1. Introduction
Nowadays, rapid development has been achieved in modern wireless communication technology and the electronic equipment has been applied widely, which does improve the convenience of our life. However, electromagnetic radiation has become an inevitable by-product[1,2] which not only causes the leakage of important information[3], but also harms human health[4,5]. In order to reduce electromagnetic radiation in the environment, wave-absorbing materials have been widely used in construction[6], and cement-based materials, as common building materials, have good environmental adaptability, low cost, and easy availability[7]. Foam concrete, as a kind of cement-based material with rich pores inside, is often used for building partitions and roof systems[8]. If absorbents are added in it, it can make buildings have functions of thermal insulation, fire protection and electromagnetic wave
absorption.
According to transmission line theory, effective impedance matching is a prerequisite for materials to absorb electromagnetic waves[9]. Traditional cement-based materials have a dense structure and poor impedance matching, making it difficult for electromagnetic waves to enter[10]. With low density and low dielectric constant, porous materials such as expanded polystyrene[11,12], hollow glass beads[13], and expanded perlite[14,15] are often incorporated into cement-based materials to improve impedance matching with free space[16,17]. However, foam concrete itself can extend the transmission length of incident waves and simplify the preparation process. The increase in the number of electromagnetic wave reflection, scattering, and interference also strengths the loss of electromagnetic wave energy[18].
Furthermore, the dielectric effect and magnetic loss effect of absorbents are key factors affecting the absorption performance of electromagnetic waves[19,20]. In terms of loss mechanism, the absorbents added to cement-based materials can be divided into magnetic media type[21,22], resistive type[23,24], dielectric type, and novel materials combining various loss methods[25,26]. As the resistive absorbent, graphite and carbon fiber can attenuate and absorb electromagnetic waves through the electronic polarization or interface polarization of the medium, and the cost is low and the conductivity and alkali resistance are excellent[27]. Xin et al.[28] incorporated graphite into foam concrete to study the effects of graphite content, foaming agent content, surface roughness, and age and moisture content of the specimen on electromagnetic wave absorption performance. The results show that when 15wt.% graphite is added, the percolation threshold is reached. When it exceeds 15wt.%, the reflectivity of electromagnetic waves is greatly increased but the porosity and compressive strength are reduced. Shi et al.[29] combined iron powder and graphite with MnZn ferritic cement-based materials. The research shows that adding graphite can decrease the minimal reflection loss of gelled composites, and the mechanical strength can meet the requirements of most construction applications. Wang et al.[30] mixed graphite and carbon fiber into cement, and the maximum absorption peak (with the reflectivity of -5.67 dB) appeared in the 12.5-18 GHz frequency range. Numerical simulations were conducted using ANSYS to verify the experimental conclusions.
Currently, there are few studies and applications of foam concrete in the field of absorbing electromagnetic waves[31]. In addition, due to the internal pore structure, the mechanical properties of foam concrete are more susceptible to the influence of absorbing agent, and its absorbing function will be limited in practical engineering due to the defects of the material itself[32].
In this study, foam concrete is used as the matrix, and graphite and carbon fiber are used as admixtures to prepare wave-absorbing foam concrete. The effects of single doped carbon fiber and graphite-carbon fiber compound doping on the mechanical-electro-magnetic properties of foam concrete are clarified. XRD, XCT and SEM tests are performed to analyze the hydration process and microstructure of foam concrete.