LI Huawei ,WANG Rong ,WANG Yulin＊ ,LIU Feiyu ,WANG Qian ,WEI Muwang

(1. School of Civil Engineering and Architecture, Wuyi University, Wuyishan 354300, China; 2. Engineering Research Center of Prevention and Control of Geological Disasters in Northern Fujian, Fujian Province Higher Education Institutes, Wuyishan 354300, China)

Abstract: To develop the microwave absorbing (MA) properties of cementitious material mixed with mine solid waste,the iron tailings cementitious microwave absorbing materials were prepared.The iron tailings was treated into different particle sizes by planetary ball mill,and the physicochemical properties of iron tailings were tested by laser particle size analyzer and scanning electron microscope (SEM).The electromagnetic parameters of iron tailings cementitious materials were characterized by a vector network analyzer and simulated MA properties,and the MA properties of iron tailings-cement composite system with steel fiber as absorber was studied.Based on the design of the single-layer structure,optimum mix ratio and thickness configuration method of double-layer structure were further studied,meanwhile,the mechanical properties and engineering application were analyzed and discussed.The results show that the particle size of iron tailings can affect its electromagnetic behavior in cementitious materials,and the smaller particles lead the increase of demagnetisation effect induced by domain wall motion and achieve better microwave absorbing properties in cementitious materials.When the thickness of matching layer and absorbing layer is 5 mm,the optimized microwave absorbing properties of C1/C3 double-layer cementitious material can obtain optimal RL value of -27.61 dB and effective absorbing bandwidth of 0.97 GHz,which attributes to the synergistic effect of impedance matching and attenuation characteristics.The double-layer microwave absorbing materials obtain excellent absorbing properties and show great design flexibility and diversity,which can be used as a suitable candidate for the preparation of favorable microwave absorbing cementitious materials.

Key words: microwave absorbing properties;iron tailings;electromagnetic parameters;single-layer structure;double-layer structure;impedance matching

1 Introduction

With the rapid development of modern electronic industry and microwave technology,as the most convenient and quick information carrier,the electromagnetic wave has been widely used in daily equipment,industry,agriculture,health care and 5G mobile communication industry[1,2].Though electromagnetic wave transmits information and facilitates people’s lives,it also brings electromagnetic radiation pollution as human’s “fifth-largest pollution”[3,4].To alleviate the adverse effect of microwave on people’s health and lives,researchers try to make buildings have the microwave absorbing (MA)function and investigate the microwave properties of cementitious materials by adding absorbents.However,traditional cementitious materials show poor absorbing properties and narrow bandwidth[5-8].To solve this urgent problem,mine solid wastes that have the potential ability of electromagnetic loss are used as fine aggregates in cementitious materials,which can significantly reduce the influence of the electromagnetic wave of electronic equipment interference and human health[9,10,11,12].Thus,the development of cementitious materials containing low-cost and environmentfriendly iron tailings which has the potential capability of microwave absorption,attracts more and more attentions and research interest[13].

In general,the composition and structure design of MA materials have to meet the following requirements:firstly,external electromagnetic waves could enter the material as much as possible;secondly,it should have strong absorption while the electromagnetic waves enter the material;thirdly,the self-weight of materials should be reduced as much as possible;at last,it is expected to have broad bandwidth[14].Iron tailings are one of the most common types of solid mine waste,in which the compositions of quartz and magnetic metal components have preferable microwave transmission and electromagnetic loss capability[15-17].However,cementitious MA materials that only consist of mine solid wastes cannot achieve broad bandwidth absorption.As an alternative method to achieve it,many researchers enhanced the microwave-absorbing performance of cementitious MA materials by adding different absorption characteristic absorbers,such as graphene,carbon nanotubes,carbon black,ferrite and conductive fibers,or by the structural design methods of double-and multi-layer microwave absorbing materials[18-23].Previous researchers indicated that solid wastes like iron tailings,copper tailings,steel slag and fly ash,which had a potential capability for the preparation of MA materials,and the MA properties of a cementitious material mix with these filling materials were enhanced due to the composition of quartz and magnetic constituents[9,12,15,24].Yaoet al[17]prepared a novel glass-ceramic using iron tailings as primary raw materials and achieved a total bandwidth of 13.61 GHz (RL value less than -3 dB) at the frequency of 2-18 GHz.Daiet al[24]studied the function of electromagnetic radiation protection of building materials mixed with steel slag,analyzed the influence of iron oxide composition in steel slag on microwave absorption properties and reported that the steel slag cement-based materials showed the electromagnetic wave-absorbing properties at 10-18 GHz.Meanwhile,the minimum reflection loss (RL) reached -11.5 dB at the thickness of 25 mm.Besides,particle size and particle morphology of filling materials also affect the MA properties due to the shape anisotropy,which influences resonance frequency[25,26].Zhaoet al[26]prepared three different diameters of Ni samples and investigated the electromagnetic properties.Results showed that the smaller sample exhibited better electromagnetic absorption performances due to the efficient complementarity between the complex permittivity and permeability of materials and the nanoscale size.Qiet al[27]prepared two kinds of nickel particles with different geometrical morphologies,proved that complicated geometrical morphologies had a noticeable promotion effect on increasing the refracted and reflected chance on the incident electromagnetic wave,and enhanced the MA properties.On the other hand,the double-layer structure design of cementitious materials with better impedance matching properties is beneficial for broadening bandwidth.Li[28]designed the double-layer structure of cementitious MA materials,and mainly considered the effects of matching layer,absorbing layer and thickness on the MA properties.Results showed that the method of double-layer design with gradual impedance significantly enhanced the MA properties and the determination of matching thickness was mainly related to the composition of each layer.Renet al[29]designed and fabricated the double-layer structured cement-based composites,in which hollow glass microspheres were used as the matching layer and single-particle sand and carbon powder as absorbing layer,and results showed that double-layer structure design significantly enhanced the absorbing properties.The optimal double-layer sample had the minimum RL value of -12.4 dB at 8.9 GHz with an effective bandwidth of 5.2 GHz.

In the present work,we propose to enhance the cementitious MA materials by using iron tailings with different particle sizes and steel fibers (as microwave absorbers).A single-layer MA material was adopted and a graded interface was utilized to produce gradual impedance change,and at the same time,the doublelayer MA material was designed to achieve preferable MA properties by adjusting the matching thickness of each layer.

2 Expermental

2.1 Materials

A ground granulated iron tailings provided by Minsheng Mine Co.,Ltd was used in this study.Table 1 shows the chemical compositions of raw iron tailings and cement (examined by X-Ray Fluorescence Spectrometer,Malvern ZETIUM),and the data of chemical compositions indicate that the iron tailings contain a large number of low dielectric constant quartz components and Fe2O3with magnetic loss ability.Fig.1 shows the mineralogical analyses of raw iron tailings,all the sharp peaks can be identified as CaO,FeO,SiO2,hematite and magnetic,and the results are consistent with Table 1.The steel fibers with length of 8 mm and diameter 0.12 mm were applied (provided by Shanghai Zhenqiang Fiber Co.,Ltd),the tensile strength is 3 000 MPa,and the average density is 7.85 g/cm3.The ISO standard sand was supplied by Xiamen Aisiou Standard Sand Co.,Ltd.

Fig.1 Mineralogical analyses of iron tailings

Table 1 Chemical composition of iron tailings and cement/%

2.2 Experiments

2.2.1 Preparation of iron tailings with different particle sizes

Different particle sizes of iron tailings were ground by a YXQM-2L planetary ball mill (Changsha Miqi Instrument Co.,Ltd.) for 30 and 60 minutes.Three kinds of zirconia balls were used in the planetary ball mill,with weight ratio of 8 mm: 3 mm: 1 mm=3: 5: 2.The iron tailings were milled for the specified time with motor speed of 400 rpm.After that,separated the zirconia balls,and different particle sizes of iron tailings (median particle size=12.75 μm and 5.64 μm)were obtained.

The particle size distribution of iron tailings was determined by a laser particle size analyzer (England Malvern Instruments Ltd,Matersizer 2000) which worked in the range of 0.02-2 000 μm,and the results were shown in Fig.2.It shows that theD50of iron tailings is 12.75 μm after 30-minute grinding and 5.64 μm after 60-minute grinding,indicating that the median particle size can be ground for the smaller size with the grinding process.

Fig.2 Particle size distribution of iron tailings

2.2.2 Microstructure

The morphology of iron tailings with different particle sizes was observed by a field-emission SEM(TESCAN Ltd,MIRA4),and the results were shown in Fig.3.SEM images show that the morphological characteristics of iron tailings is much smaller and more regular after grinding than before grinding.

Fig.3 SEM images of iron tailings: (a)(b) for raw iron tailings before grinding;(c)(d) for iron tailings with median particle size of 12.75 μm after 30-minute grinding;(e)(f) for iron tailings with median particle size of 5.64 μm after 60-minute grinding

2.3 Test methods

2.3.1 Mixture preparation

Four kinds of different mixtures were prepared to investigate the influence of different particle size of iron tailings on the microwave absorbing properties of mortar.The proportion of mixture is showed in Table 2,in which the substitution ratio of tailings for cement keeps 30 %,the water to binder ratio is 0.5,and the binder to sand ratio is 1:3.The specimen is of hollow-cylinder shape with inner diameter 10.06 mm,outer diameter 23.15 mm and height 20 mm.All the specimens were tested at the curing age of 28 d.The specimens containing the raw iron tailings (without grinding),median particle size of 14.25 μm and 5.64 μm are denoted A,B and C respectively.

Table 2 The mix proportion of cement mortar

2.3.2 Microwave absorbing properties

The electromagnetic parameters of the composites were measured by the coaxial line transmission/reflection method using a 3656B vector network analyzer (China Electronics Technology Instruments Co.,Ltd.),and testing frequency range was 0.1-5 GHz.The instrument can directly obtain real electromagnetic parameters of cement-based materials without involving other mixtures (such as paraffin).The complex permittivity and complex permeability in the entire frequency range (0.1-5 GHz) are both obtained,and the RL is calculated by Matlab software.

2.3.3 Setting time,fluidity and mechanical properties

The test of fluidity was characterized by a mini slump in accordance with the Chinese National Standard GB/T 8077-2012.The test of setting time was measured with the requirement of Chinese National Standard GB/T 1346-2011.

The mechanical properties of the iron tailings and steel fiber composites were measured in triplicate by a compressive testing machine (HUT-106D) at 28 d,and the loading speed was 2.4 KN/s.According to Chinese National Standard GB/T 17671-2020,three mortar specimens are tested,and the average value is calculated as the mechanical properties result.

3 Results

3.1 Microwave measurements of single-layer cementitious materials

3.1.1 Electromagnetic parameters of cementitious materials

In general,the complex permittivity and the complex permeability are important parameters to characterize the MA properties and mechanism of MA materials[30].Conventionally,the real part of electromagnetic parameters (ε′ and μ′) represent the storage capacity of materials for electromagnetic energy,and the imaginary part of electromagnetic parameters (ε" and μ") represents the dissipate capacity of materials for electromagnetic energy[22,31].As we can be observed in Fig.4,the value of ε′,ε",μ′ and μ" of the sample PC are about 4.3,0,1 and 0 with slight fluctuations in the test frequency range,which represented the sample PC had almost no dielectric loss and magnetic loss ability.In the cementitious materials mix with different particle size iron tailings,due to their natural composition characteristics of magnetism,the ε′,ε",μ′ and μ" produce a fluctuation to a certain degree,significantly beyond 3.2 GHz,which proves the iron tailings have a specific influence on the electromagnetic properties of materials.As for the cementitious materials mix with different particle sizes of iron tailings,compared with raw iron tailings (sample A0),it is obviously found that the electromagnetic parameters of samples B0 and C0 offset to higher frequency.

Fig.4 Electromagnetic parameters of cementitious materials mixed with different particle size iron tailings: (a) the real part of complex permittivity;(b) the imaginary part of complex permittivity;(c) the real part of complex permeability;(d) the imaginary part of complex permeability

where εrand μrare the complex permittivity and the complex permeability of materials,Ris the particle size of the particles.According to Eq.(1),whenRdecreases,the resonant wavelength decrease leads to the frequency increase,which results in the electromagnetic parameters moving to the higher frequency in macroscopic.

3.1.2 Microwave absorbing properties of single-layer cementitious materials

Based on the analysis above,we can found that the electromagnetic parameters of cementitious MA materials can be affected by iron tailings with different particle sizes.To further enhance the MA properties of cementitious,different contents of steel fiber were used as absorbers (group 1,2 and 3).When the incident electromagnetic wave under normal incidence enters the metal back plate,the RL is calculated via transmission line theory[34,35].The RL curvesvsfrequency was calculated by the following Eq.(2) and Eq.(3),and the single-layer electromagnetic wave absorbing model was depicted in Fig.5.

Fig.5 Electromagnetic wave absorbing model of single-layer material

whereZ1is the normalized input impedance,Z0is the impedance of free space,μr=μ′-jμ" is the complex permeability,and εr=ε′-jε" is the complex permittivity,jis the imaginary unit,dis the thickness of cement mortar absorber,fis the electromagnetic wave frequency in free space,andcis the velocity of light.

The calculated RL of group1,2 and 3 with various thicknesses are presented in Fig.6.Fig.6(a) and Fig.6(b) show the single-layer MA properties of group 1 and 2,and it can be observed that the optimal RL value reached -37.7 dB at 2.19 GHz with an effective bandwidth (RL ＜ -10 dB) of 0.53 GHz (10.8 % of the whole test band range) from 2.12 to 2.37 GHz and 3.48 to 3.76 GHz for sample C1 at a thickness of 35 mm.Sample C2 has the minimum RL value at 15 mm thickness which reaches -35.65 dB at 1.75 GHz with an effective bandwidth of 0.45 GHz (9.2%) from 1.70 to 1.82 GHz and 3.35 to 3.68 GHz.Compared with sample C2-35 mm,sample C3-15 mm has a more tremendous advantage in using thickness.For group 3,as shown in Fig.6(c),all the samples give fragile MA properties compared with group 1 and group 2,and the minimum RL value only reaches -21.05 dB at 1.10 GHz with an effective bandwidth of 0.13 GHz(2.7%).It should be ascribed to its overhigh content and electrical conductivity of steel fiber leading to an overhigh complex permittivity,and the Faraday Cage effect induced poor impedance mismatching with free space.The smaller particles expose more absorbing components and form a complete conductive path inside the material,resulting in better microwave absorption properties.Although the cementitious materials mixed with 5.64 μm iron tailings exhibits the better MA capability than the sample mixed with raw and 12.75 μm iron tailings,its MA capability is still hard to meet the design requirements of advanced cementitious MA materials,which requires to reduce the using thickness within a reasonable range as well as achieve wider bandwidth.Consequently,the enhancement of impedance matching and attenuation characteristics is the key to better MA properties,and there is much work that needs to do for broadening the effective bandwidth.

Fig.6 Microwave absorbing properties of cementitious materials with different thickness: (a) group 1,(b) group 2,(c) group 3

3.2 Microwave measurements of doublelayer cementitious materials

3.2.1 Design method of double-layer cementitious materials

For the single-layer cementitious material,its limited regulating parameters lead to poor MA performance.In order to simultaneously achieve more vital RL value,light-weight and reasonable using thickness and broader bandwidth,the doublelayer MA materials are designed and optimized,based on the characteristic and analysis of singlelayer cementitious MA materials above.A doublelayer design is to attempt to enhance the efficiency of MA properties,expand the bandwidth,and reduce the thickness of materials.It is required to optimize the parameters of double-layer materials,including the thickness,electromagnetic characteristic,and sequence of each layer[36].Previous research proved that gradual impedance is the optimal method for the double-layer MA materials[37,38].However,the value of electromagnetic parameters for a specific material cannot be varied.Therefore,the thickness of each layer will be the focus of the design.

For the double-layer absorbing structure,the electromagnetic wave absorbing model is showed in Fig.7.Layer one is closing to the metal plate,and layer two is facing the free space.According to the impedance conversion equation of transmission line theory,the design model of the double-layer materials was based on the design of the single-layer[39,40].Set the second-layer impedance as Z2,and the calculate method of Z2is as follow:

Fig.7 Electromagnetic wave absorbing model of double-layer material

whereZ1is the input impedance of the first layer,is the characteristic impedance of the second layer,is the propagation constant of the second layer,respectively.d2is the thickness of cement mortar absorber of the second layer,μ2and ε2are the complex permeability and complex permittivity of the second layer of material,fis the electromagnetic wave frequency in free space,andcis the velocity of light.

Therefore,the comprehensive reflection coefficient (Γ2) of double-layer material is determined by Eq.(5):

The comprehensive reflection loss (RL) of the double-layer is calculated by Eq.(6):

An evaluation routine ofZ2and RL for the doublelayer microwave absorbing materials is redacted by MATLAB software.

3.2.2 Microwave absorbing properties of double-layer cementitious materials

In order to achieve the lightweight characteristic,two kinds of specified thicknesses (d1+d2=10 and 15 mm) are measured.According to the gradual impedance design method of double-layer structure,the matching layer selected the material with lower complex permittivity;the absorbing layer selected the material with higher complex permittivity.Fig.8(a)shows the MA properties of double-layer structure with various thicknesses,the matching layer (C1) and the absorbing layer (C2).When the thicknesses ofd1andd2are 7 and 3 mm,the minimum RL value is -33.11 dB at 2.87 GHz with an effective bandwidth of 0.81 GHz(16.2 %).From here,it can be found that the doublelayer materials have more tremendous advantages than single-layer materials in terms of effective bandwidth.The RL value of double-layer structure formed with the matching layer (C2) and the absorbing layer (C3) with various thicknesses is illustrated in Fig.8(b).When the matching layer and absorbing layer are 5 mm,the optimal RL value is -39.56 dB at 2.02 GHz,coupled with an effective bandwidth to be 0.72 GHz (14.8 %),and the minimum RL value is better than C1/C2,but the effective bandwidth is unpleasant.

Fig.8 Microwave absorbing properties of double-layer cementitious materials: (a) matching layer (C1) and absorbing layer(C2);(b) matching layer (C2) and absorbing layer (C3);(c)matching layer (C1) and absorbing layer (C3)

Besides,we attempted to enlarge the impedance matching gradient,and the results are shown in Fig.8(c).As the matching layer and the absorbing layer are 5 mm,the minimum RL value of C1/C3 doublelayer material reaches -27.61 dB at 2.81 GHz,an effective absorbing bandwidth to be 0.97 GHz (19.4%,from 2.10 to 2.17 GHz,2.46 to 2.99 GHz,3.00 to 3.06 GHz and 3.47 to 3.75 GHz).Noticeably,the absorption frequency bandwidth is almost twice that of the single-layer structure.This result confirms the effectiveness of double-layer structure in the design of absorbing materials.To further understand the absorbing characteristic of C1/C3 double-layer structure and reasonable thickness design method,the visible 3D profiles of the double-layer structure with various thicknesses of matching layer is showed in Fig.9.Apparently,the double-layer structure of C1 as matching layer and C3 as absorbing layer with 10 mm total thickness has a wider effective bandwidth and a more vital absorbing capability,which meet the requirement of the advanced MA materials.

Fig.9 The visible 3D profiles of the C1/C3 double-layer structure with different thickness of matching layer: (a) d1+d2=10 mm;(b) d1+d2=15 mm

3.2.3 Impedance matching analysis

No matter the single-or double-layer structure,impedance matching is the prerequisite of MA materials,which determines the absorbing capability of materials.In order to achieve excellent MA properties,the input impedance of the materials should be as close as possible to the intrinsic impedance of the free space[41].According to the relevant literature reported,while the value of Zin/Z0is in the range of 0.8-1.2,the reflectivity of the incident electromagnetic wave reaches the lowest on the surface of MA materials.As for the impedance matching characteristic,the comparison of normalized input impedance is relative to the free space impedance (Zin/Z0) of the MA materials using iron tailings and steel fiber composites as singleor double-layer,as shown in Fig.10.It can be seen from the results of normalized input impedance,the value ofZin/Z0of double-layer samples are less than singlelayer samples.Combined with the RL and impedance matching of the related test samples,the single-layer samples (C1-35 mm and C2-15 mm) had an appropriate impedance matching in an extraordinarily narrow range,which led to an effective absorbing performance only in a narrow frequency range.In comparison with the single-layer samples,the double-layer samples(C1-7 mm/C2-3 mm,C2-5 mm/C3-5 mm and C1-5 mm/C3-5 mm) display more superior characteristics of impedance matching,especially in the higher frequency,which provides the incident electromagnetic wave for a more efficient channel and enters into the absorbing materials as much as possible,thus giving rise to a more substantial electromagnetic microwave absorbing capability.

Fig.10 Normalized input impedance of the single-layer and doublelayer of iron tailings and steel fiber composites

3.3 Workability,mechanical properties and the engineering application

Workability and mechanical properties are the important basic performance of the cement mortar.As we can see from Table 3,the fluidity of C1,C2 and C3 are 158,149 and 132 mm respectively,which is a common phenomenon after the addition of steel fiber.Consequently,to avoid the low fluidity,superplasticizer should be properly used for regulation in practical engineering.For the setting time,because steel fiber has no effect on the hydration process of iron tailingscement paste,the content of steel fiber should not be considered as a factor to affect the setting time.Moreover,the initial setting time and final setting time of the composites are 217 and 308 min,and shows a longer initial setting time and shorter final setting time,which is favorable in the construction field[42].Furthermore,the compressive strength decreases with the increase of steel fiber content,while the flexural strength shows the opposite variation.Overall,all the samples can meet the requirements of M30 masonry mortar defined by Chinese National standard GB/T 25181-2019.Due to the increase in interface and defects,the mechanical properties of double-layer structure may be slightly lower than that of single-layer structure[29].However,if the interfacial defects between the matching layer and the absorbing layer of the double-layer absorbing material can be minimized,the mechanical properties will not be excessively reduced.

Table 3 Workability and mechanical properties of iron tailings and steel fiber composites

In this study,both experimental studies and theoretical analysis show that iron tailings with smaller particle size exhibit favorable electromagnetic loss capability and can cooperate with steel fiber absorber to show excellent MA properties.Moreover,the workability and mechanical properties can meet the requirements for M30 cement mortar.In addition,the double-layer design method shows great design flexibility and diversity,which can be used not only for masonry construction,but also for building surface to resist electromagnetic microwave pollution.Importantly,the main purpose of this study is to study the low-frequency range of absorption band below 5 GHz,which can be applied to daily industrial and civil construction.However,the research frequency range of most current studies on cementitious MA materials is below 18 GHz[8,10,11,13,28,29],and there is a lack of detailed analysis of the low-frequency band.The double-layer structure design method and its low-frequency microwave absorption characteristics in this paper can supplement the current research vacancy of cementitious MA materials,which has certain theoretical significance and application value.In the further studies,we expect to further expand this concept to MA concrete,so as to obtain wider practical engineering application.

4 Conclusions

a) Iron tailings greatly enhance the electromagnetic properties of cementitious material and its particle size can affect the electromagnetic behavior of materials.As the particle size of iron tailings decreases from 78.62 to 5.64 μm after 60-minute grinding,the electromagnetic parameters of cementitious material moved to the higher frequency and impact the higher frequency bands in the frequency range of 0.1-5 GHz.

b) The steel fiber absorber can improve the electromagnetic loss capability of the MA properties of iron tailings cementitious material remarkably.For the single-layer cementitious materials,the optimal RL value was -37.7 dB at 2.19 GHz with an effective bandwidth of 0.53 GHz of sample C1 with a thickness of 35 mm.In comparison with singlelayer structures,double-layer structures exhibit more excellent microwave absorbing properties in absorbing bandwidth.The thicknesses of the matching layer (C1)and absorbing layer (C3) are 5 mm,the minimum RL value reached -27.61 dB at 2.81 GHz with an effective bandwidth of 0.97 GHz.

c) Based on the analysis of single-layer and double-layer cementitious materials of normalized input impedance,the double-layer structure obtained excellent impedance matching,which could be used as a suitable candidate for the preparation of favorable cementitious MA materials.

d) The double-layer structure cementitious MA materials show favorable workability and mechanical properties.From the viewpoint of excellent mechanical strength,the design of double-layer structure cementitious materials are attractive structural materials for buildings.

Conflict of interest

All authors declare that there are no competing interests.