This paper reports a study undertaken to achieve a compatible and affordable technique for the high-quality dispersion of carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) in an aqueous suspension to be used in multifunctional cementitious composites. In this research work, two noncovalent surfactants with different dispersion mechanisms (Pluronic F-127 (nonionic) and sodium dodecylbenzene sulfonate (SDBS) (ionic)) were used. We evaluated the influences of various factors on the dispersion quality, such as the surfactant concentration, sonication time, and temperature using UV-visible spectroscopy, optical microscopic image analysis, zeta potentials, and particle size measurement. The effect of tributyl phosphate (TBP) used as an antifoam agent was also evaluated. The optimum suspensions of each surfactant were used to produce cementitious composites, and their mechanical, microstructural, electrical, and thermal behaviors were assessed and analyzed. The best dispersed CNT+GNP aqueous suspensions using Pluronic and SDBS were obtained for concentrations of 10% and 5%, respectively, with 3 hours of sonication, at 40°C, with TBP used for both surfactants. The results also demonstrate that cementitious composites reinforced with CNT+GNP/Pluronic showed better mechanical performance and microstructural characteristics due to the higher quality of the dispersion and the increasing hydration rate. Composites prepared with an SDBS suspension demonstrated lower electrical and thermal conductivities compared to those of the Pluronic suspension due to changes in the intrinsic properties of CNTs and GNPs by the SDBS dispersion mechanism.
Various types of multifunctional cementitious composites employing carbon fillers have attracted widespread attention due to their potential applications, including monitoring, thermal management, and transportation [
The high aspect ratio and large specific surface area of GNPs lead to a high contact area between composites and nanofillers and cause a maximized transferred stress and phonon transference from the matrix to the nanofillers. Hence, GNP-reinforced composites can be expected to exhibit better mechanical, thermal, and electrical behavior even in comparison to CNTs [
Recently, several studies have been conducted to measure the CNT+GNP hybrid effects on multifunctional polymeric composites. The results show an improvement in the mechanical, microstructural, electrical, and thermal performance in comparison to their individual usage. Hence, the synergic effects of CNT+GNP combinations can decrease the required concentration of carbon nanomaterials (CNMs) by reducing the percolation threshold and subsequently significantly alleviating concerns regarding the costs and formation of porosities [
Therefore, our present study proposes a compatible and affordable method for the high-quality dispersion of CNT+GNP in multifunctional cementitious composites. Various chemical, biological, and physical techniques, such as high shear mixing, ultrasonication, ball milling, plasma, calendaring and irradiation methods, and noncovalent and covalent functionalization, have been extensively studied to modify and improve GNP and CNT dispersion in various matrices [
Currently, Pluronic F-127 (PF-127) is receiving increased attention among the different surfactants due to its lower toxicity and higher biocompatibility compared to other surfactants [
Pluronic F-127 surfactants also exhibited a stronger ability to disperse MWCNTs in basic aqueous or extremely acidic media compared to ionic surfactants [
Hence, the effects of Pluronic F-127 and SDBS on the stability of prepared suspensions of 1% CNT+GNP (0.5 wt% CNT and 0.5 wt% GNP) were evaluated for various concentrations, ultrasonication times, and temperatures and for the presence of tributyl phosphate (TBP, as an antifoam agent) in order to propose a compatible and affordable method for high-quality dispersions of CNT+GNP in multifunctional cementitious composites. We evaluated the effects of the optimally dispersed CNT+GNP suspensions on the microstructure and mechanical, thermal, and electrical behaviors of the cementitious composites, to ensure the efficiency of the technique and the absence of adverse effects.
Carbon nanotubes and graphene nanoplatelets were purchased from CNPLUS Company (USA). Table
GNP and CNT characteristics.
GNP | ||||||||||
Surface area (m2 g–1) | Density (g/cm3) | Carbon content (%) | Tensile modulus (Gpa) | pH value (30°C) | Tensile strength (GPa) | Layers | Dimension | Form | Part number | |
120–150 | 0.6 | >99.5 | 1000 | 7–7.65 | 5 | <20 | Thickness | Diameter | Gray powder | TGN201 |
4–20 nm | 5–10 | |||||||||
MWCNT | ||||||||||
Surface area (m2 g–1) | Density (g/cm3) | Color | Outside diameter (nm) | Length ( | Ash (wt%) | Carbon content (%) | Part number | |||
350 | 0.27 | Black | <8 | 30–10 | <1.5 | >98 | GCM327 |
Carbon nanotube (CNT) and graphene nanoplatelet (GNP) Raman analysis.
We characterized the morphologies of GNPs and CNTs in different modes using a scanning electron microscope (SEM) (Figure
Morphology of (a) the CNT and GNP dry mix, (b) CNTs, and (c) GNPs.
The chemical structures of (a) Pluronic F-127, (b) sodium dodecylbenzene sulfonate (SDBS), and (c) tributyl phosphate (TBP).
Ordinary Portland cement type I and CEN Standard sand (EN 196-1 and ISO 679:2009) from the SNL company were used to prepare cementitious composites (mortar). The particle size distribution of the sand and chemical composition of the ordinary Portland cement (OPC) are shown in Tables
Particle size distribution of the sand [
Mesh size (mm) | 0.08 | 0.16 | 0.5 | 1 | 1.6 | 2 |
Cumulative retained (%) | 0 |
Chemical composition and properties of ordinary Portland cement.
SiO2 | Al2O3 | Fe2O3 | MgO | CaO | Na2O | TiO2 | K2O | MnO | P2O5 | SO3 | LOI | Fineness (m2/kg) | Specific gravity |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
19.94 | 4.76 | 3.38 | 1.31 | 63.93 | 0.17 | 0.24 | 0.44 | 0.075 | 0.063 | 2.54 | 2.97 | 360 | 3.15 |
We dispersed 1% of CNT+GNP (wt of water) in aqueous solution by 5%, 10%, and 15% of each surfactant. The surfactant was first added to the water and mixed for 1 hour using a magnetic stirring mixer with a speed of 800 rpm/min. CNT+GNP was then added to the solution and stirred for one additional hour. The samples were then placed in a sonicator bath (Crest Ultrasonicator, CP 230T) for 1, 2, and 3 h, at 45 kHz frequency and 80 W power, at different temperatures.
A digital temperature regulator (NESLAB RTE-111 MP) was used to adjust the temperature during the ultrasonication process with a circulation system through a radiator and sensors. The TBP was completely dissolved in water before the addition of the surfactant.
Plain and CNT+GNP-reinforced specimens were prepared by mixing optimized CNT+GNP aqueous suspensions with ordinary Portland cement and standardized sand, using a laboratory mixer, following the EN 196-1:1994 standard. In all samples, a cement-to-water ratio of 0.5 was used. At first, the required amount of cement (450 g) was poured in the mixer’s stainless steel bowl in order to prepare the mortar mixtures. Then, the prepared CNT+GNP suspension with 125 g of water was added to the cement and the required amount of sand (1350 g), and the remaining 100 g of water was poured into the mixing machine’s hopper. The mixer was then run for 1.5 min, with a stainless steel blade rotational speed of 140 m/min, followed by a 30 s timeout, and then run at 285 m/min for another 2.5 min. The mixture was placed into
Different techniques were used for characterizing various parameters of the prepared CNT+GNP suspensions.
Due to the additional absorption caused by 1D van Hove singularities, the dispersed carbon nanotubes and graphene nanoplatelets in aqueous suspensions are active in the region of UV-Vis [
The zeta potential and bundle size of the carbon nanomaterials (CNM) were determined using the Zetasizer Malvern Nano ZS system in aqueous suspensions. The dynamic light scattering (DLS) technique has been used for measuring the bundle size of CNM in aqueous suspensions; however, this technique is more accurate for perfectly spherical particles [
where
In order to monitor the carbon nanotubes and graphene nanoplatelet agglomeration presence in the aqua suspensions and quantify the area of agglomeration, we used optical microscopy. For this purpose, a water suspension drop was analyzed through deposition on a glass plate and covered with a coverslip. Visual inspection was performed with 10x, 20x, and 50x magnifications, while for quantification analysis of the agglomeration area, 10x magnification was used. In this route, three drops were taken as samples from each aqueous suspension; the photos were prepared accurately by microscope software to completely cover the drop area. The samples’ optical micrographs were investigated with ImageJ software. The total area of agglomeration was computed by the summation of the total agglomerate area measured for a sample, divided by the total analyzed image area for each drop. The final amount of the agglomeration area was obtained from the average of three drops and expressed as a percentage with standard deviation.
Flexural and compressive testing was carried out according to the BS EN 196-1:1995 standard. The apparent porosity, dry bulk density, and capillary water absorption of the samples were measured as stated by ASTM C20, BS EN 1015-10:1999, and BS EN 1015-18 standards. For evaluating the consistency of fresh cementitious composites, the diameters of the paste were measured by using a flow table at two perpendicular directions according to the EN 1015-3 standard. The fracture surfaces of the specimens were characterized by scanning electron microscopy (SEM-FEG, Nano SEM NOVA 200, FEI) using an accelerated voltage of 10 kV and the secondary mode of the electron after coating with an Au-Pd thin film (30 nm), in a high-resolution sputter coater (Cressington 208HR), to investigate the microstructure.
We used a thermogravimetric analyzer (TGA, PerkinElmer) in a nitrogen atmosphere (100 mL/min), at a heating rate of 10°C/min up to 1000°C and energy dispersive X-ray analysis (EDX) with 1.8 nm at 3 kV (Helix detector) of vacuum resolution. The EDX tests were carried out from the cement hydration product accumulation. The graphs were obtained by means of at least five different points of analysis for each specimen. The nondestructive ultrasonic test was performed for microstructural evaluation according to the BS EN 12504-4 standard from Pundit Lab, using the ultrasonic test device H-2984 through two probes along the longitudinal transverse axis. The electrical resistance of the specimens was measured with an Agilent 34461A multimeter, after drying at 34°C for 72 hours. A portable thermal camera, model FLIR 60BX, with a resolution of
The UV-Vis absorption spectra diagrams for the CNT+GNP suspensions prepared with Pluronic F-127 and SDBS surfactants are displayed in Figures
Wavelength and maximum absorbance of CNT+GNP suspensions prepared with Pluronic F-127.
Wavelength and maximum absorbance of CNT+GNP suspensions prepared with SDBS.
Parveen et al. [
Bath sonication dispersing techniques exfoliate GNPs or debundle nanotubes into individual plates or tubes and/or thinner bundles, which consequently are stabilized through electrostatic repulsions and/or the steric force of surfactants. During sonication, mechanical vibrations overcome the van der Waals interactions between the GNPs or bundles of CNTs and provide high positional shear, particularly to the end of the GNP sheets or nanotube bundles. Once gaps or spaces at the bundle ends are formed, molecules of the surfactant are absorbed onto the surface of the nanoparticles instantly leading to their exfoliation. According to the unzipping mechanism [
Hence, short-time ultrasonication in surfactant molecules with bulky hydrophobic groups (Pluronic) showed lower debundling efficiency due to being hindered in their penetration into the intertube or sheet regions. In the case of nonionic surfactants, these bulky hydrophilic groups demonstrated advantages for CNT and/or GNP dispersions likely due to the enhanced steric stabilization caused by longer polymeric groups. The results also demonstrate that an increase in temperature to a certain range (40°C) improved the dispersion of nanoparticles for both suspensions prepared by Pluronic and SDBS, likely by increasing the solvability of the surfactant molecules and the mobility of electrons. However, excessive temperature rises (more than 45°C) caused bond breakage as well as a change in the critical micelle concentration and led to temporary reagglomeration.
In a previous study conducted by Yan et al. [
By comparing these results with the present studies, the amount of the required surfactant for dispersing CNT and GNP particles in the hybrid combination appears much greater than the individual cases, in both types of ionic and nonionic surfactants. This is likely due to the combinations of carbon nanoparticles with different geometries (1D and 2D), which leads to an increase in the aspect ratio, as well as the specific surface area. Therefore, in this case, the contact surface between the CNMs was larger than that in the observed CNMs individually, which causes stronger van der Waals attractions and hinders the dispersive efficiency of the surfactant at low concentrations.
The effects of the CNM aspect ratio on the dispersion quality were also demonstrated by Goodman et al. [
In nonionic surfactants, such as AL and Pluronic, in contrast to ionic surfactants, such as SDS, ASBS, and CTAB, the
This mechanism can be applied to GNPs and CNTs by introducing a removable and temporary surface load through permitting the molecules of the surfactant to adsorb via the hydrophobic tails on the nanoparticle surface. An ion normally dissociates itself from the head of hydrophilic groups and serves as a counterion. Then, the adsorbed molecular ions have interactions with water [
Regarding graphene dispersion, the latter condition is not always justified as some surfactants have better performance at concentrations below the critical micelle level while not exceeding the concentration of graphene [
The quality of the dispersion and the individualization/exfoliation degree of GNPs and CNTs are normally measured by the zeta potential. In contrast to carbon nanotubes, this occurs for graphene nanoplatelets within two separate ionic surfactant groups: sulfides, such as SDBS, and other ionic surfactants. Since the zeta potential reflects the electrostatic potential at the edge of the bound ion layers, we assume that maximizing the surface charge in this layer can lead to the zeta potential increase [
In Pluronic, as a nonionic surfactant, the lack of Coulomb repulsion for preventing GNP and CNT agglomeration is compensated for by steric effects. Therefore, the main factor determining the dispersibility of CNTs and GNPs in aqueous suspension is the presence of branched and long polar (PEO) chains. This leads to efficiency enhancement by increasing the molecular weight of the surfactant [
Morphology of dry CNT+GNP extracted agglomerate from an aqueous suspension: (a) prepared by Pluronic F-127 and (b) prepared by SDBS.
However, in dry mixes of CNT and GNP without the presence of surfactants, they are arranged in the form of individual spheres without interactions due to the van der Waals attraction forces between the CNTs and repulsion electrostatic forces between the GNPs and CNTs (Figure
For the evaluation of the agglomerate total area, we analyzed CNT+GNP aqueous suspension optical micrographs and expressed the results as percentages (Table
Agglomerate total area of various CNT+GNP aqueous suspensions.
SDBS | Total area of | Pluronic F-127 | Total area of agglomerates (%) |
---|---|---|---|
1%CG-1H-05%S | 1%CG-1H-5%P | ||
1%CG-2H-05%S | 1%CG-2H-5%P | ||
1%CG-3H-05%S | 1%CG-3H-5%P | ||
1%CG-1H-10%S | 1%CG-1H-10%P | ||
1%CG-2H-10%S | 1%CG-2H-10%P | ||
1%CG-3H-10%S | 1%CG-3H-10%P | ||
1%CG-1H-15%S | 1%CG-1H-15%P | ||
1%CG-2H-15%S | 1%CG-2H-15%P | ||
1%CG-3H-15%S | 1%CG-3H-15%P | ||
1%GC-05%S-1H+TBP | |||
1%GC-05%S-2H+TBP | |||
1%GC-05%S-3H+TBP | 1%CG-3H-10%P+TBP | ||
1%GC-05%S-3H+TBP+45°C | 1%CG-3H-10%P+TBP+45°C | ||
1%GC-05%S-3H+TBP+40°C | 1%CG-3H-10%P+TBP+40°C | ||
1%GC-05%S-3H+TBP+25°C | 1%CG-3H-10%P+TBP+25°C | ||
Aqueous suspension without a surfactant | |||
0.5%CG-1H | |||
0.5%CG-2H | |||
0.5%CG-3H |
CG: CNT+GNP (%); S: SDBS (%); P: Pluronic F-127 (%); H: time of sonication (h); ±SD: standard deviation.
Hence, the optimum surfactant concentrations were 15% and 5% for Pluronic and SDBS, respectively, which provided the lower agglomeration area. However, these concentrations were higher than their critical micelle concentrations [
Interestingly, the presence of TBP enhanced the dispersion of nanoparticles, likely due to the TBP molecular structure that contains three methyl branches. A methyl group is a hydrophobic alkyl functional group derived from methane (CH4) by removing one hydrogen atom (CH3-), and methyl groups are extremely reactive and capable of binding covalently with other carbon atoms due to the free capacity of eight radical electrons [
The optical micrograph analysis also showed the effect of temperature and the increasing sonication time on the nanoparticle dispersion improvement, which was obtained from the UV-visible spectroscopy. The aqueous suspension of 1% CNT+GNP with 10% Pluronic and 3-hour sonication time, at 40°C, with TBP, and the suspension of 5% SDBS with the same amount of TBP, temperature, and sonication time exhibited very low agglomerate areas (only 5.6% and 1.2%, respectively) indicating very competent dispersion qualities (Figure
Optical micrographs of CNT/GNP suspensions: (a) Pluronic: 1%CG-3H-10%P+TBP+40°C; (b) SDBS: 1% CG-3H-5%S+TBP+40°C.
As mentioned before, the stability of colloidal suspensions is frequently determined by their zeta potential, which indicates the magnitude of the electrostatic interactions between colloidal particles. Particles with a zeta potential higher than +15 mV or less than -15 mV are expected to be stable due to electrostatic considerations. However, colloidal suspensions with zeta potentials between +15 and -15 mV can also be stable if they are stabilized uninterruptedly [
The zeta potentials of the CNT+GNP suspensions are listed in Table
Zeta potentials of the CNT+GNP suspensions.
Sample name | Zeta potential (mV) |
---|---|
3H-05%S | -48.6 |
3H-10%S | -17.7 |
3H-15%S | -29 |
5%S-3H+TBP | -56.4 |
5%S-3H+TBP+25°C | -35.5 |
5%S-3H+TBP+40°C | -71.9 |
5%S-3H+TBP+45°C | -67.3 |
3H-5%P | -2.9 |
3H-10%P | -9.3 |
3H-15%P | -5.7 |
3H-10%P+TBP | -14.6 |
3H-10%P+TBP+25°C | -13.1 |
3H-10%P+TBP+40°C | -23.1 |
3H-10%P+TBP+45°C | -19.9 |
The dispersions of 1% CNT+GNP in 5%, 10%, and 15% SDBS with 3-hour sonication showed zeta potentials of -48.6, -17.7, and -29 (mV), respectively. However, these amounts for Pluronic suspensions were approximately -2.9, -9.3, and -5.7 (mV), respectively. The addition of 5% of TBP over SDBS, with 3-hour sonication, obtained a -56.4 (mV) zeta potential, which supports the positive effect of TBP to improve CNT and GNP dispersion. Increasing the temperature to 40°C led to increasing the zeta potential to -71.9 (mV), which shows more colloidal particle stability for this suspension. However, the excessive increase in temperature decreased the quality of the CNM dispersion. Similarly, increasing the temperature up to 40°C for the 10% Pluronic prepared through 3-hour sonication with TBP led to the formation of more stable colloidal particles with a -23.1 (mV) zeta potential.
The high negative zeta potential for SDBS suspensions is due to the high negative surface charges of this surfactant. Consequently, due to the nonionic chemical structure of Pluronic, the CNT+GNP suspensions prepared by this surfactant presented lower negative zeta potential. Owing to nonelectrostatic and steric interactions, the Pluronic suspensions could also be stable. The polyoxyethylene (PEO) hydrophilic groups extend into the water while hydrophobic chains of polyoxypropylene (PPO) interact with the surfaces of the GNPs and CNTs. The exfoliation and stabilization of separated GNPs and CNTs happen as a result of steric hindrances that were induced by the long PEO chain.
The functionalized CNM zeta potential was relatively higher due to -COOH functional groups, which leads to the surface negative charge on GNPs and CNTs.
The average size of the CNM bundles and the polydispersity index (PDI) of the CNT+GNP suspensions, determined through DLS by measuring the hydrodynamic diameter, are presented in Table
The CNM hydrodynamic diameters in different suspensions characterized by DLS.
Sample name | Ave bundle size (nm) | PDI |
---|---|---|
3H-05%S | 491 | 0.571 |
3H-10%S | 671 | 0.707 |
3H-15%S | 588 | 0.649 |
5%S-3H+TBP | 448 | 0.514 |
5%S-3H+TBP+25°C | 474 | 0.589 |
5%S-3H+TBP+40°C | 393 | 0.462 |
5%S-3H+TBP+45°C | 422 | 0.503 |
3H-5%P | 512 | 0.576 |
3H-10%P | 328 | 0.488 |
3H-15%P | 389 | 0.51 |
3H-10%P+TBP | 296 | 0.412 |
3H-10%P+TBP+25°C | 311 | 0.438 |
3H-10%P+TBP+40°C | 219 | 0.315 |
3H-10%P+TBP+45°C | 287 | 0.397 |
The small average size of CNM bundles in Pluronic suspensions, in comparison with SDBS ones, indicates the efficiency of the -COOH functional groups on the unbundling process. A lower particle size (219 nm) and lower PDI (0.315) were obtained for 10% of Pluronic, with 3-hour sonication with TBP at 40°C, showing low agglomeration and high quality of the dispersion. The average particle size and the amount of PDI in the SDBS-dispersed suspensions, at optimum conditions, were around 393 nm and 0.462, respectively.
After obtaining the best dispersed mixed design for hybrid CNT+GNP aqueous suspensions with each surfactant, the mechanical and microstructural properties of the reinforced cementitious composites were evaluated to determine their effects. The mechanical and microstructural parameters of the cementitious composites in different cases are shown in Figure
The compressive and flexural strength of cementitious composites after 28 days of hydration.
Test results of the cementitious composites according to the standard procedures.
Name of sample | Flow values1 (mm) | Flexural strength2 (N/mm2) | Compressive strength2 (N/mm2) | Apparent porosity3 (%) | Ultrasonic wave time passing4 ( | Dry bulk density5 (kg/m3) | |
---|---|---|---|---|---|---|---|
Longitudinal | Transverse | ||||||
Plain mortar | 183.2 | 6.9 | 43.9 | 19 | 36.40 | 9.70 | 2125 |
Plain mortar+5%S+TBP | 191.7 | 7.1 | 44.9 | 15.9 | 36.21 | 9.51 | 2130 |
1%CG+5%S+3H+TBP+40°C | 118.4 | 7.8 | 47.8 | 13.6 | 34.10 | 9.34 | 2135 |
Plain mortar+10%P+TBP | 198.2 | 7.2 | 44.2 | 13.9 | 35.88 | 9.38 | 2130 |
1%CG+10%P+3H+TBP+40°C | 131.5 | 8.3 | 51.1 | 11.4 | 32.97 | 8.93 | 2140 |
1BS EN 1015-3; 2BS EN 196-1:1995; 3ASTM C20; 4BS EN 12504-4; 5BS EN 1015-10:1999.
The general trend of the results showed an improvement in the flexural and compressive strength of CNT+GNP/Pluronic-incorporated cementitious mortars. These results demonstrate more homogeneous and denser microstructures for the CNT+GNP/Pluronic-reinforced cementitious composites. The positive effects of the CNTs and GNPs on the improvement of the cementitious composite microstructures and their mechanical behaviors, including their flexural and compressive strength, have been reported in many previous studies [
Therefore, we concluded that the differences between the cementitious composite mechanical and microstructure parameters in the cases of CNT+GNP/Pluronic and CNT+GNP/SDBS were due to the dispersion quality and the secondary effects of the surfactant on the hydration process and/or on the nanoparticle features and/or structures. However, considering the mechanical and microstructural results for the nonreinforced composites prepared with only Pluronic and SDBS (without CNMs), the presence of these two surfactants did not demonstrate adverse effects on the cementitious composite features, at least after 28 days of hydration and for the tested concentrations.
Large agglomerations of CNTs and GNPs, due to lower quality dispersions in CNT+GNP/SDBS-reinforced cementitious composites, led to the formation of microporosities. The CNM agglomerated between the growing units of C-S-H to create large gaps among them, which prevented their physical connection (Figure
SEM morphology of cementitious composites reinforced by (a) CNT+GNP/SDBS or (b) CNT+GNP/Pluronic.
SEM morphology of a CNT+GNP/SDBS sample after the mechanical strength measurement.
However, well-dispersed GNPs were more able to anchor the neighboring C-S-H clusters and bridge the voids between them. The large specific surface area of GNPs led to a high contact area with the hydration products and maximized the stress transferring, delaying the microcrack and macrocrack propagation. In contrast to SDBS, the CNT+GNP/Pluronic-reinforced cementitious composites contained lower porosities due to the better dispersion of the nanoparticles.
The results of the capillary water absorption shown in Figure
Capillary water absorption of cementitious composites.
However, the rates of capillary water absorption for CNT+GNP-reinforced composites prepared by both Pluronic and SDBS were lower than those for the plain composite.
The presence of both surfactants increased the flowability of the plain mortar. According to Table
It is generally accepted that the cement paste fluidity is directly pertinent for the performance of hardened cement (such as the pore structure and strength) [
Generally, cement hydration products have a strong tendency to form on the surface of CNTs and GNPs due to the large specific surface area accompanied by a high amount of oxygen functional groups, which serve as nucleation sites [
High-magnification SEM images (Figure
The results of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) for different cementitious cases are shown in Figure
Thermal analysis of the cementitious composite: (a) TGA analysis and (b) DSC analysis.
The results indicate that the amounts of hydration products in the CNT+GNP/Pluronic-reinforced specimens were more than those in the CNT+GNP/SDBS specimens; however, the hydration product amounts in both cases were more than those in the plain sample.
In the case of the reinforced cementitious composites with MWNTs, Konsta-Gdoutos et al. [
Energy-dispersive X-ray spectroscopy (EDX) of cementitious composites and their normalized elemental analysis.
Despite higher carbon concentration in the place of cement hydration product accumulation for CNT+GNP/SDBS-reinforced specimens compared to CNT+GNP/Pluronic specimens, lower amounts of hydration products were observed. The reason for this is likely the changing nanoparticle features and/or structures due to the SDBS dispersion mechanism. This statement could be verified by evaluating the electrical behavior.
The electrical resistances of specimens after 28 days of hydration were measured using a digital multimeter (Table
The electrical resistance and humidity of cementitious composite specimens.
Sample | Electrical resistance ( | Improvement (%) | Moisture (%) |
---|---|---|---|
Plain mortar | 8446.00 | — | 8.90 |
1%GC-5%S-3H+TBP+40°C | 2388.00 | 71.73 | 6.78 |
1%CNT-5%S-3H+TBP+40°C | 2847 | 66 | 7.7 |
1%GNP-5%S-3H+TBP+40°C | 3949 | 53.2 | 8.04 |
1%GC-10%P-3H+TBP+40°C | 765.00 | 90.94 | 4.97 |
1%CNT-10%P-3H+TBP+40°C | 2506 | 70.3 | 6.2 |
1%GNP-10%P-3H+TBP+40°C | 3011 | 64.3 | 7.93 |
The electrical resistances of the reinforced specimens with CNT+GNP/SDBS, CNT/SDBS, and GNP/SDBS were improved by 71.73%, 66%, and 53.2%, respectively, in comparison to that of the plain mortar, while the results are 60.94%, 70.3%, and 64.3%, respectively, for the Pluronic-reinforced specimens. According to the previous section’s discussion, CNT+GNP/SDBS-reinforced specimens contained more and larger CNM agglomerations compared to CNT+GNP/Pluronic specimens due to the lower dispersion quality. Although the existence of a certain agglomeration quantity can act as a key factor in raising the conductivity significantly and reducing the percolation threshold value [
Therefore, the decreasing electrical conductivity of the CNT+GNP/SDBS-reinforced composites is likely due to the nanoparticle features and/or structure changes provided by the SDBS dispersion mechanism.
In previous studies, significant deterioration in the electrical properties was also reported due to disturbances of the graphene/CNT surface
The synergic effects of hybrid CNT+GNP specimens decreased the electrical resistance of the cementitious composites more than what was observed for individual CNT or GNP utilization, as was expected.
A comparison of the CNT+GNP effects on the electrical conductivity of cementitious composites with previous studies that used individual CNT, graphene, or other carbon-based particles (Table
Comparison of the electrical resistances for carbon nanoparticle-reinforced cementitious composites.
Dispersion method | Weight fraction (%) | Nanoparticle type | Electrical resistivity ( | References |
---|---|---|---|---|
Superplasticizer and ultrasonication | 5 | GNP | 21.02 | [ |
Dry mechanical mixing | 5 | GNP | 78.20 | [ |
Superplasticizer and high-speed mixer | 5 | GNP | 27.96 | [ |
PVP and ultrasonication | 0.7 | CNT | 24 | [ |
Mixing by superplasticizer during mortar blending | 10 | Carbon black | 4.53 | [ |
Mixing by superplasticizer during mortar blending | 2 | Short carbon fiber | 2.4 | [ |
Pluronic F-127 and TBP, 3 h sonication, 40°C | 1 (half by half) | GNP+CNT | 15.3 | Present study |
SDBS and TBP, 3 h sonication, 40°C | 1 (half by half) | GNP+CNT | 47.76 | Present study |
In this study, for the first time, thermal photography was used for the evaluation of CNT+GNP dispersion in hardened specimens through the difference between the thermal diffusion coefficient and the thermal conductivity. Thermal photography of cementitious composite specimens of the same size (
Thermography of cementitious composites at different times: (a) immediately after heating (both cross section and side view), (b) after one hour, and (c) after two hours.
In this work, we conducted an extensive experimental research study on hybrid CNT+GNP dispersion techniques using a high concentration of noncovalent surfactants and aqueous suspensions. For this purpose, a nonionic Pluronic F-127 and ionic SDBS surfactants were studied in 1 wt% of CNT+GNP (0.5% CNT and 0.5% GNP) dispersions, to analyze the effect of factors, including the surfactant concentration, sonication time, temperature, and the use of an antifoam agent (TBP).
We evaluated the effects of optimized CNT+GNP suspensions prepared by SDBS and Pluronic on the mechanical, electrical, and thermal properties of cementitious composites by various tests. UV-spectroscopy and optical microscopy image analysis indicated that high-quality CNT+GNP dispersions with a low agglomeration area (less than 1.2%) were achieved with 10% Pluronic (wt% of nanoparticles), with 3-hour sonication, at 40°C, with the presence of tributyl phosphate (50 wt% of the surfactant as an antifoam agent). The optimum percentage of SDBS (5 wt%) led to a lower dispersion quality and a higher agglomeration area (5.6%).
Increasing the temperature up to 40°C, as mentioned before, played a positive role in the dispersion of suspensions prepared with Pluronic or SDBS; however, higher temperatures led to the reagglomeration of CNT+GNP. The presence of TBP significantly reduced the agglomeration area. Increasing the sonication time improved the dispersion in both CNT+GNP/Pluronic and CNT+GNP/SDBS aqueous suspensions, which was also confirmed by the zeta potential and particle size measurements. SEM photographs showed that CNTs and GNPs in prepared aqueous suspensions using Pluronic presented better interactions between each other, even after water elimination, when compared to CNT+GNP/SDBS suspensions.
Cementitious composites reinforced with hybrid CNT+GNP showed enhanced mechanical, microstructural, electrical, and thermal properties, for both the Pluronic and SDBS dispersion cases. However, the general result trends demonstrate higher efficiency for CNT+GNP/Pluronic due to the compatible mechanism and the dispersion quality.
The results obtained for the dry bulk density, water content, ultrasonic nondestructive test, and capillarity water absorption also indicate denser microstructures for the CNT+GNP/Pluronic-reinforced composites due to the lower agglomeration caused by the high-quality dispersion. The TGA, DSC, and EDX results show that dispersed CNT+GNP/Pluronic increased the hydration rate due to the nucleation agent effects of CNTs and GNPs and the oxygen functional groups of graphene, which acted as growing points for hydration products. SEM image analysis showed that CNMs were completely embedded in the hydration products, bridging them. However, this is not observed for CNT+GNP/SDBS due to the lower quality dispersion and also to the changing CNM features and structures caused by the SDBS dispersion mechanism. For the same reason, CNT+GNP/SDBS-reinforced specimens also presented lower electrical and thermal conductivities compared to those reinforced with CNT+GNP/Pluronic.
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The authors declare that they have no conflicts of interest.
This work was supported by the European Commission-Shiff2Rail Program under the project “IN2TRACK2–826255-H2020-S2RJU-2018/H2020-S2RJU CFM-2018.” The authors are thankful to Gonzalo Mármol, Paulo Lopes, Vânia Pais, and Rui Rodrigues for their inputs to analyze the experimental results.