## 3 Results and Discussion
### EMI Shielding Properties  
The effectiveness of electromagnetic interference (EMI) shielding for the created samples was evaluated based on shielding efficiency (SE) across the frequency spectrum of 3.2-7.0 GHz. The total SE can be delineated as the aggregate of reflection at the surface (\(\mathrm{SE_{R}}\)), absorption (\(\mathrm{SE_{A}}\)), and multiple reflections (\(\mathrm{SE_{M}}\)), as detailed below [40]:  
\[\mathrm{SE_{T}}\left(\mathrm{dB}\right)=\mathrm{SE_{R}}\left(\mathrm{dB} \right)+\mathrm{SE_{A}}\left(\mathrm{dB}\right)+\mathrm{SE_{M}}\left( \mathrm{dB}\right) \tag{2}\]  
Typically, internal multiple reflections within the material can be disregarded if the SE exceeds 10-15 dB. Therefore, we estimated the average \(\mathrm{SE_{T}}\) as  
\[\mathrm{SE}=\mathrm{SE_{T}}\triangleq 10\log_{10}\left(\frac{P_{\mathrm{inc} }}{P_{\mathrm{tm}}}\right)=10\log_{10}\left(\frac{1}{\mathrm{T}}\right)= \mathrm{SE_{R}}+\mathrm{SE_{A}} \tag{3}\]  
where  
\[\mathrm{SE_{R}}=10\mathrm{log_{10}}\left(\frac{1}{1-R}\right) \tag{4}\]  
\[\mathrm{SE_{A}}=10\mathrm{log_{10}}\left(\frac{1-R}{\mathrm{T}}\right) \tag{5}\]  
Figure 10: Results from compression tests for cement boards (CBs) with varying % content of OD@CCF additive. (**a**) Representative stress–strain curves and (**b**) compressive strength versus OD@CCF content. \(\text{SE}_{\text{R}}\) and \(\text{SE}_{\text{A}}\) represent the reflection and absorption components of SE, respectively. Enhanced SE indicates more effective shielding. SE, also represented as \(\text{SE}_{\text{T}}\), with A, T, and R signifying absorption, transmission, and reflection, respectively, is typically measured as the logarithmic ratio of incident power (Pinc) to transmitted power (Ptrn) [41, 42] and thus expressed in decibels (dB). Figure 11 depicts the \(\text{S}_{\text{21}}\) (transmission; Figure 11a) and the \(\text{S}_{\text{11}}\) parameters (reflection; Figure 11b) of the GB/OD@CCF composite samples for a series of octadecane samples shape-stabilized in carbon foam, with different OD@CCF concentrations, in the frequency range of 3.2-7.0 GHz. Figure 11b clearly illustrates that the reflection of the samples is almost zero for all the additive concentrations in the range of 0.00-30%. Consequently, the EMI shielding effect attributable to reflection (\(\text{SE}_{\text{R}}\)) is minimal, rendering the total shielding effectiveness (**SE**) as merely \(\text{SE}_{\text{T}}=\text{SE}_{\text{A}}\) [refer to Equation (3)]. This observation aligns with findings from other research teams investigating carbon-based composite materials [43]. Figure 12 depicts the absorption (Figure 12a) and total shielding efficiency (\(\text{SE}_{\text{T}}\)) (Figure 12b) spectra of the GB/OD@CCF composite samples for a series of concentrations of octadecane shape-stabilized in carbon foam, OD@CCF, in the frequency range of 3.2-7.0 GHz. It is obvious that absorption serves as the primary shielding mechanism, a conclusion supported by the progressively rising absorption levels observed with increasing OD@CCF content in the gypsum board composites. At low OD@CCF loadings, the electrically conductive network within the gypsum board composites is incomplete, resulting in only a minor difference in electrical conductivity between the gypsum samples and air. This causes a large impedance match and allows a significant number of electromagnetic waves to pass through the samples [44]. As shown in Figure 12b, the \(\text{SE}_{\text{A}}\) remains below 5-6 dB for OD@CCF concentrations up to 10%, but  
Figure 11: Transmission \(\text{S}_{\text{21}}\) (**a**) and reflection \(\text{S}_{\text{11}}\) (**b**) coefficients from 3.2 to 7.0 GHz (C-band) for different gypsum board samples. Figure 12: Electromagnetic radiation absorption (**a**) and \(SE_{A}\) (**b**) from 3.2 to 7.0 GHz (C-band) for different gypsum board samples. increases to between 10 and 12.5 dB for concentrations ranging from 20% to 30%.