Thermal and Electrical Properties of Cement-Based Materials Reinforced with Nano-Inclusions

Abstract

This study explores the influence of various nano-inclusions on the electrical and thermal properties of cement-based materials. Specifically, it investigates the incorporation of Multi-Walled Carbon Nanotubes (MWCNTs) and Graphene Nanoplatelets (GNPs) as reinforcement materials in cement composites. These advanced nanomaterials enhance the mechanical strength, durability, and functional properties of cementitious matrices. A series of experimental tests was conducted to evaluate the thermal and electrical behavior of nano-reinforced concrete, employing nondestructive evaluation techniques, such as Infrared Thermography (IRT) and Electrical Resistivity measurements. The results indicate that increasing the concentration of nanomaterials significantly improves both the thermal and electrical conductivity of the composites. Optimum performance was observed at a CNT dosage of 0.6% and a GNP dosage of 1.2% by weight of cement in cement paste, while in concrete, both nanomaterials showed a significant decrease in resistivity beginning at 1.0%, with optimal performance at 1.2%. The study also emphasizes the critical role of proper dispersion techniques, such as ultrasonication, in achieving a homogeneous distribution of nanomaterials within the cement matrix. These findings highlight the potential of carbon nanotubes (CNTs) and GNPs to enhance the multifunctional properties of cement-based materials, paving the way for their application in smart and energy-efficient construction applications.

1. Introduction

Cement-based materials, such as cement paste, mortars, and concrete, are the most widely used substances on the planet after water. In the construction field, they are inherently utilized due to properties such as high compressive strength, low cost, and the availability of raw materials. However, these materials exhibit poor resistance to tensile stresses; when subjected to tensile loads, they tend to develop cracks and undergo rapid fracture [1]. Cementitious composites are often modified through the incorporation of admixtures or additives to achieve properties unattainable with conventional concrete formulations. Reinforcement with steel fibers, nanomaterials, or their synergistic combination is commonly employed to significantly enhance tensile-flexural strength and long-term durability. [2,3,4,5]. The most widely used nanomaterials in recent years include CNTs and GNPs, which serve as effective reinforcements in cementitious systems, significantly enhancing their thermal and electrical performance. [6,7,8].

CNTs were discovered by Iijima in 1991 and can be visualized as a modified form of graphite. CNTs consist of multiple layers of carbon atoms that are bonded in a hexagonal pattern on flat sheets. Weak van der Waals forces hold these sheets together, while the bonds within each sheet are strong and covalent. If a single sheet is rolled up, it forms a single-walled nanotube (SWNT), whereas if multiple sheets are rolled together, they form multi-walled nanotubes (MWNTs) [9,10]. Due to the highly attractive van der Waals forces acting on their surfaces, CNTs tend to entangle together without external stimulus, forming a tortuous shape. This behavior, combined with their hydrophobic surfaces, leads to the formation of CNT agglomerates. This agglomeration poses a significant challenge, especially when a powder of nanotubes is introduced into a liquid matrix material [11,12]. To avoid the formation of agglomerates, it is necessary to disperse the nanotubes using a certain amount of surfactants. Moreover, incorporating surfactants into the cement helps to homogenize the suspension and enhance the mechanical properties of the concrete [13,14,15].

In 1962, Hans-Peter Boehm coined the term “graphene,” derived from a combination of “graphite” and the suffix “-ene,” to describe monolayer sheets of carbon [16]. Graphene is the first two-dimensional crystalline material to be isolated by humans, as its third dimension is virtually non-existent. In 2004, Andre Geim, Kostya Novoselov [17] and co-workers at the University of Manchester in the UK, discovered graphene, a single-atom-thick form of carbon with a hexagonal, lattice-like structure. They achieved this breakthrough by delicately cleaving a graphite sample using sticky tape. This innovative method allowed them to isolate this fragile material for the first time [18,19]. Graphene dispersion is crucial for preventing agglomerates in concrete and achieving homogeneous distribution of nanoparticles. Research indicates that, similar to CNTs, graphene can be effectively dispersed within the concrete matrix using ultrasonication energy [20]. This method ensures that the nanoparticles are uniformly distributed, enhancing the mechanical properties and durability of the concrete [21,22,23,24]. GPNs are recognized as the strongest material found in nature, and their tensile strength is 100 times greater than that of steel [25]. Remarkable are the unique chemical characteristics and the improved properties of GPNs and CNTs, such as the mechanical strength, specific surface area, electrical and thermal conductivity [26,27,28,29].

Sanchez and Sobolev [30] review the current state in the concrete nanotechnology field, highlighting recent key advances, particularly in nano-engineering and nanomodification of cement-based materials using a broad range of nanomaterials. Chen et al. [31] and Siddique and Mehta [32] focused οn the impact of CNTs on cement paste and examined CNT-reinforced mortar, respectively. A. Sedaghat et al. [33] studied the incorporation of graphene nanoparticles into cement paste and observed interesting modifications in the paste’s microstructural, morphological, electrical, and thermal properties. The composite’s thermal diffusivity and electrical conductivity increased proportionally with increased graphene concentration. D.A. Exarchos et al. [12] investigated the thermal and electrical properties of nano-modified materials using nondestructive techniques in real time, to monitor their structural integrity. L. Coppola et al. [28] observed that adding CNTs to the cement matrix alters the electrical resistivity of cementitious composites under various stress conditions, including static and dynamic loads. They presented data on the pressure-sensitive behavior under compressive stress of cement pastes and mortars containing different percentages of MWCNTs. M. Devasena, J. Karthikeyan [34] investigated the effect of graphene oxide on the physical properties of concrete, specifically to determine the optimum quantity of graphene oxide required to achieve maximum strengths. Test results indicated that the inclusion of graphene oxide in concrete enhanced the compressive, split tensile, and flexural strength.

Nowadays, a significant development in nanotechnology in cement-based materials is presented. Nano-reinforcements are being increasingly applied in civil engineering and various other industries [35]. New generation nanomaterials, such as CNTs and GNPs, are among the most utilized in cement-based applications. These nanomaterials are sought after for their ability to induce exotic properties, such as enhanced electrical and thermal properties, to the matrix material [36].

2. Experimental Study

2.1. Materials

The primary components of the mixture were ordinary Portland cement type II 42.5 t, tap water, natural sand, aggregates with specific granulometry, and nano-reinforcement. The water-to-cement ratio was maintained at 0.65 for both types of cement-based materials. This ratio was selected to ensure sufficient workability and promote uniform dispersion of CNTs and GNPs. This ratio also facilitated thermal and electrical testing by minimizing viscosity-related mixing issues and ensuring consistent hydration behavior across specimens. The nano-reinforcements comprised MWCNTs and GNPs. MWCNTs 4060, characterized by a long aspect ratio, long length, and a purity greater than 97%. The graphene was in the form of nanoplatelets (av-PLAT-7), with key characteristics including a lateral size (LD50) of 7.2 μm, an average thickness of 3 nm, and an average number of layers of 5–10 (multilayer). A water-based superplasticizer, Viscocrete Ultra 300 (Sika AG, Baar, Switzerland), was employed to disperse the nano-reinforcements effectively. The weight ratio of nano-reinforcement to dispersant agent was 1:1 for all concrete mixtures. Additionally, Viscocrete Ultra 600 superplasticizers with characteristics similar to Viscocrete Ultra 300 were utilized to maintain the workability of fresh concrete within acceptable values.

In order to assess the morphology of the specific batches of CNTs and GNPs nano-powders, a Scanning Electron Microscope (SEM, Zeiss SUPRA 35VP, Carl Zeiss AG, Oberkochen, Germany) was used. SEM images in Figure 1 reveal distinct morphological characteristics of the nanomaterials under investigation. The CNTs form an entangled network of tubular nanostructures with diameters ranging from approximately 40 to 60 nm, as specified in the datasheet. These nanotubes exhibit a relatively high aspect ratio and tend to form bundles or aggregates, a feature commonly attributed to van der Waals interactions between individual tubes. In contrast, the GNPs display a layered, sheet-like morphology with lateral dimensions significantly larger than their thickness, typically ranging from 3 to 8 μm as specified in the datasheet. The GNP structures appear as stacked or wrinkled flakes, indicative of their two-dimensional nature and potential for high surface area exposure. The observed morphological differences between CNTs and GNPs highlight their complementary structural properties, which can be strategically leveraged in hybrid composite systems to tailor electrical and thermal performance. A summary of all properties for CNTs and GNPs is provided in

Table 1. Properties of CNTs and GNPs according to datasheets and literature [37,38].

Properties
CNTs	GNPs
Elastic Modulus: 950 GPa [35,36]	Elastic Modulus: 950 GPa [35,36]
Tensile Strength: 11–63 GPa [35,36]	Tensile Strength: ~130 GPa [35,36]
Diameter/Thickness: 15–40 nm	Diameter/Thickness: ~0.08 nm
Electron Conductivity: 6.3 × 107 S/m	Electron Conductivity: 107–108 S/m
Thermal Conductivity: 406 Wm−1K−1	Thermal Conductivity: 3000–5300 Wm−1K−1
Main Range of Diameter: 40–60 nm	Lateral Size (LD50): 7.2 μm
Purity: ≥95%	Average Thickness: 3 nm
Ash: <3%	Oxygen Content (XPS): <1%
Length: 5–15 μm	Average Number of Layers: 5–10
Special Surface Area: 40–300 m2/g	BET: 70 m2/g

2.2. Preparation of Nano-Included Suspension

The dispersion process of CNTs and GNPs to create a stable suspension is illustrated in Figure 2. The initial step involves producing a solution under magnetic stirring, which includes tap water, the plasticizer Viscocrete Ultra 300, and the CNTs/GNPs. This nano-included aqueous solution is then subjected to ultrasonication for 90 min at 50% of the maximum ultrasound wave amplitude. The sonication is performed using a Hielscher UP400S 24 kHz device (Hielscher Ultrasonics GmbH, Teltow, Germany), which has a power capacity of 400 Watts and is equipped with a Ø22 mm cylindrical sonotrode. The final step before proceeding to cement paste and concrete production is the desiccation process, which utilizes a vacuum pump to remove entrapped air and achieve a stable suspension effectively. These parameters were defined as the most suitable in a previous study of our team [23].

2.3. Procedure of Cement Paste and Concrete Mixtures

Seven cement paste and concrete laboratory mixtures, one plain and six with different CNT and GNP concentrations, were prepared, respectively (28 mixtures in total). The concentrations of nano-reinforcement were 0.2%, 0.4%, 0.6%, 0.8%, 1.0%, and 1.2% by weight of cement. Hence, according to the “BS EN 196-1” standard protocol, three specimens of each mixture were produced. The prismatic specimens were fabricated with dimensions of 10 × 10 × 40 mm³ and 100 × 100 × 400 mm³, respectively. Before the final mixtures, several preliminary trial mixtures were conducted to achieve consistent and reproducible fresh-state properties. The target workability was set at approximately 160–170 mm spread diameter for cement pastes and 240 mm for concrete, as measured by the mini-slump test. Air content was controlled to remain below 4% in pastes and below 6% in concrete to ensure minimal porosity and reliable thermal and electrical measurements, as is common practice.

2.3.1. Cement Paste

The preparation of the nano-reinforced cement pastes was carried out in accordance with the requirements of the internationally certified and recognized standard BS EN 196-1:2005 [39,40]. For this reason, the proportions of the materials used were made in accordance with those imposed by the standard, and the only thing that changed each time was the percentage of additives. For enhanced workability of the final product and to maintain the additive percentage within acceptable levels, a small amount of Viscocrete Ultra 600 superplasticizer was added. Then the mixture was placed in oiled molds and kept for 24 h, as shown in Figure 3. Following demolding, the specimens were cured by immersion in water maintained at 20 °C in a humidity-controlled environment (100% RH) for a period of 28 days.

2.3.2. Concrete

The experimental procedure includes the preparation of five concrete mixes with a target concrete quality of C20/25 (28 days of cylindrical compressive strength of 20 MPa and cubic compressive strength of 25 MPa). To eliminate any residual moisture, both fine and coarse aggregates were oven-dried at 120 °C for 48 h prior to mixing. The production process was performed according to the standard test method BS ΕΝ-206 [41], as presented in Figure 4. The specimens were produced in a mixing machine with a total mixing time of 3 min at constant speed. Immediately after mixing, the properties of fresh concrete were evaluated. The workability was measured through slump according to ASTM C143 standard [42], using the standard slump test apparatus and the air content was determined according to ASTM C 231 standard [43]. Following these tests, the fresh concrete was cast and cured following the same procedure previously described for the cement paste specimens.

2.4. Nondestructive Materials’ Characterization

2.4.1. Thermal Behavior Procedure

In the present study, the thermal behavior of the specimens was evaluated using Infrared Thermography (IRT). The experimental set-up comprised an infrared (IR) camera and the test specimen. A CEDIP Jade 510 IR (CEDIP Infrared Systems, Croissy-Beaubourg, France) camera was employed, operating in the mid-wave infrared range (4–7 μm) with a spatial resolution of 320 × 240 pixels, enabling real-time monitoring of the specimen. Optimal field-of-view conditions were ensured by positioning the IR sensor at an approximate distance of 50 cm from the sample. Prior to testing, all specimens were conditioned in a laboratory oven at 120 °C for 48 h to ensure complete moisture removal and to achieve a uniform thermal state. For the experiment, the specimens were subsequently positioned on a base within a temperature-controlled environment maintained at 22 °C. The IR camera continuously recorded the cooling process, capturing the temperature decline from 56 °C to 35 °C, in order to evaluate the thermal behavior of the samples. Figure 5 depicts the typical IRT set-up utilized for evaluating the thermal behavior of cement-based materials.

2.4.2. Electrical Resistivity Measurements

Surface DC electrical resistivity measurements of the cement-based materials were conducted using a custom-designed electrical probe equipped with 22 circular pin electrodes. To ensure optimal contact between the probe and the specimen surface, each electrode was internally fitted with a pre-loaded spring mechanism and terminated with a conductive rubber pad at the tip. A high-precision digital electrometer (Keithley 6517B, Tektronix Inc., Beaverton, OR, USA) was connected to the probe, offering resistance measurement capabilities up to 10¹⁸ Ω and an ultra-high current resolution of 10 × 10⁻¹⁸ A. The circular probe was manually lowered to make uniform contact with the specimen surface (Figure 6). During testing, each specimen was placed on an electrically insulating platform to eliminate the possibility of electrical leakage. The resistivity measurement process involved applying a known voltage and recording the resulting current, allowing for the evaluation of the material’s resistance to leakage currents. For each concentration, two specimens were tested, and the average surface resistivity was calculated based on five consecutive measurements per specimen.

3. Results and Discussion

3.1. Infrared Thermography

The thermal response of cement paste and concrete specimens reinforced with varying concentrations of CNTs and GNPs was assessed using infrared thermography (IRT). The temperature–time profiles obtained from IRT, shown in Figure 7 and Figure 8, offer insights into the heat dissipation behavior of the nanomodified cementitious systems.

For the cement paste (Figure 7), CNT-reinforced samples exhibited progressively steeper and earlier thermal decay curves with increasing CNT content from 0.2% to 1.2%. The most pronounced deviation from the control sample was observed at concentrations of 0.8%, 1.0%, and 1.2%, suggesting the formation of an interconnected thermal conduction network. These findings are consistent with the known high thermal conductivity of CNTs and their ability to form one-dimensional pathways that facilitate efficient heat transfer within the cement matrix.

In contrast, the GNP-modified cement paste showed a more gradual evolution in thermal response. Although increases in GNP concentration also led to faster temperature dissipation compared to the plain sample, the effect was less intense and more uniform across concentrations. The absence of sharp changes in the thermal curves implies that GNPs require higher loadings to initiate effective conductive networks, likely due to their planar geometry and limited interconnectivity at lower concentrations.

Similar trends were observed in concrete specimens (Figure 8). CNT-reinforced concrete demonstrated a clear correlation between nanoparticle concentration and thermal conductivity, with samples containing 1.0% and 1.2% CNTs showing significant improvement in heat dissipation. This again points to the formation of a thermally conductive network at these concentrations.

The thermal behavior of GNP-reinforced concrete was more subtle, but a notable change in the thermal decay profile emerged at the 1.0% concentration. A ‘jump’ in the heat dissipation trend suggests that this is the threshold at which GNPs begin forming effective conduction networks in the more heterogeneous concrete matrix. Below this concentration, the influence of GNPs appears to be limited.

These observations support that both CNTs and GNPs enhance the thermal conductivity of cement-based materials, but their efficiency depends on dosage and geometry. CNTs show stronger thermal effects at lower concentrations due to their fibrous structure and network-forming capability. GNPs, though highly conductive in-plane, require higher concentrations to overcome their tendency to stack and to compensate for their two-dimensional morphology, especially in coarse concrete systems. Overall, infrared thermography proves to be a valuable non-destructive method for assessing thermal performance and detecting the percolation behavior of nanomodified cementitious composites.

3.2. Electrical Resistivity

The surface electrical resistivity of cement paste samples reinforced with varying concentrations of CNTs and GNPs is presented in the bar graph in Figure 9. The vertical axis is logarithmic, ranging from 10¹² to 10⁸ Ω·Sq, which allows for the visualization of changes across multiple orders of magnitude.

For CNT-reinforced cement paste, resistivity remains unchanged at low concentrations of 0.2% and 0.4%, maintaining values near 10¹² Ω·cm, similar to the plain control sample. A substantial decrease is first observed at 0.6%, where resistivity drops sharply by approximately three orders of magnitude to ~10⁹ Ω·Sq. This marks the onset of effective conductive network formation through percolation. The resistivity remains around this level at 0.8%, rises slightly at 1.0%, and then decreases modestly again at 1.2%, but no further significant improvement beyond the initial drop at 0.6% is observed.

In contrast, GNP-reinforced samples show no meaningful change from 0.2% through 0.6%, with resistivity values remaining close to 10¹² Ω·Sq. A slight decrease is visible at 0.8%, dropping marginally to ~10^11.8 Ω·Sq, indicating only minimal impact. However, at 1.0%, the resistivity returns to levels comparable with the plain sample, suggesting that conductive pathways have not yet formed. A significant reduction occurs only at 1.2%, where resistivity drops sharply to ~10^8.5 Ω·Sq, the lowest recorded value among all tested mixtures. This delayed but substantial decrease indicates that GNPs require a higher concentration to surpass the percolation threshold and form an interconnected conductive network.

Overall, the results suggest that CNTs are more effective at forming conductive pathways in cement paste at moderate concentrations, starting from 0.6%. GNPs, on the other hand, require higher loading (1.2%) to achieve comparable or superior conductivity, although they ultimately produce the lowest resistivity when sufficiently dispersed. The distinct behavior of these nanomaterials is attributed to their different morphologies and dispersion characteristics, with CNTs benefiting from their high aspect ratio at lower dosages, and GNPs requiring higher concentrations to offset their tendency toward agglomeration and limited interconnectivity at lower loadings.

Figure 10 presents the electrical resistivity of concrete samples with increasing concentrations of CNTs and GNPs, shown on a logarithmic scale from 10¹⁸ to 10¹⁰ Ω·Sq. The plain concrete sample shows a resistivity of approximately 10¹⁷ Ω·Sq. At 0.2%, GNP-reinforced concrete exhibits a moderate decrease to around 10^16.5 Ω·Sq, indicating an early but limited improvement in conductivity. CNTs at 0.2% remain close to the plain sample.

From 0.4% to 0.8%, both materials show gradual reductions in resistivity, reaching values around 10^16.3–10^16.4 Ω·Sq. A dramatic decrease occurs at 1.0%, where resistivity drops by roughly five orders of magnitude—to ~10^11.3 Ω·Sq for CNTs and ~10^11.4 Ω·Sq for GNPs—signaling the onset of percolation and conductive network formation. At 1.2%, both materials continue to improve slightly, reaching ~10^10.7 and ~10^10.6 Ω·Sq, respectively.

The result is significant given that concrete has inherently low conductivity, and such percolation behavior allows the material to transition into a multifunctional system.

This capability is essential for innovative structural materials, where conductivity is linked to performance and functions such as damage detection, self-monitoring, and structural health diagnosis.

The similar behavior of CNTs and GNPs at higher concentrations suggests that in concrete, the formation of conductive pathways is more influenced by the bulk matrix structure than by the nanomaterial geometry and dispersion alone. More specifically, the heterogeneous, porous, and aggregate-rich matrix disrupts dispersion and connectivity, effectively equalizing the impact of their different morphologies. Unlike in cement paste —where CNTs percolate earlier due to their high aspect ratio—the structural complexity of concrete forces both nanomaterials to reach a higher, similar threshold to form stable conductive networks.

4. Conclusions

This study presents a comprehensive experimental study of the electrical and thermal behavior of cement-based materials reinforced with carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs), across a range of dosages from 0.2% to 1.2% by weight of cement. The comparison between the cement paste and concrete matrices revealed that the effectiveness of these nanomaterials in forming conductive and thermally active networks is highly dependent on the matrix characteristics and dispersion conditions.

In cement paste, CNTs exhibited a sharp decrease in electrical resistivity at 0.6%, reducing resistance by approximately three orders of magnitude, due to efficient percolation and network formation. GNPs showed a significant drop only at 1.2%, indicating a higher percolation threshold. Regarding thermal conductivity, both nanomaterials increased heat transfer capacity with increasing concentration, but CNTs were more effective, especially at lower dosages. GNPs required higher loadings to produce comparable thermal effects, which reflects their planar geometry and dispersion limitations.

In concrete, the distinction between CNTs and GNPs diminished. Both materials displayed a similar percolation threshold for electrical conductivity at 1.0%, with resistivity dropping by five to six orders of magnitude. Thermal conductivity also improved modestly with nanomaterial addition, though the heterogeneous nature of concrete reduced the magnitude of enhancement. CNTs again provided a slightly stronger thermal effect than GNPs, particularly at intermediate dosages.

These findings underscore the dual functionality of CNTs and GNPs in enhancing both electrical and thermal performance of cement-based systems. While CNTs generally outperform GNPs at lower dosages due to their high aspect ratio and superior intrinsic conductivity, the differences become less critical in concrete, where matrix complexity dominates behavior. The results emphasize that material morphology, dispersion strategy, and matrix type must all be carefully considered in designing multifunctional composites.

From a practical standpoint, the study offers clear guidance: in real concrete applications, both CNTs and GNPs must be used at approximately 1.0% by weight of cement to achieve meaningful improvements in both electrical and thermal transport. This has direct relevance for developing smart, responsive building materials for applications such as self-sensing, thermal regulation, fire monitoring, and electromagnetic shielding.

However, some limitations must be acknowledged. The study focused on electrical and thermal behavior under controlled, static conditions. Mechanical properties, environmental durability, and dynamic responses (e.g., thermal cycling) were not evaluated. Only one type of CNT and GNP was used, and their dispersion was optimized in lab-scale settings. Field-scale dispersion challenges remain an open issue.

Future work should explore hybrid reinforcement systems, long-term durability under combined loads, and in situ monitoring technologies. Additional studies on the integration of these nanomaterials with smart sensing networks or energy-harvesting systems could help unlock their full potential. Furthermore, a detailed microstructural analysis of network formation—both electrically and thermally—could aid in optimizing next-generation multifunctional cement-based composites.