Carbon-fiber-reinforced cement-based composites are widely used as electromagnetic microwave shielding materials. However, high temperature on fire can affect not only their mechanical properties but also microwave-absorbing abilities. In this paper, the specimens of the composites with different carbon-fiber content were prepared. The mechanical properties were measured when the composites were cooled down to room temperature from 600°C naturally and by spraying water. The reflectivity of the composites with different cooling methods against microwave was evaluated through an arch reflectivity measurement system in 2.0–18.0 GHz frequency range at the temperature of 20°C, 300°C, and 600°C. The porosity, morphology, and the compositions were analyzed by mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and x-ray diffraction (XRD). Results show that mechanical properties decrease and porosity increases dramatically at the temperature of 600°C. The reflectivity varies with the increase of porosity at low frequencies, but increases at high frequencies with reflecting waves when the temperature rises. At the same temperature, the increasing carbon-fiber content enhances the reflectivity with reflecting electromagnetic microwaves. After high temperature, the cooling exerts little effect over the microwave reflectivity of the composites.
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