as highway bridges, pedestrian bridges, long-span bridges, girders and decks, railway sleepers, protective facilities, stadiums, and architectural elements such as façade panels [3,4,5]. The insitu application for rehabilitation or strengthening projects has also been an exciting field of application [6,7]. Although the direct cost and the specificity of HPC/UHPC applications are still limiting their use, the expected lower indirect costs and construction time may provide a mark of economic sustainability when compared with ordinary concrete. Even though the production of HPC/UHPC requires more cement per unit volume than ordinary vibrated concrete and, consequently, more embodied CO2 and energy consumption, the slender HPC/UHPC elements provide overall material savings, weight reduction, and significant gains in terms of durability. Moreover, HPC and UHPC elements or structures should involve lower-maintenance interventions and faster construction [1,7,8,9,10]. More recently, the development of HPC in contemplating the massive incorporation of supplementary cementitious materials as partial cement replacements started to be pursued as a means to reduce embodied CO2 and costs while providing other environmental benefits [11,12].
An exciting opportunity for HPC and UHPC is the intrinsic smart properties (such as self-sensing, self-healing, and thermo-responsive) [13]. This will transform passive structural elements into stimulus-responsive elements, as already observed in aerospace, biomedical, and semiconductor industry fields. The addition of specific constituents in small dosages to cement-based materials has already shown potential to provide exceptional properties, such as thermal parameters self-adjusting by using phase change materials, self-healing through super absorbent polymers or damage self-sensing employing nano-iron particles, carbon nanotubes, or carbon microfibers, among others [14,15,16,17]. So, a self-sensing capacity and high performance contribute to advancing smart materials for SHM applications [18].
In the context of self-sensing abilities, carbon nanotubes (CNTs) have been under the scientific community’s attention due to their exciting electrical and mechanical properties [19,20]. CNTs are quasi-one-dimensional carbon nanomaterials with diameters ranging from 5 to 100 nm and lengths varying between 500 nm and 15 μm [20,21,22] and, as such, with aspect ratios ranging from 30 to some thousand. CNTs can be classified as single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) [23]. One-layer graphene sheets rolled up are considered SWCNT, and MWCNTs are rolled up by multilayer graphene sheets [23]. Generally, CNTs present a modulus of elasticity of approximately 1 TPa, a tensile strength between 65 and 93 GPa, and a yield strain of 10–20% [24]. Moreover, the electrical conductivity can range from 102 to 10−4 S/cm [24]. Those engineering properties may prevent the initiation of microcrack formation in the cementitous matrix, even at very early ages [25]. Previous studies revealed that CNTs, namely MWCNTs, improved the mechanical behaviour and durability properties of cement-based composites [26,27,28,29,30,31,32,33,34,35,36,37]. However, most of those studies concern conventional cement-based paste or mortar; those conclusions cannot be directly drawn to advanced concrete material families such as HPC or UHPC.
Research efforts have also been made to embed nanomaterials within HPC/UHPC [38,39]. The main findings follow here. The nanoinclusion of CNT may reduce the porosity of UHPC, especially in the fibre–matrix interface, significantly increasing compressive strength and flexural strength [40]. According to Huang et al. [40], the nano inclusion of carbon-based nanoparticles reduced, even more, the porosity of UHPC (from 5.5% to 4%), especially in the fiber–matrix interface (ranging from 6–12.5% to 4–7%), increasing significantly the compressive strength (15–50%) and the flexural strength in 80–90% [40]. In other works, as independently reported by Yoo et al. [41] and Lee et al. [42], tensile self-sensing capacity can be provided to UHPC by incorporating 0.5 in volume (%) of CNTs and 2 volume (%) of micro steel fibres. The microstructure analysis of UHPC with CNTs revealed a more complex hydration reaction; however, a stiffer and denser structure was observed [43,44]. Meng and Khayat [45] suggested that the “filler effect” of CNT graphite nanoplatelets (GNP) increased the cement degree of hydration and reduced the porosity of UHPC, and, consequently, produced gains in compressive strength. When microcracks form, the “bridging effect” of the CNT and GNP also improves the mechanical performance. The addition of CNT and GNP was 0.05% to 0.5%, concerning the mass of the binder [45]. Other work suggested that adding 0.067% CNT improved 70% in terms of UHPC flexural strength compared to the same UHPC without CNTs [46].
The nanomaterial dispersion into a cementitious material mixture significantly influences the product’s final properties. As previously mentioned, CNTs present a high aspect ratio, specific surface area, hydrophobic feature, and strong van der Waals forces. Thus, the dispersion in aqueous solutions may be complex. As such, one of the main challenges of using nanomaterials in cementitious matrixes is the workability decrease due to CNT free water adsorption [39]. This is particularly valid for a water-to-binder ratio (w/b) such as in HPC/UHPC [39]. Poor dispersion of CNTs can create weak zones and form voids [47], reducing the reinforcing efficiency of CNTs. CNT agglomerations may cause defect sites in the hardened cementitious matrix, resulting in fragilities that impact the final mechanical strength. The CNT hydrophobic surface may also create a weak CNT–matrix interfacial bond, compromising the desirable CNT bridging effect [46].
When CNTs are added to the fresh cement-based mixture, the regular mixer and procedures used to prepare cement-based materials may not be applicable. In fact, CNT incorporation needs extra steps, namely, they need to be dispersed in a solution (surfactant and deionised water, for instance) before mixing into cement material. As such, several CNT dispersion methods have been under scrutiny. The main findings are based on ultrasonic methods using sonicators, chemical dispersion using surface treatments such as surfactants, and incorporation of silica fume [48]. The ultrasonic techniques and the mixing of surfactants are the basis of most dispersion methods and seem to provide enough dispersion. Concerning methodologies for the dispersion of CNT into ionic cementitious composites, Mendonza et al. [49] observed that the negative charge of hydroxyl-functionalised CNT can interact with Ca(OH)2, influencing the CNT dispersion and generating a re-agglomeration effect. In the same line, Silvestro et al. [50] assessed the carboxyl-functionalised CNT stability dispersion in contact with a simulated cement pore solution for six hours through UV–visible spectroscopy, also identifying that an alkaline environment affects CNT stability, gradually reducing the dispersed CNT concentration over time. Given this, using NaOH as a dispersing agent can be a promising alternative for the dispersion of CNT for application in cementitious composites. Furthermore, if the dispersion and homogeneity of CNT dispersion with NaOH are guaranteed without sonication, it will also represent an advance for the production of cementitious nanocomposites on a large scale.

2. Research Significance and Objectives

From the literature review, the authors highlight that producing HPC with very low w/c and CNT incorporation is still a challenge. Moreover, for scalable application of CNT into cement composite materials, the dispersion methodologies need to be simplified and friendly and potentially without the necessity of high-cost instruments or limitations regarding the CNT content [51].
Previous research [20] indicates that MWCNT (optimum content between 0.06–1% cement weight) is preferred for cement materials due to cost, physical properties, and higher conductivity; however, the incorporation of MWCNT in HPC is still lacking. This way, the current work aimed to develop an HPC nanomodified composite with different contents of multi-wall carbon nanotubes. The effect of MWCNT concentration, from 0% to 0.6% of cement weight, was evaluated on HPC output properties, namely, the flowability, mechanical strength, electrical resistivity, and microstructure observation. A tentative, simplified, and more cost-effective method based on dispersion through a high-pH hydroxide solution was also adapted to disperse the MWCNT before incorporation in fresh HPC.

3. Materials and Methods

3.1. Raw Materials and Mix Design

The HPC was a non-property ternary blend produced with commercially available materials in Portugal, namely, Portland cement CEM I 42.5 R (according to EN 197-1 [52]), limestone filler (LF), and dry micro undensified silica fume (SF). The aggregate fraction of HPC corresponds to siliceous natural sand with a maximum dimension of 1 mm, a 2570 kg/m3 density, and 0.5% water absorption. To guarantee self-compacting behavior, a liquid superplasticiser (Sp) was used (density of 1070 kg/m3 and solid content of 29.5%).
Table 1 summarises the main element analysis and the physical properties of cementitious materials employed in the current research. Figure 1 presents the particle size distribution (PSD) of cementitious materials (by laser method for cement and limestone and SEM method for silica fume) and sand (sieving method according to EN 933-1 [53]). SEM images in the secondary electron mode of cement, silica fume, and limestone particles are depicted in Figure 2. It can be perceived that cement particles are angled and present a wider size range. In contrast, silica fume assumes a perfectly spherical shape with a narrow size distribution, and limestone presents a polyhedron shape with a PSD between cement and silica. SEM observations corroborate PSD analysis (Figure 1).
The functionalised MWCNTs were supplied by CTNano (UFMG), and, according to SEM observation, shown in Figure 3, they presented a spaghetti shape with a diameter near 20 nm. The main properties of MWCNT are shown in Table 2.
The HPC mixtures’ proportions are presented in Table 3 and were designed according to the authors’ previous work [54,55,56]. Four HPC mixtures were produced, namely, a control mixture (HPC-0%) without MWCNT, and three HPC mixtures incorporating 0.2%, 0.4%, and 0.6% of MWCNT regarding the mass of cement and called HPC-0.2%, HPC-0.4%, and HPC-0.6%, respectively. As can be perceived, the mixture proportions were kept constant among all HPCs, except the MWCNT content. Moreover, the superplasticiser dosage (Sp) was adjusted.

3.2. MWCNT Dispersion and Specimen Production

Considering that an alkaline environment positively influences the MWCNT stability and maintains the dispersed MWCNT concentration over time in cementitious mixtures [20,49,57,58], a dispersion method based on NaOH was developed based on previous work [20] and the experience of the authors. A NaOH solution was prepared to disperse the MWCNT, consisting of the magnetic stirring of 50 mL of distilled water and 10 g of NaOH for 30 min. Afterwards, the MWCNT was dispersed in the NaOH aqueous solution by magnetic stirring for 15 min.
All HPCs were prepared in batches of 1.20 L using a mixer following EN 196-1 [59]. The mixing sequence followed the authors’ previous work [55], except for the first step for HPC with MWCNT. In that case, the NaOH solution with dispersed MWCNT was added to the mixing water and mixed for 5 min at a speed of 140 rotations per minute (low speed).
After production, the flowability of the fresh composites was evaluated using a mini-slump test (described in Section 3.3). Then, 40 × 40 × 160 mm³ prismatic specimens were cast for each HPC mixture to access electrical resistivity at 2, 7, 14, 21, and 28 days (Section 3.4) and mechanical strength at 28 days (Section 3.5). In addition, small cylindrical specimens with a diameter of 50 mm and height 30 mm were cast and water-cured for SEM observations (Section 3.6). The HPC-0% achieved self-compacting ability; no vibration or compaction process was needed. Concerning HPCs incorporating CNTs, the self-compacting capacity was not reached (as discussed in Section 4.1), and mechanical vibration was employed by allocating the specimens 30 s on the vibration tables.

3.3. Flow Test

Immediately after production, the slump-flow test evaluated fresh HPCs’ flowability using the stainless steel mini-cone suggested by EFNARC recommendations [60]. The mixture design of HPC families requires a high content of very fine materials (cement and supplementary cementitious materials), in the current work 1140 kg/m3, and very low w/b (0.145 in this work, see Table 3). Usually, HPC fresh mixtures show high viscosity and low risk of segregation. Thus, no additional test, such as the t-funnel test, was performed in the fresh state.

3.4. Electrical Resistivity

In the present work, two electrodes assessed the electrical resistivity under an alternating current technique, following the procedure suggested by Polder [61]. Thus, the specimens’ faces were allocated stainless steel mesh at casting. As mentioned in Section 3.2, the electrical resistivity was evaluated on prismatic specimens (40 × 40 × 160 mm³), which were maintained under water curing in controlled temperature conditions (T = 20 ± 2 °C) and only removed to perform electrical resistivity measurement at 2, 7, 14, 21 and 28 days.
A sinusoidal current of 100 Hz with a peak voltage of ±10 V was applied to the specimens, and the response was recorded with the multimeter at 0.0001 amp resolution. Through Equation (1), based on Ohm’s Law, the electrical resistivity of the HPC samples was calculated considering the specimens’ geometry. The electrical resistivity of each HPC series (see Table 3) is the average result of three identical specimens.
ρ = V × A I × L
where ρ is the resistivity in Ω·m, V is the voltage in Volts, A is the cross-section area in m2, I is the electrical current in Amper, and L is the distance between electrodes in m.

3.5. Mechanical Strength

The mechanical strength, flexure and compressive, were assessed according to NP EN 196-1. In brief, for each HPC mixture (see Table 3), three prismatic specimens (40 × 40 × 160 mm³) were produced. The specimens remained for 24 h in stainless steel moulds and then were water cured (under a controlled temperature 20 ± 2 °C), until testing time, at 28 days.

3.6. Microstructure Observation

The microstructure of HPCs with MWCNTs was analysed using an FEI-Quanta 450 high-resolution field emission gun scanning electron microscope (FEG-SEM). Samples were kept under water curing at a controlled temperature of 20 ± 2 °C and just removed before SEM observation. Before introducing it in the SEM apparatus, HPC samples were coated with an Au thin film by sputtering.

4. Results and Discussion

4.1. Flowability

The flow spread diameters of all HPC mix compositions are depicted in Figure 4. The reference mixture with no MWCNT, HPC-0% (see Figure 4a), presented a flow diameter of 280 mm and thus self-compacting ability. The HPC mixtures incorporating MWCNT, HPC-0.2%, HPC-0.4%, HPC-0.6%, showed a minimum flow of 100 mm, which is the cone-down diameter; see Figure 4b–d. The MWCNT compromised the HPC’s workability, even with a higher superplasticiser dosage (see Table 3). This seemed justified by MWCNT’s water absorption characteristics, the high aspect ratio, and the high specific surface area, and, consequently, the strong van der Waals forces. The HPC developed (HPC-0%) did not require any vibration due to its self-compacting ability (Figure 4a). However, external mechanical vibration was needed for the HPC–MWCNT series due to no-flow value occurrence. Otherwise, specimens would not be sufficiently compacted [13,62] and would be inadequate for resistivity and mechanical tests.

4.2. Electrical Measurements

Figure 5 depicts the electrical resistivity progress of HPC specimens between 2 and 28 days. As expected, the electrical resistivity develops with time. This evolution is known in cement-based materials as a result of the hydraulic reaction of cement and water and, eventually, the pozzolanic reaction of some SCM with calcium hydroxide, such as the silica fume employed in the current work, providing a denser microstructure, in which the pore volume and interconnectivity reduce with time [63,64,65]. Electrical resistivity ranged from 341 to 485 Ω m at 28 days, demonstrating a very compact cementitious matrix. As a reference, Polder [61] stated that electrical resistivity from 300 to 1000 Ω m is likely in a ten-year dense-aggregate concrete with silica fume submerged at 20 °C [61]. Moreover, it seems that electrical resistivity would evolve after 28 days, as suggested by Figure 5.
In the first two days of curing, the electrical resistivity observed was equivalent to 31.24 Ω m for the HPC without MWCNTs, while the HPC with 0.2% and 0.4% of MWCNT presented electrical resistivity of 39.37 Ω m and 39.53 Ω m, respectively. The HPC at 28 days of curing presented a value of 484.58 Ω m, while the samples with 0.2%, 0.4%, and 0.6% of MWCNT showed electrical resistivity of 341.41 Ω m, 363.44 Ω m, and 360.34 Ω m, respectively. The sensible difference between the electrical resistivity of the sample with the addition of 0.2%, 0.4%, and 0.6% of MWCNT at 28 days indicates that the dispersion method was not enough, especially with the content of 0.4% and 0.6% of MWCNT.
Figure 6 shows the relationship between the electrical conductivity of the sample and the MWCNT content. A higher conductivity was observed at 2 days of curing due to the still water availability in the cementitious matrix. For the HPC without MWCNTs, the conductivity is equivalent to 32 mS/m. From 7 days to 21 days of curing, a slight variation occurred among the HPCs produced. However, between days 21 and 28 of curing, a more significant reduction in the conductivity of HPC without MWCNTs was observed with 0.0021 S/m, while the HPC with 0.2%, 0.4%, and 0.6% presented 0.0029 S/m, 0.0027 S/m, and 0.0028 S/m, respectively. The increase in conductivity observed in the composites produced with MWCNTs compared to the composite without MWCNTs occurs through electrical percolation [66]. The conductivity is related to the addition of MWCNTs in the cement composite. The nanotubes added to the cement mixture develop a connection network that allows lower resistance in the electrical current until a percolation limit is reached [43,67].

4.3. Mechanical Strength

Figure 7a,b present the mechanical strength of HPCs in flexure and compression, respectively, including standard deviation (red bars). A reduction in the compressive strength of the HPC with MWCNTs occurred, from 104 MPa in the HPC without nanotubes, to 78 MPa, 80 MPa, and 78 MPa for HPCs with 0.2%, 0.4%, and 0.6% of MWCNT incorporation, respectively. Although some works show that adding nanomaterials in cementitious composites promotes an increase in their mechanical properties [40,43,68], this effect is not always observed. A reduction of mechanical strength with the increase of nanotube content can be justified by the significant increase in water absorption, causing a reduction in the amount of water for cement hydration as the MWCNT content increases in the mixture. The mechanical strength reduction in the HPC families is also prone due to the low w/c, indicating a smaller amount of water available for the hydration reactions [69]. The dispersion method obtained also seemed insufficient to disperse the MWCNT, at least for HPC families. It may influence the composite’s mechanical properties by increasing the samples’ porosity [70,71]. These effects can also justify the reduction in flexural strength, in which the HPC without MWCNTs presented 20 MPa and reached values of 16 MPa, 18 MPa, and 17 MPa, for the contents of 0.2%, 0.4%, and 0.6% of MWCNT incorporation, respectively.
Previous research by Cui et al. [71] revealed that the compressive strength of HPC incorporating CNTs decreased compared to the same HPC without CNTs. Moreover, the compressive strength was affected by the CNT dispersion method adopted. HPC specimens with CNTs previously dispersed by ultra-sonication and the addition of surfactant showed the best compressive strength results, followed by ultrasonically dispersed CNTs without surfactant and, lastly mechanically dispersed CNTs. Thus, the dispersion method of CNTs may significantly affect the compressive strength, and if CNTs are not still properly dispersed before incorporation in HPC, mechanical behaviour can decrease. This seemed to be related to CNT agglomeration, which created defect sites in the cementitious matrix and may even act as a crack initiator. Zhang et al. [72] observed MWCNT agglomeration using SEM, which justified it as a major cause of the mechanical and damping properties of HPC with MWCNT incorporation.
Moreover, ineffective or poorly dispersed CNTs can increase the viscosity of fresh mortar at lower shear stress compared to the same mortar with no CNT. The high or excessive viscosity entraps additional air into the fresh mortar, leading to a higher porosity in the hardened CNT–UHPC, which also has side effects on mechanical strength [71]. The MWCNTs’ size also has a role in both compressive and flexural strength. Previous studies revealed that shorter-size MWNTs with OD above 8 nm increased the compressive strength since they can better fill the nanopore space within the cement matrix more efficiently if uniformly dispersed. On the other hand, longer MWCNTs provide a better reinforcement effect and, consequently, an improved flexural strength [71,73,74].

4.4. SEM

SEM observations of HPC samples under study were performed, and some examples are presented in Figure 8. Although individual MWCNTs can be identified within the cementitious matrix, relatively large agglomerations of the long MWCNTs were observed in HPC-0.6%; see Figure 8d. Even though some regions of HPC samples have no MWCNTs, other regions presented agglomerated MWCNTs, which was also found by [73]. SEM observations corroborate the results obtained in the previous sections. Even though the MWCNTs reduced the resistivity, i.e., increased the conductivity, the HPC suffered high flowability loss (Section 4.1) and decreased mechanical strength when MWCNTs were employed (Section 4.3). This highlights the need for more efforts to improve the current MWCNT dispersion techniques for application in high-performance concrete material families.
The hydration products of Portland cement, namely, small fibres of calcium silicate hydrate (C–S–H) and calcium hydroxide (CH), can be seen in Figure 8. HPC samples with 0.4 and 0.6 wt.% MWCNTs showed some connections between the hydration products and the MWCNTs; this can be observed in Figure 8c,e, indicating the inclusion of multi-walled carbon nanotubes among the cementitious hydration products, as observed by [75].

5. Conclusions and Final Remarks

The current work investigated the effect of MWCNTs on the fresh, mechanical, electrical, and microstructural dimensions of HPC. The HPC was proportioned with 0%, 0.2%, 0.4%, and 0.6% of MWCNT by cement mass. In addition, a new, simplified, and more cost-effective MWCNT dispersion method (a pre-mixing method) was experimented for the first time. Significant findings of the current work can be drawn as follows:
  • The addition of 0.2–0.6% MWCNT in HPC reduced the workability of HPC even with a higher dosage of superplasticiser.
  • Adding 0.2–0.6% MWCNT in HPC reduced the electrical resistivity compared to the HPC without nanotubes. The electrical resistivity reached 484.58 Ω m for the HPC without MWCNTs at 28 days of curing, while the samples with 0.2%, 0.4%, and 0.6% MWCNT presented 341.41 Ω m, 363.44 Ω m, and 360.34 Ω m, respectively.
  • The use of 0.2–0.6% MWCNT in HPC caused a decrease of 20% and 30% in flexural and compressive strength, respectively.
  • SEM observations revealed some regions without MWCNTs, regions with individual MWCNTs within an HPC cementitious matrix, and relatively large agglomerations of long MWCNTs, namely in HPC-0.6% samples.
As such, the dispersion method employed in the current work was not effective, at least for HPC families, with typically low w/b. Still, efforts are needed to improve the current technique for efficient MWCNT dispersion and incorporation in HPC. In addition, smaller dosages of MWCNTs are suggested for future research works.

Author Contributions

Conceptualisation, E.M., A.M.M. and M.V.; methodology, E.M., A.M.M., M.V. and L.P.M.S.; validation: A.M.M., I.S. and L.P.M.S.; formal analysis; E.M., A.M.M. and L.P.M.S.; investigation, E.M., A.M.M., I.S., M.V. and L.P.M.S.; data curation, E.M., A.M.M. and I.S.; writing—original draft preparation, E.M., A.M.M., I.S. and L.P.M.S., writing—review and drafting, E.M., A.M.M. and M.V.; funding acquisition, E.M., A.M.M. and M.V. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Base Funding—UIDB/04708/2020 and Programmatic Funding—UIDP/04708/2020 of the CONSTRUCT—Instituto de I&D em Estruturas e Construções—funded by national funds through the FCT/MCTES (PIDDAC) and by FCT—Fundação para a Ciência e a Tecnologia through 2021.01765.CEECIND attributed within the 4th edition of the Individual Call to Scientific Employment (CEEC Individual call). This work was financially supported by FUNCAP, Project 01172921/2022—Cientista Chefe Cultura and Project 09672717/2020, by the National Council for Scientific and Technological Development (CNPq)—Project 302054/2022-7 and CAPES Foundation. Luís P. M. Santos was funded by CAPES scholarship (88882.463158/2019-01) PNPD/CAPES.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available upon request from the corresponding author.

Acknowledgments

The authors acknowledge CONSTRUCT—Instituto de I&D em Estruturas e Construções, FCT—Fundação para a Ciência e a Tecnologia, FUNCAP—Fundação Cearense de Apoio ao Desenvolvimento Científico e Tecnológico, CAPES—Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, and CNPQ—Conselho Nacional de Desenvolvimento Científico e Tecnológico. Luis Santos thanks the CAPES scholarship (88882.463158/2019-01) PNPD/CAPES. Collaboration and materials supplied by Secil, Omya, Chryso Portugal, and Sika Portugal are gratefully acknowledged.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. PSD of silica fume, limestone, cement, and sand employed to produce HPC.
Figure 1. PSD of silica fume, limestone, cement, and sand employed to produce HPC.
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Figure 2. Secondary electron mode SEM micrographs acquired from (a) cement; (b) silica fume; and (c) limestone.
Figure 2. Secondary electron mode SEM micrographs acquired from (a) cement; (b) silica fume; and (c) limestone.
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Figure 3. Secondary electron mode SEM micrograph of MWCNT employed in current work.
Figure 3. Secondary electron mode SEM micrograph of MWCNT employed in current work.
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Figure 4. Slump test of HPC mixtures under study.
Figure 4. Slump test of HPC mixtures under study.
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Figure 5. Electrical resistivity development of HPCs under study between 2 to 28 days.
Figure 5. Electrical resistivity development of HPCs under study between 2 to 28 days.
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Figure 6. Electrical conductivity development of HPCs between 2 to 28 days.
Figure 6. Electrical conductivity development of HPCs between 2 to 28 days.
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Figure 7. Mechanical strength of HPCs at 28 days: (a) flexure; (b) compressive.
Figure 7. Mechanical strength of HPCs at 28 days: (a) flexure; (b) compressive.
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Figure 8. SEM observation of HPC with MWCNT.
Figure 8. SEM observation of HPC with MWCNT.
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Table 1. Chemical and physical properties of cementitious mixture employed to produce HPC.
Table 1. Chemical and physical properties of cementitious mixture employed to produce HPC.
Chemical Composition (%)Physical Properties
LOISiO2Al2O3Fe2O3CaOMgONa2OK2OSO3ClFree LimeDensity (kg/m3)Specific Surface (g/cm)
Cement2.719.45.123.2862.31.570.130.63.220.051.2831104395 *
Silica fume<3>90---------220019632 **
Limestone filler---0.02990.3--<0.05<0.001-26805400 *
* Blaine method; ** BET method.
Table 2. Main properties of the multi-wall carbon nanotubes used in the current work.
Table 2. Main properties of the multi-wall carbon nanotubes used in the current work.
ParameterValues
Length0.5–15 µm
Average length4.5 μm
Diameter range8–45 µm
Average diameter20 nm
Specific mass2.1 g/cm3
Purity≥95%
Specific surface area40–300 m2/g
Functionalisation9%
Table 3. HPC mixture proportions and ratios.
Table 3. HPC mixture proportions and ratios.
Constituent Materials Dosage (kg/m3)Main Ratios
Mix IDCementLFSFMWCNTSpWaterw/bSp/b (%)MWCNT/c (%)
HPC-0.0%790.40311.4339.52-30.01650.1452.63-
HPC-0.2%790.40311.4339.521.5837.51650.1453.290.2
HPC-0.4%790.40311.4339.523.1637.51650.1453.290.4
HPC-0.6%790.40311.4339.524.7437.51650.1453.290.6
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Mesquita, E.; Matos, A.M.; Sousa, I.; Vieira, M.; Santos, L.P.M. Studying the Incorporation of Multi-Walled Carbon Nanotubes in High-Performance Concrete. Sustainability 2023, 15, 12958. https://doi.org/10.3390/su151712958

AMA Style

Mesquita E, Matos AM, Sousa I, Vieira M, Santos LPM. Studying the Incorporation of Multi-Walled Carbon Nanotubes in High-Performance Concrete. Sustainability. 2023; 15(17):12958. https://doi.org/10.3390/su151712958

Chicago/Turabian Style

Mesquita, Esequiel, Ana Mafalda Matos, Israel Sousa, Mylene Vieira, and Luís P. M. Santos. 2023. "Studying the Incorporation of Multi-Walled Carbon Nanotubes in High-Performance Concrete" Sustainability 15, no. 17: 12958. https://doi.org/10.3390/su151712958

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