Preparation and Properties of a Composite Carbon Foam as Energy Storage and EMI Shield Additive for Advanced Cement and Gypsum Boards

1. Introduction

The ongoing need for sustainable building solutions has spurred the search for novel materials capable of improving energy efficiency and tackling emerging issues like electromagnetic interference (EMI) shielding. Traditional construction materials like cement, gypsum, concrete, and brick have long been staples in wall construction. However, recent attention has shifted towards developing materials that not only fulfill structural requirements but also exhibit thermal energy storage capabilities [1]. This focus on thermal energy storage aligns with the wider push to enhance energy efficiency and sustainability in building practices. Moreover, the demand for buildings equipped to counteract the effects of electromagnetic waves has grown more urgent in our increasingly technology-driven surroundings. The rapid evolution of electronic devices has raised concerns about electromagnetic radiation, as it can disrupt device functionality and potentially pose risks to human well-being [2]. In light of these challenges, integrating advanced materials with diverse properties has emerged as a promising avenue toward achieving multifunctionality in building solutions.

Within this context, the incorporation of phase change materials (PCMs) into building elements such as gypsum and cement boards has garnered considerable interest. This interest stems from their capacity to store and release thermal energy, thereby facilitating effective temperature control within enclosed environments [1,3,4]. The cost-effectiveness and adaptability of PCM products have catalyzed inventive approaches in the construction sector. They are progressively acknowledged for their capacity to improve energy efficiency and regulate thermal conditions within constructed spaces [1,5,6]. The adoption of these materials has undergone extensive examination within the engineering and scientific spheres. This scrutiny is largely attributed to their ability to absorb and release thermal energy throughout phase transitions, presenting an encouraging avenue for improving indoor temperature management [1]. Nonetheless, the practical implementation of PCMs encounters hurdles stemming from their inherent drawbacks, including low thermal conductivity and the possibility of leakage when in a liquid state during phase transition [7]. To effectively tackle these obstacles, the adoption of a shape-stabilizing (SS) support matrix has emerged as a highly effective strategy. This matrix serves as a safeguarding enclosure for the PCM, guaranteeing its containment and stability in a molten state, particularly at higher temperatures. As a result, it prevents leakage and reduces corrosion in structural materials [8]. Carbon-based porous materials, such as carbon-based foam, expanded graphite, graphene oxide, graphene aerogel, and activated carbon, present a promising platform for incorporating functional additives like PCMs. This integration results in enhanced thermal conductivity and electromagnetic interference (EMI) shielding capabilities, offering compelling opportunities for various applications [8,9,10,11,12].

Significant studies in the literature have delved into integrating carbon-based materials with shape-stabilized PCMs into composite cement or gypsum boards [1,13]. Chin et al. showed that concrete panels incorporated with paraffin-oil palm kernel shell-activated carbon exhibit increased thermal lag and reduced peak temperature during the phase transition of composite PCM [14]. Qian and Li, reported the preparation of cement composites with n-octadecane stabilized on diatomite/carbon matrix [15]. The heat-storage cement mortar developed has been found to mitigate fluctuations in interior temperature, showcasing significant potential for energy savings and enhanced thermal comfort in the built environment. Chen et al. reported that the incorporation of SAT-urea eutectic salts encapsulated with modified commercial-activated carbon in gypsum, could slowly reduce the thermal conductivity and significantly improved the thermal inertia of the composites [16]. Finally, in our recently published work [17], the incorporation of activated carbon/RT18HC on gypsum boards led to composite gypsum boards with enhanced properties on thermal storage and EMI shielding applications, compared to the net gypsum board.

In this study, we present a novel investigation into the advancement of building materials with dual functionalities by incorporating gypsum and cement boards with shape-stabilized organic phase change materials (ssPCM). This ssPCM consists of n-octadecane (OD) stabilized on carbon red-mud foam (CCF). Our previous research has extensively examined the thermal and electromagnetic interference (EMI) shielding properties of OD@CCF composites, demonstrating their effectiveness in thermal management and EMI reduction in electronic devices [8]. Our current study focuses on assessing the impact of varying percentages of this additive on the thermal storage capacity and EMI shielding abilities of gypsum and cement boards. We conducted experiments under real-world conditions, simulating realistic weather scenarios using a custom-designed environmental chamber. By integrating n-octadecane encapsulated in carbon red-mud foam into the matrix of these building components, we aim not only to efficiently regulate indoor temperatures but also to enhance structural resilience against electromagnetic interference. This approach offers a comprehensive solution to the dual challenges of energy conservation and electromagnetic shielding in modern architectural designs.

2. Materials and Methods

2.1. Materials

The materials used to produce Ceramic Carbon Foam (CCF) and shape stabilized n-octadecane on CCF (OD@CCF) were commercially sourced and are extensively detailed in our previous publication [8]. The raw materials required for cement production were supplied by Energy Houses (ENHSS, Serres, Greece), a collaborating business partner based in Greece. Gypsum and starch were provided by KNAUF (Amfilochia, Greece), a globally recognized company, to ensure that the final product closely resembled commercial standards.

2.2. Preparation of CCF and OD@CCF Hybrid

Comprehensive details regarding the experimental procedures for preparing these materials can be found in our previously published research [18]. The CCF matrix was synthesized through the polymeric foam replication technique, utilizing a polyurethane sponge as a template, resin as a carbon source, and red mud as a filler.

2.3. Preparation of Cement Boards

The experimental procedure devised for manufacturing the cement boards received guidance from a Greek collaborative partner, Energy Houses (ENHSS, Serres, Greece), which not only supplied the necessary raw materials but also offered technical insights throughout the production process. In accordance with both company and international standards, the raw materials included cement powder, polypropylene fibers, acrylic polymer emulsion-resin, perlite, water, and composites containing encapsulated n-octadecylamine with foams (OD@CCF) in varying proportions. The cylindrical morphology composites (D=30 mm, h=30 mm) were subsequently fragmented into smaller pieces to facilitate their uniform dispersion within the cement boards. The experimental procedure involved weighing the aforementioned raw materials in specified quantities for each composition (detailed in Table 1) and then subjecting them to continuous manual stirring for at least fifteen minutes to ensure thorough homogenization. Following this, the mixture was applied to a wooden mold and allowed to dry at room temperature for a minimum of twenty-four hours, resulting in specimens measuring 200 x 200 x 37 mm. The cement board without additives is denoted as CB, while those with composite additions are referred to as CB/OD@CCF-x, where x represents the volume percentage of OD@CCF additive.

2.4. Preparation of Gypsum Boards

To produce the gypsum boards, all solid materials were thoroughly mixed manually until a uniform consistency was achieved. Water was then added while vigorously stirring the mixture. The blending process continued for approximately five minutes before pouring the resulting slurry into a mold, resulting in gypsum boards with dimensions of 200 x 200 x 12.5 mm. The slurry was left undisturbed in the mold for a minimum of 24 hours at room temperature to allow for drying. The specific compositions of the prepared gypsum boards are detailed in Table 2. The reference gypsum board is denoted as GB, while composite boards are labeled as GB/OD@CCF-x, where ‘x’ represents the volumetric percentage of OD@CCF additive.

2.5. Measurements of Density

The bulk densities (ρ) of the gypsum and cement boards were initially estimated in the original specimens using the formula ρ = m/V, where ‘m’ represents weight, and ‘V’ stands for the volume of the boards. However, to enhance measurement accuracy, a modified Archimedes water displacement method was employed, following a standardized procedure [19]. This method involved initially measuring the bulk volume and then determining the bulk density for small-sized porous specimens. Before immersion into a cylinder with known dimensions, the samples were sealed with wax. Both density measurement techniques yielded similar ρ-values, showing no significant deviations.

2.6. X-ray Computed Microtomography (Micro-CT)

X-ray computed microtomography (micro-CT) imaging was employed to investigate the internal structure of composites with varying concentrations of additive (10%, 20%, and 30%). Radiographic imaging was carried out on a Bruker SkyScan 1275 (Billerica, MA, USA) scanner, equipped with a distortion-free 3 Mp active flat-panel detector. An accelerating voltage of 60 kV and a current of 60 μA were applied, while a 1 mm thick Al filter was used. The object distance was 20 mm leading to a pixel size of 10 μm. A 360° scan was performed with a 0.20° rotation step corresponding to 25 minutes scan duration. The reconstruction was performed on Bruker’s NRecon – v2.1.0.1 software. While CTan and Dataviewer were used for the image analysis.

2.7. Thermal Properties

Differential Scanning Calorimeter (DSC) measurements were performed under N₂ atmosphere using instrument DSC 214 NETZCH Polyma instrument (Selb, Germany). Rate of heating and cooling was 2 ^oC/min, while temperature ranged from 0 ^oC to 40 ^oC and 40 ^oC to 0 ^oC respectively. The thermal transport properties of gypsum board specimens, including thermal conductivity (k) and heat capacity (Cp), were assessed using a C-Therm TCi Thermal Conductivity analyzer (Fredericton, Canada). The Modified Transient Plane Source (MTPS) method (ASTM D7984) [20] was employed, wherein a one-sided thermal sensor applied a low-energy current pulse for 0.8 seconds to the specimen. This ensured that the measurement of thermal conductivity (k) was not influenced by thermal convection. Some of the heat generated was absorbed by the specimens, while the remaining portion was utilized to raise the temperature at the sensor-sample interface. The resulting temperature variation indicated a resistance change in the sensor, leading to a voltage drop. The voltage data were utilized to assess the thermal transport properties of the investigated material. Thermal conductivity (k) and thermal effusivity (e) were measured directly, providing a detailed overview of the heat transfer properties of the sample material. The final values were derived from an average of 5 measurements for the specimen using the polymer test method and pyrex glass as a standard (k=1.143W/m∙K). The heat capacity (Cp) was then calculated using the equation: Cp =e²/k∙ρ where ρ is the density of each specimen.

2.8. Mechanical Properties

The modulus of rupture, MOR of gypsum boards (GB, GB/OD@CCF), was determined via three-point bending strength tests (Autograph AGS-H; Shimadzu, Kyoto, Japan), according to ASTM C293/C293M-16 [21]. More specifically, the cross-sectional dimensions of each specimen were measured (the accuracy was ± 0.01 mm). Then, the gypsum specimen (200 mm x 50 mm x 12.5 mm) was centered at the gap (the span was 100 mm) between the supporting rods of the test machine. The velocity of the bending load was 2.0 mm/min, and the load until fracture was measured. The flexural strength (σ) was calculated using the equation (1)

$$MOR=\frac{3WL}{2b{d}^2} (1)$$

where, W (N) is the fracture load, and L, W, and h are the span between the supporting rods, the specimen width, and the thickness, respectively. Five specimens from each subgroup (n = 5) were tested.

The mechanical properties of the cement boards (CB, CB/OD@CCF)were evaluated by measuring the compressive deformation behavior of cubic specimens x=y=z ~50 mm, using the same testing machine. The compressive strength (CS=Load / Area(N/mm²) of every cement board was conducted according to standard ASTM C109/C109M-20 [22]. Compressive stress- stain (%) curves were derived from the load- displacement curves, and subsequently compressive strength and modulus of elasticity of the samples were calculated. The results are the average of measurements made on, at least, 5 specimens.

2.9. Thermal Performance Measurements

We utilized a custom-built environmental chamber [23] for conducting thermal performance measurements. The chamber, illustrated in Figure S2, was crafted from polystyrene foam panels and had internal dimensions of 1000 mm length, 1000 mm width, and 1000 mm height. An opening in the top section of the side panel accommodated the installation of a fan heater and a portable air conditioner.

Below the main chamber, four smaller chambers were positioned as test rooms, each with internal dimensions of 200 mm length, 200 mm width, and 200 mm height. These test chambers, also constructed from polystyrene foam panels, featured a design with a hollow top side panel to facilitate the placement of test samples.

Temperature measurements were conducted using Type K thermocouples placed at the center of each test room, along with additional thermocouples attached to both the outer and inner surfaces of each test sample. Additionally, a centrally placed thermocouple within the environmental chamber allowed for comprehensive temperature monitoring.

The chamber’s internal temperature was manipulated by the fan heater and portable air conditioner, causing elevation and decrease, respectively, from the top of the chamber. This setup enabled the assessment of thermal reactions in the test samples under varying temperature conditions. Temperature measurements were recorded at intervals of 5 minutes.

2.10. Electromagnetic Interference (EMI) Shielding Properties

The EMI shielding effectiveness properties were evaluated using a P9372A Keysight Streamline Vector Network Analyzer (Keysight, Santa Rosa, CA, USA) and two sets of microwave standard 15 dB gain waveguides (WR 187 and WR 147, respectively, obtained from Advanced Technical Materials Inc. (ATM), Patchogue, NY, USA) covering a broad C frequency band in the range of 3.2–7.0 GHz (C-band), which is used for long-distance radio telecommunications, such as satellite communications transmissions, Wi-Fi devices, cordless telephones, weather radar systems, etc. In particular, every sample was placed in the middle of each set of waveguides, and its scattering parameters (S-parameters; S11, S12, S22, S21) were recorded.

3. Results and Discussion

In our earlier research research [18], we extensively examined the structural and physicochemical attributes of both the Carbon-Red mud foam matrix (CCF) and the composite OD@CCF. To summarize, our analysis revealed that CCF possesses a remarkably porous structure, featuring elliptical and spherical pores ranging from 50 to 500 μm. The cell walls exhibit partial graphitization of carbon alongside various oxide phases. Of particular note, the hybrid foam OD@CCF exhibited exceptional efficiency in encapsulating paraffins, with a loading capacity of 48.8 %w.t. of octadecane. In the forthcoming sections, we will delve deeper into the characteristics of gypsum and cement composite boards integrated with OD@CCF additive at different concentrations.

3.1. Density Results and Microtomography Analysis of Composite Boards

Figure 1 and Figure 2, illustrate the images of both the reference gypsum or cement board (GB or CB) and the GB/OD@CCF or CB/OD@CCF composite boards, respectively.

Table S1 presents the results of density measurements, while Figure 3 illustrates the plotted data against the percentage content of OD@CCF. For GB/OD@CCF composites. The data in Figure 3a reveals a slight decrease in gypsum board density with increasing additive content. Specifically, integrating 30% OD@CCF into gypsum boards reduced the density from 0.96 to 0.92 g cm^-3, showing a relatively consistent trend considering the error margins. This decrease aligns with previous findings, where the density of the current shape stabilizer, carbon foam, was measured at 0.54 g cm^-3 [18], significantly lower than the initial density of the reference gypsum board (0.96 g cm^-3). Incorporating such lightweight additives contributes to the overall density reduction of the composite boards, although they still meet the minimum density requirement of 0.60 g cm^-3, as specified by European regulation UNE-EN 13279-1 for gypsum binders and plasters [24,25]. Similar density reduction trends during additive incorporation have been observed in the literature. For instance, in our prior research [17], adding 30% activated carbon-PCM to gypsum boards led to a density decrease from 0.96 to 0.81 g cm^-3. Additionally, Jameel et al. [26] noted a density decrease in gypsum boards enhanced with chopped carbon fibers, reaching from 1.258 to 1.098 g cm^-3 for a 0.3% volume fraction.

Conversely, in CB/OD@CCF composites, density increases with rising additive content (Figure 3b), attributed to the higher density of PCM compared to the components of cement boards. The solid-phase density of the current PCM, raw octadecane, stands at 0.88 g cm^-3 [27], contrasting with the measured density of the reference cement board at 0.51 g cm^-3. A similar trend was observed by Ye et al. [28], who reported an increase in cement board density from 0.32 to 0.43 g cm^-3 upon adding 40% w.t. RT28HC stabilized on expanded perlite.

To substantiate the findings and investigate the cement and gypsum structure, X-ray computed microtomography (μCT) was conducted. These results offer crucial insights into the correlation between additive concentration and the microstructural characteristics of the composites. Microscale images (Figure 4 and Figure 5) show distinctive features corresponding to varying levels of additive incorporation. The colored scale bar in the images represents different X-ray attenuation values, indicating distinct physical density values of the wall-board components, ranging from lower (black) to higher (white) indices. The black spots in all images signify lower intensity areas (air voids), representing the porous regions of the boards. In the case of the cement boards, the images for CB reference board, predominantly feature black- and purple-colored areas (Figure 4a). For 10 % additive OD@CCF, the microtomography images (Figure 4b) display a reduction in black and purple areas, accompanied by the emergence of new yellow and green areas (higher index components). These areas are more intense in the images of composite boards with higher OD@CCF content, 20 and 30 %w.t (Figure 4c,d, respectively). These latter components likely correspond to OD@CCF particles, indicating their homogeneous integration into the cement board even at the higher loading (30 %). These findings are also, in agreement with the density measurements, where the density of the cement board increases with increment of OD@CCF loading.

In the case of gypsum boards, the corresponding microtomography images are shown in Figure 5. The images for GB reference board, predominantly feature yellow- and green- colored areas as well as few purple- and back- colored areas (Figure 5a). For 10 % additive OD@CCF, the microtomography images (Figure 5b) display more black- colored areas (lower index components, air voids) accompanied by the emergence of blue (higher index components). After a careful observation of black areas, it is obvious that consists also from purple- colored areas and these components may attributed to OD@CCF particles having lower intensity than gypsum components. Similar features are also shown in the images of composite boards with higher OD@CCF content, 20 and 30 %w.t (Figure 5c,d, respectively) and OD@CCF particles seems to be homogenously dispersed in gypsum matrix, even at higher loadings (30 %).

3.2. Thermal Properties

Figure 6 displays the Differential Scanning Calorimetry (DSC) curves for (a) composite gypsum boards GB/OD@CCF and (b) composite cement boards CB/OD@CCF with varying percentages of OD@CCF content. The coresponding curves for pure octadecane are presented in Figure S1. The melting (Tm) and solidification (Ts) temperatures, along with their corresponding enthalpies (ΔHm and ΔHs), are detailed in Table S1. The analysis reveals that for both gypsum and cement boards, the melting temperature of octadecane falls within the range of 26.4-28.3°C, while the solidification temperature ranges from 22.7-24.5°C. Additionally, the melting enthalpy increases from the lowest to the highest OD@CCF loading, as anticipated (6.3 J/g to 22.5 J/g for gypsum boards and 6.0 J/g to 14.8 J/g for cement boards). Conversely, the solidification enthalpy decreases from -5.2 J/g to -22.0 J/g for gypsum boards and from -8.3 J/g to -13.5 J/g for cement boards. These findings are plotted in Figure 7.

The results from thermal conductivity (k) and specific heat capacity (Cp) measurements for reference gypsum and cement boards and their composites GB/OD@CCF and CB/OD@CCF are presented in Table S2. Additionally, Figure 8 illustrates the graphical relationship between thermal conductivity and specific heat capacity as the percentage content of OD@CCF additive increases. The data reveals noticeable variations in these parameters corresponding to the incremental increase in additive content. Specifically, a positive correlation is evident, indicating an improvement in both thermal conductivity and specific heat capacity with higher additive content. This positive correlation is interpreted as the OD@CCF component making a positive contribution to the heat transfer capabilities within the composite gypsum and cement boards. The data suggests that as the percentage of OD@CCF additive increases, there is a concurrent enhancement in both thermal conductivity and specific heat capacity, highlighting the positive influence of the additive on the heat transfer properties of the composite material. This improvement can be attributed to two main factors. Firstly, carbon foam contributes to an overall enhancement in the thermal conductivity of the composites. Secondly, the well-dispersed and inter-connected carbon foam particles within the gypsum or cement matrix create additional channels for heat transfer [29]. Similar findings are reported in the literature. For instance, in our previous work [17] the incorporation of activated carbon/RT18HC in gypsum boards increased thermal conductivity and specific heat capacity from 0.33 W/(m·K) and 1455 (J/Kg·K), to 0.53 and 1833 (J/Kg·K), respectively for 30% w.t. additive. Zhang et al. [30] noticed that diatomite/paraffin incorporation into gypsum boards reinforced with 1% carbon fibers, resulted in increament of thermal conductivity from 0.86 to 0.93 W/(m·K). In addition, according to the same team, expanded Graphite/Paraffin Gypsum-Based Composites with 20% w.t. additive, increased thermal conductivity from 0.742 W/(m·K) to 1.137 W/(m·K) [31]. Concerning cement composite boards, in our case, the thermal conductivity values are measured in the range of 0.06-0.08 W/(m·K), which are in agreement with the literature for perlite-concrete thermal conductivity properties (0.07-0.08 W/(m·K)) [32], showing ~33 % increment for 30 % additive. According to previous published works, carbon based nanomaterials can significantly improve cement’s thermal conductivity [33,34,35]. Also, Ye et al. [28] reported the increment of cement board’s thermal conductivity by adding expanded perlite stabilized RT28HC and specifically for 40 % w.t. additive reached to 0.149 W/(m·K) from 0.11 W/(m·K). Octadecane exhibits 0.2 W/(m·K) [27], which also can improve in our case, the lower thermal conductivity of perlite-cement boards.

3.3. Mechanical Properties

The results of mechanical testing on gypsum and cement boards with varying OD@CCF content are presented in Figure 9 and Figure 10, respectively. Analysis of load-deformation data from bending tests on GB/OD@CCF boards (Figure 9) indicates a clear relationship between additive percentage and composite board mechanical properties. At lower concentrations (up to 20%), a positive correlation is observed, indicating that OD@CCF inclusion enhances material strength likely due to improved intermolecular interactions and reinforcement effects. However, with increasing OD@CCF content, a decline in mechanical strength is evident, possibly due to potential disruption of the gypsum matrix by higher OD@CCF concentrations. As shown in Figure 9b, the optimal OD@CCF content for gypsum boards is found to be 10%. At this concentration, the modulus of rupture measures at 3.14 MPa, significantly higher than the 2.64 MPa for the reference gypsum board. This suggests that mechanical strength of the composite board peaks at 10% OD@CCF content in bending tests.

The results of compression tests on cement boards (CB) with varying percentages of OD@CCF additive are presented in Figure 10. The corresponding parameters from these tests are outlined in Table S3. A minor reduction in compressive strength is noted with increasing OD@CCF loading, decreasing from 1.09 MPa to 0.96 MPa for 10% and 20% loading, respectively. However, with a higher OD@CCF loading of 30%, this reduction is more pronounced, resulting in a compressive strength of 0.81 MPa.

3.4. EMI Shielding Properties

The performance of the EMI protection of developed samples was assessed from the point of view of the EMI protection (SE) efficiency in the frequency range of 3.2–7.0 GHz.

SE_T can be expressed as the sum of reflection from the surface (SE_R), absorption (SE_A) and multiple-reflection (SE_M) as follows [36]:

SE_T(dB) = SE_R(dB) + SE_A(dB) + SE_M(dB) (3)

Generally, multiple-reflection at internal interfaces inside the material can be excluded if SE > 10-15 dB. Thus, we calculated the average SE_T as

$$SE=S{E}_T\triangleq 10{\log }_{10}\left(\frac{P_{inc}}{P_{trn}}\right)=10{\log }_{10}\left(\frac{1}{T}\right)=S{E}_R+S{E}_A (4)$$

where

$$S{E}_R=10{\log }_{10}\left(\frac{1}{1-R}\right) (5)$$

$$S{E}_A=10{\log }_{10}\left(\frac{1-R}{T}\right) (6)$$

SE_R, SE_A refer to the reflection and absorption SE, respectively.

The higher the SE the better the shielding. The SE (also denoted as SE_T, with A, T, R indicating the absorption, transmission and reflection, respectively) is usually quantified in terms of the logarithm of the incident power P_inc over the transmitted power P_trn [37,38] and thus expressed in decibels (dB).

Figure 11 depicts the S₂₁ (transmission; Figure 11a) and the S₁₁ parameters (reflection; Figure 11b) of the GB/OD@CCF composite samples for a series of shape stabilized octadecane stabilized in carbon foam OD@CCF concentrations, in the frequency range 3.2-7.0 GHz. Figure 11b clearly illustrates that the reflection of the samples is almost zero for all the additive concentration from 0.00-30%. As a result, the EMI shielding effect due to reflection (SE_R) is also negligible and in all cases the total SE is simply SE_T = SE_A [see equation (4)], in agreement with other research groups working on carbon based composite materials [39].

Figure 12 depicts the absorption (Figure 12a) and the total shielding efficiency (SE_T) (Figure 12b) spectra of the GB/OD@CCF composite samples for a series of shape stabilized octadecane stabilized in carbon foam OD@CCF concentrations, in the frequency range 3.2-7.0 GHz.

As one can see, the dominant shielding mechanism is absorption, which is also supported by the continuously increasing absorption levels with increasing OD@CCF content in the gypsum board composites.

When the loading amount of OD@CCF was low, the interconnected electrically conductive network was incomplete, and there is a small difference in electrical conductivity between gypsum board samples and air, leading to a large impedance match and a large number of electromagnetic waves transmitted into the samples [40]. Indeed, as one can easily observe in Figure 12b, for OD@CCF concentrations up to 10%, the SE_A does not exceed 5-6 dB, while, for 20% up to 30%, the SE_A varies from 10 to 12.5 dB, suggesting the absorption-dominant EMI shielding mechanism of our composite samples, in agreement with the literature [39].

At this point, it should be noted that, the addition of GB/ OD@CCF-20 with the GB/OD@CCF-30 samples was studied in order to verify higher additives’ concentration could reach a SE_T of ~ 15-20 dB which is considered sufficient for EM shielding, e.g., electronic devices, etc. [41].

Similar samples to those described above were studied, making cement boards that can be used outdoors. Figure 13 depicts the S₂₁ (transmission; Figure 13a) and the S₁₁ parameters (reflection; Figure 13b) of the CB/OD@CCF composite samples for a series of shape stabilized octadecane stabilized in carbon foam OD@CCF concentrations, in the frequency range 3.2-7.0 GHz.

Like previously, the dominant shielding mechanism is absorption, which is also supported by the continuously increasing absorption levels with increasing OD@CCF content in the cement board composites.

Figure 14 depicts the absorption (Figure 14a) and the total shielding efficiency (SE_T) (Figure 14b) spectra of the CB/OD@CCF composite samples for a series of shape stabilized octadecane stabilized in carbon foam OD@CCF concentrations, in the frequency range 3.2-7.0 GHz.

It is worth noticing that the SE_A levels of the cement board samples are higher that the corresponding ones of the gypsum samples, reaching a value of ~19.5 dB for the combination of CB/OD@CCF-20 with the CB/OD@CCF-30 samples (compared to ~18 dB for GB/OD@CCF-20 & CB/OD@CCF-20. Concrete and cementitious composites in general, are very poor electrical conductors, but still conductors, at least compared to gypsum [42,43]; This is actually the reason for their enhanced EMI effect. Moreover, it has been shown that the dry resistance of cement specimens is related to the porosity inside the material and the free-moving ions [44]. As shown in Figure 4 and Figure 5, pore sizes inside the gypsum samples are much smaller, compared to cement boards; As a result some of the big channels available for free ion movement in CB/OD@CCF composite boards (Figure 4), close in GB/OD@CCF samples, and consequently the mean path of ion movement increases [45]. In turn, this changes the electrical conductivity of gypsum (compared to cement) boards and increases the resistance, which decreases EMI accordingly.

3.5. Thermal Performance Measurements

To assess the thermal performance of the test samples, the upper environmental chamber underwent an 8h heating phase, followed by an 8h cooling period. Figure 15 displays the temperature curve of the environmental chamber and the temperature curves measured at the lower surface of the samples, specifically the surface within the testing room of each sample, namely gypsum boards GB/OD@CCF-10, GB/OD@CCF-30, and the reference gypsum board GB.

In the first 45 minutes of heating, the rate of temperature increase (RTI) of the GB/OD@CCF-10 and GB/OD@CCF-30 samples is identical to that of the reference sample. At this point, the temperature is 28°C. After this point, the phenomenon of phase change of the stabilized PCMs begins. The absorption of the required heat for the n-octadecane change from solid to liquid is depicted by the decrease in the RTI of the two samples and the appearance of a “plateau” in their temperature curves (phase change zone). Interestingly, the duration of this plateau in each sample is obviously related to their different paraffin loading. In GB/OD@CCF-10, the phase change zone extends to 90 minutes, while in GB/OD@CCF-30 it extends to 135 minutes. When all the paraffin has become liquid, a brief sharp increase in the RTI of the two samples compared to that of the reference sample is observed, followed by a smoothing of these rates and their equalization with the reference. The difference in the absolute temperatures of the two samples compared to the reference is also noteworthy. GB/OD@CCF-10 has a difference of approximately 2°C, and GB/OD@CCF-30 has a difference of approximately 7°C, highlighting the effect of the loading percentage on the effectiveness of the advanced gypsum board to maintain a lower temperature within a space. The response of the samples to the subsequent sudden cooling of the environmental chamber confirms the positive effect of the higher paraffin content on the properties of the gypsum board. The rate of temperature decrease of the GB/OD@CCF-30 sample is noticeably lower than that of the GB/OD@CCF-10 sample in the phase change zone. Furthermore, the final temperature of GB/OD@CCF-30 is approximately 2°C higher than that of the reference.

The temperature curves of the indoor spaces of the test rooms are shown in Figure 16 along with the temperature curve of the environmental chamber. Specifically, the temperature variation is recorded in the environmental chamber and in the test rooms (indoor temperature) of the reference cement board CB, and the cement board CB/OD@CCF-30 respectively.

As the temperature of the environmental chamber increases, a difference in the rate of temperature increase (RTI) is observed between the reference sample CB and the CB/OD@CCF-30 sample. This difference, evident near 28°C, can be attributed to the thermal energy absorbed by the CB/OD@CCF-30 sample for the phase change of the stabilized PCM from solid to liquid. Notably, the phase change zone stabilizes the temperature in the test room around 28°C for approximately 3 hours, unlike the reference cement board CB test room temperature, which steadily rises to around 33°C during the same period. Subsequently, after about 5.5 hours, the RTI in both rooms becomes similar. The absolute temperature difference between the two rooms remains constantly at approximately 2.3°C. It is evident that the cement board with the stabilized n-octadecane has the capability to maintain the room cooler for a longer period compared to the plain reference cement board.

4. Conclusions

To summarize, this research represents a significant step forward in the development of cement and gypsum building boards infused with shape-stabilized n-octadecane within carbon-based foams, aiming to improve energy storage and electromagnetic interference shielding capabilities. Our investigation underscores the critical importance of optimizing the concentration of shape-stabilized n-octadecane in these composite materials. Determining the optimal concentration is pivotal for achieving the desired balance between enhanced energy storage, EMI shielding, and mechanical properties.

Key findings from this study include:

i) Incorporating OD@CCF into gypsum and cement boards enhances thermal energy storage properties, resulting in improved thermal conductivity and specific heat capacity. Notably, temperature differentials of up to 7°C were observed between reference gypsum and composite board inner surfaces, with a 2.3°C improvement in the indoor test room temperature for composite cement boards.

ii) EMI shielding properties also improve with increasing OD@CCF content, with the combination of boards containing 20% and 30% OD@CCF achieving EMI shielding absorption levels of approximately 18 dB and 19.5 dB for gypsum and cement boards, respectively.

iii) Mechanical properties remain largely unaffected with up to 20% OD@CCF additive in both gypsum and cement boards. Particularly for gypsum boards, the modulus of rupture increases by approximately 19% compared to the baseboard. However, mechanical degradation is observed with higher additive loadings.

Overall, these positive results pave the way for practical applications and future innovations in construction materials. They offer a sustainable and technologically advanced solution for enhancing energy storage and EMI shielding in building environments.