Multifunctional Graphene–Concrete Composites: Performance and Mechanisms

Abstract

Concrete is a cornerstone material in the construction industry owing to its versatile performance; however, its inherent brittleness, low tensile strength, and poor permeability resistance limit its broader application. Graphene, with its exceptional thermal conductivity, stable lattice structure, and high specific surface area, presents a transformative solution to these challenges. Despite its promise, comprehensive studies on the multifunctional properties and underlying mechanisms of graphene-enhanced concrete remain scarce. In this study, we developed a novel concrete composite incorporating cement, coarse sand, crushed stone, water, and graphene, systematically investigating the effects of the graphene dosage and curing duration on its performance. Our results demonstrate that graphene incorporation markedly improves the material’s density, brittleness, thermal conductivity, and permeability resistance. Notably, a comprehensive analysis of scanning electron microscopy (SEM) images and thermogravimetric (TG) data demonstrates that graphene-modified concrete exhibits a denser microstructure and the enhanced formation of hydration products compared to conventional concrete. In addition, the graphene-reinforced concrete exhibited a 44% increase in compressive strength, a 0.7% enhancement in the photothermal absorption capacity, a 0.4% decrease in maximum heat release, a 0.8% increase in heat-storage capacity, and a 200% reduction in the maximum penetration depth. These findings underscore the significant potential of graphene-reinforced concrete for advanced construction applications, offering superior mechanical strength, thermal regulation, and durability.

1. Introduction

Concrete is recognized for its durability, low cost, and favorable structural properties, making it a predominant building material worldwide. It is extensively utilized in infrastructure, commercial buildings, and residential construction. However, the continuous growth of the concrete industry has led to an increasing demand for cement and aggregates, resulting in heightened CO₂ emissions. Consequently, the development of high-performance, multifunctional concrete is essential for advancing a sustainable and resource-efficient construction industry [1,2,3,4].

Currently, extensive research has been conducted on the mechanical properties of concrete incorporating steel slag, fibers, and other materials. These studies provide innovative insights into the application of novel concrete in modern construction [5,6]. Lin H et al. investigated the effects of various mixtures of high-toughness polypropylene (HTPP) fiber on the flexural strength, compressive strength, and stress–strain behavior of ceramite concrete. They analyzed the action mechanism of HTPP fiber reinforcement in ceramic concrete using scanning electron microscopy. This study provides important theoretical support for the structural analysis and design of this type of concrete [7]. Nguyen T T H et al. incorporated steel slag material into concrete as a substitute for coarse aggregate and investigated the relationship between the compressive strength of steel slag concrete and age. The results indicated that the compressive strength of the concrete increased rapidly within the first 7 days before gradually slowing down [8]. Steel-fiber-reinforced concrete (SFRC) is widely recognized for its excellent properties. To investigate the influence of steel fibers on the mechanical properties of concrete, steel fibers were incorporated into beams to evaluate the compressive strength of the concrete [9]. Abbass W et al. conducted a comprehensive evaluation of the compressive strength and flexural strength of concrete incorporating various lengths and diameters of steel fibers, as well as different water–cement ratios. Both compressive and flexural strengths were found to be significantly improved. Additionally, they proposed an analytical model to describe the stress–strain relationship under compression in steel-fiber-reinforced concrete (SFRC) [10]. Ozer Zeybek et al. partially replaced cement with waste glass powder (WGP) and examined the effects of varying proportions of WGP on the compressive strength, splitting tensile strength, and flexural strength of concrete. They determined the optimal WGP ratio and developed empirical equations to predict these mechanical properties of the concrete [11]. Currently, the rapid economic development is driving a continual increase in total energy consumption, with energy use related to heating and operating buildings becoming the third largest contributor to social energy consumption [12,13]. Currently, energy savings in buildings can primarily be achieved through innovations in building materials and structures, as well as the comprehensive utilization of renewable energy sources. As a renewable energy option, solar energy is both clean and efficient. By integrating solar energy storage into building envelope structures, it is possible to significantly reduce energy consumption and meet the energy demands of buildings [14,15,16]. However, solar energy is intermittent and unpredictable, making it unavailable throughout the day. Therefore, the efficient conversion of solar energy and effective energy storage solutions are essential [17]. Therefore, the photothermal performance of novel concrete materials has become a research hotspot. Photothermal materials, which exhibit exceptional light- and heat-absorption capabilities, enable the efficient conversion of solar energy into thermal energy. This process not only diminishes reliance on conventional energy sources but also contributes to a reduction in greenhouse gas emissions. Additionally, such materials facilitate the regulation of indoor temperatures, thereby lowering the energy consumption of air conditioning systems and enhancing overall energy efficiency. Al-Tamimi et al. utilized hemp concrete, which possesses low thermal conductivity and high specific heat capacity, to minimize energy loss through the building envelope. They employed modeling analysis using ANSYS Fluent software15.0 to investigate the influencing factors on energy storage and loss [18]. Additionally, the impermeability of concrete, being a porous material, can be improved through the incorporation of slag, polymers, and ceramisite [19,20,21]. Pang Chen et al. utilized alkali-activated slag cementing material (AASCM), characterized by rapid hardening, early strength development, high strength, and a low carbon content, for the repair of damaged concrete [22]. Through tests on water penetration, chloride ion penetration, and microanalysis, it was demonstrated that alkali-activated slag cementing material (AASCM) enhances the impermeability of repaired specimens. Graphene possesses high thermal conductivity, a stable lattice structure, excellent heat-transfer capability, and significant strength, making it an effective thermal-conductivity enhancer. Its remarkable photothermal, chemical, and electrical properties have led to its widespread application across various fields [23,24]. Shah et al. investigated the microstructure, mechanical properties, and durability of concrete enhanced with graphene and graphene-based compounds as additives. Their analysis revealed that these additives significantly improve the mechanical properties and longevity of concrete [25]. Ji et al. reviewed recent advancements in the synthesis and manufacturing of graphene nanocomposites across various energy-storage systems. They also discussed the latest developments in the production and application of energy-storage materials, along with the future prospects and potential challenges associated with energy-storage applications [26]. Li Z et al. investigated the performance of conductive grid asphalt concrete by incorporating graphene and carbon fibers as conductive phase materials. The study evaluated key mechanical and durability properties, including high-temperature stability, low-temperature crack resistance, and water stability, through standardized tests such as high-temperature rutting tests, low-temperature bending tests, and immersion Marshall tests. The results demonstrated that the addition of graphene significantly improved the high-temperature stability, low-temperature crack resistance, and water stability of the conductive grid asphalt concrete [27]. Wang S et al. developed a porous cement-based photothermal conversion material by depositing a graphene layer as a photothermal conversion coating on the surface of porous cement substrates. The study systematically evaluated the material’s photothermal conversion efficiency and water evaporation performance. The results revealed that the graphene-based photothermal conversion layer exhibited remarkable efficiency in both photothermal conversion and water evaporation, demonstrating its potential for solar energy utilization [28]. Meng S et al. investigated the impact of graphene and graphene oxide on the hydration kinetics of cementitious systems. Employing advanced analytical techniques, including simulated potential measurements, scanning electron microscopy (SEM), and isothermal calorimetry, the study demonstrated that the highly mobile ions associated with graphene and graphene oxide significantly accelerate the cement hydration process. These findings provide critical insights into the mechanistic role of carbon-based nanomaterials in enhancing the early-age properties of cementitious materials, offering potential applications for optimizing the construction material performance [29].

The novelty of this paper lies in the integration of graphene into concrete as a photothermal material. This study investigated the microstructure, mechanical properties, thermal properties, and impermeability of the concrete, exploring the effects of the graphene content and curing duration on these properties. Furthermore, the working mechanism of graphene-reinforced concrete was elucidated. The results demonstrated that the incorporation of an appropriate amount of graphene into concrete enhances its mechanical properties, photothermal performance, and impermeability.

2. Experimental Program

2.1. Characterization of Materials

The cement utilized in this study was P·O42.5R-grade ordinary Portland cement and the type of cement was limestone Portland cement, produced by Anhui Conch Cement Co., Ltd. located in Wuhu, China. The coarse sand (particle size ranging from 1 to 5 mm) and crushed stone (particle size ranging from 12 to 15 mm) were sourced from standard manufacturing facilities. Graphene was supplied by Tianjin Jiayin Nano Technology Co., Ltd., located in Tianjin, China. The components and chemical composition of the concrete prepared in this study are shown in

Table 1. The chemical composition of each component in concrete.

Components of Concrete	Chemical Composition
cement	calcium oxide, silicon dioxide, aluminum oxide, iron oxide
coarse sand	silicon dioxide
crushed stone	silicon dioxide, iron oxide
graphene	carbon
tap water	water

.

A specific quantity of graphene was selected for subsequent analyses, and the results of these analyses are presented as follows: transmission electron microscopy (TEM) was employed to observe the submicroscopic or ultrastructural characteristics of the material, facilitating a deeper understanding of its properties. X-ray powder diffraction (XRD) was used to analyze the crystal structure, phase composition, grain size, crystallinity, lattice parameters, crystal orientation, texture, and stress of the materials. X-ray photoelectron spectroscopy (XPS) enabled the determination of the chemical properties and surface composition of the samples by measuring the binding energy of electrons. Additionally, Raman Spectroscopy was utilized to investigate the molecular structure and composition of the substances.

2.2. Sample Preparation

The instruments utilized in this study included an analytical balance manufactured by Shanghai Shangping Instrument Co., Ltd., located in Shanghai, China, an ultrasonic material emulsion disperser (model TH-100), and an adjustable high-speed homogenizer (model FSH-2A), with daily tap water as the dispersing medium.

According to the specifications for the target test block, the mass ratios of cement, coarse sand, stone, and water was as follows: m (cement):m (coarse sand):m (stone):m (water) = 1:0.58:1.39:0.47 (see Figure 1a). The total volume of the concrete mixture was 0.02 m³, resulting in a total mass of approximately 52.6 kg. The graphene contents for the target test blocks were set at 4.728 g, 8.944 g, 14.184 g, and 18.912 g. Accordingly, four groups of dispersion liquids with varying graphene concentrations were prepared, corresponding to mass ratios of cement to graphene of 1:0.002, 1:0.004, 1:0.006, and 1:0.008, respectively. To prepare the dispersion solution, each amount of graphene was first weighed using an analytical balance and placed in a 500 mL beaker. Subsequently, 400 mL of water was added, and the mixture was subjected to vibration using a 325 W ultrasonic material emulsifier for 5 min. This was followed by stirring with an adjustable high-speed homogenizer at a speed of 8000 rpm for 3 min. Finally, an additional 1038 mL of water was added, and the mixture was stirred until homogeneous, resulting in a graphene dispersion (see Figure 1b). The graphene dispersion was then mixed with the concrete, stirred at 1000 rpm for 5 min, and poured into a mold that had been coated with a release agent. The mixture was slightly above the upper edge of the mold before being placed on a shaking table for vibration compaction. The concrete test blocks were prepared within 3 h post-dispersion configuration (see Figure 1c). To prevent water evaporation after the test blocks were formed, they were immediately covered with an impermeable film. The curing conditions for the test blocks were maintained at a temperature of 18 °C and a relative humidity of 90%, with curing durations of 3 days, 7 days, and 28 days.

As illustrated in Figure 2, a total of 20 test blocks with dimensions of 100 mm × 100 mm × 100 mm were prepared for this experiment. Each set comprised four test blocks, including one group without graphene and four groups containing suspensions with varying graphene concentrations. The first three test blocks in each group underwent compressive testing, while the fourth test block in each group was used for photothermal and impermeability testing.

The experimental procedure is systematically illustrated in Figure 3, which presents a comprehensive flowchart delineating the sequential steps and methodological framework employed in this study.

2.3. Characterization of Microstructure in Concrete

To investigate the influence of graphene on the microstructure of hydrated concrete, scanning electron microscopy (SEM) was employed to compare the microstructural features of ordinary concrete and concrete specimens incorporating varying graphene dosages after 28 days of curing. The brand and model of the instrument used was a Zeiss G360 provided by the Quantum Detection Technology Center of Zhengzhou City, China. The SEM analysis was performed on cross-sectional thin sheets (approximately 5 mm × 2 mm in size), with sampling focused on interfacial transition zones between the crushed stone and mortar, as well as the bonding regions of coarse aggregates. Additionally, thermogravimetric analysis (TG) was conducted to evaluate the impact of graphene on the durability of concrete. For this purpose, concrete fragments with different graphene contents were ground into a homogeneous powder (particle size < 100 μm). The TG measurements were carried out under a nitrogen atmosphere, with a heating rate of 20 °C/min over a temperature range of 40–1000 °C. To ensure reproducibility, each sample type was analyzed in triplicate. The instrument used is a thermogravimetric analyzer (TGA/DSC 3+, HKG, Beijing Hengjiu Experimental Equipment Co., Ltd. in Beijing, China).

2.4. Mechanical Tests

The compressive strength of the test blocks was evaluated on the 3rd, 7th, and 28th days of curing. A rock pressure testing machine (model UH-200, manufactured by Youhongyan Measurement and Control Technology Co., Ltd. in Shanghai, China) was employed to measure the compressive strength of the cubic test blocks. The loading rate was set at 0.25 MPa/s, with force loading as the applied method, and the oil temperature was maintained at 29 °C. Testing was terminated upon reaching 30% of the peak force after failure. The failure process was monitored and recorded for both before and after 40% of the peak strength of the test block.

2.5. Photothermal Experiment

In this study, five samples with identical curing times but varying graphene contents were subjected to photothermal experiments. As illustrated in Figure 4a, the experimental setup included a thermocouple thermometer (model: RE-Y2101B) from People Electric Appliance Group Co., Ltd. in Yueqing City, Zhejiang Province, China, a temperature sensor, and a reflective indoor heater (rated power: 700 W, model: RSN22-S07J) from Guangdong Rongsheng Electric Appliance Co., Ltd. in Guangzhou City, China. According to Figure 4b, temperature sensors were embedded at the front, back, top, and left sides of each test block to monitor temperature variations from multiple directions. The distance between the reflective indoor heater and the test block was maintained constant throughout the experiment. Initially, the infrared light source was preheated for 1 min, after which the test block was irradiated for 10 min, with temperature recordings taken every 30 s. Following the irradiation, the reflective indoor heater was turned off, and temperature readings continued every 30 s for an additional 10 min. To minimize potential experimental errors, the tests were conducted concurrently over three consecutive days.

2.6. Impermeability Test

Currently, various methods are available for evaluating the impermeability of concrete [30]. In reference [31], a pore grid model was established using X-ray tomography (CT) to analyze the microstructure of concrete. The permeability coefficient obtained through numerical simulation was compared with experimental values derived from permeability tests. Reference [32] examined the permeability height of concrete via anti-seepage tests. Additionally, reference [33] emphasized that effective impermeability is crucial for the long-term durability and mechanical properties of concrete, as it helps mitigate issues such as cracking and other detrimental phenomena.

3. Results and Discussion

Subsequently, the crystallinity, layer count, and structural integrity of graphene will be systematically characterized using the aforementioned analytical techniques. Following this, the experimental data will be employed to conduct a comprehensive evaluation of the concrete’s microstructure, alongside its mechanical, photothermal, and impermeability properties, to elucidate the structure–property relationships in graphene-modified concrete systems.

3.1. Material Characterization

As shown in Figure 5a, the two-dimensional lamellar structure of the graphene material is clearly visible, exhibiting areas of light and dark contrast. This variation is attributed to interactions between the graphene and the substrate, as well as thermal stress and other factors that lead to the formation of folds and ripples.

X-ray diffraction (XRD) analysis was conducted using a 20 kV X-ray diffractometer (model DX-2700BH) produced by Dandong Haoyuan Instrument Co., Ltd. located in Dandong City, China, with a 2θ range from 10 to 70 degrees. As depicted in Figure 5b, a narrower half-height width typically indicates a larger grain size and fewer lattice defects, suggesting that the graphene possesses a higher degree of crystallinity. Furthermore, a sharp diffraction peak is observed at an angle of 26.3°, corresponding to the graphene crystal surface. According to calculations based on Bragg’s law, the interlayer spacing of the graphene is approximately 0.34 nm.

Raman spectroscopy is an effective technique for assessing the quality of graphene by characterizing its bonding structure. Raman analysis was conducted using a LabRam HR 800 Raman spectrometer produced by HORIBA Jobin Yvon Company located in Paris, France, with a scanning range of 0 to 4000 cm⁻¹. As illustrated in Figure 5c, the Raman spectrum of graphene exhibits three characteristic peaks: the D peak at 1318 cm⁻¹, the G peak at 1580 cm⁻¹, and the 2D peak at 2628 cm⁻¹ [34]. The D peak is relatively weak and corresponds to the vibrations of sp³ hybridized carbon atoms, which are typically associated with defects or edges in the graphene lattice. In contrast, the G peak is the sharpest and most intense, representing the fundamental lattice vibration mode of graphene, arising from the in-plane vibrations of sp² hybridized carbon atoms. The 2D peak is a symmetric single peak that results from a biphonon resonance process; it possesses high intensity and serves as a key indicator for determining the number of graphene layers. The intensity ratios (I(D)/I(G)) and (I(2D)/I(G)) are used to assess the quality of graphene. The intensity ratio of the D-band to the G-band, I(D)/I(G) = 0.47, indicates a low defect density and high crystallinity, confirming the better quality of the CVD-grown graphene. Furthermore, the intensity ratio of the 2D-band to the G-band, I(2D)/I(G) = 0.48, suggests the multi-layer structural characteristics of the graphene studied [35,36]. S multilayer graphene exhibits high potential in the field of photothermal conversion due to its broadband light absorption, efficient electron phonon coupling, and controllable number of layers.

X-ray photoelectron spectroscopy (XPS) analysis was conducted using a PHI Quantera XPS system to determine the elemental composition of the graphene. As presented in Figure 6, curve fitting of the high-resolution XPS spectra revealed peak intensities for the elements O, N, and C at 20,455.58, 54,383.66, and 215,663.8, respectively, with corresponding binding energies of 532.36 eV for O, 399.26 eV for N, and 284.83 eV for C. The C1s spectral peak is particularly significant, as it represents the 1 s electron binding energy of carbon atoms in graphene. The symmetry and sharpness of the C1s peak indicate that the carbon atoms are in a highly uniform sp² hybridization state and exhibit good crystallinity.

3.2. Microstructure of Concrete

As illustrated in Figure 7, the cross-sectional morphology of ordinary concrete exhibits significant roughness and pronounced cracking at both 200 μm and 20 μm scales. At a higher resolution (2 μm scale), the hydration products display a predominantly loose and disordered sheet-like structure. In contrast, graphene-modified concrete demonstrates a smoother cross-section with no observable cracks and a denser microstructure at the same magnification levels (200 μm and 20 μm). At the 2 μm scale, the graphene-modified specimens reveal a higher abundance of well-defined plate-like crystals and a reduced presence of unbound hydration products. These observations collectively indicate that graphene incorporation enhances the interfacial bonding between coarse aggregates and cement paste, thereby improving the microstructural integrity and compactness of the composite material.

As shown in Figure 8a, the thermogravimetric analysis reveals distinct weight-loss stages for concrete specimens within specific temperature ranges, consistent with previous studies [37]. Between 150 and 400 °C, the primary mass loss originates from the dehydration of calcium silicate hydrate gel (C-S-H), the dominant hydration product of cement. Subsequently, from 400 to 600 °C, the decomposition of calcium hydroxide (CH) accounts for the observed mass reduction, while the decarbonation of calcium carbonate (CaCO₃) predominates between 600 and 1000 °C. The residual mass analysis demonstrates that graphene-modified concrete exhibits higher proportions of both CH and C-S-H compared to ordinary concrete, indicating enhanced hydration product formation and a denser microstructure. Figure 8b further illustrates that at a cement-to-graphene mass ratio of 0.006, the derivative thermogravimetry (DTG) peak intensity decreases most significantly, suggesting graphene’s inhibitory effect on localized rapid pyrolysis induced by microcrack propagation. The DTG curve exhibits a prominent peak near 600 °C, confirming graphene’s substantial influence on the thermal stability of concrete. The shift in peak characteristics underscores graphene’s role in modulating the decomposition kinetics of concrete at elevated temperatures. In summary, microstructural characterization reveals that graphene-modified concrete exhibits a denser microstructure and a higher abundance of hydration products compared to conventional concrete, as evidenced by comprehensive analytical techniques.

3.3. Mechanical Property

As illustrated in Figure 9a, the test block doped with graphene exhibited fewer cracks and maintained better structural integrity after failure, suggesting that graphene enhances the strength and toughness of concrete, thereby inhibiting the propagation of macroscopic cracks and preventing the transition of microscopic cracks to the macroscopic level.

Figure 9b presents the measurements of the height of the damaged test blocks, which were recorded as 8.3 cm, 9.0 cm, 9.5 cm, 10.0 cm, and 8.2 cm, respectively. From Figure 5c, it is evident that, in terms of compressive failure, the test block numbered 2-1 displayed a larger size, improved compactness, and superior integrity compared to the other test blocks after failure, indicating its excellent compressive performance.

In Figure 9c, the failure mode of the test block without graphene predominantly initiated at the edges and corners, with cracks progressively extending toward the opposite end, resulting in the detachment of large fragments during failure. In contrast, the damage observed in the graphene-doped test block began with only a few small cracks; the extent of crack propagation was limited, and the overall integrity of the block remained good upon failure, with only a minor number of small fragments detaching.

In summary, the incorporation of an appropriate amount of graphene significantly enhanced the strength and structural density of concrete. In future research, computer vision techniques will be employed to automatically quantify the crack propagation, pore structure, and surface defects in graphene–concrete composites, ensuring more objective and reproducible results. For instance, binary image segmentation will be performed using a convolutional neural network (CNN) based on the DeepLab architecture, enabling pixel-level crack detection with high precision [38]. Additionally, EfficientNet can be integrated as a feature extractor within the target detection framework, significantly improving the efficiency and accuracy of crack recognition [39].

The stress–strain curve provides a comprehensive representation of the mechanical properties and deformation characteristics of materials under stress. As illustrated in Figure 10a, the stress–strain curve of the undoped graphene test block exhibits a relatively low slope in the initial rising section, whereas the curve for the graphene-doped test block demonstrates a steeper slope in the same region. This indicates that the incorporation of a certain amount of graphene enhances the elastic modulus of concrete, allowing it to effectively bear and transfer stress, thereby improving the compressive strength of the material. Three replicate specimens were prepared for each graphene content level, and the mean compressive strength derived from these replicates was adopted as the representative compressive strength for the corresponding graphene content. According to Figure 10b, after 28 days of curing, the compressive strength of the graphene-modified concrete increased by as much as 44% compared to the control group (66.82 MPa was 20.48 MPa higher than 46.34 MPa in the control group). However, it is crucial to maintain an appropriate concentration of graphene dispersion; excessively high concentrations can lead to a reduction in the strength of the test block. This reduction occurs because, while graphene effectively transfers stress and enhances the toughness of concrete, it can also promote crack bridging and improve hydration, thereby increasing the compressive strength. Nonetheless, the relationship between the graphene content and compressive strength is complex, influenced by multiple mechanisms. When present in excessive amounts, graphene tends to agglomerate, leading to uneven dispersion. Thus, it is evident that the correlation between the graphene content and concrete strength is not linear.

3.4. Photothermal Property

The amount of graphene incorporated into concrete significantly influences its photothermal conversion capacity; however, this increase in graphene content also enhances heat conduction, leading to faster heat dissipation. Consequently, a competitive interaction exists between these two properties, necessitating an examination of the impact of graphene content on the thermal characteristics of concrete.

Table 1 (here is the number of a test block in Figure 2), which did not contain graphene, was examined during both the placement and removal of the light source. As illustrated in Figure 11, during the initial 10 min exposure to the light source, the temperatures of the front, upper, and rear surfaces of the test block exhibited a continuous increase. In contrast, the temperature of the left surface displayed a gradual initial rise followed by stabilization. Following the removal of the light source in the subsequent 10 min interval, the temperatures of the rear and upper surfaces continued to decline. Concurrently, due to internal heat conduction within the test block, the temperatures of the left and front surfaces demonstrated a slow but steady increase.

Since the front surface of the test block is oriented away from the light source, monitoring its temperature variation is essential for assessing the heat-absorption characteristics of the material. To provide a direct reflection of this thermal performance, the difference (ΔT) between the peak temperature attained by the front surface of each test block during irradiation and the initial temperature (21.2 °C) will be analyzed. According to Figure 12a and

Table 2. Peak temperature of the front surface.

Number	Peak Temperature of the Front Surface (°C)
	First Test	Second Test	Third Test	Average Value
1-4	25.6	25.2	25.4	25.4
2-4	25.5	25.4	25.6	25.5
3-4	26.2	26.2	26.2	26.2
4-4	25.9	25.8	25.7	25.8
5-4	27.2	27.2	27.5	27.3

, with an increasing graphene content, the peak temperature of the front surface of the test blocks demonstrated an upward trend. This indicates that graphene significantly enhances the heat-absorption capacity of the test blocks, with the heat-absorption capacity of the graphene-modified concrete reaching up to 0.7% higher than that of the control concrete. (The ΔT exhibited an increase of 1.9 °C, rising from 4.2 °C to 6.1 °C, and the temperature value converted to Kelvin is 1.9 K, rising from 277.35 K to 279.25 K.)

The investigation of surface temperature variations in the test blocks facilitates the analysis of differences in the heat-release performance among the various samples. Below, we analyze the peak temperature of the upper surface of the test blocks. As illustrated in Figure 12b and

Table 3. Peak temperature of the upper surface.

Number	Peak Temperature on the Upper Surface (°C)
	First Test	Second Test	Third Test	Average Value
1-4	20.0	20.2	20.1	20.1
2-4	19.6	19.5	19.4	19.5
3-4	18.8	19.2	19.0	19.0
4-4	19.5	19.6	19.4	19.5
5-4	19.3	19.7	19.5	19.5

, the peak surface temperature of the graphene-doped test blocks is significantly lower than that of the undoped test blocks. This finding suggests that graphene enhances the dissipation of heat from the test block to lower-temperature regions, with the graphene content in test blocks 3 and 4 proving particularly effective for heat release. Consequently, the heat-release capacity of the graphene-modified concrete is reduced by up to 0.4% relative to that of the control concrete. (The temperature exhibited an decrease of 1.1 °C, fallng from 20.1 °C to 19.0 °C, and the temperature value converted to Kelvin is 1.1 K, fallng from 293.25 K to 292.15 K.).

To assess the thermal-storage performance of the test blocks over an extended duration, the blocks were subjected to irradiation from a light source for 1 h, and the temperatures of the front and back surfaces were recorded at 5 min intervals. Following the removal of the light source, temperature readings were taken every minute. This temperature-variation study aids in analyzing the differences in heat-storage performance across the different test blocks. As shown in Figure 13a, the peak surface temperatures of test blocks from groups 2 to 4 are significantly higher than those of the test blocks in group 1. This indicates that a certain amount of graphene improves the heat-storage capacity of concrete. During the cooling phase, the rear surface temperature of the first group of test specimens exhibited the lowest final value. In contrast, the rear surfaces of the second, third, fourth, and fifth groups demonstrated comparatively higher final temperatures. This disparity further corroborates that the incorporation of graphene significantly enhanced the heat-storage capacity of the concrete, as evidenced by the delayed thermal dissipation in the modified specimens. Figure 13b and

Table 4. The Difference Between Peak Temperature of the Rear Surface and Initial Temperature.

Number	The Difference Between Peak Temperature of the Rear Surface and Initial Temperature (°C)
	First Test	Second Test	Third Test	Average Value
1-4	17.7	17.8	17.6	17.7
2-4	19.6	19.7	19.5	19.6
3-4	20.1	20.0	19.9	20.0
4-4	19.8	19.6	19.4	19.6
5-4	16.7	16.9	16.8	16.8

indicate that the maximum heat-storage capacity is achieved when the mass ratio of cement to graphene is 1:004, resulting in a heat-storage capacity that is up to 0.8% greater than that of the control concrete. (The temperature exhibited an increase of 2.3 °C, rising from 17.7 °C to 20.0 °C, and the temperature value converted to Kelvin is 2.3 K, rising from 290.85 K to 293.15 K.).

In summary, graphene exhibited remarkable photothermal-conversion performance when utilized for wall insulation. Its high specific surface area enabled concrete to absorb and store a significant amount of heat while efficiently dispersing that heat across one side of the wall, thereby preventing local heat accumulation. However, excessive amounts of graphene can result in a reduced effective path for photothermal conversion.

3.5. Impermeability

Concrete is inherently porous and hydrophilic. In practical applications, water infiltration can lead to issues such as skin peeling of walls, resulting in indoor dampness and other negative effects. Consequently, it is essential to investigate the influence of graphene on the permeability of concrete test blocks [30].

Under the combined influence of surface tension and capillary action, water ascends beyond the initial water level. The vertical distance between the initial water level and the maximum water level attained is defined as the penetration depth. As illustrated in Figure 14a, this study employed the methodology described in the literature by immersing five completely dried test blocks in water, ensuring a specified distance between the blocks. The initial water level was set 2.5 cm above the base of the test blocks. The effect of the graphene dosage on the impermeability of concrete was systematically investigated by quantifying the maximum penetration depth and the corresponding penetration area of the test specimens over a specified duration. This analysis provided critical insights into the relationship between the graphene concentration and the material’s resistance to fluid ingress. Both the peak water level and the penetration depth are recorded after a 7-day period, with the maximum penetration depth denoted as k (see Figure 14b) [33].

As illustrated in Figure 14c, the surface corresponding to the maximum penetration depth (k) for each test specimen is selected as the analysis plane. The area infiltrated by water on this surface is designated as the seepage area (S). The seepage area is quantitatively assessed to facilitate a comparative analysis of the seepage performance across different test specimens, thereby elucidating the impact of graphene on the permeability resistance of concrete. The seepage area is calculated using the following formula:

$$S=S_1+S_2$$

(1)

$$S_1=h·d$$

(2)

$$S_2=s·n$$

(3)

where (S) represents the water seepage area, (S₁) denotes the corresponding seepage area at the initial water level, as illustrated in Figure 14c, (S₂) represents the seepage area after the test block was immersed in water for 7 days, showing an increase relative to S₁, as shown in Figure 14c, (h) is the initial infiltrating height of the test block and (d) is the test block side length, (s) represents the area of a square with a side length of 1 cm, and (n) represents the number of squares in S₂ with a side length of 1 cm. To facilitate the calculation of S₂, we divide the region into a number of squares of area (s). Squares smaller than half of s are ignored, and squares larger than half of s are counted as 1.

As shown in

Table 5. Test result of block impermeability test.

Number	K (cm)				S (cm2)
	First Test	Second Test	Third Test	Average Value	First Test	Second Test	Third Test	Average Value
1-4	1.4	1.4	1.7	1.5	28.5	30.5	30.0	29.5
2-4	1.0	1.1	0.9	1.0	25.0	26.0	24.0	25.0
3-4	0.4	0.6	0.5	0.5	24.5	25.5	25.0	25.0
4-4	1.0	0.6	0.8	0.8	27.0	26.0	25.0	26.0
5-4	1.0	1.0	0.7	0.9	30.5	29.5	30.0	30.0

, the penetration depth (k) of graphene-doped test blocks is significantly lower compared to that of undoped test blocks. Moreover, the water seepage area of test blocks 2-4, 3-4, and 5-4 is markedly reduced relative to that of test blocks 1-4 and 5-4. These findings indicate that graphene effectively fills the internal pores of concrete, imparting improved permeability resistance. There exists an optimal dosage range for graphene to enhance the impermeability of concrete. Within this range, graphene is uniformly dispersed, and the most pronounced improvement in impermeability is observed at a cement-to-graphene mass ratio of 1:0.04, where the maximum penetration depth is reduced by approximately 200%. However, when the mass ratio of cement to graphene increases to 1:0.08, the permeability resistance of the test blocks diminishes, leading to a significant increase in the seepage area.

4. Conclusions

In conclusion, this study investigated the performance of graphene-reinforced concrete, which is influenced by multiple mechanisms, including heat conduction, bridging, and stress dispersion. The incorporation of an optimal amount of graphene into concrete significantly enhanced its mechanical, photothermal, and impermeability properties. Compared to the control group, graphene-modified concrete exhibits a denser microstructure and the enhanced formation of hydration products. In addition, the compressive strength of graphene-reinforced concrete increased by 44%. In terms of photothermal performance, the heat-absorption capacity improved by up to 0.7%, while the heat-release capacity decreased by up to 0.4%, resulting in an overall increase in the heat-storage capacity by up to 0.8%. When applied to building walls, this material can effectively reduce the heat and cold loads of the building. Regarding impermeability, the maximum penetration depth of graphene-reinforced concrete was significantly reduced by approximately 200%, making it suitable for use in building walls and other applications where water infiltration needs to be minimized. These findings demonstrated the multifunctional benefits of graphene-reinforced concrete, offering significant advancements in both structural and environmental performance. Despite the relatively high cost of graphene materials, their utilization in this study and potential future applications remain minimal. Furthermore, graphene can be selectively incorporated into the surface layer of the wall, thereby optimizing material efficiency. To reduce costs, the preparation process should be streamlined to the greatest extent possible.