Microwave absorption and mechanical properties of double-layer cement-based composites containing different replacement levels of fly ash

2.1 Materials

According to Chinese Standard GB175-2007, Ordinary Portland Cement supplied by Hainan Tianya Cement Corporation in China was used. FA was obtained from Shanghai Baogang power plant. According to GB/T 1596-2005 standard, it was classified as grade II. Table 1 presents the chemical compositions of FA and cement. The size of PET fragment was 3–8 mm and it had an irregular shape. Ni-Zn ferrite (Ni0.5Zn0.5Fe2O4) was analytically pure and purchased from Shenzhen HC Magnetic Materials Co., LTD. N234 CB was milled in a ball milling for a duration of 3 h before it was used.

2.2 Sample preparation

The cement-based plates used for the RL measurement were prepared as follows. To prepare the absorbing layer, first, the cement and PET fragment were spread and dry mixed for 2 min by a mortar mixer. Second, FA, absorbent powder, and water were put into it and then mixed for 10 min with a water/cement ratio of 0.35. Finally, the mixture of cement was poured into the oiled molds whose size is 200×200×5 mm³, vibrated simultaneously to remove any air entrapped and was smoothed by virtue of a float. When the initial setting of the layer at the bottom was done, the matching layer which was as thick as the bottom layer was added. After a day, the samples were firstly demolded and then cured in water for 28 days at 25±2°C. The arched testing method was applied to measure the RL by virtue of a HP8720B vector network analyzer in an anechoic chamber. The test frequency ranges from 2 to 18 GHz. The mixed proportions of the samples are presented in Table 2.

The samples used for electromagnetic parameter measurement were prepared by mixing 50 vol.% FA power and 50 vol.% paraffin and were later compressed into toroid-shaped specimens whose inner diameter was 3 mm, outer diameter was 7.0 mm, and thickness was 2 mm. The electromagnetic parameters of FA were examined by virtue of the coaxial flange way with a HP8720B analyzer of vector network whose range of test frequency was 2–18 GHz. The microstructures and the mineralogical composition of FA were examined by XRD and SEM.

The size of specimens for the mechanical properties were 40×40×40 mm³ and 160×40×40 mm³, respectively. The compressive and flexural strength of specimens at the curing periods of 7 days and 28 days were tested according to GB/T17671-1999. Three samples were tested for each mix. and the average value was used as the final compressive and flexural strength.

3.1 Features of FA powder

The XRD spectra of FA are illustrated in Figure 1. A series of sharp XRD peaks were found in the crystalline planes, which are identified as mullite, quartz, and minor quantities of hematite and lime. Additionally, a pronounced broad hump of amorphous vitreous materials can be observed in the background between 16° and 35° 2θ.

Figure 1:

XRD pattern of fly ash.

The morphology and chemical compositions of FA particles are controlled mainly by the source of coal, the combustion temperature, and cooling rate. As shown in Figure 2, according to SEM photograph, FA samples comprise irregular unburned carbon, cenospheres, and solid spheres. The present study has also observed mineral aggregates containing corundum, quartz, and magnetite particles [17]. SiO₂ and Al₂O₃ are rich in most particles, which are electromagnetically transparent and contribute about 82% of weight. So FA with the cenosphere structure serves to offer sufficient transmission paths to incident wave, which could be employed to modulate the impedance matching of cement composites.

Figure 2:

SEM image of fly ash.

3.2 The electromagnetic properties of FA

The storage capacity of magnetic and electric energy is embodied in real parts of complex permeability and permittivity, and the loss of energy is symbolized by the imaginary parts. In this paper, in the absorbing layer CB is employed as absorbent because of its large permittivity and excellent dielectric loss character. Ni-Zn ferrite is used to be magnetic fillers in the absorbing layer or the matching layer. The electromagnetic characteristics of CB and Ni-Zn have been explained elsewhere [18]. Here the complex permeability and permittivity of FA have been studied specifically, as shown in Figure 3A and B. The real parts of complex permeability and permittivity have been almost constant, which are 1.01 and 3.15 within the range of 2–18 GHz. Also, the imaginary parts of permeability and permittivity are 0.01 and 0.07, which means that the microwave absorption property of FA is largely due to the dielectric loss instead of magnetic loss. Its dielectric loss is mainly ascribed to the conduction of unburned carbon, dielectric relaxations of metal oxides, and the complicated interaction of defects and particles with incident wave in FA [13]. Furthermore, the main components of SiO₂ and Al₂O₃ with special porous structure in FA can ameliorate the impendence matching between the atmosphere and the double layers of cement composite.

Figure 3:

Complex permeability (A) and complex permittivity (B) of FA in 2–18 GHz range.

3.3 Electromagnetic wave absorbing property

3.3.1 Theory

3.3.1.1 The impedance characteristics of double-layer cement composites

On the basis of the electromagnetic wave transmission line theory, when it comes to a double-layer absorber comprising the matching layer as the first layer as well as the absorption layer as the second layer, as shown in Figure 4, the input impedance Z_in of the absorber backed by a perfect conductor is given by Equation (1) [19], [20]:

Figure 4:

Schematic illustration of the double-layer cement composite consisting of a matching layer and an absorbing layer.

(1)Zin=Z1Z1 tanh(k1d1)+Z2 tanh(k2d2)Z1+ Z2 tanh(k1d1) tanh(k2d2)

where Zi=μ0μi/ε0εi, ki=ωεiε0μiμ0

d₁, Z₁, and k₁ in the matching layer, as well as d₂, Z₂, and k₂ in the absorption layer, are the layer thickness, propagation constant, and characteristic impedance, respectively. Z_in is the input impedance; ε₀ and μ₀ are the permittivity and permeability of vacuum. The propagation constant and characteristic impedance are expressed by the complex permeability μ (μ=μ′−jμ″) and the complex permittivity ε(ε=ε′−ε″). The RL is shown below [19], [20]:

(2)RL=201g|Zin−Z0Zin+Z0| (dB)

where Z₀ is the feature impedance of a vacuum, Z0=μ0ε0=120πΩ. As is deduced from Equations (1) and (2), the absorption characteristic of the absorber with double layer is improved effectively when multiple parameters are modulated, including complex permeability μ, complex permittivity ε, and the thickness d of the absorbing layer and the matching layer to make Z_in more proximate to Z₀.

3.3.1.2 Scattering and absorption of FA and PET

On the one hand, PET is a transparent material to electromagnetic wave, and there are many porous particles in FA considered to be electromagnetically transparent too. Cement is uniformly coated on PET and porous particles, which formed a core-shell structure as shown in Figure 5A and B, of which the cores are the cavities of PET fragment and porous particles in FA. Also, the shells are actually cement walls, as is demonstrated in Figure 5A and B. The incident wave could be ameliorated by the scattering, dielectric losses, and complex refraction by cement matrix when propagating in the specimen.

Figure 5:

Reflection and scattering of the hollow spheres in FA on electromagnetic wave. (A) Single hollow sphere. (B) Multiple hollow sphere.

Figure 5A and B show the phenomenon of reflection and multiple scattering of electromagnetic wave in single cavity and multiple granules. When electromagnetic wave makes its entry to the absorber along x-axis direction, the interface between air and absorber is x=0; the energy of scattering wave at x point I(x) is shown below [21]:

I(x)=I0exp(−nk4(|εr−1|2+|μr−1|2)6π⋅x)

where I₀ is the energy of incident, n is the number of hollow beads in one unit volume, k=ωε0εrμ0μr is wave number in composite basis, and μ_r and ε_r are the permeability and permittivity of cement composites. It can be seen from the formula that the scattering attenuation is in close correlation with the ratio of porous beads, the permittivity and the permeability. Also, the multiple scattering among cavity increases the attenuation times in single hollow cavity. As a result, through the introduction of absorbents in the cement walls as well as the rise of hollow cavity content, more energy is ameliorated by enhancing the attenuation probabilities.

3.3.2 The influence of FA in matching layer on absorption performance

Table 2 demonstrates the design of absorbing layer and matching layer of samples 1#~9#, in which the thickness of the matching layer as well as the absorption layer are the same, 5 mm. The RL of double-layer samples with different levels of FA replacement cement in matching layer is demonstrated in Figure 6, and relevant data are shown in Table 3. From Figure 6 and Table 3, we can see that the rise of FA content improves the microwave absorption. The optimal value of RL achieves −15.4 dB, and the effective absorption band (less than −8 dB) arrives at 5.9 GHz, when the volume fraction of FA replacement cement reaches 60%. By contrast, for the FA-free cement plate the minimum RL is −10.3 dB and the effective asorption bandwidth (better than −8 dB) is only 2.2 GHz.

Figure 6:

Reflection loss of double-layer cement samples with different replacement level of FA in matching layer.

Table 3:

Detailed data of double-layer cement composites with different replacement levels of FA in the matching layer.

Sample	Optimal RL (dB)	Absorption band for RL<−8 dB/GHz	Bandwidth for RL<−8 dB/GHz
1	−10.3	4.3–4.9, 10.9–12.5	2.2
2	−12.1	4.1–5.0, 15.6–18.0	3.3
3	−14.0	3.9–5.1, 13.7–18	5.7
4	−15.4	4.0–4.9, 10.5–15.5	5.9

Apparently, the additional FA is conducive to lowering the peak value and expanding the absorbing bandwidth. Compared with free space, cement has higher permittivity and compact structure that leads to impedance mismatch and limits incidental wave transmission. Therefore, owing to the low complex permittivity, FA as well as PET are introduced to modulate the impedance matching feature of the cementitious composites. At the same time, the space network structures consisting of PET fragments and the cenospheres in FA bring about channels for the transmission of electromagnetic wave. In conclusion, the FA in the matching layer is of vital importance to reduce the peak values and expand the absorption bandwidth.

3.3.3 The influence of FA in absorbing layer on absorption performance

Figure 7 and Table 4 exhibit the RL curve and the corresponding data in detail of samples 5#, 6#, and 7# with the same matching layers of 50% FA and 5% PET. The absorbing layers are filled with 4% CB, 3% PET, and 10% (5#), 20% (6#), and 30% (7#) FA, respectively.

Figure 7:

Reflection loss of double-layer cement samples with different replacement level of FA in absorbing layer.

Table 4:

Detailed data of double-layer cement composites with different replacement levels of FA in the absorbing layer.

Sample	Optimal RL/dB	Absorption band for RL<−8 dB/GHz	Bandwidth for RL<−8 dB/GHz
5	−18.4	3.2–4.0, 10.5–15.5	5.8
6	−18.1	3.5–3.6, 10.8–15.5	4.8
7	−18.4	7.9–9.3, 11.0–15.6, 17.6–18	6.4

It can be seen that as the FA content (varied from 10%, 20%, to 30%) in the absorbing layers increases, the peak positions (about 13 GHz), peak value (−18.4, −18.1, and −18.4 dB), and the absorption band for RL<−8 dB (11.0–15.6, 10.8–15.5, and 10.5–15.5 GHz) are found to exhibit no obvious changes in the high-frequency region, while the value of low-frequency absorption peaks changes and the peak positions get closer to the high-frequency region. It is due to the fact that increasing FA content gives rise to a low permittivity and increasing matching frequency, of which the scale-dependent effective medium theory is a proof [22]. The improved absorption properties can be ascribed to the matched characteristics impedance of the absorbing layer. Otherwise, the incident wave would be reflected partly on the interface of the absorbing layer as well as the matching layer. The effective absorption bandwidth will worsen as a result of the strong reflection, like sample 6#. So there would be an optimal ratio of FA in the absorbing layer and the matching layer to obtain minimum RL value, which reaches −22.3 dB at 13.2 GHz. Also, the effective absorption bandwidth (<−8 dB) arrives at 6.4 GHz at the FA ratio of 50%:30%.

3.3.4 The influence of Ni-Zn ferrite on absorption performance

Figure 8 shows the influence on the absorbing performance of Ni-Zn ferrite in the absorbing layer (8#) and the matching layer (9#), and Table 5 presents the detailed relevant data. The optimal RL is −22.2 dB. Also, the bandwidth (RL<−8 dB) is 7.2 GHz for 9#. Nonetheless, the value of effective absorption bandwidth (RL<−8 dB) is 6.1 GHz, which peaks at −17.0 dB for 8#. The comparisons indicate that in the matching layer Ni-Zn ferrite can palpably improve absorption in the peak value as well as the bandwidth, but Ni-Zn ferrite in the absorbing layer serves to decrease absorbing properties compared to the Ni-Zn ferrite free sample (7#). Ni-Zn ferrite is considered as a promising magnetic loss absorbent, which boasts high ε and μ. In the present study, an increase was observed in the composite ameliorating performance when Ni-Zn ferrite was filled in the matching layer. The reflections of incident wave are also suppressed by the closer ε and μ. As a result, the peak value and the bandwidth are greatly improved. However, the introduction of Ni-Zn ferrite in the absorbing layer has negative influence on absorption, which is possibly ascribed to the mismatching impedance of the matching layer and the absorption layer.

Figure 8:

Effect of Ni-Zn ferrite on reflection loss of double-layer cement samples.

Table 5:

Detailed data of double-layer cement composites with Ni-Zn ferrite in different layers.

Sample	Optimal RL/dB	Absorption band for RL<−8 dB/GHz	Bandwidth for RL<−8 dB/GHz
7	−18.4	7.9–9.3, 11.0–15.6, 17.6–18	6.4
8	−17.0	4.5–4.8, 10.6–16.4	6.1
9	−22.2	3.2–4.0, 10.7–17.1	7.2

3.4 Effects on mechanical performance of the filling proportion of FA

Table 6 exhibits the compressive strength of samples consisting of various replacement levels of FA ranging from 20.52 MPa to 35.44 MPa on 7 days and ranging from 31.40 MPa to 47.24 MPa on 28 days. The compressive strength of mixtures showed a reduction that goes hand in hand with the rise of FA replacement level. The FA, CB-blended samples had a lower compressive strength than the cementitious mixtures of only FA replacement. The reduction in the compressive strength of the cementitious mixer was stopped by a prolonged curing period. For example, when the curing period lasted 7 days, it turned out that the compressive strength of the mixer with 60% FA was reduced by 42.01% in comparison to the FA free cement. However, in the same samples when the curing period lasted 28 days, the compressive strength was found to be down by 23.26%. This could be because at the early curing period, the pozzolanic reaction activity of FA was low, and during the later stages of the curing period, the reaction of calcium hydroxide (CH) as a result of the primary cement hydration and FA generated a secondary hydration [9], [23], [24].

The flexural strength of samples decreases first and then rises with the replacement of cement by FA at the curing ages of 7 days and 28 days. The 20% FA replacement cement exhibits the highest flexural strength (7.31 MPa at 7 days and 9.12 MPa at 28 days) compared to others. For 4% CB blended mortars, both flexural and compressive strength are reduced to a lower value compared to samples without.