Crystalline Nanoparticles and Their Impact on Electromagnetic Radiation Absorption in Advanced Clay Building Materials

Abstract

Given the increasing human exposure to electromagnetic radiation of various frequen-cies, mostly in the microwave range, awareness of potential health problems caused by this radiation has begun to grow. New building materials are being developed and tested to prevent or limit the penetration of microwave radiation, especially those frequencies that are used in mobile telephony. In contrast with the majority of the available literature on the investigation of concrete (cement) materials, in this paper, clay composite materials with the addition of nanoparticles of antimony(III)–tin(IV) oxide, zinc ferrite, iron(III) oxide, and two crystal modifications of titanium dioxide (rutile and anatase) were prepared in order to examine their effect on the absorption of electro-magnetic radiation. Nanomaterials are characterized by different physical and chemical methods. Specific surface area (B.E.T.), thermal properties (TGA/DSC), phase composition (PXRD), morphology (SEM), and chemical and mineralogical composition (EDX, and ED–XRF,) were determined. Thermal conductivity of clay composites was tested, and these materials showed a positive effect on the thermal conductivity (λ) of the composite: a reduction of 10–33%. The reflection and transmission coefficients of microwave radiation in the frequency range used in mobile telephony (1.5–4.0 GHz) were determined. From these data, the absolute value of radiation absorption in the materials was calculated. The results showed that the addition of the tested nanomaterials in a mass fraction of 3 to 5 wt.% significantly increases the absorption (reduces the penetration) of microwave radiation. Two nanomaterials, Sb₂O₃·SnO₂ and TiO₂ (rutile), have proven to be particularly effective: the reduction in transmission is 30–50%. The results of the test were correlated with the crystal structures of the examined nanomaterials. The inclusion of titanium dioxide and antimony-doped tin oxide into the clay led to a significant enhancement in microwave electromagnetic radiation absorption, which can be attributed to their interaction with the dielectric and conductive phases present in clay-based building materials.

1. Introduction

Since the 1990s, growing awareness has emerged regarding the potential health risks associated with increased exposure to electromagnetic (EM) radiation across various frequencies. The impact of EM radiation on human health depends on several factors, including its frequency, intensity, duration of exposure, and the specific type of radiation involved [1]. Some studies have suggested a correlation between EM radiation exposure and a range of health issues, including neurological disorders, leukemia, cancer, brain tumors, and other serious conditions [2,3,4].

The electromagnetic spectrum includes all types of electromagnetic waves and is typically divided into regions based on wavelength or frequency: radio waves, microwaves, infrared, visible light, ultraviolet, X–rays, and γ–rays [5]. High–energy radiation (γ–rays, X–rays, and certain ultraviolet wavelengths) has enough energy to remove electrons from atoms by a process known as ionization. In contrast, radiation with longer wavelengths lacks sufficient energy to cause ionization. Accordingly, electromagnetic radiation is broadly categorized into two types: ionizing and non-ionizing radiation.

Most research on protection against ionizing radiation focuses on concrete as the primary shielding material. Efforts to enhance its shielding effectiveness often involve modifying the material to increase its electrical conductivity. A variety of additives are incorporated into concrete to reduce the penetration of ionizing radiation, including metals, carbon-based materials, ceramics, conductive polymers, and other composite materials [6,7]. These modifications aim to improve the material’s attenuation properties while maintaining its structural integrity and practicality in construction applications. An increasing number of studies have explored the incorporation of conductive materials into clay at various wt.%, primarily to assess their effectiveness in shielding against non-ionizing radiation. Although some recent studies [8,9] have begun to investigate the microwave frequency range (1.5–6.0 GHz), the available literature remains limited. In particular, there is a lack of systematic comparisons involving different types of conductive materials and their wt.% within this frequency range. On the other hand, this frequency range is of special interest because multiple generations of mobile telephony (from 2G to 5G) operate in it: LTE 1800 (1.80–1.88 GHz), LTE 2100 (2.11–2.17 GHz), LTE 2600 (2.62–2.69 GHz), and NR3500 (3.40–3.80 GHz). Suitable additives for the development of electromagnetic (EM) shielding building materials include metals, carbon-based materials, ceramics, cement or concrete, conductive polymers, and other composite materials [7]. Among these, metals are the most commonly used due to their high electrical conductivity and strong interaction with the electric field component of EM radiation. Metals have long been employed as shielding materials, either as additives (e.g., aluminum sheets used in electronic enclosures) or as surface coatings. Magnetic ceramics, such as iron(II, III) oxide (Fe₃O₄ or magnetite), are also effective shielding materials due to their ability to absorb EM radiation [10,11,12,13]. Unlike magnetic metals, these ceramics offer improved corrosion resistance, making them attractive for long-term applications. The synergistic use of Fe₃O₄ and a conductive component has proven particularly effective. In contrast, typical ceramics alone are generally inadequate for EM shielding because of their low conductivity, poor polarizability, and limited magnetic properties [14]. As a result, research has primarily focused on ceramic–metal composites. Polymers are the least effective shielding materials, unless modified to be conductive. In general, for any shielding material, the presence of magnetic properties significantly enhances performance by enabling interaction with the magnetic field component of electromagnetic radiation [15].

Various additives were used as fillers in the clay, including two types of titanium dioxide (TiO₂–rutile and TiO₂–anatase), zinc ferrite (ZnFe₂O₄), maghemite (γ-Fe₂O₃), and antimony(III)–tin(IV) oxide (Sb₂O₃·SnO₂). These additives were selected based on their demonstrated electromagnetic (EM) shielding capabilities when used in cementitious composites as partial replacements for cement. Zinc ferrite, a member of the spinel ferrite family (e.g., CoFe₂O₄, MnFe₂O₄, ZnFe₂O₄), exhibits excellent electromagnetic properties and high magnetic permeability. Its incorporation into mortar has been shown to increase reflectivity relative to reference samples [16]. In contrast to ferrites, metal oxides such as TiO₂ (although non-ferromagnetic) exhibit dielectric loss and are therefore considered to be effective electromagnetic absorbers [17]. Maghemite, known for its strong magnetic properties, has also been shown to enhance EM shielding when used in both structural materials and coatings. This is supported by several studies reporting improved shielding performance with the addition of γ-Fe₂O₃ [18,19,20]. Although limited research exists on the use of Sb₂O₃·SnO₂ in cementitious systems, its high electrical conductivity has been well documented, making it a promising candidate for electromagnetic shielding applications [21].

The aim of this research is to enhance protection against microwave radiation by increasing absorption in the clay-based composite materials that contain several nanomaterials. The selected nanomaterials were chosen based on their documented electromagnetic properties as reported in the available literature. These composite materials exhibited a pronounced improvement in their ability to absorb microwave electromagnetic radiation, indicating enhanced shielding or attenuation performance.

2. Materials and Methods

2.1. Materials

As a basic material, brick clay supplied by a major fired brick products manufacturer was used. Commercially available nanoparticles used in this study were two crystal modifications of titanium dioxide (TiO₂–rutile and TiO₂–anatase), iron(III) oxide (γ-Fe₂O₃), zinc ferrite (ZnFe₂O₄), and antimony(III)–tin(IV) oxide (Sb₂O₃·SnO₂) (

Table 1. Materials used in the experimental work.

Material	Chemical Formula	Manufacturer
Clay	-	Wienerberger, Đakovo, Croatia
Titanium dioxide R050	TiO2 (rutile)	MK Nano, Canada
Titanium dioxide A050	TiO2 (anatase)
Iron(III) oxide	γ-Fe2O3	Iolitec, Germany
Zinc Ferrite	ZnFe2O4	Nanografi, Turkey
Antimony(III) Tin(IV) oxide (ATO)	Sb2O3·SnO2	EPRUI, China

).

Clay was supplied by the manufacturer as standard production blocks 375 × 250 × 238 mm (mass 15.1 kg) prior to firing. Blocks were broken into smaller pieces, ground up in a rotary mixer, and sieved through the standard sieve No. 18 (1.0 mm opening). All other materials were used as received. Potable water without purification was used for sample preparation.

Samples for measurements were prepared in the form of 150 mm disks (diameter) of 20 mm thickness (three samples of the same composition, approx. mass 650 g each). Pure clay disks were prepared as a standard while composite disks were prepared with the addition of 3 and 5 wt.% of selected nanomaterials. The proportions of nanomaterials were selected based on a study [9], and preliminary tests were carried out with addition of varying amounts of nanomaterials (1–10 wt.%). It was shown that the 3 wt.% and 5 wt.% additions of nanomaterials are optimal with respect to cost and the properties obtained. All samples were prepared in a wet form (25 wt.% water) and, after thorough mixing, compacted in a steel mold. After the preparation, the samples were subjected to a drying process in a chamber (room) with controlled air humidity. Drying clay samples is a very sensitive process, especially in the case of such thin specimens. A very important factor is gradual drying, in order to prevent cracks from forming as a result of too rapid drying. Therefore, they were first dried for two days in the air with 70% relative humidity in a humidity-controlled room, followed by two more days of drying in the air at 30% relative humidity. The final step was drying in a laboratory oven at 105 °C for 24 h to completely remove the water from the samples.

Once the water had been completely removed, the firing process followed. The firing program was adopted from Wienerberger d.o.o., a company primarily engaged in brick production (Figure S1). After 13 h, the kiln was turned off, and the disks were left to cool naturally until they could be handled by hand (Figure 1).

2.2. Methods

Materials listed in Table 1 were characterized by different physical and chemical methods.

Their specific surface area was determined by absorbing ultrapure nitrogen according to the Brunauer–Emmett–Teller (B.E.T.) method on a Quantachrome© Nova 4200e device (Boynton Beach, FL, USA).

The thermal properties of the material were examined using a Mettler-Toledo System 1 instrument (Mettler-Toledo, Columbus, OH, USA) equipped for simultaneous thermogravimetric and differential scanning calorimetric (TGA/DSC) analysis. Approximately 10–15 mg of the sample was weighed and placed in aluminum(III) oxide (Al₂O₃) crucibles (Mettler-Toledo, Columbus, OH, USA). The measurements were performed under the oxygen atmosphere (O₂ flow, 195 mL min⁻¹) in the temperature range of 50–1000 °C, with a heating rate of 10 °C min⁻¹. Data acquisition and processing were conducted using STARe Software version 10.0 [22].

The surface morphology and elemental composition of the material were analyzed using a Thermo Fisher Scientific Apreo C scanning electron microscope (SEM, Waltham, MA, USA) equipped with an energy-dispersive X-ray (EDX) analyzer. The measurements were carried out at a working distance of 10.3 mm, under accelerating voltages of 10 and 20 kV. The examined areas measured 5 and 10 µm², respectively. A small quantity of the sample was mounted on carbon adhesive tape affixed to an aluminum stub prior to analysis. No metal sputtering was applied.

The crystalline phases of the materials were determined by powder X-ray diffraction (PXRD). Diffractograms were recorded on a PANalytical Aeris Research Diffractometer (Malvern PANalytical, Malvern, UK) using CuKα radiation (40 kV, 15 mA, λ = 1.54178 Å) at 295 K in Bragg–Brentano θ–θ geometry. Phase identification was performed by matching the obtained diffraction patterns with the literature. The diffraction data were analyzed using PANalytical HighScore Plus 3.0 software (Malvern PANalytical, Malvern, UK).

The elemental composition of clay was determined using an energy-dispersive X-ray fluorescence spectrometer (ED–XRF). The instrument model NEX CG manufactured by Rigaku (Tokyo, Japan) was used.

Thermal conductivity, λ (Wm⁻¹K⁻¹) of clay samples and composite materials was measured using a FOX 200 instrument (TA Instruments, New Castle, DE, USA). The thermal conductivity of a material is defined as the amount of heat that passes through a 1 m thick layer with a surface area of 1 m² per unit of time, given a temperature difference of 1 K. It is calculated according to Equation (1).

$$\lambda ={\displaystyle \frac{Q·d}{A·\mathsf{\Delta }T·t}}$$

(1)

In many technical disciplines, including construction, thermal resistance (R, m² K/W) is commonly reported. It is defined as the ratio of a material’s thickness to its thermal conductivity (Equation (2)).

$$R={\displaystyle \frac{d}{\lambda }}$$

(2)

The penetration of microwave radiation through the prepared clay composite disks was measured. The measuring device (Figure 2) consists of two aluminum cylindrical waveguides with two monopole antennas built in, between which there is a steel plate where the tested sample is placed. The antennas are connected via coaxial cables to the spectrum analyzer Anritsu MS2038C—Handheld Vector Network Analyzer and Spectrum Analyzer (Morgan Hill, CA, USA).

The measurements were conducted to determine the area of protection against microwave radiation penetration within the frequency range used in mobile telephony (1.5–4.0 GHz). The measured transmission (S₂₁) coefficients were recorded across this range, and based on the obtained data, the absolute values of radiation absorption in the tested materials were calculated. However, the following frequency bands are discussed in more detail in this paper: 2G (1.75–1.85 GHz), 3G (2.10–2.30 GHz), 4G (2.60–2.70 GHz), and 5G (3.40–3.80 GHz). The device assembled in this configuration is capable of measuring all four S parameters (Figure 3). The output signals from Antennas 1 (I₀₁) and 2 (I₀₂) are directed toward the sample, where a portion of the electromagnetic energy is reflected (described by the reflection coefficients, S₁₁, S₂₂) while portion is absorbed within the sample (denoted as A), and the remaining energy is transmitted through the sample and detected by the opposite antennas, which measure the transmission coefficient (S₂₁, S₁₂). The absorption (A) within the sample at each specific frequency can be calculated using the reflection and transmission coefficients, according to Equation (3).

$$A=10log\left[1-{\left|S_{11}\right|}^2-{\left|S_{21}\right|}^2\right]$$

(3)

In this study, the base material utilized was industrial brick clay obtained from a leading manufacturer of fired clay products. In addition to the clay matrix, several commercially available nanoparticulate additives were incorporated: Sb₂O₃·SnO₂, ZnFe₂O₄, γ-Fe₂O₃, and two polymorphic forms of titanium dioxide: anatase (TiO₂ A050) and rutile (TiO₂ R050). Although these nanomaterials are widely available on the market, a comprehensive physicochemical and structural characterization was performed before their use. This step was essential to ensure the reproducibility and consistency of their intrinsic properties, thereby providing a reliable foundation for the subsequent experimental procedures and analysis.

3. Results and Discussion

3.1. Materials Characterization

Results of specific surface area measurement by B.E.T. method as well as calculated and measured particle sizes for materials used are given in

Table 2. Specific surface area and calculated particle size of the materials based on B.E.T. data and particle size determined from electron microscopy micrographs using ImageJ software (1.54r) [23].

Materials	Specific Surface Area/m2g−1	Particle Size/nm *	Particle Size/nm ***
Clay	39	**	**
TiO2 R050	25	50	74 ± 13
TiO2 A050	64	50	42 ± 7
γ-Fe2O3	39	30	49 ± 7
ZnFe2O4	74	15	45 ± 8
Sb2O3·SnO2	60	50	24 ± 5

* Manufacturer’s data; ** Supplied in blocks and mechanically ground; therefore, the specific surface area depends on the processing method. *** Data determined by measuring 20 particles from electron microscopy micrographs using ImageJ software [23].

.

The thermogram of the clay sample (Figure 4a) exhibits, as expected, a two-step mass loss. In the first step, a mass loss of 4.2 wt.% is observed in the temperature range of 50–160 °C, while the second step shows a 3.5 wt.% mass loss occurring between 350 and 650 °C. These losses correspond to the release of physically adsorbed water in the first step, and to dehydroxylation in the second, i.e., the removal of interlayer water from the clay structure. According to the literature [24], all minerals of the kaolinite group display a characteristic mass loss in the 400–700 °C range, associated with transformation during which kaolinite is converted into metakaolinite.

Figure 4b,c presents thermogravimetric curves for two titanium dioxide samples with different crystal modifications (anatase and rutile), both having a nanometer particle size (~50 nm), measured over a temperature range of 25 to 1000 °C. As shown, no significant mass loss was observed. A minor initial loss between 100 and 150 °C (0.6–2.1 wt.%) is attributed to the presence of a small amount of adsorbed moisture. Based on these results, it can be concluded that TiO₂ exhibits excellent thermal stability [25]. Additionally, the DSC curve (heat flow) is given as an insert in Figure 4c, showing the phase transformation (anatase to rutile) at around 830 °C.

As expected, the nanomaterial samples γ-Fe₂O₃, ZnFe₂O₄, and Sb₂O₃·SnO₂ did not exhibit significant mass loss over the measured temperature range (Figure 4d–f). The observed mass losses, all below 6%, can be attributed to surface-adsorbed water (moisture) [26,27,28].

As observed with SEM, the clay sample (Figure 5a,b) contains particles of varying sizes, which is expected given that the material was supplied in block form and subsequently mechanically crushed and sieved using a standard 1 mm mesh.

Figure 6 shows SEM micrographs of the materials used as additives to clay. The observed particle morphologies and sizes are consistent with the particle size data provided by the manufacturer.

The results of elemental analysis obtained by SEM-EDX are consistent with the empirical formulas of the analyzed materials. The corresponding data, expressed in atomic percentages, are provided in the Supplement (Table S1).

The X-ray diffractogram of clay is presented in the Supplement (Figure S2), while all the other X-ray diffractograms with Rietveld analysis are presented in Figure 7a–e.

The results presented in Figure 7 indicate that all analyzed materials are phase-pure, as confirmed by the agreement between the observed and calculated PXRD patterns in the Rietveld refinement plots [29,30,31,32,33]. The observed peaks in the X-ray diffractogram of clay (Figure S2) correspond to SiO₂ (black) [34], and γ-Fe₂O₃ (blue) [35]. These X-ray diffraction results are consistent with the elemental analysis data presented in

Table 3. Mineralogical composition of the clay sample as determined by ED–XRF.

Clay
Oxides	wt.%
SiO2	64.73
Al2O3	22.70
CaO	2.40
Fe2O3	4.59
SO3	0.06
MgO	1.39
K2O	2.16
Na2O	1.02
TiO2	0.96
MnO	0.01
P2O5	<0.01

, which summarizes the most common elements (expressed as oxides) in clay, as determined by energy-dispersive X-ray fluorescence spectrometry ED–XRF [36].

3.2. Measurements on Composite Disks

Composite disks (clay + 3 and 5 wt.% of selected nanomaterials) prepared as described in Section 2.1. were used in thermal conductivity measurements as well as for microwave penetration investigations (Section 3.3).

Figure 8 shows the measured thermal conductivity values (λ, Wm⁻¹K⁻¹) for the pure clay sample. The results listed in

Table 4. Measured thermal conductivity values (λ) and corresponding calculated thermal resistance values (R) for clay and composite samples.

Sample	wt.%	λ (W m−1 K−1)	R (m2 K/W)
Clay		0.2146 ± 0.010	0.0890
TiO2—anatase	3	0.2002 ± 0.006	0.0962
	5	0.1970 ± 0.009	0.0987
TiO2—rutile	3	0.2231 ± 0.006	0.0848
	5	0.2458 ± 0.013	0.0773
ZnFe2O4	3	0.2187 ± 0.004	0.0853
	5	0.2279 ± 0.004	0.083
γ-Fe2O3	3	0.2305 ± 0.009	0.0835
	5	0.2208 ± 0.003	0.0864
Sb2O3·SnO2	3	0.2185 ± 0.011	0.0866
	5	0.2656 ± 0.010	0.0737

demonstrate the influence of nanomaterial additions in 3 wt.% and 5 wt.% on the thermal conductivity of clay-based composite materials.

Considering the λ values reported in the literature [37,38,39,40,41,42] for standard clay solid bricks—ranging from 0.2 to 0.8 Wm⁻¹K⁻¹—and comparing them with the thermal conductivity measured in this study (0.2146 ± 0.010 Wm⁻¹K⁻¹ for the clay sample, see Table 4), it can be concluded that the measured value is relatively low. Consequently, the corresponding thermal resistance (R) is relatively high. This low thermal conductivity is particularly noteworthy.

Results of thermal conductivity measurements for pure clay and clay-based composite materials incorporating 3 wt.% and 5 wt.% of various additives TiO₂ (anatase and rutile), ZnFe₂O₄, γ-Fe₂O₃, and Sb₂O₃·SnO₂ are presented in Table 4. Representative thermal conductivity values (λ) were taken as those measured at 25 °C in accordance with the HRN ISO 8302:1998 standard [43]. These values, together with the corresponding thermal resistance (R), calculated based on the precisely measured sample thickness, are also presented in Table 4.

3.3. Electromagnetic Radiation Characterization of Clay and Clay-Based Composites

The measurements of the electromagnetic radiation (EMR) characteristics (S₂₁ and |A|) of clay and clay-based composites with 3 and 5 wt.% of additives are presented in Figure 9 and Figure 10. Calculated areas under the curves that correspond to 2G–5G ranges are presented in

Table 5. Integrated area under S₂₁ curves (Figure 9) in the specific frequency ranges (2G–5G).

Sample	wt.%	2G Area (1.75–1.85 GHz)	3G Area (2.10–2.30 GHz)	4G Area (2.60–2.70 GHz)	5G Area (3.40–3.80 GHz)
Clay		1.156	1.548	1.065	2.428
TiO2 R050	3	2.294	3.243	1.411	3.465
	5	2.084	2.188	1.501	5.408
TiO2 A050	3	2.367	3.379	1.471	3.988
	5	1.577	2.569	1.342	5.725
Fe2O3	3	2.416	3.331	1.434	3.009
	5	2.134	2.344	0.815	3.737
ZnFe2O4	3	2.408	3.347	1.513	3.628
	5	1.982	2.293	0.881	4.323
Sb2O3·SnO2	3	2.312	3.377	1.462	3.478
	5	1.861	3.278	1.616	5.624

(S₂₁) and

Table 6. Integrated area under |A| curves (Figure 10) in the specific frequency ranges (2G–5G).

Sample	wt.%	2G Area (1.75–1.85 GHz)	3G Area (2.10–2.30 GHz)	4G Area (2.60–2.70 GHz)	5G Area (3.40–3.80 GHz)
Clay		0.495	0.628	0.363	1.857
TiO2 R050	3	0.181	0.434	0.167	0.643
	5	0.176	0.469	0.121	0.492
TiO2 A050	3	0.181	0.467	0.169	0.735
	5	0.176	0.558	0.163	0.641
Fe2O3	3	0.180	0.455	0.169	0.733
	5	0.167	0.482	0.272	0.545
ZnFe2O4	3	0.177	0.432	0.160	0.553
	5	0.109	0.215	0.093	0.220
Sb2O3·SnO2	3	0.178	0.476	0.157	0.596
	5	0.151	0.276	0.143	0.281

(|A|).

The electromagnetic spectral measurements presented in Figure 9 (transmission coefficient S₂₁) and Figure 10 (|A|), together with Table 5 and Table 6 (integrated areas under the curves in the 2G–5G frequency ranges), reveal several notable trends.

The clay sample exhibits strong microwave absorption (Figure 10a) and low transmission (S₂₁, Figure 9a) in the 1.5–4.0 GHz range, confirming its inherent ability to attenuate electromagnetic radiation. This behavior can be attributed to the clay’s mineralogical and chemical composition, particularly the relatively high content of γ-Fe₂O₃ [44], which contributes to dielectric and magnetic losses.

Within the 2G range, all samples containing nanomaterial additives exhibit larger S₂₁ areas compared with the clay reference (1.156), indicating enhanced microwave attenuation and improved shielding effectiveness (Table 5). According to Table 6, all samples exhibit lower absorption compared to the reference clay.

Overall, these results indicate that increasing the additive concentration does not necessarily lead to better attenuation performance or higher electromagnetic shielding efficiency. Instead, the effect appears to depend on both the additive type and its dispersion within the matrix.

In the 3G range, most composite samples exhibit higher S₂₁ values (Figure 9, Table 5) relative to the clay reference (1.548), implying lower transmission and therefore a reduction in microwave penetration. The largest S₂₁ areas are observed for Sb₂O₃·SnO₂ 5 wt.% (3.377), TiO₂ A050 3 wt.% (3.379), and ZnFe₂O₄ 3 wt.% (3.347), while the TiO₂ R050 5 wt.% sample shows the lowest value (2.188).

Interestingly, the corresponding absorption values (Figure 10, Table 6) for these composites are slightly lower than that of the clay reference (0.628), suggesting that the addition of nanomaterials tends to reduce transmitted signal intensity (S₂₁), but not necessarily through enhanced absorption (|A|).

Within the 4G range, the S₂₁ transmission coefficients (Figure 9, Table 5) are generally comparable to the clay reference (1.065), although certain composites show improved attenuation of microwave penetration, specifically TiO₂ (3 and 5 wt.%), Fe₂O₃ 3 wt.%, ZnFe₂O₄ 3 wt.%, and Sb₂O₃·SnO₂ (3 and 5 wt.%).

A comparison of the absolute absorption values (Figure 10, Table 6) reveals that the clay (0.363) still shows higher absorption than the composite materials (0.093–0.272). This observation suggests that the principal mechanism responsible for reducing electromagnetic penetration in the composites is not direct absorption, but rather reflection or impedance mismatch introduced by the nanophase inclusions.

In the 5G frequency region, transmission coefficient values (Figure 9, Table 5) increase notably across all samples compared to lower-frequency ranges (2G–4G), indicating more pronounced attenuation of microwave penetration at higher frequencies. The most effective shielding is observed for TiO₂ R050 5 wt.% (5.408), TiO₂ A050 5 wt.% (5.725), and Sb₂O₃·SnO₂ 3 wt.% (5.624). These results are particularly relevant because the 5G band represents the frequency range currently most used in modern mobile communications.

Similarly to lower frequencies, the clay reference sample exhibits the highest absorption (1.857), while all composites display lower absorption values (0.220–0.735). This again confirms that attenuation in these systems arises predominantly from reflection and scattering processes rather than intrinsic absorption.

Overall, the combined analysis of transmission (S₂₁) and absorption (|A|) data across the 2G–5G frequency range demonstrates that the inclusion of nanomaterial additives modifies the interaction between electromagnetic radiation and the clay matrix. While most compositions show enhanced shielding performance, the dominant mechanism of attenuation appears to shift from absorption to reflection and scattering as the additive type and concentration vary. The results highlight the complex interplay between composition, microstructure, and frequency-dependent electromagnetic behavior in clay-based composites.

3.4. Discussion

Table 7 presents the previously published works in which the authors have used the same additives, but in different systems, and/or measurements have been performed differently.

Titanium dioxide, TiO_2, naturally occurs in three polymorphic forms: rutile, anatase, and brookite (Figure S3) [45].

Brookite has a rhombohedral structure and, because of its instability, is rarely used, while both rutile and anatase belong to the tetragonal crystal system. However, their lattice structures are different and that is the reason for some of their significant differences in properties, stability, and eventually in applications.

The unit cell of anatase contains four TiO₂ molecules; the crystal structure is more open (less compact), and consequently its density is lower (3.8–3.9 g cm⁻³) than that of rutile (4.2–4.3 g cm⁻³). Although stable at room temperature, anatase transforms (irreversibly and exothermally) at high temperature (approx. 750–800 °C) to the thermodynamically more stable rutile form. Each unit cell of rutile contains two TiO₂ molecules and their close-packed arrangement results in a higher density and greater stability when compared to anatase. Its significantly higher dielectric constant (114 vs. 48 for anatase) and higher electrical conductivity, which increases markedly at higher temperatures, makes a rutile potentially better choice in electronic applications [46]. The addition of conductive materials, especially with particles in the nanometer range, into generally low conductive base materials like clay or concrete, has been proven to be an effective way for increasing shielding properties against the penetration of electromagnetic radiation (including microwaves).

Most studies in the literature [47] investigate the addition of titanium dioxide (alone or in combination with other additives) to concrete. One such report involves the preparation of a composite aggregate by dispersing nanosized TiO₂ (80 wt.% anatase + 20 wt.% rutile) into clay, followed by calcination [48]. Measurements of the electromagnetic properties of such TiO₂/clay composites show a high relative dielectric constant. The authors then prepared concrete samples by using three different aggregates: natural (gravel + sand), haydite (granulated clay), and a TiO₂/clay composite. The results showed some. improvement in the absorption of electromagnetic radiation (8–18 GHz) for the samples with haydite (~7.5 dB), which is attributed to the highly porous material, the scattering of electromagnetic waves, and the resonance absorption in voids. The highest adsorption (10–14 dB) was exhibited for the samples with TiO₂/clay composite aggregate. The authors concluded that titanium dioxide, as a semiconductor characterized by its high relative dielectric constant, can be used for the absorption of microwave radiation.

One other study reports the results of microwave radiation (2–18 GHz) absorption on cement samples with the addition of 2.32 vol.% of rutile and anatase [49]. All the samples with either TiO₂ polymorph showed a significant increase in radiation absorption, especially at some specific frequencies (up to 242.97% at 16 GHz).

Our results show that the addition of either anatase (TiO₂ A050) or rutile (TiO₂ R050) into clay effectively decreases the penetration of microwave radiation in the frequency range of 1.5–4.0 GHz by increasing its absorption (Figure 10b–e). Minor differences, i.e., samples with rutile show higher absorption, can be attributed to its higher dielectric constant and higher electrical conductivity. On the other hand, similarities in behavior can be explained by the method used to prepare the composite disks: heat treatment at a temperature of 850 °C (firing) would cause phase transformation of at least a part of anatase into rutile.

Iron(III) oxide exists in two main crystalline modifications, namely the minerals hematite (α-Fe₂O₃) and maghemite (γ-Fe₂O₃) (Figure S4) [45]. Hematite is a weakly ferromagnetic mineral that crystallizes in the hexagonal crystal system and is one of the most common iron oxides found in nature. Maghemite, on the other hand, is a ferrimagnetic mineral at room temperature with a cubic crystal structure. Each unit cell contains 32 oxygen ions (O²⁻), 21$\frac{1}{3}$ iron (Fe³⁺) ions, and 2 vacancies. The cations are randomly distributed among 8 tetrahedral and 16 octahedral sites. Both hematite and maghemite nanoparticles exhibit superparamagnetic behavior at room temperature [50,51]. Due to the characteristic magnetic properties of iron(III) oxide, researchers have suggested that its nanoparticulate form could potentially be used for protection against electromagnetic radiation.

Sayyed and colleagues [52] prepared cement mortar samples (composed of cement, water, and sand) as a reference sample, along with five additional samples containing hematite in 5, 10, 15, 20, and 25 wt.%. They measured the absorption of radioactive radiation for three different radioisotopes (¹³⁷Cs, ⁶⁰Co, and ²⁴¹Am) and found that the addition of Fe₂O₃ had a positive influence on the reduction in radiation transmission. The mortar containing 25% Fe₂O₃ was the most effective in attenuating radiation. The study also concluded that sample thickness positively affects radiation attenuation.

Investigations into electromagnetic radiation absorption in composite materials made from clay and functional aggregates (Fe₂O₃, SiC, and Fe₃O₄) showed significant electromagnetic wave attenuation (up to −12.13 dB) in the frequency range of 8–18 GHz. These materials also demonstrated notable compressive strength [53].

The influence of Fe₂O₃ addition to cement mortar was further explored in detail by Ng and colleagues [54,55] by examining the transmission coefficient (S₂₁) for samples with thicknesses of 4 cm and 6 cm and Fe₂O₃ mass fractions of 2, 4, 6, 8, and 10 wt.%. Their results indicated that, within the frequency range of 3.4 to 3.6 GHz (5G), the highest increase in transmission losses (S₂₁) was observed in the sample containing 2 wt.% Fe₂O₃. The authors emphasized that their measurements were conducted in an open configuration (Antennas 1 and 2, along with the sample, were placed in an open space) and recommended waveguide-based measurements, which have been carried out in the present study.

Antimony(III) tin(IV) oxide (Sb₂O₃∙SnO₂) is a transparent conductive oxide widely employed in electronic and optoelectronic applications due to its combination of high electrical conductivity and excellent optical transparency in the visible spectral range. These properties render Sb₂O₃∙SnO₂ particularly suitable for integration into devices that require both optical and electrical functionalities, such as touchscreens, flat-panel displays, photovoltaic cells, and electrochromic devices [56,57,58,59,60,61].

This material is typically synthesized via a high-temperature solid-state reaction between antimony(III) oxide (Sb₂O₃) and tin(IV) oxide (SnO₂), often represented by the empirical formula Sb₂O₃∙SnO₂, most often by the acronym ATO [62].

The material is classified as a wide-bandgap semiconductor, characterized by a substantial energy gap between the valence and conduction bands, which enables efficient visible light transmission while maintaining conductive behavior. Its electrical and optical properties can be systematically tuned by adjusting the Sb–Sn ratio, enabling optimization for specific applications. In addition, Sb₂O₃∙SnO₂ exhibits notable chemical and thermal stability, as well as resistance to environmental degradation, further supporting its use in various operational environments.

Given its favorable properties, recent studies have investigated the potential of Sb₂O₃∙SnO₂ as a functional additive in ceramic and clay-based matrices for electromagnetic (EM) radiation shielding applications in the frequency range of 1.5–4.0 GHz. Clay samples containing 10 wt.% Sb₂O₃∙SnO₂ were prepared and analyzed in wet, dried, and sintered forms. The most significant result was observed at 2.82 GHz, where a maximum attenuation of the S₂₁ transmission coefficient reached −57.57 dB. These findings suggest that Sb₂O₃∙SnO₂ can significantly enhance the EM-shielding effectiveness of clay composites and may be considered a promising additive for developing multifunctional ceramic materials capable of attenuating non-ionizing electromagnetic radiation [9].

There is a notable lack of published literature concerning the incorporation of Sb₂O₃∙SnO₂ into cementitious or clay-based materials. However, based on manufacturer specifications indicating favorable electrical properties, Sb₂O₃∙SnO₂ has been considered a promising candidate for applications involving electromagnetic radiation shielding.

Zinc ferrite (ZnFe₂O₄) is a spinel-type ferrite in which Zn²⁺ ions preferentially occupy the tetrahedral (A) sites, while Fe³⁺ ions are located in the octahedral (B) sites of the spinel lattice (Figure S5) [45].

This material possesses a unique combination of magnetic and electrical properties arising from its chemical composition and spinel crystal structure. It is a ferrimagnetic material, exhibiting a net magnetic moment even in the absence of an external magnetic field. Owing to its favorable electrical conductivity and low magnetic permeability, zinc ferrite is widely considered to be suitable for high-frequency applications, including inductors, transformers, microwave devices, and magnetic recording media [63,64].

The functional properties of zinc ferrite can be finely tuned by varying the stoichiometry and synthesis parameters, enabling control over its magnetic, electrical, and structural characteristics to meet the demands of specific applications. Moreover, zinc ferrite combines the magnetic behavior typical of ferrites with the chemical stability and corrosion resistance associated with zinc-containing materials, making it attractive for a variety of electronic and electromagnetic applications [65,66,67,68].

Despite its potential, a review of the existing literature indicates that no systematic studies have been reported on the incorporation of zinc ferrite into cementitious materials for the purpose of electromagnetic radiation (EMR) shielding, particularly within the microwave frequency range of 1–6 GHz. This represents a significant gap and an opportunity for further investigation into the use of ZnFe₂O₄ as a functional additive in construction materials aimed at mitigating non-ionizing radiation.

Li et al. [69] investigated the effect of incorporating zinc ferrite into concrete mixtures and reported a notable improvement in electromagnetic wave absorption, with absorption values exceeding 6 dB. However, their measurements were conducted in the high-frequency range of 78.5–98.5 GHz, which limits their relevance for typical building and shielding applications.

To better understand the shielding performance of ferrite materials in more applicable frequency ranges, Zhang and Sun [16] developed two-layer cement-based composites incorporating Mn-Zn ferrite (Mn_0.5Zn_0.5 Fe₂O₄) as microwave absorbers. The design of the composites was based on impedance matching theory and the principles of electromagnetic wave propagation. Compared to single-layer structures, the two-layer cement panels demonstrated enhanced microwave absorption, with a reduction in reflectivity of 6–8 dB, and a maximum reflection loss of 15 dB achieved with a 30 wt.% ferrite content. Furthermore, the reflectivity remained below –10 dB across the frequency range of 11.4–18 GHz, indicating strong absorption performance.

These results confirm that the addition of ferrite to cementitious composites can effectively reduce microwave reflectivity, with higher ferrite content corresponding to greater attenuation. Such ferrite-filled composites exhibit strong potential as microwave-absorbing construction materials for electromagnetic shielding applications in buildings [70]. Based on the available literature, it can be concluded that the previously discussed materials: iron(III) oxide, titanium dioxide (anatase and rutile crystal modifications), zinc ferrite, and antimony(III) tin(IV) oxide demonstrate significant potential as functional additives in construction materials designed to enhance the electromagnetic shielding effectiveness of buildings.

Table 7. Comparison of electromagnetic shielding properties and measurement parameters for materials, taken from the literature and this study.

Base Material	Additive/Size/Content	EMR Measurements/Frequency Range	Conclusion	Ref.
- Concrete (Portland cement + aggregate + water)	- Nano TiO2 with clay in aggregate (30:70%). - 80% anatase and 20% rutile (aggregate).	- Electromagnetic wave reflection loss (reflectivity). - Frequency range: 8–18 GHz.	- Addition of nano TiO2 improves microwave absorbing ability of the concrete.	[48]
- Ordinary Portland cement	- Anatase (5, 10, 15 nm). - Rutile (20–30, 50, 500, 1500 nm). - Rutile + anatase (20–30 nm). - 2.32 vol.%.	- Electromagnetic wave reflection loss (reflectivity). - Frequency range: 2–18 GHz.	- Addition of TiO2 improves microwave absorbing ability (increases reflectivity) of the cement paste.	[49]
- Brick clay	- Anatase (50 nm). - Rutile (50 nm). - 3 and 5 wt.%.	- Electromagnetic spectra measurements in a closed system (waveguides): reflection and transmission parameters measured; absorption calculated. - Frequency range: 1.5–4.0 GHz.	- Addition of TiO2 improves significantly shielding properties.	Present work
- Cement mortar (cement + sand + water)	- Hematite Fe2O3. - 100 × 15 nm. - 5, 10, 15, and 25 wt.%.	- γ-rays attenuation (I/I0). - Energy: 0.06–1.333 MeV. - Frequency range: 1.45 × 1010–3.22 × 1011 GHz.	- Increasing the ratio of Fe2O3 nanoparticles can lead to a remarkable improvement in the gamma ray shielding.	[52]
- Cement mortar (cement + sand + water)	- Hematite (Fe2O3) 38 µm with clay in aggregate (10:90%). - 10 wt.%.	- Electromagnetic wave reflection loss (reflectivity). - Frequency range: 8–18 GHz. - Reflectivity less than −10 dB.	- Excellent absorption performance.	[53]
- Cement mortar (cement + sand + water)	- Micro-sized Fe2O3. - 2, 4, 6, 8, 10 wt.%.	- Two horn antennas in shielded chamber (open system). - Frequency range: 3.40–3.60 GHz. - Measurements of transmission coefficient S21.	- Propagation loss is increasing with the amount of Fe2O3.	[54,55]
- Brick clay	- Maghemite Fe2O3 (50 nm). - 3 and 5 wt.%.	- Electromagnetic spectra measurements in a closed system (waveguides): reflection and transmission parameters measured; absorption calculated. - Frequency range: 1.5–4.0 GHz.	- Conclusion: addition of maghemite improves shielding properties.	Present work
- Concrete (cement + aggregate + water)	- Ferrite (ZnFe2O4). - 12.92 and 24.10 wt.%.	- Wave radar reflectance measurer. - Frequencies: 78.5, 85.0, 92.5, and 98.5 GHz. - Measurements of absorption rate (dB). - Absorption rate between 8 and 12 dB.	- Absorption rate increases with increasing content of ferrite.	[69]
- Cement mortar (cement + sand + water)	- Mn-Zn ferrite. - 10 and 30 wt.%.	- Electromagnetic wave reflection loss (reflectivity). - Frequencies: 8–18 GHz. - Reflection loss because of EM absorption. - Higher ferrite content, lower reflection loss.	- Double-layer composites have excellent absorption properties.	[16]
- Brick clay	- ZnFe2O4 (46 nm). - 3 and 5 wt.%.	- Electromagnetic spectra measurements in a closed system (waveguides): reflection and transmission parameters measured; absorption calculated. - Frequency range: 1.5–4.0 GHz.	- Addition of ZnFe2O4 improves shielding properties.	Present work
- Brick clay	- Sb2O3∙SnO2 (ATO). - 5% wt.%.	- Electromagnetic spectra measurements in a closed system (waveguides): transmission parameter (S21) measured. - Frequencies: 1.5–6 GHz.	- The specimens with Sb2O3∙SnO2 at most frequencies result in the lowest transmission.	[9]
- Brick clay	- Sb2O3∙SnO2 (ATO). - 3 and 5 wt.%.	- Electromagnetic spectra measurements in a closed system (waveguides): reflection and transmission parameters measured; absorption calculated. - Frequency range: 1.5–4.0 GHz.	- Addition of Sb2O3∙SnO2 improves significantly shielding properties in lesser amounts.	Present work

Table 7. Comparison of electromagnetic shielding properties and measurement parameters for materials, taken from the literature and this study.

Base Material	Additive/Size/Content	EMR Measurements/Frequency Range	Conclusion	Ref.
- Concrete (Portland cement + aggregate + water)	- Nano TiO2 with clay in aggregate (30:70%). - 80% anatase and 20% rutile (aggregate).	- Electromagnetic wave reflection loss (reflectivity). - Frequency range: 8–18 GHz.	- Addition of nano TiO2 improves microwave absorbing ability of the concrete.	[48]
- Ordinary Portland cement	- Anatase (5, 10, 15 nm). - Rutile (20–30, 50, 500, 1500 nm). - Rutile + anatase (20–30 nm). - 2.32 vol.%.	- Electromagnetic wave reflection loss (reflectivity). - Frequency range: 2–18 GHz.	- Addition of TiO2 improves microwave absorbing ability (increases reflectivity) of the cement paste.	[49]
- Brick clay	- Anatase (50 nm). - Rutile (50 nm). - 3 and 5 wt.%.	- Electromagnetic spectra measurements in a closed system (waveguides): reflection and transmission parameters measured; absorption calculated. - Frequency range: 1.5–4.0 GHz.	- Addition of TiO2 improves significantly shielding properties.	Present work
- Cement mortar (cement + sand + water)	- Hematite Fe2O3. - 100 × 15 nm. - 5, 10, 15, and 25 wt.%.	- γ-rays attenuation (I/I0). - Energy: 0.06–1.333 MeV. - Frequency range: 1.45 × 1010–3.22 × 1011 GHz.	- Increasing the ratio of Fe2O3 nanoparticles can lead to a remarkable improvement in the gamma ray shielding.	[52]
- Cement mortar (cement + sand + water)	- Hematite (Fe2O3) 38 µm with clay in aggregate (10:90%). - 10 wt.%.	- Electromagnetic wave reflection loss (reflectivity). - Frequency range: 8–18 GHz. - Reflectivity less than −10 dB.	- Excellent absorption performance.	[53]
- Cement mortar (cement + sand + water)	- Micro-sized Fe2O3. - 2, 4, 6, 8, 10 wt.%.	- Two horn antennas in shielded chamber (open system). - Frequency range: 3.40–3.60 GHz. - Measurements of transmission coefficient S21.	- Propagation loss is increasing with the amount of Fe2O3.	[54,55]
- Brick clay	- Maghemite Fe2O3 (50 nm). - 3 and 5 wt.%.	- Electromagnetic spectra measurements in a closed system (waveguides): reflection and transmission parameters measured; absorption calculated. - Frequency range: 1.5–4.0 GHz.	- Conclusion: addition of maghemite improves shielding properties.	Present work
- Concrete (cement + aggregate + water)	- Ferrite (ZnFe2O4). - 12.92 and 24.10 wt.%.	- Wave radar reflectance measurer. - Frequencies: 78.5, 85.0, 92.5, and 98.5 GHz. - Measurements of absorption rate (dB). - Absorption rate between 8 and 12 dB.	- Absorption rate increases with increasing content of ferrite.	[69]
- Cement mortar (cement + sand + water)	- Mn-Zn ferrite. - 10 and 30 wt.%.	- Electromagnetic wave reflection loss (reflectivity). - Frequencies: 8–18 GHz. - Reflection loss because of EM absorption. - Higher ferrite content, lower reflection loss.	- Double-layer composites have excellent absorption properties.	[16]
- Brick clay	- ZnFe2O4 (46 nm). - 3 and 5 wt.%.	- Electromagnetic spectra measurements in a closed system (waveguides): reflection and transmission parameters measured; absorption calculated. - Frequency range: 1.5–4.0 GHz.	- Addition of ZnFe2O4 improves shielding properties.	Present work
- Brick clay	- Sb2O3∙SnO2 (ATO). - 5% wt.%.	- Electromagnetic spectra measurements in a closed system (waveguides): transmission parameter (S21) measured. - Frequencies: 1.5–6 GHz.	- The specimens with Sb2O3∙SnO2 at most frequencies result in the lowest transmission.	[9]
- Brick clay	- Sb2O3∙SnO2 (ATO). - 3 and 5 wt.%.	- Electromagnetic spectra measurements in a closed system (waveguides): reflection and transmission parameters measured; absorption calculated. - Frequency range: 1.5–4.0 GHz.	- Addition of Sb2O3∙SnO2 improves significantly shielding properties in lesser amounts.	Present work

4. Conclusions

Unlike most studies on electromagnetic radiation protection, which focus on concrete-based materials, this work investigates clay-based composites for microwave attenuation in the 1.5–4.0 GHz range, relevant to mobile telecommunications (2G to 5G). The clay matrix, rich in γ-Fe₂O₃, shows strong intrinsic absorption, especially at lower frequencies.

Spectral analysis reveals that nanomaterial additives modify the shielding behavior in a frequency-dependent manner. In the 2G range, all composites show improved attenuation (higher S₂₁ areas), but lower absorption than pure clay, indicating that reflection and impedance mismatch are the dominant mechanisms. This trend continues in the 3G and 4G ranges, with reduced transmission but limited absorption. In the 5G range, TiO₂ (rutile and anatase, 5 wt.%) and Sb₂O₃·SnO₂ (3 wt.%) demonstrate the most effective shielding.

Overall, additive concentration alone does not guarantee better performance. Shielding efficiency depends on additive type, dispersion, and microstructure. These findings confirm that clay composites with all investigated additives are promising candidates for EMI shielding, offering a sustainable alternative for modern communication technologies.