1 Introduction

Volcanic ashes are non-generic expressions that represent the fine crumbs of pyroclastic materials [1]. Regular volcanic ashes are pyroclastic wreckages, which are less than 2 mm [2]. They have fairly low reactivity in general; however, they have variable chemical, mineralogical, and physical properties [3]. Their physicochemical properties can play a crucial role in the assessment of their technological applications [4]. Volcanic ashes consist of many oxides; some oxides can be found as major components such as Al2O3, SiO2, Fe2O3, CaO, MgO, Na2O, and K2O in addition to other oxides in minor percentages [5,6,7]. The minerals of the volcanic ashes grow within the magma under the earth’s surface. Therefore, the mineralogical compositions depend on both the eruption conditions and magma chemistry [1, 4]. They have widely varying physical properties such as structural, thermal, and optical properties. They have different degrees of crystallinities and vary from entirely amorphous to almost crystalline material [1, 4]. Based on their chemical and mineral compositions, their color can vary from black to poorly red [8].

Many of the volcanic ashes have been used as ceramic materials [4, 9], cement and concrete [10, 11], and geopolymer materials [3]. Many studies showed that the volcanic ashes in Saudi Arabia are excellent supplementary cementitious materials for about 30% replacement of ordinary Portland cement [10,11,12]. These volcanic ashes were produced by active volcanoes in the region of the Arabian Gulf; therefore, there are abundances of volcanic materials that could be used in many applications [12], which could be an economical source.

Scoria is one of the volcanic ashes that commonly used in applications due to its morphology and microstructure [8, 13,14,15]. Scoria has been utilized for hundreds of years in worldwide as a building material [16]. Many researchers investigated the effect of scoria as alternative materials for cement [17, 18], thermal activation [19], and the fabrication of lightweight concrete [20]. It was reported that scoria could act as a friendly, economical, and ecological building material [21]. It has promising applications, and there is a global abundance of different unexploited sediments [10,11,12,13]. Nevertheless, the studies and research on the volcanic ash generally and scoria particularly are still limited in comparison to the fly ash that has been subjected to many studies [22,23,24,25,26,27].

To the best of our knowledge, the effect of heat treatments on structural, electrical conductivity, dielectric constant, and dielectric loss of scoria has not been studied yet. Therefore, this work aimed to investigate the effect of heat treatment on the structural, electrical, and dielectric properties of scoria. For this purpose, the thermal properties of raw scoria were explored using thermogravimetric analysis as well as differential scanning calorimetry analysis (TGA–DSC). According to the thermal results, the raw scoria was thermally treated at different temperatures. The structural phases and morphological features were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively, while the mineralogical characterizations were studied using X-ray fluorescence spectrometry (XRF). In addition, chemical functional groups were explored using Raman/Fourier-transform infrared (FTIR) spectroscopy. The electrical and dielectric properties of raw and treated scoria were investigated at temperature range up to 1100 K at different frequencies range, starting from 500 Hz up to 4 MHz.

2 Materials and methods

2.1 Collection and processing the samples

The samples of scoria were collected from Harrat Rahat located in the western region of Saudi Arabia. Harrat Rahat is a fertile region with numerous pyroclastic scoria [28,29,30]. The raw material of scoria has particle sizes ranging between 0.01 and 32 mm and poor red color. The sample was ground to normal powder using a crusher unit, then smoothened using a smoothing machine Pulverizers agate (ROCKLABS, New Zealand) to attain a fine powder. The thermal properties were investigated in static air to estimate the heat treatment temperatures of scoria. The thermal measurement was performed with a heating rate of 5 °C/min at the temperature range starting from room temperature up to 1300 °C using TGA–DSC (NETZSCH, Germany). According to the thermal analysis results, three samples of scoria were tested; one was as raw, and the other two were thermally treated in a muffle furnace at temperatures of 800 and 1000 °C with a heating rate of 5 °C/min for 1 h. Finally, the samples were named as Raw, 800, and 1000.

2.2 Characterization techniques

The chemical analyses of the samples were carried out using XRF with SPECTRO XEPOS (AMETEK Inc., USA) to determine the oxides elements (wt%). For XRF analysis, Lithium Borate (0.5% LiBr, 32.83% LiBO2, and 66.67% Li2B4O7) is required as most oxides are best dissolved in lithium borate salts. One gram of scoria powder was added in the platinum crucible and mixed homogenously with 8 g of Lithium Borate then transferred to the fusion system for 20 min till the mixture converted to glass disk. The crystalline phases were determined by XRD with a Bruker D8 Advance XRD (Bruker, USA), Cu Kα Ni-filtered radiation with a wavelength of 1.54 Å. The samples were scanned in an angular range (2θ) from 20° to 70°. Cards of the International Centre for Diffraction Data (ICDD) were utilized to identify the phases of the samples. The morphologies of the samples were scanned by SEM model JEOL 7600f (JEOL, Japan). The FTIR spectra of the samples were recorded using Nicolet (iS50 FTIR) FTIR spectrometer (Thermo Fisher Scientific, USA). Raman spectra of the scoria samples were recorded by Micro Raman microscope model DXR (Thermo Fisher Scientific, USA) equipped with diode-pumped solid-state (DPSS) laser 532 nm for sample’s excitation. Raman system has a high-precision motorized stage in X and Y dimensions with a software-controlled Z focus. A 50X objective was used to collect the back-scattered radiation at 10 mW excitation laser power.

2.3 Electrical and dielectric measurements

The scoria samples were prepared as pellets to measure electrical and dielectric properties. Pellets were fabricated using a manual hydraulic press with a diameter and thickness of 13 and 2 mm, respectively. For fabrication of the pellets, the powders of the samples were mixed with 3.5 wt% of polyvinyl alcohol (PVA) as a binder. More details about the fabrication procedure can be found in the published works [31,32,33]. The pellets were heated at 800 °C for 2 h in air atmosphere. Both two faces of the pellets were polished to make perfect electrical contact. Then, they were coated by a high-performance silver paste (TED PELLA INC, USA) to form electrodes. After that, the pellets of the scoria samples were dried at 150 °C for 1 h. HIOKI LCR bridge as an impedance analyzer model Hioki 3532-50 LCR Hi-Tester (Hioki, Japan) was utilized to carry out the ac electrical and dielectric measurements. The measurements were performed using constant voltage (CV) mode at different frequencies starting from 500 Hz up to 4 MHz, and temperatures range up to 1100 K, with a heating rate of 2 °C/min. Through an RS-232C serial port, the measurements were taken digitally at a regular temperature step of 10 °C.

3 Results and discussion

3.1 Structural properties

The chemical compositions of the raw scoria were determined by XRF for both major and minor elements (expressed as % oxides). The raw scoria contains SiO2, Al2O3, CaO, and Fe2O3 as major elements and K2O, MgO, MnO, Na2O, P2O5, TiO2, and SO3 as minor elements (Table 1).

Table 1 Chemical compositions (wt.%) of the collected raw scoria used in this study
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The structural and mineral properties of raw scoria were investigated using XRD. Figure 1 shows the XRD pattern of raw scoria that was obtained from Harrat Rahat in the western region of Saudi Arabia. As can be seen, the XRD pattern of raw scoria indicates the presence of a high degree of crystallinity and exhibits different peaks with 2θ in the range between 20° and 70° that is characteristic of various minerals present in raw scoria. Based on the literature [34,35,36,37,38] and ICCD cards No; 00-001-1053, 01-070-6267, 00-003-0451, and 00-023-1406, four phases were found, and referred to albite (NaAlSi3O8), pargasite (NaCa2Mg4Al3Si6O22(OH)2), hematite (α-Fe2O3), and ilmenite (FeTiO3). These phases were indexed to their peaks, as shown in Fig. 1. The major minerals composition of scoria is albite and pargasite, and the minor minerals are ilmenite and hematite. Albite, pargasite, and hematite are insulators, while ilmenite is a conductive mineral [39, 40]. The structural refinements and mineral quantifications of raw scoria were carried out by applying the Rietveld analysis to the XRD pattern. The diffraction peaks were shaped using a B-spline function. The space group and lattice parameters of every phase were calculated and found in good agreements with PDF files. Using whole powder pattern fitting (WPPF) and based on the full width at half maximum (FWHM), the mean crystallite sizes were calculated using Scherrer’s equation:

$$D = \frac{{0.94\lambda }}{{\beta _{{1/2}} \cos \theta }}$$
(1)

where β1/2 is FWHM of the diffraction band, θ is the angle of diffraction, D is the crystalline size, and λ is the X-ray wavelength. The weighted percentages of crystalline phases were calculated only from the crystalline content of the sample. The results obtained by XRD analysis are listed in Table 2.

Fig. 1
figure 1

XRD pattern of raw scoria obtained from Harrat Rahat in the western region of Saudi Arabia

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Table 2 Mineral phases present in scoria with their quantifications, lattice parameters, crystallite sizes, and space groups obtained by XRD analysis
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The thermal treatment is an important factor. It leads to many structural transformations such as decomposition, oxidation, hydroxyl migration, hydroxylation, surface reconstruction, and formation of new phases [41,42,43,44]. Therefore, thermal analyses were conducted on the powder of raw scoria. Figure 2 shows TGA–DSC curves that were recorded in static air with a heating rate of 5 °C/min. It is obvious from TGA that the scoria is almost thermally stable up to 1300 °C, with a weight loss of about 1.5%. The loss in the weight could be attributed to the trapped gases that release and come out of scoria particles during thermal heating. DSC exhibited a strong endothermal peak at a temperature of 1200 °C. It could be attributed to densification–vitrification with fusion and structural breakdown of scoria mineral compositions [9, 44]. From DSC analysis, scoria seemed to be thermally stable up to 1000 °C. However, there is a very weak endothermic peak at around 966 °C with an onset temperature of 820 °C, the expansion of this peak is shown in the inset of Fig. 2. From the mineral compositions obtained by XRD for raw scoria, hematite and ilmenite are two mineral phases, which can form a solid solution [45, 46]. Therefore, this weak endothermic peak could be attributed to the formation of the solid solution of (hematite–ilmenite).

Fig. 2
figure 2

Thermal analysis of the raw scoria powder (black line) DSC curve and (blue line) TGA. The inset shows the endothermic peak at 966 °C

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According to the thermal analysis results, two samples of raw scoria were then thermally treated at temperatures of 800 °C and 1000 °C in static air. Therefore, three samples of scoria (Raw, 800, and 1000 scoria samples) will then be studied by XRD, SEM, and FTIR/Raman spectroscopies. In addition, their electrical and dielectric properties will be investigated.

Figure 3 shows XRD patterns of Raw, 800, and 1000 scoria samples. The XRD pattern for the powder of sample 800 was found to be identical to the Raw sample with no change in the peak positions, peak intensities, or crystal structure. The only eminent variation is the XRD pattern for the powder of sample 1000 exhibited a significant decrease in the all XRD peaks intensities with no change in the peak positions or crystal structure. The crystallite sizes were found to be somewhat increased with increasing the thermal temperature. The calculated mean crystallite sizes for all samples are listed in Table 3. The reduction in the XRD patterns at sample 1000 could be most likely due to the effect of trapped gases, which come out from the lattice and could have generated defects in the lattice, or from the impurities that might happen because of interference between the atoms of mineral phases (i.e., hematite, ilmenite, albite, and pargasite), which drive the impurities in each phase. Besides, the dissolution of crystalline phases could happen due to the high temperature (1000 °C).

Fig. 3
figure 3

XRD patterns of Raw, 800, and 1000 scoria samples

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Table 3 Crystallite sizes of mineral phases present in Raw, 800 and 1000 scoria samples
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Figure 4a–d displays SEM micrographs of raw scoria before grinding, raw scoria after grinding, 800, and 1000, respectively. As can be seen in Fig. 4a, morphological properties of raw scoria sample before grinding showed no specific shapes; it exhibited numerous particles with huge aggregate and mixture of nano/microsized, which intercalated with each other to form bulk scoria. Raw scoria sample after grinding showed similar morphologies only with small aggregate and some fragments with nanosized (Fig. 4b), which resulted from the grinding process. Sample 800 that was thermally treated at a temperature of 800 °C showed that the particles and fragments started to accumulate again (Fig. 4c), while the 1000 sample that was treated at temperatures of 1000 °C exhibited a dense matrix without clear particles’ boundaries (Fig. 4d). This can be understood from the formation of (hematite–ilmenite) solid solution that could weld between particles of all mineral compositions.

Fig. 4
figure 4

Scoria SEM images of a Raw before grinding b Raw after grinding c 800 d 1000

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Figure 5 displays the FTIR analysis of Raw, 800, and 1000 scoria samples. A broad and intense absorption peak in the range between 1250 and 800 cm−1 can be noticed in all samples, this peak centered at 996 cm−1. In addition, two shoulder peaks can be distinguished at 1125 and 1025 cm−1. These peak and shoulder peaks were attributed to the asymmetric stretching vibration of Si‒O‒T (T: Si or Al) bonds [6, 7, 43, 47,48,49] in the mineral phases of albite and pargasite. FTIR spectra for all scoria samples exhibited nearly the same peaks without significant shifting (Fig. 5). The intensities of peaks in the FTIR spectrum of sample 1000 get decreased comparing with Raw and 800 scoria samples, this is the only difference between the FTIR spectra of scoria samples. This decrease may result from the change in the structure of Si‒O‒T (T: Si or Al), where heat treatment and formation of (hematite–ilmenite) solid solution affected the molecular structure of mineral phases albite and pargasite. FTIR results are in good agreement with that obtained by XRD.

Fig. 5
figure 5

FTIR spectra of Raw, 800, and 1000 scoria samples

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Raman spectra of Raw, 800, and 1000 scoria samples are shown in Fig. 6. As can be seen, Raman spectra exhibit different peaks. The strongest peak centered at 1321 cm−1 characterizes the hematite mineral phase [50, 51], the peak located at 672 cm−1 was attributed to the ilmenite and pargasite mineral phases [51, 52], while the shoulder peak situated at around 602 cm−1 was referred to as the hematite mineral phase [50, 51]. The peak in the range between 540 and 460 cm−1 referred to albite mineral phase [53, 54], the peak centered at 402 cm−1 was attributed to the hematite mineral phase [50, 51], while the peaks centered at 288 and 223 cm−1 were attributed to hematite/albite [50, 51, 54] and hematite/pargasite mineral phases [51], respectively. Finally, the shoulder peak at 161 cm−1 was referred to albite mineral phase [53, 54]. As can be seen in Fig. 6, all tested samples have the same peaks nearly without significant shifting; the reduction in the intensities of the Raman peaks of the 1000 sample is the only noticeable difference comparing with the Raw and 800 scoria samples. This result is in a full agreement with those obtained by XRD and FTIR. It can be noticed the peaks that belong to either hematite or ilmenite get decreased more comparing with the peaks that belong only to either albite or pargasite.

Fig. 6
figure 6

Raman spectra of Raw, 800, and 1000 scoria samples

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3.2 Electrical and dielectric properties

The electrical and dielectric properties of Earth's materials such as scoria is affected by several factors, including (i) the major element composition (mineral phases), (ii) temperature, (iii) porosity, (iv) grain boundaries, (v) impurities, (vi) the water content and the degree of partial melting. Alternating current conductivity (σac) of Raw, 800, and 1000 scoria samples were investigated in temperatures starting from room temperature up to 1100 K at frequencies of 500 Hz, 1 kHz, 2 kHz, 5 kHz, 10 kHz, 20 kHz, 50 kHz, 100 kHz, 200 kHz, 500 kHz, 1 MHz, 2 MHz, and 4 MHz. The ac conductance (G) of every sample pellet was measured; then, σac was calculated using the following relations [55, 56]:

$$\sigma _{{{\text{ac}}}} = G{\mkern 1mu} \frac{d}{A}$$
(2)

where d and A are the thickness and the cross-sectional area of sample, respectively. Figure 7 shows the ln σac as a function of 1000/T at different applied frequencies for all scoria samples. As can be seen in this figure, all scoria samples (Raw, 800, and 1000 samples) showed nearly the same electrical behavior. Two distinct regions can be identified in the conductivity spectra of scoria. The first region seems to be stable in which the electrical conductivity is almost without any changes. The scoria samples showed stable low electrical conductivities, although the temperatures were raised to nearly 700 K, so it can be said they were electrically insulators. Scoria is composed mostly of silicate minerals (albite, pargasite), which are essentially insulators, with a minor of ilmenite and hematite minerals. Therefore, the samples were of low electrical conductivity. This can take place when thermal energies are not enough to activate the hopping of carriers and ions [33]. The second region is at a higher temperature range, and this region started from a temperature of 700 K till 1100 K. The electrical conductivities in this region increased linearly with temperature.

Fig. 7
figure 7

Electric conductivity of Raw, 800, and 1000 scoria samples, ln σac versus reciprocal of absolute temperature at different applied frequencies

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As a rule, the increase in the electrical conductivity with increasing temperature pointed out the semiconducting properties of the material [57]. Therefore, at higher temperatures, all scoria samples showed semiconductor behaviors. This can be understood from that the thermal energies at higher temperatures become enough to induce more defects of oxygen vacancies [58, 59], which drive to increase the electrical conductivities. Subsequently, the conductivities are dominated by the process of thermal activation [60, 61]. Figure 8 shows the curves of the ln σac as a function of applied frequency at different temperatures for Raw, 800, and 1000 scoria samples that exhibited gradual increases with frequencies at low temperatures. Thus, the conductivities of all scoria samples seem to be frequency-dependent, suggesting that the mechanism of conduction could happen through hopping of the charge carriers at lower temperatures [62,63,64]. Here, the frequency works as a force that eases the charge carriers to transfer between different localized states and liberate the trapped charges from their trapping centers [32]. However, at a higher temperature region, the curves of the ln σac are linear. Thus, the conductivities of all scoria samples seem to be frequency-independent, where the large vibrations of the lattice can scatter the charge carriers and abolish the effect of frequency [32]. The clear frequency independence at higher temperatures suggests that the mechanism of conduction becomes polarons conduction [62].

Fig. 8
figure 8

ln σac as a function of applied frequency at different temperatures for Raw, 800, and 1000 scoria samples

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The polarization of any materials is shared out by four different polarization mechanisms viz. electronic polarization, ionic polarization, space charge polarization, and orientation polarization [65]. The complex dielectric permittivity (ε*) of the materials is suitable for understanding the polarization of materials. The complex dielectric permittivity is given by the relation [60, 66, 67]

$$\varepsilon {\text{*}} = \varepsilon ^{\prime} - j\varepsilon ^{\prime\prime}$$
(3)

where the real part (ε′) and the imaginary part (ε″) of the complex dielectric permittivity are called dielectric constant and dielectric loss, respectively. The dielectric constant points out the storage of energy or the polarizing ability of the material, whereas the dielectric loss points out the loss of energy in a material. The ratio between dielectric constant and dielectric loss is called dielectric loss tangent, tan δ = ε″/ε′. Both dielectric constant and dielectric loss can be, respectively, calculated by the following equations [65, 67, 68]:

$${\mkern 1mu} \varepsilon ^{\prime} = \frac{{Cd}}{{\varepsilon _{o} A}}$$
(4)
$${\mkern 1mu} \varepsilon ^{\prime\prime} = \varepsilon ^{\prime}\tan \delta$$
(5)

where ε˳ is the free space permittivity, and C is the measured capacitance of the sample. Figures 9 and 10 display the dielectric constant and dielectric loss, respectively, versus absolute temperatures at different applied frequencies for Raw, 800, and 1000 scoria samples. It can be seen two behaviors, the first behavior started from room temperature up nearly to 700 K in which the curves of the dielectric constant and dielectric loss showed no alterations with changes the temperature at all applied frequencies. In this range of temperature, dielectric constant and dielectric loss appeared to be temperature-independent and frequency-independent. This points out that the dielectric properties of scoria samples have excellent thermal stability in these temperature and frequency ranges. The second behavior started from 700 up to 1100 K in which the curves of the dielectric constant and dielectric loss increased, particularly at low frequencies. In this region, the dielectric constant and dielectric loss seemed to be temperature-dependent; however, they decreased as the frequencies increased. The dielectric constants and dielectric loss appeared to be lower at lower temperatures suggesting the contribution of electronic and ionic components to all polarizabilities could be constant and small at lower temperatures [69]. However, at higher temperatures and lower frequencies, the rise of dielectric constant, suggesting the orientation of dipoles, might become easier [57, 69]. In addition, the increase in the temperature seems to be a pumping force that can liberate the trapped charge carriers and subsequently increase the polarization [57]. However, at higher temperatures, the decrease of dielectric constant with increases the frequencies indicating all the dipoles could not follow well the fast-applied field (higher frequencies) to reorient themselves quickly that finally leads to reduce the dielectric constants [33, 62]. As can be seen in Fig. 9, the dielectric constants of 800 and 1000 scoria samples somewhat get increased, indicating the thermal treatment could enhance the polarization of scoria.

Fig. 9
figure 9

Dielectric constant (ε′) as a function of absolute temperature at different applied frequencies for Raw, 800, and 1000 scoria samples

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Fig. 10
figure 10

Dielectric loss (ε″) as a function of absolute temperature at different applied frequencies for Raw, 800, and 1000 scoria samples

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For all scoria samples, Fig. 11 displays dielectric loss tangent curves versus absolute temperatures at different applied frequencies. The curves showed stable behavior up to the temperature of 700 K, then an increase in the dielectric loss tangent with increasing temperature. However, at the higher temperature, the dielectric loss tangent decreased as frequency increased. This can be understood from the dielectric constant that showed the same features, and it was reported [68] when the dielectric constants show an increase (the energy storage abilities increase), also dielectric losses show an increase (energy dissipations increase). From curves of dielectric loss tangent, relaxation temperature can be noticed at around of 953 K (680 °C) at all applied frequencies for only Raw and 800 scoria samples. This relaxation temperature became very weak in sample 1000. This relaxation temperature is a hematite Néel temperature [70, 71] in which the hematite goes from canted antiferromagnet (weakly ferromagnetic) to paramagnetic phase transition. This transition appeared very weak in the 1000 scoria sample that could be because of the formation hematite–ilmenite solid solution that might affect the structural properties, as found in the results of XRD and Raman spectroscopy.

Fig. 11
figure 11

Dielectric loss tangent (tan δ) as a function of absolute temperature at different applied frequencies for Raw, 800, and 1000 scoria samples

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4 Conclusion

Scoria volcanic ash whose minor minerals include hematite and ilmenite has been investigated in this study. No significant changes have been detected in the structural properties of the scoria when treated with a temperature of 800 °C. However, when treating the sample to a temperature of 1000 °C, a decrease in the intensities of XRD and Raman spectrum peaks were observed, and this was attributed to the formation of the hematite–ilmenite solid solution. As for electrical measurements, all scoria samples were electrically and dielectrically stable. They showed stable and low values of conductivity and dielectric constant up to a temperature of 700 K. Therefore, it can be concluded that this type of scoria is suitable for electrical insulation applications.