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Article

The Valorization of Marble Waste to Synthesize a Novel Calcium Niobate–Magnesium Niobate Composite and an Investigation of Its Thermophysical Properties

by
Pedro Guilherme Sousa Passalini
1,
Andrey Escala Alves
1,
Thallis Custódia Cordeiro
2,
Roberto da Trindade Faria, Jr.
2 and
José Nilson França Holanda
1,*
1
Group of Sustainable Ceramic Materials, Laboratory of Advanced Materials, State University of Northern Fluminense, Campos dos Goytacazes 28013-602, Brazil
2
Laboratory of Physical Sciences/PPGCN, State University of Northern Fluminense, Campos dos Goytacazes 28013-602, Brazil
*
Author to whom correspondence should be addressed.
Processes 2025, 13(4), 1014; https://doi.org/10.3390/pr13041014
Submission received: 14 January 2025 / Revised: 15 March 2025 / Accepted: 26 March 2025 / Published: 28 March 2025
(This article belongs to the Special Issue Municipal Solid Waste for Energy Production and Resource Recovery)
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Abstract

:
Marble waste is produced on a large scale in many countries, resulting in serious pollution problems. This investigation aimed to study the valorization potential of marble waste from the ornamental rock industry used in the synthesis of a novel calcium niobate–magnesium niobate composite powder prepared by a solid-state reaction between 1000 °C and 1200 °C. The chemical and mineralogical characteristics of the marble waste were determined. Structural and morphological characterizations of the synthesized calcium niobate–magnesium niobate composite powders were conducted by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The thermophysical properties were measured using open photoacoustic cell and photothermal techniques. Structurally, at all synthesis temperatures, the calcium niobate–magnesium niobate powders were found to be composed of a complex mixture of CaNb2O6/Ca2Nb2O7/MgNb2O6/CaMg0.33Nb0.67O3. In addition, the calcium niobate–magnesium niobate composite powders exhibited low values of thermal diffusivity (1.88–2.15 × 10−7 m2/s) and thermal conductivity (0.12–0.16 W/mK). The findings of this investigation highlight the potential of marble waste as a promising sustainable source of carbonate for obtaining calcium niobate–magnesium niobate composite powder, which has thermophysical properties that should be explored in low-thermal-conductivity applications.

1. Introduction

In recent years, lead-free single-phase perovskite ceramics, such as those from Ca-Nb-O, Mg-Nb-O, and Ca-Mg-Nb-O solid-solution systems, have aroused considerable research interest due to their unique properties (piezoelectric properties, photocatalytic activity, photoluminescence properties, and microwave dielectric properties) [1,2,3,4,5,6]. In particular, these perovskite ceramics exhibit a combination of highly attractive dielectric properties, including high relative permittivity, low dielectric loss, a high quality factor, and a low temperature coefficient of resonant frequency.
Calcium niobate ceramics correspond to stoichiometric compounds of the Ca-Nb-O system, which includes four single-phase intermediate compounds: CaNb2O6 (CN, calcium metaniobate), Ca2Nb2O7 (C2N, calcium pyroniobate), Ca3Nb2O8 (C3N), and Ca4Nb2O9 (C4N) [2]. On the other hand, magnesium niobate ceramics correspond to stoichiometric compounds of the Mg-Nb-O system, which includes four single-phase intermediate compounds: MgNb2O6, Mg4Nb2O9, Mg3Nb6O11, and Mg5Nb4O15 [3].
Dielectric ceramic composites of the ceramic–ceramic type are of great practical and scientific relevance because they present versatile properties and have a wide range of applications [7,8,9,10,11,12,13]. For example, they can be applied in the microwave field to produce wireless technology devices such as resonators, oscillators, and communication filters. Currently, wireless technology devices are vital in terrestrial communication, satellite communication, and environmental monitoring, among others [14,15,16]. However, it is verified in the available literature that the synthesis of calcium niobate–magnesium niobate-type ceramic composites has not yet been tested. In addition, the relevant properties of calcium niobate–magnesium niobate-based perovskite ceramics, such as their thermophysical properties, remain unknown. Therefore, the development of a novel ceramic composite combining phases of calcium niobate and magnesium niobate may result in interesting properties of practical importance for several application fields [17,18].
Calcium niobate and magnesium niobate perovskite ceramics are synthesized by several methods, including a solid-state reaction, solvothermal, hydrothermal, co-precipitation, sol–gel, the reaction-sintering process, the autoigniting combustion method, and polymerized complex methods [1,2,3,4,5,6]. In addition, these perovskite ceramics are synthesized using conventional high-purity calcium and magnesium sources, which are generally expensive [1,2,3,4,5,6]. However, the use of solid waste as a sustainable source of calcium to replace conventional calcium sources in the synthesis of calcium niobate has only recently been employed. In fact, Kamkum et al. [19,20] successfully tested chicken eggshell waste as a source of CaCO3 (calcite) to synthesize single-phase calcium niobate ceramics of the Ca4Nb2O9 and CaNb2O6 types. Alves et al. [21] also effectively tested the use of seashell waste (aragonite and calcite) to obtain a biphasic calcium niobate ceramic of the Ca4Nb2O9/Ca2Nb2O7 type.
Niobium (Nb) is a rare metal belonging to the group of refractory metals (i.e., high-melting-point metals); these are very abundant in Brazil, which has about 98% of world’s reserves [22]. Nb has a vast field of applications, including microalloyed steels, super alloys, thin films, medical implants, titanium and aluminum alloys, copper alloys and superconductors, ceramic capacitors, and electrolytic capacitors [23]. On the other hand, niobium pentoxide (Nb2O5) is widely used as a precursor source of niobium in the synthesis of niobate-based ceramics, including calcium niobate and magnesium niobate [2,3].
The ornamental rock industry is responsible for the processing of several natural rock types, resulting in considerable economic and social development. In 2020, around 155 million tonnes of natural ornamental rocks were processed worldwide [24]. The leading countries in the global production of natural ornamental rocks are China (first), India (second), Turkey (third), Brazil (fourth), Iran (fifth), and Italy (sixth). In particular, Brazil has produced around of 8 million tonnes of ornamental rocks, equivalent to 5.2% of global production [24]. In this scenario, the Brazilian ornamental rock industry extracts a significant amount of marble rock (corresponding to approximately 25% of the total Brazilian production of ornamental rocks) and transforms it into finished products of great beauty and economic value. Both the extraction and finishing processes to obtain the final product generate significant amounts of marble waste [25,26]. In fact, up to 800,000 tonnes of waste marble can be produced annually. Due to causing environmental pollution and public health problems, marble waste needs to be disposed of using sustainable approaches [27,28]. On the other hand, marble waste is primarily composed of carbonates in the form of calcite (CaCO3) and dolomite ((CaMg(CO3)2)) [29]. Therefore, marble waste can be a sustainable and cheap option, capable of simultaneously providing sources of calcium and magnesium for obtaining calcium niobate–magnesium niobate-type ceramic composites. To the best of our knowledge, there are no previous investigations on the valorization of marble waste in the development of calcium niobate and magnesium niobate perovskite ceramics, including their composites. In this context, the recycling of marble waste to produce novel dielectric ceramic composites can be a sustainable alternative for its valorization, contributing to the circular economy.
The purpose of this investigation was to assess, for the first time, the feasibility of synthesizing calcium niobate–magnesium niobate composite powders using marble waste as a dual source for calcium and magnesium. Additionally, we aimed to determine the thermophysical properties of the synthesized composites. Prior to synthesis, the marble waste underwent characterization using XRF, XRD, FTIR, and SEM. A solid-state reaction was chosen as the preferred synthesis method due to its well-established practicality and promising results when employing micron-sized powders. The synthesis process involved heating the precursor powder mixture (marble waste and niobium pentoxide, 4:1) at temperatures ranging from 800 °C to 1200 °C, producing a multiphase composite composed of calcium niobate and magnesium niobate. This was further confirmed by XRD and FTIR analysis. SEM analysis revealed that the nano-sized particles within the composite exhibited a polygon-like morphology. The thermophysical properties of the synthesized composite were then evaluated, revealing low thermal conductivity, which makes it a potential candidate for thermal insulation applications. Furthermore, it is worth noting that the approach developed in this investigation is in full agreement with the precepts of the 12th United Nations Sustainable Development Goal (SDG 12), which emphasizes the need to manage solid waste in a sustainable and economical way.

2. Materials and Methods

2.1. Starting Materials

Marble waste and niobium pentoxide were used as starting raw materials in the present investigation. The marble waste sample, provided by an ornamental rock industry located in southeastern Brazil (Cachoeiro de Itapemirim-ES) in the form of shards (Figure 1), was crushed, dry-ground into powder, and sieved to a fraction of <325 mesh (<44 µm ASTM). High-purity commercial niobium pentoxide powder (Nb2O5, 99.9%, Sigma-Aldrich, St. Louis, MO, USA) with a particle size < 44 µm was used.

2.2. Synthesis of Calcium Niobate–Magnesium Niobate Composites

The calcium niobate–magnesium niobate composites, obtained using marble waste as a simultaneous precursor source of CaCO3 and CaMg(CO3)2 via the solid-state reaction route, involved the following steps: (i) wet mixing and homogenization of the starting powders were carried out for 18 h in the stoichiometric formulation of marble waste (CaCO3 + CaMg(CO3)2):Nb2O5 (4:1) [19]; (ii) drying took place at 110 °C for 8 h; (iii) the homogenized mixture was placed in an alumina crucible and calcined in air at 1000 °C, 1100 °C and 1200 °C for 8 h in a muffle furnace with a heating rate of 10 °C/min; and (iv) the composite powders obtained were disaggregated using a porcelain mortar and pestle.

2.3. Characterization of Marble Waste and Composites

The chemical analysis of the marble waste sample was conducted using an energy-dispersive X-ray fluorescence spectrometer (model EDX 700, Shimadzu, Kyoto, Japan). The loss on ignition (LoI) value was determined from the equation LoI (%) = Md − Mc/Md, where Md is the mass of the sample (g) dried at 110 °C and Mc is the mass of the sample (g) calcined at 1000 °C for 2 h.
Thermal analysis (TG-DTA) of the starting stoichiometric formulation was performed using a simultaneous thermal analyzer (model STA 409E, Netzsch, Selb, Germany) from room temperature (~25 °C) to 1000 °C, under an air atmosphere and heating rate of 10 °C/min.
The structural characterization of marble waste and synthesized calcium niobate–magnesium niobate powders was performed by X-ray diffraction (XRD) analysis with a conventional diffractometer (model XDR-7000, Shimadzu, Kyoto, Japan). The XRD analysis was carried out using Cu-Kα radiation, a 0.05° step, a 5 s count, a 40 kV voltage, a 30 mA current, and 2θ from 5° to 80°. Crystalline phases were identified by comparing the diffraction peak intensities and interplanar distances with those described in the PDF Cards for each mineral.
The average crystallite size (δ) of the synthesized powders was calculated using Scherer’s equation, given by δ = 0.9 (λ/βcosθ), where λ = 0.15406 nm, β is the full-width at half maximum of the calcium niobate–magnesium niobate composite lines, and θ is the diffraction angle.
The functional groups present in the marble waste sample and synthesized powders were identified by Fourier transform infrared (FTIR) analysis using a spectrophotometer (model Spectrum 400 FTIR, PerkimElmer, Woodbridge, ON, Canada) in the range of 400 cm−1 to 4000 cm−1.
The morphological analysis of the marble waste and synthesized powders was carried out by scanning electron microscopy through secondary electron images obtained using SEM equipment (model SSX-550, Shimadzu, Kyoto, Japan) at 15 kV, after covering the sample with a thin layer of gold.

2.4. Determination of Thermophysical Properties

To study the thermophysical properties, the synthesized powders were compacted by uniaxial pressing at 100 MPa to produce disk specimens (10 mm in diameter and 280 to 320 µm in thickness) with smooth and flat surface.
The thermal diffusivity (α) of the calcium niobate–magnesium niobate powders was measured by the open photoacoustic cell (OPC) technique at room temperature; the experimental details were published elsewhere [30,31]. The specific heat capacity (ρc) was determined using the photothermal technique of temperature increase caused by continuous illumination of the specimen by a He-Ne laser in a vacuum [30,31,32].
The thermal conductivity (k) and thermal effusivity (ε) of the calcium niobate–magnesium niobate composite specimens were determined using equations that relate them to the thermal diffusivity and specific thermal capacity, which are described as k = α.ρc and ε = (k.ρc)1/2, respectively.

3. Results and Discussion

3.1. Characterization of Marble Waste

Table 1 gives the chemical composition of the marble waste. Chemically, it was composed primarily of 42.749% wt.% of calcium oxide (CaO) and 15.371 wt.% of magnesium oxide (MgO). Traces of other oxides were also found. The high LoI value of 41.160 wt.% is indicative of the presence of large amounts of carbonate minerals (calcite and dolomite). These results are in good agreement with the literature [29,33,34].
Figure 2 shows the XRD pattern of the marble waste sample. The marble waste powder had all of its diffraction peaks indexed to trigonal structured calcite (CaCO3; unit cell parameters: a = 4.9890 Å, b = 4.9890 Å, c = 17.0620 Å, R-3c (167) space group; PDF Card: 00–005-0586) and trigonal structured dolomite (CaMg(CO3)2; unit cell parameters: a = 4.8069 Å, b = 4.8069 Å, c = 16.0020 Å, R-3 (148) space group; PDF Card: 01-073-2324) structures, with predominance of calcite. This result confirms the chemical composition data (Table 1). Furthermore, the mineralogical composition of the marble waste used in this work is in accordance with that described in the literature [29]. Thus, marble waste derived from raw marble rock fragments presents chemical and mineralogical characteristics that make it attractive for supplying both calcium and magnesium, which are essential in the synthesis of calcium niobate–magnesium niobate composite powders.
Figure 3 displays the FTIR spectrum of the marble waste sample. The results indicate that all detected vibration bands are characteristic of the functional groups of calcite and dolomite [35,36]. The vibration bands centered at 712 cm−1, 858 cm−1, 1424 cm−1, 1830 cm−1, and 2530 cm−1 are associated with the presence of calcite. Bands are also observed at 730 cm−1, 881 cm−1, 1438 cm−1, 2626 cm−1, 2898 cm−1, and 3019 cm−1, indicating the presence of dolomite. Therefore, the results of the FTIR analysis are supported by those of the structural characterization of the marble waste, which indicate the presence of calcite and dolomite, as seen in Figure 2.
Figure 4 shows a SEM micrograph of marble waste powder particles. It can be observed that the processing of raw marble waste formed a fine powder with particles of irregular morphology, due to the crushing process. It is also noted that most of the marble waste particles were found in the size range of <50 µm.

3.2. Thermal Behavior of the Starting Formulation

Figure 5 displays the TG-DTA curves of the starting formulation consisting of marble waste (CaCO3/CaMg(CO3)2):Nb2O5 (4:1). The DTA curve indicates four endothermic events and one exothermic event as the temperature increases: (i) an endothermic event at ~180 °C associated with the evolution of physically adsorbed water; (ii) an endothermic event at 781.1 °C related to the decomposition of dolomite to form calcite (CaMg(CO3)2 → CaCO3 + MgO + CO2) [37]; (iii) two endothermic events at 825.0 °C and 853.5 °C related to the decomposition of calcite to form CaO (CaCO3 → CaO + CO2) [37]; and (iv) an exothermic event around 900 °C related to the solid-state reaction between components originating from marble waste (CaO and MgO) and Nb2O5, resulting in the formation of a ceramic composite based on calcium niobate–magnesium niobate. The TG curve showed a total mass loss of 27.126%, which is associated with the decomposition of carbonates (dolomite and calcite), as observed in the endothermic events described in the DTA curve.

3.3. Structural Characterization of Synthesized Composites

The XRD patterns of the powders synthesized by the solid-state reaction process are shown in Figure 6. It was found that the solid-state reactions between marble waste (CaCO3 + CaMg(CO3)2) and Nb2O5 resulted in the simultaneous formation of several ternary (Ca/Nb/O and Mg/Nb/O) and quaternary (Ca/Mg/Nb/O) compounds. There were no remnants of the starting raw materials, indicating a complete reaction between them at the synthesis temperatures used. This indicates that intense structural reorganization occurred between the marble waste and niobium pentoxide during the calcination process. More specifically, all synthesized powders were indexed with diffraction peaks corresponding to the formation of a complex calcium niobate–magnesium niobate composite-type orthorhombic structured calcium metaniobate (CaNb2O6; unit cell parameters: a = 5.7479 Å, b = 14.9866 Å, c = 5.2263 Å, Pcan (60) space group; PDF Card No. 00-039-1392)/monoclinic structured calcium pyroniobate (Ca2Nb2O7; unit cell parameters: a ≠ b ≠ c, P21 (4) space group; PDF Card No. 01-074-0390)/orthorhombic structured columbite-(Mg) (MgNb2O6; unit cell parameters: a = 5.7000 Å, b = 14.1930 Å, c = 5.0320 Å, Pcan (60) space group; PDF Card No. 00-033-0875)/monoclinic structured calcium magnesium niobium oxide (CaMg0.33Nb0.67O3; unit cell parameters: a = 9.5983 Å, b = 5.4633 Å, c = 16.8213 Å, P21/c (14) space group; PDF Card No. 00-061-0438). The variation in the synthesis temperature between 1000 and 1200 °C did not change the phase composition. However, changes occurred in the intensities of the diffraction peaks. It can be noted that the quaternary compound peaks became more prominent as the synthesis temperature increased, whereas the peak intensity of CaNb2O6 decreased. There is no evidence of the formation of Ca4Nb2O9 at 1200 °C, differently from what was previously reported by Kamkum et al. [19] and Alves et al. [21]. This indicates that the development of the quaternary phase (CaMg0.33Nb0.67O3) within the structure may have hindered the formation of compounds rich in Ca or Mg (Ca4Nb2O9 or Mg4Nb2O9) at high temperatures.
The average crystallite size calculated by Scherrer’s equation was found in the nano-size range, with 40.83 nm (1000 °C), 34.41 nm (1100 °C), and 34.07 nm (1200 °C), respectively. However, the average crystallite size tended to stabilize in the temperature range between 1100 and 1200 °C. This finding may be related to the greater consolidation of the ternary (Ca/Nb/O and Mg/Nb/O) and quaternary (Ca/Mg/Nb/O) phases, as observed in the changes in the intensities of the diffraction peaks in Figure 6. This result is in agreement with those obtained for biphasic calcium niobate [21]. Thus, this novel nano-sized calcium niobate–magnesium niobate composite powder may be highly relevant for enhancing sintering driving force, resulting in lower sintering temperature. In addition, nano-sized dielectric ceramic powders are currently highly desirable as they play an important role in improving properties in many applications.

3.4. FTIR Analysis of Synthesized Composites

The FTIR spectra of the calcium niobate–magnesium niobate composite powders are shown in Figure 7. It can be seen that the FTIR spectra presented similar vibration bands, regardless of the synthesis temperature. This result was highly correlated with the XRD analysis (Figure 6). The FTIR spectra of the as-synthesized powders exhibited vibration bands at wavelengths between 400 cm−1 and 900 cm1, which are characteristic of calcium niobate and magnesium niobate compounds [19,38,39]. The band detected at 1409 cm−1 is due to the CO3−2 group. The band centered at ~2250 cm−1 is usually assigned to CO2 gas adsorbed from the environment during the calcination step [38]. Finally, the vibration band centered at ~ 3640 cm−1 can be attributed to the adsorbed water molecules. Therefore, the FTIR results are in good agreement with the thermal analysis (Figure 5) and XRD analysis (Figure 6).

3.5. Morphological Analysis of Synthesized Composites

SEM images of calcium niobate–magnesium niobate composite powder particles taken at 6000× are shown in Figure 8. It can be seen that the synthesis temperature had little effect on the particle morphology. The synthesized powders presented a polygon-like particle morphology, which is typical of calcium niobate and magnesium niobate compounds [4,19,38]. It was also observed that the powders presented a high degree of agglomeration. In fact, the formed agglomerates showed a plate-shaped morphology with variable sizes < 2 µm, which were produced in the calcination step due to the solid sintering effect.

3.6. Evaluation of Thermophysical Properties of Synthesized Composites

The thermophysical properties of the synthesized calcium niobate–magnesium niobate composite powders are summarized in Table 2. Thermophysical properties are of fundamental relevance for understanding the thermal performance of novel nano-sized materials [40]. In particular, each of the studied thermophysical properties has unique importance in understanding the thermal behavior and practical application of a dielectric perovskite ceramic material. In this context, the key thermophysical properties for the application of dielectric perovskite ceramic materials directly related to temperature changes are thermal diffusivity (measures how quickly heat propagates through a material), specific thermal capacity (measures the amount of heat required to raise the temperature of a material), thermal conductivity (measures the ability of a material to conduct heat), and thermal efficiency (measures the ability of a material to store and restore heat flow).
Thermal diffusivity values between 1.73 and 2.15 m2/s, specific heat capacity between 6.65 and 8.50 J/Km3, thermal conductivity between 0.12 and 0.16 W/mK, and thermal effusivity between 1.02 and 1.17 kWs1/2/Km2 were found. Therefore, the calcium niobate–magnesium niobate composite powders derived from marble waste presented low values of thermal diffusivity and thermal conductivity. In addition, an increase in the synthesis temperature resulted in only a small variation in the thermophysical properties values. This result is in line with the XRD patterns (Figure 6) and FTIR spectra (Figure 7).
These thermophysical property values are being reported for the first time. In addition, available data on the thermophysical properties of calcium niobate and magnesium niobate perovskite ceramics at room temperature are scarce. For this reason, no comparison was made with data from the available literature related to calcium niobate and magnesium niobate ceramics, including their composites. This gap constitutes a major technical–scientific challenge for practical applications of these materials that require thermal performance. However, the low values of thermal diffusivity and thermal conductivity can be further explained by the structural and microstructural aspects of the complex calcium niobate–magnesium niobate composite ceramic synthesized. The complex multiphase composition (CaNb2O6, Ca2Nb2O7, MgNb2O6, and CaMg0.33Nb0.67O3) introduces multiple phase boundaries that act as barriers to phonon transport, effectively increasing phonon scattering and reducing thermal conductivity [41]. The presence of mixed cations (Ca2+, Mg2+, and Nb5+) in the structure disrupts lattice vibrations due to ionic size mismatches, leading to lattice distortions and enhanced phonon scattering. Defects and dislocations introduced during synthesis act as scattering centers that hinder phonon propagation, further decreasing thermal conductivity [42]. The nanometric crystallite size (34.07–40.83 nm) results in an increased density of grain boundaries, contributing to phonon scattering [43]. Moreover, the polygon-like particle morphology and high degree of agglomeration create irregular interfaces, further limiting thermal transport by impeding heat flow across particle boundaries [44].
Based on the thermophysical property values obtained in this investigation, the novel calcium niobate–magnesium niobate composite powders exhibited low thermal conductivity characteristics, indicating that they may be promising materials for thermal barrier coating applications [45].

4. Conclusions

This study successfully introduced a novel nano-sized calcium niobate–magnesium niobate composite powder derived from marble waste, synthesized via the solid-state reaction process. This is an innovative approach to the eco-friendly valorization of mineral resources, such as marble waste, as an alternative raw material, which can help minimize the environmental impacts caused by the ornamental rock industry. The main outcomes are as follows:
  • Marble waste demonstrated excellent chemical compatibility and serves as a sustainable and cost-effective source of calcium and magnesium for the synthesis of dielectric perovskite ceramics.
  • The synthesized powders consisted of a composite phase mixture of CaNb2O6, Ca2Nb2O7, MgNb2O6, and CaMg0.33Nb0.67O3, with crystallite sizes ranging from 34.07 nm (1200 °C) to 40.83 nm (1000 °C).
  • The morphological analysis revealed that the powders exhibited a polygon-like particle shape with a high degree of agglomeration.
  • The thermophysical characterization indicated that the nano-sized calcium niobate–magnesium niobate composite ceramic possesses low thermal conductivity, making it a promising candidate for thermal insulation applications.
Future research should explore alternative synthesis conditions, evaluate microwave properties, and further investigate the potential applications of this novel composite perovskite ceramic.

Author Contributions

Conceptualization, J.N.F.H.; methodology, P.G.S.P., T.C.C. and R.d.T.F.J.; validation, A.E.A. and J.N.F.H.; investigation, P.G.S.P., T.C.C., R.d.T.F.J. and J.N.F.H.; writing—original draft preparation, P.G.S.P.; writing—review and editing, A.E.A. and J.N.F.H.; supervision, J.N.F.H.; funding acquisition, J.N.F.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Foundation for Research Support of the State of Rio de Janeiro (Grant No. E-26/201.137/2022) and National Council for Scientific and Technological Development (Grant No. 306147/2023-8).

Data Availability Statement

All data generated during this investigation are included in the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

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Processes 13 01014 g001
Figure 1. Sample of waste from used raw marble shards.
Figure 1. Sample of waste from used raw marble shards.
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Figure 2. The XRD pattern of the marble waste sample.
Figure 2. The XRD pattern of the marble waste sample.
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Figure 3. The FTIR spectrum of the marble waste sample.
Figure 3. The FTIR spectrum of the marble waste sample.
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Figure 4. An SEM micrograph of marble waste powder particles.
Figure 4. An SEM micrograph of marble waste powder particles.
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Figure 5. TG-DTA curves of the starting niobate formulation.
Figure 5. TG-DTA curves of the starting niobate formulation.
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Figure 6. The XRD patterns of the synthesized calcium niobate–magnesium niobate composite powders.
Figure 6. The XRD patterns of the synthesized calcium niobate–magnesium niobate composite powders.
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Figure 7. The FTIR spectra of the calcium niobate–magnesium niobate composite powders.
Figure 7. The FTIR spectra of the calcium niobate–magnesium niobate composite powders.
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Figure 8. SEM micrographs of calcium niobate–magnesium niobate composite powder particles: (A) 1000 °C and (B) 1100 °C.
Figure 8. SEM micrographs of calcium niobate–magnesium niobate composite powder particles: (A) 1000 °C and (B) 1100 °C.
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Table 1. Chemical analyses and loss on ignition of the raw materials (wt.%).
Table 1. Chemical analyses and loss on ignition of the raw materials (wt.%).
CaOMgOAl2O3SiO2Na2OP2O5* LoI
42.74915.3710.1400.4000.1200.0641.160
* LoI—loss on ignition.
Table 2. The thermophysical properties of the calcium niobate–magnesium niobate composite ceramics.
Table 2. The thermophysical properties of the calcium niobate–magnesium niobate composite ceramics.
Temperature (°C)Thermal Diffusivity (×10−7 m2/s)Specific Heat Capacity (×105 J/Km3)Thermal Conductivity (W/mK)Thermal Effusivity (kW√s/Km2)
10002.15 ± 0.027.00 ± 0.150.15 ± 0.011.02 ± 0.01
11001.73 ± 0.036.65 ± 0.180.12 ± 0.050.90 ± 0.05
12001.88 ± 0.028.50 ± 0.210.16 ± 0.021.17 ± 0.01
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MDPI and ACS Style

Passalini, P.G.S.; Alves, A.E.; Cordeiro, T.C.; Faria, R.d.T., Jr.; Holanda, J.N.F. The Valorization of Marble Waste to Synthesize a Novel Calcium Niobate–Magnesium Niobate Composite and an Investigation of Its Thermophysical Properties. Processes 2025, 13, 1014. https://doi.org/10.3390/pr13041014

AMA Style

Passalini PGS, Alves AE, Cordeiro TC, Faria RdT Jr., Holanda JNF. The Valorization of Marble Waste to Synthesize a Novel Calcium Niobate–Magnesium Niobate Composite and an Investigation of Its Thermophysical Properties. Processes. 2025; 13(4):1014. https://doi.org/10.3390/pr13041014

Chicago/Turabian Style

Passalini, Pedro Guilherme Sousa, Andrey Escala Alves, Thallis Custódia Cordeiro, Roberto da Trindade Faria, Jr., and José Nilson França Holanda. 2025. "The Valorization of Marble Waste to Synthesize a Novel Calcium Niobate–Magnesium Niobate Composite and an Investigation of Its Thermophysical Properties" Processes 13, no. 4: 1014. https://doi.org/10.3390/pr13041014

APA Style

Passalini, P. G. S., Alves, A. E., Cordeiro, T. C., Faria, R. d. T., Jr., & Holanda, J. N. F. (2025). The Valorization of Marble Waste to Synthesize a Novel Calcium Niobate–Magnesium Niobate Composite and an Investigation of Its Thermophysical Properties. Processes, 13(4), 1014. https://doi.org/10.3390/pr13041014

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