## 3 Results and Discussion
### Characterization of Gypsum/RT18 HC@AC Composite Boards
#### 3.2.4 EMI Shielding Properties for GB/RT18 HC@AC Composite Boards  
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. Electromagnetic shielding literally means that when an electromagnetic field occurs in a system enclosed in a shield material, there is a minimum or ideally absent transmission over the shield material and in the system. \(SE_{T}\) can be expressed as the sum of reflection from the surface (\(SE_{R}\)), absorption (\(SE_{A}\)), and multiple reflection (\(SE_{M}\)) as follows [59]:  
\[SE_{T}(\text{dB})=SE_{R}(\text{dB})+SE_{A}(\text{dB})+SE_{M}(\text{dB}) \tag{3}\]  
Generally, multiple reflection at internal interfaces inside the material can be excluded if \(SE>10\)-15 dB [59, 60, 61]. Thus, we calculated the average \(SE_{T}\) as  
\[SE=SE_{T}\triangleq 10\ \log_{10}\biggl{(}\frac{P_{inc}}{P_{trn}}\biggr{)}=10 \ \log_{10}\biggl{(}\frac{1}{T}\biggr{)}=SE_{R}+SE_{A} \tag{4}\]  
where  
\[SE_{R}=10\ \log_{10}\biggl{(}\frac{1}{1-R}\biggr{)} \tag{5}\]  
Figure 10: Results from the impact resistance tests. (**a**) Maximum load, Fmax, and (**b**) absorbed energy, Ea, for the gypsum boards with varying % w.t. contents of RT18 HC@AC additive. Figure 9: Results from the bending tests. (**a**) Stress–strain curves and (**b**) the modulus of rupture for the gypsum boards with varying % w.t. contents of RT18 HC@AC additive. \[SE_{A}=10\ \log_{10}\biggl{(}\frac{1-R}{T}\biggr{)} \tag{6}\]  
\(SE_{R}\) and \(SE_{A}\) refer to the reflection and absorption \(SE\), respectively. The higher the \(SE\), the better the shielding. The \(SE\) (also denoted as \(SE_{T}\), with \(A\), \(T\), and \(R\) 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 \(P_{trn}\)[62; 63; 64; 65] and is thus expressed in decibels (dB). Figure 11 depicts the absorption (Figure 11a) and the reflection (Figure 11b) spectra of the GB/RT18 HC@AC composite samples for a series of additive RT18 HC@AC concentrations in the frequency range of 3.5-7.0 GHz. Figure 11b clearly illustrates that the reflection of the samples is almost zero for all of the additive concentrations from 0.00 to 30% w.t. As a result, the EMI shielding effect due to reflection (\(SE_{R}\)) is also negligible (see Figure 11d), and in all cases, the total SE is simply \(SE_{T}=SE_{A}\) (see Equation (4)), which is in agreement with the results of other research groups working on carbon allotropes [66]. Clearly, the dominant shielding mechanism is absorption, which is also supported by the continuously increasing absorption levels with an increasing RT18 HC@AC content in the gypsum board composites. When the loading amount of activated carbon was low, the interconnected electrically conductive network was incomplete, and there was a small difference in electrical conductivity between the gypsum board samples and air, leading to a large impedance match and a large number of electromagnetic waves being transmitted into the samples [67]. Indeed, as one can easily observe in Figure 11c, for RT18 HC@AC concentrations from 0.0 to 5% w.t., the \(SE_{A}\) does not exceed 2.5 dB, while for 10% w.t. up to 30% w.t., the \(SE_{A}\) varies from 2.5 to 12.5 dB, suggesting the absorption-dominant EMI shielding mechanism of our composite samples, which is in agreement with the properties of other carbon-based composite samples [66]. At this point, it should be noted that an \(SE_{T}\) of \(\sim\)15-20 dB is more than sufficient for shielding, e.g., computer devices [64], since an \(SE\) of 20 dB corresponds to the blocking of \(\sim\)99% of incident EM waves. Figure 11: Absorption (**a**) and reflection (**b**) spectra from 3.5 to 7.0 GHz (C-band) for different gypsum board samples.