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.