A Review of Microwave Absorption and Reflection by Cement‐Based Materials, With Emphasis on Electromagnetic Interference Shielding and Admixture Effects

Abstract

AbstractCement‐based materials can be modified for microwave reflection/absorption. Admixtures (particles or short fibers) include conductive, dielectric, and magnetic ones, used solely or in combination. Synergism occurs with conductive and magnetic admixtures, due to the enabled eddy current. The conductive admixture promotes absorption and reflection, whether steel rebars are present or not. Regardless of the admixtures, rebars, or aggregates, absorption loss dominates over reflection loss. Steel microfiber is more effective than carbon fiber. Fly ash enhances the absorption slightly, due to the included iron oxide. Because of the skin effect, a small admixture unit size is preferred. Thus, carbon nanofiber is more effective than carbon fiber, if it is adequately dispersed. Fine particles (such as silica fume) as an additional admixture help the dispersion. Nanoscale admixtures tend to fill the pores, so their functional effectiveness is limited. Carbon black (nanoparticles) is less effective than carbon fiber, but it is inexpensive and the combined use of carbon fiber and carbon black is effective. The high cost of the nanofibers and the difficulty of their dispersion limit their practicality. Another approach involves porous admixtures or aggregates for reducing the impedance mismatch so that reflection is lessened and more radiation enters the cement‐based material.