Electromagnetic Wave Transmission Properties Of Fly Ash / Slag Alkali Activated Composites

With the development of science and technology,electronic devices are used more and more frequently.Although bring convenience to our daily lives,electronic devices that generated a lot of electromagnetic wave radiation has brought great harm to human beings and environment.At present,methods to reduce or eliminate electromagnetic interference and electromagnetic radiation hazards include EMS（Electromagnetic Shielding）and EMA（Electromagnetic Absorbing）.Electromagnetic Shielding is not only useless for weakening or getting rid of electromagnetic waves fundamentally,but also easily causes"secondary interference"of reflected electromagnetic waves to incident electromagnetic waves.So it has not been widely promoted.However,Electromagnetic Absorption can loss electromagnetic waves by converting the electromagnetic waves entering the composite material into heat or other forms of energy.For the absorbing material,two basic conditions must be met for absorbing:1)The impedance matching of the absorbing material.2)Attenuation matching of absorbing materials.Therefore,how to improve the impedance matching of the absorbing material has become the primary problem that needs to be solved urgently.Cement is often used as a matrix material in the field of electromagnetic wave absorption research,but its large-scale use will pollute the environment and consume a lot of energy.Modification of cement-based materials requires consideration of the influence of admixtures on the mechanical properties of composite materials.The alkali-activated materials have the characteristics of high compressive strength,energy saving and environmental protection,simple production process and low energy consumption.Existing researches often focus on the basic properties of alkali-excited materials such as mechanics,hydration,and drying shrinkage,and there is very little research on their wave-transmitting properties.In order to improve transmission performance of electromagnetic wave of the alkali-activated composite material,it is necessary to improve the impedance matching performance of composite material and free space.This paper studies the wave-transmitting properties of alkali-activated materials,through the performance of the matrix material and the wave-transmitting agent.The details of the study are as follows:First,this paper studies the influence of water-cement ratio and activator modulus on the compressive strength and wave-transmitting properties of alkali-activated cementitious materials.Secondly,under the conditions of the same particle size of quartz sand or length of glass fiber,the influence of the content of wave-transmitting agent on the compressive strength,shielding efficiency and reflectivity of alkali-activated composite cementitious materials has been studied.Afterwards,based on the above research results,the influence of the compounding of quartz sand,glass fiber and expanded perlite on the compressive strength and wave-transmitting properties of alkali-activated composite cementitious materials was studied.Finally,the effects of the compound doped wave-transmitting agent and nano-Fe₃O₄ on the wave-absorbing properties of the alkali-activated composite cementitious materials were studied.This paper further explored the influence of structure design on the absorbing properties of alkali-activated composite cementitious materials through the design of double-layer structure.The parameters to be tested in this paper include compressive strength,shielding effectiveness and reflectivity.The shielding effectiveness and reflectivity of the composite materials were tested by the flange coaxial method and the bow method respectively.In this study,an alkali-activated composite absorbing material that not only satisfies the compressive strength required by engineering practice,but also has excellent absorbing properties was prepared.The main research conclusions are as follows:（1）The compressive strength of alkali-activated cementitious materials decreases as the water-cement ratio increases,and decreases first and then increases as the modulus of the activator increases.The compressive strength and wave-transmitting performance of the sample with a water-cement ratio of 0.35 are the best.Although the compressive strength of the specimen with the activator modulus of 1.40 reaches the maximum,the specimen with the activator modulus of 1.20 has the best wave-transmitting performance.（2）The research results of single-doped wave-transmitter alkali-activated cementitious materials illuminate that the compressive strength of composite materials doped with quartz sand of the same particle size first increases and then decreases with the increase of the content of quartz sand.When the quartz sand content is the same,the compressive strength of the alkali-activated composite cementitious material decreases with the decrease of the particle size.Due to the mixture of quartz sand,the shielding effectiveness and reflectivity of the alkali-activated composite cementitious material are reduced,and the wave-transmitting performance is enhanced.When mixed with 40-70 mesh quartz sand with 50vol%,the effective absorption bandwidth of the composite material reaches the maximum value of 26.59GHz,and the minimum reflectivity also reaches-14.67d B.The compressive strength of alkali-activated composite cementitious materials increases first and then decreases with the increase of glass fiber content.As the length of the glass fiber increases,the optimal content corresponding to the maximum compressive strength of the composite material gradually decreases.The incorporation of glass fiber improves the electromagnetic wave transmission performance of the alkali-activated cementitious material.The effective absorption bandwidth of the composite material mixed with 7vol%6mm glass fiber reaches the maximum value of 18.73GHz,and its minimum reflectivity peak value is-11.76d B.（3）The incorporation of expanded perlite will greatly reduce the compressive strength of the composite material,and the incorporation of quartz sand can increase the compressive strength of the composite material.When the total amount of wave-transmitting agent is 50vol%,the compressive strength of the sample increases as the volume ratio of quartz sand and expanded perlite increases.However,the wave transmission performance of the sample with a volume ratio of 1:1 is stronger than that of the sample with a volume ratio of 2:1.When the optimum content of quartz sand and expanded perlite are mixed,the influence of the optimum content of glass fibers of different lengths on the wave-transmitting performance of the composite is related to the particle size of the quartz sand,length and content of the glass fiber.For the alkali-activated composite cementitious material mixed with different particle size quartz sand at the optimum content,the glass fiber content and length for the best improvement of its wave-transmitting performance are 11vol%and 12mm,respectively.When compounded with optimal content of quartz sand and glass fiber and expanded perlite,the wave-transmitting performance of alkali-activated composite cementitious materials is greatly affected by the length of glass fiber,particle size and content of quartz sand.When the optimum content of glass fiber is mixed,the addition of 120-180 mesh quartz sand 30vol%can improve the overall wave transmission effect of the composite material most obviously in the whole frequency range.（4）In a certain frequency range,the incorporation of nano-Fe₃O₄ can enhance the absorbing performance of the alkali-activated composite cementitious material.In the frequency range of 12.00～28.00GHz,the double-layer structure design significantly increases the wave-absorbing performance of the 50vol%6-8 mesh quartz sand alkali-activated composite cementitious material.When 33.33vol%40-70 mesh quartz sand and 16.67vol%expanded perlite are incorporated,the incorporation of nano-Fe₃O₄ significantly improves the absorbing performance of the alkali-activated composite cementitious material.The double-layer structure design includes two design ideas,the surface layer is only mixed with wave-transmitting agent,the first type of bottom layer is mixed with wave-absorbing agent,and the second type of bottom layer is mixed with wave-transmitting agent and wave-absorbing agent.In the low frequency and high frequency range（1.00～2.50GHz and 19.50～40.00GHz）,the former has better absorption performance than the latter.In the middle frequency band range（2.50～19.50GHz）,the opposite is true.In summary,when the water-cement ratio is 0.35 and the modulus of the activator is 1.20,the best wave-transmitting agent combination for improving the wave-transmitting performance of the alkali-activated composite cementitious material is 30vol%120-180 mesh quartz sand,11vol%12mm glass fiber and 9vol%expanded perlite.In the low-frequency and high-frequency range（1.00～2.50GHz and 19.50～40.00GHz）,when the surface layer is doped with wave-transmitting agent and the bottom layer is doped with wave-absorbing agent,the double-layer structure design is beneficial to the improvement of the wave-absorbing performance of composite materials.In the middle frequency range（2.50～19.50GHz）,the double-layer structure design with wave-transmitting agent and wave-absorbing compound doped at the bottom layer has a better effect on improving the wave-absorbing performance of composite materials.In this study,the wave-transmitting properties of alkali-activated composite cementitious materials were improved through the design of multiple-doped wave-transmitting agent and double-layer structure.This study provides a relatively basic reference result for the study of electromagnetic wave absorption performance in the field of construction,which is conducive to promoting the diversified application of alkali-activated cementitious materials.