3.1 Evaluation of Physical Performance of Cement Paste
Fig. 6 shows the results of the flow tests. The flow was measured according to KS L 5111. The flow did not change significantly until the MCF mixing amount was 10%. However, the flow decreased rapidly at MCF mixing amounts of 15% or higher. This suggests that, because the MCFs are fine particles with a size of 100 μm or less, the flow decreased as the fine particles of MCF were adsorbed into the mixing water.
Fig. 7 shows the results of the compressive strength measurements. When MCF was mixed with cement, high compressive strengths were observed. Specimen MCF-5 showed the highest compressive strength. However, MCF-10, MCF-15, and MCF-20 showed higher compressive strengths than the plain specimen. Furthermore, the compressive strength of the specimens with an MCF admixture of 10% was lower than that of the specimen with an MCF admixture of 5%; this is because the MCF mixing amount exceeded that of the cement paste, which resulted in lower strength.
Fig. 8 shows the morphologies of the plain and MCF-5 specimens, affording an examination of the distribution of MCFs among the hydrate tissues via the SEM. In the case of the plain specimen, MCF was confirmed to be absent in the specimen, whereas in the MCF-5 specimen, MCFs existed among the hydrates as if they were interconnecting the cement paste tissues. This suggests that the compressive strength increased owing to the MCFs interconnecting the cement hydrate tissues; in this case, the tensile strength is also expected to increase.
3.2 Evaluation of EMP Shielding Performance
The final setting time is the time elapsed between the moment the water is added to the cement, and the time when the paste has completely lost its plasticity and has attained sufficient firmness to resist certain definite pressure. Fig. 9 shows the results of the evaluation of the shielding performance according to the MCF mixing amount by frequency band over the frequency range of 0.75−2.0 GHz, in accordance with U.S. MIL-STD-188-125-1. Based on the results of the shielding performance experiment, the plain specimen showed an average shielding performance of 20 dB or less, indicating that there was no shielding, which is consistent with the results in literature. The MCF-5, MCF-10, MCF-15, and MCF-20 specimens exhibited a shielding performance of at least 50 dB. The MCF-5 specimen showed the highest shielding performance of 70 dB at a frequency of 1.3 GHz. The plain and MCF-mixed specimens exhibited a difference of approximately 30 dB or higher in terms of the shielding performance.
Fig. 10 shows the average shielding performance with respect to the MCF admixtures at 0.75–2.0 GHz. The shielding performance measurements did not indicate significant differences in the shielding effectiveness for MCF mixing ratios of 5−20%. Based on these results, 5% was determined to be the optimal admixture of MCF for EMP shielding. In the future, shielding performance needs to be analyzed for MCF mixing amounts ranging from 1 to 5%.
Fig. 11 shows the EMP shielding performance based on the thickness of the plain and MCF-5 specimens. The plain specimens exhibited low shielding performance regardless of the thickness. This suggests that, for cement without a shielding additive, the thickness has no effect on the shielding performance. The MCF-5 specimens showed shielding performances of 40–60 dB, 50–75 dB, and 65–85 dB at thicknesses of 100 mm, 200 mm, and 300 mm, respectively. The thicker material demonstrated a higher shielding performance. It is believed that the inclusion of an MCF admixture with an increase in thickness generated an increase in the time and range of the transmission of the EMP spectrum, thus resulting in higher shielding performance. Furthermore, the shielding performance was analyzed when varying the direction of EMP penetration by generating EMP spectra horizontally (marked as H after the specimen name) and vertically (marked as V after the specimen name). The MCF-5–100 mm, MCF-5–300 mm, and plain specimens did not show any difference in shielding based on the direction of the EMP spectrum. However, the MCF-5–300 mm specimen showed higher shielding effectiveness for the horizontal spectrum. However, this appears to be an experimental error due to the homogeneity of the specimen or the direction of the antenna that generated the EMP spectra.
As shown in Fig. 11, the shielding performance did not improve significantly with larger MCF admixtures at a constant thickness. However, the shielding performance improved with increasing thickness for an MCF content of 5%. This suggests that, when EMP shielding is implemented via MCF addition, increasing the thickness is more effective than increasing the mixing amount.