Characterisation of the Electrical Properties of Wastes Vitrified from Canarian Island Basaltic Quarries: Original Glasses and Glass-Ceramics

Abstract

We obtained original glasses and glass-ceramics through the controlled melting and recrystallisation of basalt rocks extracted from several quarries in the Canary Islands. The electrical measurements of the resulting glasses and glass-ceramics were conducted in a complex impedance at temperatures in the 250–700 °C range. These electrical determinations made it possible to follow the nucleation and crystal growth processes. The main crystalline phases were pyroxenes, feldspar (anorthite) and magnetite, which decorate the dendritic crystallisation of pyroxenes. The magnetite is present as nanocrystals, being the component chiefly responsible for the electrical conduction properties of these glass-ceramics. Electrical conduction is facilitated by the presence of magnetite nanocrystals on the axes of dendrites of pyroxene crystals, enabling polar electron conduction in these materials. Thus, the Fe²⁺/Fe³⁺ ratio was related to the total Fe²⁺/Fe, which made it possible to express an electronic conduction model.

1. Introduction

Since the last decades of the 20th century, much research has been carried out to produce glass-ceramics from waste and even from natural rocks such as basalts [1,2]. The nucleation and crystal growth mechanisms for producing these types of glass-ceramics from original glasses after the controlled melting of basalt rocks have also been widely considered [3]. In recent years, end-use requirements have dictated the need for special coating materials with much superior properties than conventional enamels. These coatings provide protection against high temperatures and corrosive atmospheres and are commonly called glass-ceramic coatings. These coatings are polycrystalline solids prepared by controlled crystallisation of glass. The homogeneity of parent glass together with controlled conditions under which the crystals are developed, results in materials having a very fine-grained uniform structure free from porosity. This helps in developing high mechanical strength and good electrical insulation properties. Glass and glass-ceramic coatings are versatile materials for industrial and engineering applications. More recently, this research has sparked interest in Spain due to the existence of basalt exploitation quarries in the Iberian Peninsula and also in the Canary Islands for the manufacture, not only of glass-ceramics, but also of aggregates for cement-based construction materials [3,4,5].

Although the main technological applications for these basalt glass-ceramics were as abrasion-resistant tiles and glazes for pavements and pipes [2,6,7], their composition and microstructure were also considered an interesting matrix for immobilising nuclear waste [8,9]. They were also based on glasses with compositions in the complex general system Na₂O-K₂O-CaO-MgO-(FeO + Fe₂O₃)-Al₂O₃-SiO₂, which includes small contents of TiO₂, Cr₂O₃, MnO, P₂O₅ … [10]. Several researchers had previously considered mechanical properties [11,12], but few studies have been conducted on the electrical properties of these materials due to the higher content of iron oxides and the presence of magnetite as nanocrystals in the glass-ceramic microstructure.

The characterisation of their electrical properties can be of great interest. From an electrical behaviour point of view, basalt glass-ceramics can be considered as silico-aluminate materials with high iron oxide content (>15% of their weight). Although these types of materials are essentially electrical isolators due to the high proportion of alkaline metals, they give rise to important mobile carrier species (ionic or electronic). This is also due to Fe²⁺ → e⁻ → Fe³⁺.

The Electrical Impedance Spectroscopy Analysis (EISA) is, therefore, an adequate method for deducing the polarisation mechanism and controlling the nucleation and crystal growth of the original basalt glasses.

This paper presents the possibility of transforming basalt, which is a natural, abundant and low-cost raw material, into a glass-ceramic material with high added value, excellent properties and numerous applications. Therefore, the main objective of this research is to characterise an experimental series of new basalt glass-ceramics obtained from different types of waste of basalt rocks from the Canary Islands.

2. Materials and Methods

Glasses (named VO and VR) were melted from several selected Canary Island rocks at 1450 °C for 30 min in an oxidant (ambient) and reducing atmosphere, respectively, as shown in

Table 1. Analytical composition of samples obtained in a laboratory furnace and analysed by X-Ray Fluorescence (XRF).

GLASS	VO-HI06	VR-HI06	VO-LG16	VR-LG16	VO-TF2	VR-TF2	VO-FT1	VR-FT1
SiO2	46.01	45.62	50.28	50.17	45.81	46.05	47.54	46.45
Al2O3	17.04	16.34	18.29	17.69	16.91	16.95	15.75	12.52
Fe2O3	8.99	1.40	7.33	1.28	8.53	2.37	8.79	1.40
FeO	2.85	9.91	2.53	8.19	3.25	8.76	3.32	11.67
MnO	0.17	0.19	0.22	0.22	0.20	0.20	0.14	0.15
MgO	4.75	4.76	3.18	3.13	4.75	4.72	8.94	10.20
CaO	9.55	10.09	8.14	8.51	10.08	10.17	9.51	10.94
Na2O	3.61	4.31	4.27	5.09	4.23	4.51	1.77	2.05
K2O	1.84	1.98	1.96	1.99	1.43	1.46	0.78	0.64
TiO2	3.77	3.86	2.53	2.57	3.48	3.51	2.90	3.43
P2O5	1.07	1.10	1.00	1.03	1.02	1.03	0.40	0.44
CO2	0.12	0.14	0.11	0.11	0.11	0.08	0.12	0.08
SO3	0.04	0.04	0.01	0.02	0.02	0.01	0.02	0.01
LOI	0.18	0.26	0.13	-	0.17	0.14	-	0.02
TOTAL	100.00	100.00	100.00	100.00	100.00	100.00	100.00	100.00
IOX *	2.84	0.13	2.61	0.14	2.36	0.24	2.38	0.11
Fe2O3 #	3.11	3.17	4.84	4.96	5.06	5.03	3.41	3.91
FeO #	8.10	8.27	4.74	4.85	6.34	6.31	8.18	9.38

VO-series: Glasses obtained by melting original alkali basalt in an electrical furnace. VR-series: Glasses obtained by melting in a propane furnace. LOI: Losses of ignition. IOX *: Oxidation index calculated from chemical analytical data. Fe₂O₃ # and FeO #: The original weight % in iron oxides if calculated considering the same Oxidation Index as the corresponding original alkali basalt.

. For the XRD mineralogical analysis, we used the Scherrer powder method on milled samples of the basalt wastes after selecting representative samples of crackled basalt rocks. The average particle size (d50) of the resulting powder was around 10 µm. The equipment used was a Siemens diffractometer, the D5000 model (Munich, Germany) with Cu Kα working at 50 kV and 30 mA, equipped with the Diffract-AT software [3]. We conducted the chemical characterisation of the glass samples by XRF with a PW2400 X-ray spectrometer (Eindhoven, the Netherlands).

In the case of electron microscopy observations for SEM (Scanning Electron Microscopy), we used a ZEISS digitalised DSM-950 (New York, NY, USA) working at 35 kV and attached an analytical EDS TRACOR northern ZX-II system over embedded epoxi-araldite and polished samples of original glasses and the resulting glass-ceramics. Although the samples for SEM exhibit good conduction due to their high iron content, gold was sputtered on them using EMSCOPE equipment. The Transmission Electron Microscopy (TEM) observations were carried out using a TEM Hitachi-8000 (Tokyo, Japan) working at 200 kV with a Kevex EDS spectrometer and a Be window. In the case of TEM observations, the samples were previously prepared as thin foils by Ar ion thinning with a Gatan Dual-Ion Milling working at 6 kV, 20 mA and a 150° inclination angle [3,5] using previous mechanical thinned samples (SiC successive grade grain abrasive papers) of the glasses and glass-ceramics. Differential Thermal Analyses (DTA) were carried out with Mettler model TA2 (Toledo, Spain) equipment in a dry air atmosphere (7 L/h) and with a 10 °C/min heating speed until reaching 1200 °C and a 200 mV scale sensibility. The DTA exo-bands or peaks made it possible to determine the optimal zone for TTT (Temperature-Time-Transformation) treatments, as explained in the following paragraph. Then, as stated in previous publications [3,5], the sequential thermal treatments at several time-temperature one-step cycles with quenching allowed us to determine the respective TTT curves for each original basalt glass.

With this experimental information, we selected the optimal samples for the SEM and TEM microstructure analyses and for the conductivity measurements investigated therein. Prismatic samples of these basalt glasses and basalt glass-ceramics were machined and covered with evaporated gold as electrodes for impedance measurements to be observed under scanning electron microscopy and to obtain EISA (Electrical Impedance Spectroscopy Analysis) spectra. The electrical impedance measurements were carried out by means of a bridge system (HP-4192 A, Cambridge, UK) in the frequency range of 5 Hz to 2 MHz following the protocols proposed in the tutorial on electrochemical impedance spectroscopy at temperatures in the 25–70 °C range [10]. From the complex impedance spectra, the total conductivity values were determined. These values, σ (S cm⁻¹), have been represented in an Arrhenius graph, log σ (S cm⁻¹) versus 1/T (103 K⁻¹). All the conductivities for these materials fit well with almost perfectly straight lines, without any dispersion of results, which is typical of a thermally activated process according to the expression: σ = A.e − Ea/kT. Thus, with this Arrhenius representation from the respective slopes, it was possible to determine the Ea, each activation energy of the conduction process for glasses obtained in oxidant and reducing atmosphere.

3. Results

VO glasses were obtained in oxidant conditions and VR glasses in reduction conditions (Table 1). This table has been rewritten and reorganised including the chemical analysis recorded in the [5] reference. At very low temperatures, growth will not be favoured, and at very high temperatures, growth will also be slow because the system finds it difficult to dissipate the heat released by the crystallisation. It is a known fact that the nucleation and crystalline growth curves have a peak at which their rate is the highest and growth will be favoured, as heat will dissipate more easily at these temperatures. For this reason, after the vitrification of these basalt wastes, the original glasses were subjected to several cycles of thermal treatments to achieve crystallisation microstructures, specifically, at 600 °C for 2 h for the nucleation, followed by a thermal treatment at 800 °C during 16 h for the crystal growth of the final microstructure as glass-ceramics.

The role of Fe₂O₃ in crystal nucleation is attributed to the Fe³⁺ grouping in the glass structure, which, upon heating between 650 and 800 °C, reacts with the oxygen and forms magnetite. This mineral is the first phase to crystallise and acts as a nucleating agent to start the crystallisation. The original basalt glasses and glass-ceramics obtained in this study have shown high electrical resistance. The EISA spectra have been obtained at high temperatures (400–650 °C) as indicated in

Table 2. Conductivity results and estimated parameters for basalt glass samples.

Melting Atmosphere	Sample	T (°C)	Equivalent Circuit	Estimated Parameters
OXIDANT	VO-HI06	500°	(RQ)	Yo = 2.91 × 10−4; n = 0.74
	VO-TF2	650°	(RQ)(RW)	Yo = 2.43 × 10−8; n = 0.56 Yo (W) = 6.04 × 10−5
	VO-LG16	650°	R(RQ)W	Yo = 5.48 × 10−5; n = 0.30 Yo (W) = 6.62 × 10−5
REDUCING	VR-LG16	550°	R(RQ)(RQ)W	Yo = 7.85 × 10−7; n = 0.81 Yo = 5.10 × 10−5; n = 0.76 Yo (W) = 3.35 × 10−4
REDUCING	GCR-HI06	350°	Circuit Type R(RQ)W	R = 1.72 × 103
	GCR-TF2	350°	Circuit Type R(RQ)W	R = 6.19 × 103
	GCR-FT1	350°	Circuit Type R(RQ)W	Yo = 1.30 × 10−8; n = 0.73
	GCR-LG16	350°	Circuit Type R(RQ)W	Yo (W) = 4.95 × 10−4

, in such a way that the relaxation frequency increases at higher temperatures and there is a shift from a lattice to an interface process.

The shape of semicircular impedance spectra (Figure 1) was adjusted to an equivalent electrical circuit (RQ) with a resistance (R) defined by an arc crossing the real space of the x-axis associated with a Q phase, which is given by the following formula:

Q = Y₀ (jϖ)ⁿ,

(1)

where ϖ = 2πf (f = frequency).

Thus, in Table 2 below, the parameters Y₀ and n define the phase value Q.

Table 2 shows conductivity results and estimates parameters for VO obtained by melting the original wastes in an oxidant atmosphere and in a reducing atmosphere (VR) (LG16 sample).

The conductivity results of glass-ceramics (GCR) obtained after thermal treatments in a reducing atmosphere are also shown (Parameters determined by using EQUIVALENT CIRCUIT PROGRAMME-EQUIVCRT.PAS BOUKAMP) (RQ are ‘arc impedances’ and W means ‘Walburg impedances’) [13].

The nucleation step can be detected with the intermediate semicircles that appear in the EISA spectra. Figure 1 is provided as an example of this effect due to the nucleation of the crystallisation of the original glasses. In fact, in the case of glass-ceramic samples (GCs), the impedance spectra were obtained as from 300 °C measurement temperatures due to the formation of magnetite nano-crystallites.

The X-Ray Diffraction (XRD) of samples and scanning electron microscopy with microanalysis (SEM/EDS) revealed that, after thermal cycling, glasses transform into pyroxene microcrystals, showing a dendritic crystallisation decorated with magnetite nanocrystals and with a low number of dispersed anorthite feldspar crystallites [5] in the matrix of the resulting glass-ceramics.

The final total EISA spectra are shown in Figure 2 below for the glasses in oxidant atmospheres VO-HI06, VO-TF2, VO-FT1 and VO-LG16 (from the El Hierro, HI; Tenerife, TF; Fuerteventura, FT and La Gomera, LG islands).

The results show that the geometry of these spectra is related to the respective microstructures [13,14] demonstrated by SEM micrographs. EISA curves for HI06 and LG16 basalt glasses are very similar due to the respective presence of a homogeneous distribution of phase separation droplets. TF2 has a longer curve, while FT1 has a very heterogeneous microstructure with a short and lower curve.

As shown in

Table 3. Fe²⁺/total iron content ratios and activation energy for electrical conductivity (eV) obtained for all the basalt materials from the Canary Island basalt waste.

Sample	Fe2+/Fetotal	Ea (eV)	Ea (eV)
VO-HI06	0.24	0.804	0.804
VR-HI06	0.87	0.867	0.867
GCR-HI06	(*)	0.674	0.674
VO-TF2	0.28	0.801	0.801
VR-TF2	0.78	0.878	0.878
GCR-TF2	(*)	0.669	0.669
VR-FT1	0.89	0.810	0.810
GCR-FT1	(*)	0.643	0.643
VO-LG16	0.10	0.763	0.763
VR-LG16	0.86	0.820	0.820
GCR-LG16	(*)	0.687	0.687

(*) The glass-ceramic samples (named GCR) were obtained after applying heat treatments to the corresponding original VR glasses which were previously melted in a reducing atmosphere.

below, due to the simultaneous presence of Fe²⁺ and Fe³⁺ in the composition of these materials, there is an n-type electronic conductivity produced by the transference of electrons in equivalent sites, in agreement with reference [15] for other oxide materials. Table 3 also shows the activation energy values for conductivity with the procedure explained in the subsequent paragraph.

The conductivities and E_act are similar for both iron concentration states. This behaviour can be explained by a small polar mechanism, which can be described by the following equation:

σ = A·c·(1 − c)·e^−Ea/2kT

(2)

where A = constant, c = Fe²⁺/Fe_total, Ea = activation energy, and T = temperature.

Figure 3 shows an example of conductivity values with respect to the Fe²⁺/Fe total ratio for one of the basalt materials with an inverted curve for the activation energy values. The highest point of the conductivity curve (σ) determines the minimum Ea reported in Table 3 for each sample.

Figure 3 shows the maximum conductivity value predicted by equation in the case of basalt glasses (BGs) and basalt glass-ceramics (BGCs), which is usually defined in the crossing and near-crossing lines for Ea and σ (example for basalt glass samples VO-HI06 and VR-HI06).

In general, Ea values are in the range of 0.643–0.878. Specifically, these were 0.763–0.804 for VO (oxidant atmosphere), 0.801–0.878 for VR (glasses in a reducing atmosphere) and 0.643–0.687 for GCRs obtained from glasses melted in a reducing atmosphere. They are lower values than those obtained by Jurado et al. at 900 °C for similar basalt glass-ceramics, but have a completely different origin (the Canary Islands) [16].

It is noteworthy that the VR-TF1 glass has lower conductivity (one order of magnitude less) than other glasses, possibly due to its higher MgO content, replacing Fe²⁺ and breaking the 2+–3+ pairs and decreasing the small polar mechanism of this material [16,17].

The SEM and TEM micrographs (Figure 4 and Figure 5) make it possible to see the precipitation of magnetite (cubic habit) nanocrystals decorating the dendritic axis [18,19]. As is usual or frequent for dendritic growth [20], the dendritic crystallisation growth in these basalt glasses, which are enriched with iron oxides, is caused by the high diffusion of the iron species facilitating the axial growth of pyroxenes.

Similar microstructures were probed by carbon-triafol TEM replicas in similar glass-ceramics obtained from goethite residues [19].

Figure 5 shows the dendrites’ edges (asterisked * in the image) decorated with magnetite nanocrystals (less than 50 nm, which can be seen at the right of the dark triangle inserted). This is the case of the GCR-TF2 sample of Tenerife basalt waste [20].

The literature reveals that considerable knowledge and expertise has been accumulated on the process of transforming silicate waste into useful glass-ceramic products [21,22,23,24]. However, there is very little information on the electrical properties of the glass-ceramics obtained. For example, pyroxenes with sizes below 1 μm are formed as main crystal phases after the crystallisation [23]. In fact, a large amount of the hazardous waste in the batch of glass obtained is characterised by its high chemical durability. At the same time, the glass-ceramics obtained are characterised by a suitable coefficient of thermal expansion and attractive mechanical characteristics. However, the literature consulted says nothing about electrical properties. A simplified overview of the model for electric conduction in the basalt glass-ceramics investigated in this study shows a tunnel effect between the nanomagnetite crystals deposited in the dendrites of the resulting glass-ceramic material, formed after the thermally activated mechanisms of nucleation and crystal growth. In any case, each specific microstructure of each of these glass-ceramics is conditioned by the specific volume fraction of the crystal and the respective phase microstructure distribution [2,25,26,27]. There is no doubt that these materials obtained have significant possibilities for use.

Finally, we conducted an exhaustive search for information and found three interesting articles that studied the electrical properties of materials [28,29,30]. The first article [28] studied six glass compositions based on basaltic rocks and several types of industrial waste. In this case, the decrease of basalt content in glass samples decreased the conductivity. The different variations and combinations of element concentrations in the samples were directly responsible for the variation of their electrical properties. The second article [29] tested whether measurements of ionic conductivity could be used to monitor the evolution of the residual glass composition during crystallisation. In the third article [31], the impedance properties and conductivity mechanisms of volcanic basalt rocks were investigated according to their chemical composition, similar to this study. Although these analyses may seem similar to those carried out in this research, the compositions and the processes to obtain the glass-ceramic materials are different, so it is not possible to compare them or discuss their results together.

4. Conclusions

This research proves that wastes of basalt rocks from all of the Canary Islands (Spain) can be melted at 1450 °C to produce glasses and, after thermal treatments to induce nucleation and crystalline growth, glass-ceramics. Although researchers had previously detailed the processing and properties of these glasses and glass-ceramics, as well as the characterisation of their microstructure, this study has measured, for the first time, their electrical conductivity. We conclude that electrical conduction is facilitated by the presence of magnetite nanocrystals on the axes of dendrites of pyroxene crystals, enabling polar electron conduction in these materials.