Valorization of Tungsten Mining Waste and Clay Residues in the Production of Technical Ceramic Materials for Sustainable Construction and Architectural Rehabilitation

Abstract

Mining and industrial activities generate large volumes of waste, up to 99% of the extracted material, forming a major global residue source. In this context, the valorization of mining sludge for sustainable construction materials gains relevance. This study examines the fabrication of ceramic bricks incorporating mining sludge from the Panasqueira mine, evaluating sludge incorporation levels and sintering temperatures to optimize resource use and reduce environmental impacts. Bricks were produced by blending residual clays from Víznar (Granada, Spain) with Panasqueira sludge at substitution rates of 10, 25 and 50%, and fired at 800, 950 and 1100 °C. Granulometry was determined for the Víznar clay and mining sludge, while bulk density was measured for the fired bricks. The raw materials were analyzed by XRF and XRD, whereas the ceramic samples were characterized by water absorption, porosimetry, ultrasound pulse velocity, compressive strength testing, ESEM, leaching and colorimetry, to assess their chemical, physical and mechanical behaviour. Both clays and sludge are rich in SiO₂ and Al₂O₃, suitable for ceramic processing, while fluxing oxides promote vitrification and densification. Incorporating 25 and 50% sludge reduces porosity, increases ultrasonic velocity and improves mechanical strength, achieving optimal performance at 1100 °C. Moreover, firing immobilizes toxic metals and allows controlled colour development, confirming their technical performance and suggesting their potential suitability from an environmental perspective. Their microstructure and stability depend on sludge content and firing temperature, essential factors for sustainable construction and architectural rehabilitation.

1. Introduction

Extractive mining industries constitute one of the most extensively developed activities worldwide. It is estimated that in 2021 approximately 17,896 million metric tons of minerals were produced globally [1], generating millions of tons of waste annually, many of which are classified as hazardous to human health and the environment. Several authors estimate that the largest fraction of the extracted ore, between 97% and 99% of the total mass, consists of solid residues that ultimately end up in waste dumps or storage facilities [2]. This implies that the amount of useful material obtained represents only a minimal portion of the total mineral processed, and this fraction is even smaller in the extraction of noble metals such as gold [3]. Consequently, at least seven billion tons of mine tailings are generated annually worldwide, and a 2.70% annual growth in mining waste is projected for the period between 2026 and 2034 [4]. According to Eurostat statistics, the countries with the highest rates of mining waste generation are Romania, Bulgaria, and Serbia.

Mining and ceramic industries generate large volumes of waste materials, many of which are currently underutilized despite their potential as secondary raw resources [1,4]. In particular, mining sludge represents a significant environmental challenge, as it is typically stored in tailing deposits where it may generate long-term risks related to heavy metal leaching, soil occupation, and water contamination [2]. Similarly, clay residues from ceramic industries are produced in considerable quantities and are often discarded despite their suitable composition for reuse [3].

These materials are widely available and generally associated with low economic cost, as they are by-products of existing industrial processes that do not require additional extraction. Their valorization not only reduces the demand for natural raw materials but also mitigates the environmental impact associated with waste disposal. In this context, the reuse of mining sludge and clay residues in ceramic production represents a promising strategy aligned with circular economy principles, enabling the transformation of abundant and low-cost waste streams into value-added construction materials [3,4].

Despite their availability and potential, the effective integration of these waste materials into ceramic products remains a complex challenge, particularly in terms of optimizing composition and processing parameters to achieve adequate performance.

Multiple studies address the development of new materials using various wastes generated by the mining industry as secondary raw materials. Conventionally, stony waste produced during mining activities have been used in pavement or road construction [5,6,7]; as backfilling material in both open-pit and underground mines [8,9,10]; and in the manufacturing of other construction materials, such as aggregates of different types for concrete production [11], mortars [12], and various ceramic products [13,14,15,16,17]. The feasibility of using mining sludges in glass production has also been explored [18,19]. Currently, the production of geopolymeric materials from mining residues is considered the most promising technology for converting these wastes into new manufactured products due to their high silicon and aluminum content [20,21,22].

Furthermore, the construction sector demands enormous quantities of resources and materials and accounts for the largest share of waste generated within the European Union. According to Eurostat statistics, in 2020 construction contributed 37.5% of the total waste generated, followed by waste derived from mining and quarrying, which accounted for 23.4%. Together, these sectors represent more than half of the waste generated annually at the European level. Moreover, when establishing a connection between material extraction activities, the manufacture of construction products, and the construction sector itself, the sum of these actions is responsible for between 5% and 12% of total national greenhouse gas emissions [23].

In recent decades, an increased awareness of environmental sustainability has emerged globally and particularly within the regulatory framework of the European Union, notably through the communication of the New Circular Economy Action Plan. In contrast to earlier industrial waste management policies largely based on massive, sometimes uncontrolled and hazardous stockpiling, new strategies have been implemented that focus on both the reduction and the reuse of waste, which is now increasingly regarded as a suitable secondary raw material for the production of new products [24]. This shift has played a decisive role in the management, valorization, and recovery of waste across various industrial sectors.

Currently, the production of construction materials represents the main strategy for the recovery of mining wastes [25,26,27]. In this context, research efforts worldwide have surged, highlighting the potential for utilizing various by-products—previously deemed unusable—as partial substitutes for primary raw materials in brick production. These industrial waste materials can be incorporated into ceramic pastes in the form of powders and/or aggregates. A substantial body of research focuses on the development of traditional ceramic bricks incorporating organic waste as starting materials [28,29,30] as well as inorganic waste [31,32,33]. Furthermore, several authors have demonstrated the feasibility of producing bricks with satisfactory properties using different proportions of mining sludges as raw materials [34,35,36,37].

Although numerous studies have investigated the use of mining residues in construction materials, there is still a lack of systematic studies addressing the incorporation of mining sludge into traditional ceramic matrices under controlled firing conditions. In particular, the case of Panasqueira mining residues has been mainly explored in alternative systems such as geopolymers and cement-based materials, while their application in conventional ceramic production remains comparatively less developed.

In addition, the enormous consumption of natural resources in general, and of clay deposits in particular, for the manufacture of ceramic products leads to the gradual depletion of non-renewable resources [38]. Therefore, the development of strategies based on the use of by-products as substitutes for traditional raw materials becomes imperative to reduce the demand for primary mineral resources and to prevent landscape degradation [39]. In line with this, the processing of ceramic materials incorporating waste involves the design and initial mixing of the raw materials in predetermined proportions, the shaping of pieces through techniques such as modelling, moulding, or extrusion, and finally, sintering, which consists of subjecting these materials to thermal ranges between 900 and 1260 °C. As a result of this methodological process, thermally transformed materials with adequate chemical and physical properties are obtained, and in some cases, even with superior performance compared to commercial products.

The aim of this study is to obtain new ceramic materials based on the recycling of mining sludges from the Panasqueira mine (Portugal). Numerous previous investigations have highlighted the potential use of waste generated by the Panasqueira mining activity for the production of new sustainable materials with diverse applications. For instance, Castro-Gomes, J.P., et al. [40] developed geopolymer composites incorporating coarse mining residues for the production of terrazzo tile prototypes. In the doctoral thesis by [41], new ceramic materials were designed and produced using mineral waste, with the waste incorporated into the mixtures in high proportions (up to 90%). Other authors, such as [42], proposed various construction materials for civil engineering applications in which mining waste play a central role, bituminous mixtures, concretes, and other materials for prefabricated elements, all based on the concept of alkali-activated materials. Similarly, [43] demonstrated the production of geopolymeric compounds from mixtures (in different proportions) of tungsten mining sludges and brick waste using sodium silicate and sodium hydroxide as alkaline activators. Along the same lines [44], geopolymer-based foams incorporating residual mining sludges and expanded cork granules through the chemical foaming technique with aluminum powder. More recently, other researchers have explored the design and production of magnesium-based mortars cured under accelerated carbonation, capable of absorbing surrounding CO₂ and incorporating mining sludges, among other industrial residues, as aggregates [45].

This work proposes a new alternative for the valorization of sludges derived from the mining activity of Panasqueira, consisting in the production of bricks doped with these by-products. In addition to reducing the amount of clay required for ceramic manufacture, the study aims to optimize the firing temperature of the designed materials without compromising the quality of the final products. Another objective is to determine the improvement of the physical and mechanical properties of the doped ceramics according to the different percentages of mining sludges incorporated (10, 25, and 50 wt%) and/or firing temperatures applied (800, 950, and 1100 °C). Finally, this research seeks to contribute to environmental sustainability through the production of new materials based on the valorization of solid waste, reducing their disposal and accumulation in landfills.

In this context, the novelty of the present study lies in the systematic evaluation of the combined effects of sludge incorporation ratios and firing temperature on the physical and mechanical properties of ceramic materials based on Panasqueira mining residues.

The amount of mining sludges incorporated into the manufacture of the bricks was selected based on results reported by other researchers. In general, waste contents above 60–70% tend to impair the performance of the final products [46,47], which is why no more than 50% sludge was added to obtain manufactured materials with suitable properties. Likewise, the working temperatures were chosen according to previous studies on brick production using similar starting materials [39,48]. Furthermore, this selection is motivated by the need to assess potential mineralogical and textural changes as a function of the intensity of applied thermal treatment.

2. Materials and Methods

The materials used to design the sustainable bricks products consist of natural clay residues derived from ceramic industries and extractive mining waste primarily tungsten-bearing tailings together with water for homogenization and compaction of the mixture, followed by subsequent drying and firing at different ceramic transformation temperatures.

The clayey raw material used for the manufacture of the technical bricks was supplied by the company Cerámica Castillo Siles S.L., located in the municipality of Víznar (Granada Province, Spain). As ceramic additives, mining tailings sludge originating from the residual deposits of the Panasqueira mine, specifically from the Cabeço do Pião and Aldeia da Panasqueira areas, located in the Beira Interior region of Portugal, was incorporated into the formulations (Figure 1).

Geologically, the Víznar area (Granada, Spain) is composed of carbonate rock massifs dominated by limestones and dolomites associated with the Granada Basin. The region exhibits significant Neogene–Quaternary sedimentary sequences and includes various lithologies, such as Jurassic–Cretaceous limestones and dolomites, Neogene conglomerates, sandstones and sands, and Quaternary silts and clays. These clayey materials constitute the natural extractive base for the local ceramic industry [49].

The geology of the Panasqueira region (Covilhã, Portugal) consists of marine sedimentary sequences of Ediacaran and Cambrian age, dominated by schists and greywackes, affected by low-grade metamorphism and Hercynian deformation. The area includes post-tectonic granite intrusions, altered granitoids and extensive quartz vein systems hosting metallic mineralization of tungsten–wolframite, tin–cassiterite, copper and beryllium [50,51].The Panasqueira mining complex, located in the Serra da Estrela and near the Serra do Açor and the Zêzere River, is internationally recognized for its long extractive history and its production of strategic minerals since the mid-19th century and continuing to the present. Tungsten and tin have been the main exploited resources, although other mineral species such as cassiterite and beryl, as well as metals such as copper and lead, have also been documented [52,53,54,55]. Tungsten extraction constitutes the most significant mining activity in Panasqueira. This metal is considered of strategic importance for several industrial sectors, including high-strength steel production, electronics and the chemical industry. The Panasqueira tungsten deposit is one of the largest in Europe and has historically represented a key supply source for the international market [56,57,58].

2.1. Experimental Process for Traditional Bricks with Mining Sludges

The ceramic pastes were formulated by blending natural clay residues from Víznar (Granada, Spain) with mining tailings sludge from the Panasqueira mines (Covilhã, Portugal). Three additive proportions were established by weight: 10% (MC10), 25% (MC25) and 50% (MC50). Additionally, control samples composed exclusively of clay residues (CC) were prepared as reference specimens to comparatively assess the influence of the mining tailings on the properties of the newly developed ceramic materials. The four brick typologies were fired at 800, 950 and 1100 °C.

The selection of the firing temperatures (800, 950 and 1100 °C) was not arbitrary but based on both previous literature and the expected physicochemical transformations of clay-based ceramic systems [13,14,15,16,17,35,36,37]. Specifically, 800 °C represents the lower limit at which dehydroxylation processes are completed and the initial stages of sintering begin [13,31], allowing evaluation of partially consolidated structures. The intermediate temperature (950 °C) was selected to assess the progression of densification and the development of new mineral phases typically reported in conventional brick manufacturing [34,35]. Finally, 1100 °C corresponds to an advanced sintering stage, where significant vitrification and microstructural consolidation are expected [13,37], providing insight into the upper performance limits of the material.

These temperatures also fall within the typical industrial firing range for structural ceramics [28,29,30,34], ensuring comparability with previous studies while remaining suitable for the specific composition of the clay–mining sludge system investigated. In addition, preliminary trials confirmed that this temperature interval allows measurable differences in physical and mechanical properties without inducing excessive deformation or collapse of the specimens.

The main characteristics of the resulting ceramic samples are summarized in

Table 1. Ceramic sample groups showing the percentage of mining sludge added relative to the additive-free control samples, the ceramic firing temperatures, and the water volume used per kilogram of mixture. Target samples (without mining sludges) and sustainable bricks made of clay and mining sludge at 10, 20, and 50%.

	T (°C)	Control Samples	Sustainable Bricks (wt% of Sludge)
			10%	25%	50%
	800	CCL	MC10L	MC25L	MC50L
	950	CCM	MC10M	MC25M	MC50M
	1100	CCH	MC10H	MC25H	MC50H
Water (mL)		300	235	225	150

.

Control samples (CC) are composed exclusively of clay residues and are used as a reference to comparatively evaluate the effect of mining sludge incorporation on the final properties of the ceramic materials. In contrast, the MC10, MC25, and MC50 samples are designated as “sustainable bricks” due to the progressive substitution of primary raw materials with mining residues, as well as the reduction in water content required during processing. Both aspects are directly related to waste valorization and the reduction in natural resource consumption. This classification is based on technological and environmental criteria and is not solely determined by considerations related to raw material transportation.

The ceramic pastes were formulated by dry-mixing the clayey raw material with the mining sludge, except for the control samples (without sludge), to which water was subsequently added until the required plasticity was achieved. The resulting mass was manually homogenized and shaped in detachable wooden moulds with a capacity for pieces measuring 16 × 12 × 4 cm. After filling the mould, the surface was levelled using a wooden strip. Once the initial moisture loss had occurred, the moulds were removed and the shaped bricks were cut into cubic specimens of 4 × 4 × 4 cm, yielding a total of twelve standardized samples per batch for the different tests.

The hygroscopicity of the wooden mould facilitated demoulding, allowing the pieces to be cut 7 h after fabrication. Subsequently, the specimens were dried under controlled laboratory conditions (22 ± 5 °C; 45 ± 2% RH) for three days. A total of four samples per group were produced, resulting in 48 specimens overall. Prior to firing, all samples were oven-dried at 60 °C for 24 h to minimize adverse effects during firing associated with rapid moisture removal. Each series of specimens was subjected to thermal cycles in a Herotec CR-35 electric kiln (T_max = 1350 °C), following a common protocol consisting of a heating rate of 120 °C/h from room temperature to the target peak temperature, a 30 min dwell at the selected temperatures (800, 950 and 1100 °C), and free cooling to approximately 25 °C over a period of approximately 10 h under controlled kiln conditions, ensuring a reproducible and consistent cooling regime for all samples (Figure 2 and Figure 3).

2.2. Particle Size Distribution

Particle size distribution was determined by laser diffraction using a Bettersizer 2600 (Bettersize Instruments Ltd., Dandong, China), equipment belonging to the University of Granada, Spain, and operated at the Scientific Instrumentation Center (CIC), University of Granada, within the Particle Size Analysis Unit, equipped with a wet dispersion unit (BT 802), following the ISO 13320 standard [59]. Measurements were carried out in liquid medium under mechanical stirring and ultrasonic dispersion to ensure complete deagglomeration of the samples. Two raw material samples were analyzed: the clay residues from Víznar (Granada, Spain) and the mining sludge from the Panasqueira deposit (Portugal), each considered representative of the corresponding source material.

Bulk density was determined by measuring the dimensions of each specimen using a digital calliper and calculating its volume, while mass was obtained using a precision balance (±0.1 g). Density was then calculated as the ratio between mass and volume.

2.3. X-Ray Fluorescence

The elemental chemical composition of the raw materials was determined by X-ray fluorescence (XRF) using a Zetium wavelength-dispersive sequential spectrometer (PANalytical B.V., Almelo, The Netherlands; currently Malvern Panalytical following the 2017 merger), located at the Scientific Instrumentation Center (CIC) of the University of Granada (UGR) equipped with a rhodium-anode ceramic tube, a 4 kW generator, and a decoupled θ/2θ goniometer. For major-element analysis and semiquantitative analysis, the samples were prepared as fused beads and pressed pellets, respectively. Semiquantitative values were obtained using PANalytical’s Omnian software package (version 4.2). Four samples in total were analyzed using this technique. For X-ray fluorescence (XRF) analyses, approximately 3 g of representative sample were used per test. The samples were prepared exclusively as fused beads, a standard procedure for the determination of major oxides in ceramic and mineral materials, ensuring a homogeneous composition of the analyzed material.

2.4. X-Ray Diffraction

The mineralogical characterization of the raw materials and of the different ceramic typologies was performed by X-ray diffraction (XRD) using a PANalytical X’Pert PRO diffractometer. (PANalytical B.V., Almelo, The Netherlands). All samples were finely ground in an agate mortar and subsequently dry-sieved to obtain the <80 µm fraction, ensuring the homogeneity and representativeness of the analyzed material. The working conditions were: CuKα radiation, 45 kV and 40 mA, a scanning range of 3–70° (2θ), and a step speed of 0.1° 2θ·s⁻¹. Mineral phase identification was carried out, and a total of fourteen samples were examined using this technique. For X-ray diffraction (XRD) analyses, approximately 3 g of sample were used per test.

2.5. Hydric Tests

Water absorption by free immersion and the subsequent drying behaviour were determined according to standards [60,61], respectively. Three specimens per brick typology were tested, yielding a total of 36 samples. All tests were performed under controlled laboratory conditions (20 ± 5 °C; 60 ± 5% RH) using distilled water. Mass variations were recorded with a Radwag PS 6000/C/2 balance(RADWAG Balances and Scales, Radom, Poland), with a precision of ±0.1 g. A total of eighteen samples were examined in the absorption and desorption test.

The water content by weight and the absorption coefficient for each sample were calculated using Equations (1) and (2), respectively. The drying index was determined from the integral of the drying curve, following Equation (3).

$$Al={\displaystyle \frac{M_s-M_0}{M_0}}100$$

(1)

where $Al$ is the water content (%), $M_s$ is the saturated mass (g), $M_0$ is the dry mass (g).

$$Ac={\displaystyle \frac{M_s-M_0}{Vol}}$$

(2)

where $Ac$ is the volumetric absorption (g/cm³), $M_s$ is the saturated mass (g), $M_0$ is the dry mass (g) and $Vol$ is the specimen volume (cm³).

$$ID={\displaystyle \frac{{\int }_{t_0}^{t_f}f\left(\Delta M\right)dt}{IA\ast t_f}}$$

(3)

where $ID$ is the drying index (dimensionless), $t_0$ and $t_f$ are the initial and final time (s, m or h, depending on the test conditions, in this case expressed as the square root of m), $\Delta M$ is the mass variation (g) and $IA$ is the initial water content (dimensionless or %).

2.6. Mercury Intrusion Porosimetry

The pore system, its quantification, and the pore size distribution ranges were evaluated by mercury intrusion porosimetry (MIP) using an AutoPore IV 9500 V1.07 porosimeter (Micromeritics Instrument Corporation, Norcross, GA, USA), available at the C-MADE-Centre of Materials and Civil Engineering for Sustainability, University of Beira Interior, Portugal). Prior to analysis, the specimens were pre-dried with silica gel in a hermetically sealed container for seven days. The accessible porosity and the percentage pore size distribution for the twelve typologies tested (mesopores, macropores and air voids/cracks) were determined according to the procedures outlined in reference [62]. Twelve samples in total were used in the MIP test, with an approximate volume of 1 cm³.

2.7. Ultrasonic Pulse Velocity

The ultrasonic pulse transmission velocity was measured using a Tico Proceq device (Proceq SA, Schwerzenbach, Switzerland), belonging to the Research Group HUM 629, Department of Conservation and Sculpture, Faculty of Fine Arts, University of Granada, equipped with 54 kHz transducers of 50 mm diameter. The test was performed in direct transmission mode, using an aqueous gel couplant (TELIC S.A.U.). All specimens were measured, with four samples per group. Measurements were taken in the three spatial directions, both before and after firing, following standard [63], which specifies the laboratory determination of pulse velocities and ultrasonic elastic constants of rock. The propagation velocity of P-waves was recorded in m/s. A total of sixteen samples were used, with three measurements performed on each face of the hexahedral sample.

2.8. Compressive Strength

Uniaxial compressive strength was evaluated using a Shimadzu Autograph AGS-X testing machine (Shimadzu Corporation, Kyoto, Japan), belonging to the RNM Research Group, University of Granada, applying an increasing load at a crosshead displacement rate of 1 mm/min until specimen failure. The maximum load capacity of the equipment was 45 kN. The tests were performed in accordance with standard [64,65,66]. which specify the determination of compressive strength in natural and dimension stone materials. A total of thirty-six specimens were tested, corresponding to three samples per typology.

2.9. Environmental Scanning Electron Microscopy

Morphological and textural characterization was carried out using environmental scanning electron microscopy (ESEM) with a high-resolution FEI QuemScan 650FEG (FEI Company, Eindhoven, The Netherlands) operating in high-vacuum mode (<6 × 10⁻⁴ Pa), equipped with secondary electron detectors (SED) and backscattered electron detectors (BED). Chemical microanalysis was performed using a Dual EDS XFlash 6/30 X-ray system (Bruker, Karlsruhe, Germany). All instruments are available at the Scientific Instrumentation Center (CIC) of the University of Granada (UGR). Images, spectra and elemental distribution maps were processed Correlative Workflow and MAPS Tiling & Stitching software packages (Thermo Fisher Scientific, Eindhoven, The Netherlands). All samples were coated with a carbon film prior to analysis. Twenty-six samples in total were used for the ESEM study with an approximate size on the order of 1 cm³.

2.10. Leaching Test

The leaching test was carried out following the procedure described in [47]. The method is based on extracting the aqueous phase from a suspension of a previously milled solid sample and determining the concentration of chemical elements released into the aqueous medium. The procedure consists of taking a representative solid sample (20 ± 0.05 g), subsequently grinding it in an agate mortar and sieving it to 150 µm; oven-drying the material at 150 ± 5 °C to constant mass; transferring it to a 500 mL volumetric flask; and adding the leaching liquid (200 mL of deionized water).

The solid and liquid components were agitated at 400 rpm at room temperature for 60 ± 2 min. After 15 min from the end of the extraction, the suspension was filtered through an ash-free paper filter for gravimetric analysis. The filtrate (eluate) was collected, and the concentrations of leached chemical elements (μg/L) were determined by semiquantitative multi-element analysis using inductively coupled plasma mass spectrometry (ICP-MS) with a NexION 300X instrument (Perkin-Elmer, Shelton, CT, USA), available at the Center for Human and Social Sciences, Department of Art History and Heritage, Spanish National Research Council (CSIC), Madrid, Spain), which provides detection limits in the ng/L range for most elements, a linear dynamic range of 8–10 orders of magnitude, and semiquantitative precision better than 30%.

Using this procedure, both the mining sludge sample and the ceramic samples fired at 800 and 1100 °C were analyzed, for sludge incorporation levels of 10%, 25% and 50%. The leachate obtained from the mining sludge was used as the reference pattern against which the leachates from the ceramic samples were compared. The acidity or basicity of the eluates was also monitored using environmental pH sensors developed by [67] with a precision of ±0.1 pH units. pH measurements were conducted in aqueous media. Sixty samples in total were used in this test.

2.11. Colorimetry

The chromatic properties of the raw materials and the ceramic products were determined using a Konica Minolta CM-2600d/2500d spectrophotometer (Konica Minolta, Tokyo, Japan), belonging to the Research Group HUM 629, University of Granada. Measurements were performed under CIE D65 illumination [68], recording the L (lightness), a and b* (chromaticity) coordinates using the SpectraMagic NX software (version 1.x, Konica Minolta). A total of fourteen samples were analyzed, and in accordance with standard [68], mean values were obtained from six measurements taken on each face of the hexahedral specimens.

3. Results

3.1. X-Ray Fluorescence (XRF)

The elemental chemical composition of the raw materials is presented in

Table 2. Results of the elemental chemical analysis of major oxides obtained by XRF for the clay material and the mining residues, together with the semi-quantitative analysis of the sludges (wt%).

Major (%)	SiO2	Al2O3	Fe2O3	MnO	MgO	CaO	Na2O	K2O	TiO2	P2O5	Zr	LOI (%)	∑ (%)
Clay	47.46	14.77	5.55	0.07	3.78	10.11	0.67	2.58	0.77	0.12	0.019	13.86	99.74
Sludge	57.72	13.15	11.14	0.11	1.92	0.66	0.9	2.41	0.56	0.34	0.013	4.60	93.51
Semi-Q (%)	Sr	Cr	Ba	Ni	Nb	Rb	As	Y	Zn	Sn	Co
Sludge	0.009	0.013	0.019	0.009	0.001	0.035	1.139	0.002	0.759	0.046	0.010
Sludge	Cu	Ga	Cd	I	Ce	W	Bi	F	Cl	Au	SO3
	0.201	0.002	0.013	0.046	0.016	0.147	0.005	1.213	0.015	0.004	5.836

. Both materials exhibit high silicon oxide (SiO₂) contents and substantial proportions of aluminum oxide (Al₂O₃) and iron oxide (Fe₂O₃). The clay from the study area also contains notably elevated concentrations of calcium oxide (CaO), whereas magnesium oxide (MgO) and potassium oxide (K₂O) appear in comparatively lower amounts. The clay displays a higher loss on ignition (LOI) than the mining sludges (13.86% and 4.60%, respectively), primarily due to the release of structurally bound water inherent to clay minerals, as well as the decarbonation of calcium carbonates.

The major-element analysis underscores the predominance of SiO₂ and Al₂O₃, which together account for more than two-thirds of the total composition in both raw materials (clay > 60%; sludge > 70%). This chemical configuration is particularly favourable for densification and hardening during high-temperature firing, promoting the formation of stable ceramic structures. Silicon oxide acts as the principal network former in the vitrified matrix, a key component in ceramic and glass production, conferring rigidity, mechanical strength, thermal stability, and chemical durability. Aluminum oxide, in turn, functions as a network stabilizer and reinforcing agent within the silicate framework, enhancing hardness as well as thermal and mechanical resistance [69,70,71,72].

In addition, regarding the major-oxide analysis, the presence of alkali and alkaline-earth oxides is noteworthy due to their role as fluxing agents. These include Na₂O, which lowers the melting point and promotes the formation of a liquid–glass network; K₂O, which stabilizes the vitreous phase; CaO, which enhances vitrification and contributes to matrix densification; and MgO, which provides phase stability and favours the development of neoformed phases. Together with these, the previously mentioned Fe₂O₃ also acts as a flux under the oxidizing atmosphere in which the sustainable bricks were fired, in addition to contributing to the formation of intermediate mineral phases such as hematite [73,74,75,76,77,78].

Regarding the semi-quantitative elemental composition of the mining sludges, the XRF results reveal a high concentration of arsenic (As), reaching 1.139 wt%, which confirms their potentially hazardous character. Lower concentrations of chromium (Cr) and cadmium (Cd) were also detected, together with the presence of other elements of interest such as rubidium (Rb), tin (Sn), zinc (Zn), copper (Cu), and tungsten (W). It is also noteworthy that fluorine (F) exhibited the highest elemental concentration recorded (1.213 wt%). Of particular significance is the role of several minor elements as fluxing agents: strontium (Sr), which promotes the formation of liquid phases; barium (Ba), which acts both as a flux and as a catalyst of the glassy network [79]; zinc (Zn), which reduces the viscosity of the liquid phase in ceramics; rubidium (Rb), which serves as a flux and glass-network modifier, contributing to a lower viscosity of the liquid phase; and fluorine (F), whose relatively high proportion, despite being a minor constituent, acts as a flux by reducing viscosity and enhancing the vitrification of ceramic matrices [80].

The XRF analysis, together with the chemical characterization of the raw materials, both the clay itself and the mining sludges, demonstrates that the preparation of the ceramic products is, a priori, supported by highly favourable compositional properties across a wide thermal window. This allows us to conclude that these raw materials exhibit excellent suitability for ceramic technology. Individually, the raw materials can sustain conventional ceramic firing ranges between 900 and 1250 °C [81,82,83,84], largely due to their SiO₂ and Al₂O₃ contents, which can promote mullite formation, along with other components that act as densifying agents and fluxes. In addition, the proposed clay–sludge mixtures present optimal firing ranges that should fall within approximately 900–1125 °C for the 90–10% blend, 1050–1175 °C for the 75–25% blend, and 1100–1225 °C for the 50–50% blend.

3.2. X-Ray Diffraction (XRD)

The XRD results for the raw materials, clay and sludges (Figure 4), reveal that the detected phases, chlorite, illite, paragonite, kaolinite, quartz (highly predominant), calcite, microcline, dolomite, muscovite, clinoclore and magnetite, provide the compositional basis for the successful production of functional ceramic materials. This is particularly relevant for the manufacture of sustainable bricks derived from the recycling of the aforementioned clay and sludge materials. These mineralogical assemblages are consistent with the chemical composition of the raw materials and supply essential components for ceramic manufacturing. In particular, kaolinite and illite, together with muscovite and paragonite, constitute the principal contributors of Al₂O₃ and SiO₂, which under optimal firing conditions can promote the formation of secondary mullite and an aluminosilicate glassy phase [85,86].

Moreover, the significant contribution of microcline (K) and paragonite (Na) is noteworthy, as they act, respectively, as sources of K-feldspar and sodium, both functioning as fluxes within the mixture and consequently lowering the sintering–vitrification temperature. Their presence promotes the formation of an adhering liquid phase among the various compounds and grains [87,88]. Similarly, chlorite, clinoclore and illite, due to their variable iron contents, favour the formation of hematite depending on the firing temperature. The influence of hematite is not limited to modifying the colour of the ceramic products; rather, it also behaves as a neoformed phase capable of occupying pores in the final material, thereby enhancing ceramic technological performance, in a manner comparable to the role of mullite [89,90]. Furthermore, the carbonates present in calcite and dolomite may contribute to lowering the melting point of the mixture.

Finally, the substantial presence of quartz leads to the formation of secondary silicates, contributes to stoichiometric balance, minimizes deformation and shrinkage in ceramic materials, and enhances the mechanical strength of the final ceramic products [91,92].

Figure 4. Mineralogy of the raw materials (clay and sludges) obtained from X-ray diffraction analysis. According to [93,94]. Chl = chlorite; Ilt = illite; Pg = paragonite; Kln = kaolinite; Qz = quartz; Cal = calcite; Mc = microcline; Dol = dolomite; Ms = muscovite; Clc = clinoclore; Mag = magnetite.

Figure 4. Mineralogy of the raw materials (clay and sludges) obtained from X-ray diffraction analysis. According to [93,94]. Chl = chlorite; Ilt = illite; Pg = paragonite; Kln = kaolinite; Qz = quartz; Cal = calcite; Mc = microcline; Dol = dolomite; Ms = muscovite; Clc = clinoclore; Mag = magnetite.

The results obtained for the CC ceramic samples (control ceramics without tungsten-mining additives) indicate that the clay-based raw materials contain the following minerals (Figure 5): chlorite, illite, paragonite, kaolinite, quartz, calcite, microcline, and dolomite. As a consequence, slight, moderate, and substantial mineralogical transformations occur across the three thermal ranges applied for the preparation of these reference ceramics, produced without mining additives. At 800 °C, only illite, quartz, calcite, and hematite are detectable, with a very limited amorphous phase. These features are likely related to the dehydroxylation of the constituent clays (illite, kaolinite, and paragonite), leaving remnants of illite that remain only partially transformed, either as thermally unaltered residues or as material lacking long-range order, that is, displaying a partially amorphous structure. Calcite is still present at this temperature because decarbonation is incomplete, while hematite is observed, likely derived from the breakdown of chlorites and/or micas [95]. Quartz remains thermally stable at this temperature.

In summary, the ceramic products fired at 800 °C exhibit very limited ceramic maturity in the strict sense, due to the almost complete absence of stable crystalline phases and vitreous material. At 950 °C, illite is still detected as a remnant phase, indicating incomplete dehydroxylation; sanidine appears as an alkali feldspar, likely derived from the mobilization of microcline and paragonite; calcite persists due to the refractory nature of carbonates, which prevents full decomposition at this temperature, or possibly as a product of reprecipitation; and both hematite and quartz remain stable.

In the control ceramic (CC) fired at 1100 °C, the mineralogical assemblage becomes significantly simplified, comprising exclusively quartz, anorthite [96], diopside [97], and hematite, the latter acting as a stable or recrystallized phase derived from chlorite breakdown. Illite and sanidine disappear, likely incorporated into the feldspathic amorphous matrix and vitreous phase, while other phases emerge. Anorthite forms from the decomposition of calcite and dolomite, which at this temperature react with aluminosilicates originating from illite and/or kaolinite, producing a ceramic body with high firing maturity and a high degree of sintering, thereby imparting hardness and thermal stability to the final product.

Similarly, diopside develops as a result of ceramic metamorphism involving Ca (from calcite), Mg (from calcite or chlorite), and Si (from quartz or even kaolinite). Its presence at this temperature reflects the calcareous chemical character of the precursor clays, in this case from the clay residue [98,99].

With regard the ceramics incorporating mining residues (Figure 5), particularly those composed of 10% residue and 90% clay, the mineral phases identified are as follows: at 800 °C, illite, quartz, orthoclase, calcite and hematite; similarly, at 950 °C, illite, quartz, orthoclase, calcite and hematite are still present. However, at 1100 °C, illite, orthoclase and calcite disappear, giving way to new or persistent phases such as quartz, anorthite, diopside and hematite.

The overall behaviour is relatively similar to that observed in the control ceramics, with the distinctive feature of the incorporation of orthoclase at the two lower temperature ranges (800–950 °C), derived from other potassium-bearing minerals such as microcline, illite, muscovite or paragonite. Illite persists up to the intermediate temperature (950 °C), reflecting incomplete dehydroxylation and the presence of clay minerals that are not yet thermally altered, together with calcite that has not undergone full decarbonation. Likewise, hematite appears at all temperatures as a pore-filling phase, originating from the breakdown of chlorite, micas and magnetite present in the mining residues, the latter derived from Fe and associated with redox processes during firing.

The appearance of anorthite at 1100 °C is consistent with reactions involving Ca, Al and Si, together with a relative dolomitization reflected by the formation of diopside, an association of Ca, Mg and Si that forms through reactions between the clay minerals and free silica, producing plagioclase and pyroxene crystals. Orthoclase and illite dissolve into the liquid and glassy phase due to the dissolution or volatilisation of potassium during firing. Quartz remains stable across all temperatures, while the presence of anorthite and diopside at 1100 °C indicates that the resulting ceramic body exhibits optimal technological properties, including increased densification, reduced porosity, high hardness and enhanced thermal shock resistance [100,101,102].

For the ceramics containing 25% mining residues and 75% clay residues, the mineral phases identified are as follows: at 800 °C, illite, quartz, orthoclase, calcite and hematite; at 950 °C, illite, quartz, orthoclase, anorthite, calcite and hematite; and at 1100 °C, quartz, anorthite, mullite, diopside and hematite. On the one hand, the higher proportion of tungsten-mining residues increases the availability of carbonates and magnesium in the thermal reactions. On the other hand, at the lowest temperature (800 °C), the behaviour resembles that observed in the 10% mixture, where the resulting material does not yet exhibit fully developed ceramic characteristics. Illite remains undecomposed; orthoclase persists due to the presence of potassium, undergoing recrystallization and exhibiting partial thermal stability; calcite is still present due to incomplete decarbonation; and hematite is derived from Fe-bearing components in the starting materials.

At 950 °C, in addition to the newly formed mineral phases, early anorthite appears as a consequence of the increased availability of Ca, the likely smaller particle size and, consequently, greater specific surface area, and the onset of a liquid phase due to potassium released from orthoclase and illite acting as fluxes within the mixture [103,104].

At 1100 °C, mullite emerges alongside anorthite and diopside, indicating the development of ceramic quality. Mullite formation is attributed to the high reactivity of Al and Si, likely derived from minerals such as kaolinite. In this residue proportion, the early formation of anorthite, occurring at temperatures lower than expected (≈1000 °C), is indicative of improved ceramic technological performance, manifested through increased densification, higher refractoriness, and greater hardness. Together with mullite, these phases confer optimal mechanical strength and high refractory performance to the ceramic products [3,105].

In the mixture containing 50% tungsten-mining residues and 50% clay residues, the mineral phases identified are as follows: at 800 °C, illite, quartz, orthoclase, calcite and hematite; at 950 °C, illite, quartz, anorthite, calcite and hematite, where orthoclase is replaced by anorthite at this temperature; and at 1100 °C, the phase assemblage matches that of the 25-residue mixture, consisting of quartz, anorthite, mullite, diopside and hematite. The results at 800 °C and 1100 °C are therefore essentially identical to those observed in the mixture with 25% mining residues, with the only differences in neoformed phases occurring at the intermediate temperature of 950 °C. This behaviour is attributed to the reactive sensitivity of Ca and Mg toward the formation of a liquid phase within the 850–1000 °C interval, likely enhanced by the smaller particle size and corresponding increase in specific surface area. The partial formation of a liquid phase acts as a nucleation medium for adjacent materials, and overall, the higher proportion of tungsten-mining residues modifies the metastability of the mixture. Whereas the 25% mixture retains orthoclase and illite, with incipient anorthite not yet detectable by X-ray diffraction, the 50% mixture suggests a higher availability of Ca and Mg, together with increased nucleation–diffusion effects facilitated by the emerging liquid phase [106], and greater interparticle contact resulting from the larger specific surface area.

Taken together, the system exhibits enhanced chemical reactivity and a favourable transition toward thermodynamically more stable phases, such as anorthite, at the expense of metastable phases, notably orthoclase.

It appears evident that as the proportion of tungsten-mining residues increases, up to a theoretical limit at which an excess of fluxing agents could lead to over-melting of the ceramic body, and when medium-to-high firing temperatures (950–1100 °C) are applied, the resulting materials exhibit enhanced ceramic performance. This improvement is associated with the formation of a larger amount of stable and densifying phases, in which anorthite and diopside contribute to increased mechanical and thermal resistance while reducing porosity and promoting compaction in the sustainable ceramic bodies.

Similarly, greater chemical reactivity is observed, and consequently increased sintering, due to the higher availability of Ca and Mg, which act as fluxes within these temperature ranges, promoting the formation of a liquid phase and thereby yielding higher density, improved mechanical strength and reduced porosity. Finally, the metastability of the resulting ceramic materials diminishes, giving rise to thermodynamically more stable phases, which enhances durability and strengthens the sustainable value of the final products [107,108,109].

The absence of mullite in the ceramics produced with tungsten-mining residues and clay residues in a 10–90% proportion indicates a relative drawback in terms of ceramic technological quality (i.e., ceramics with lower refractoriness and earlier vitrification). This absence may be attributed to an alumina deficiency within the kaolinite–illite–muscovite system, and even paragonite, which may be partially diluted by the excess of calcium, magnesium and carbonates in general, thereby limiting the availability of essential aluminosilicates required for mullite nucleation. It may also reflect a greater tendency for the system to form anorthite and diopside instead of mullite, as the Ca and Mg present exert a partial sequestration effect on Al and Si. Furthermore, the early development of a liquid–glassy (amorphous) phase may inhibit the crystallization capacity necessary for mullite growth [85,110,111,112,113].

Table 3. Mineralogy of the materials studied: raw materials, ceramics without additives (CC), and ceramics containing different proportions of tungsten-mining residues relative to clay (10, 25, and 50%), determined by X-ray diffraction (XRD).

	CLAY	SLUDGE	CC			MC10%			MC25%			MC50%
			800	950	1100	800	950	1100	800	950	1100	800	950	1100
Chlorite	X
Illite	X	X	X	X		X	X		X	X		X	X
Paragonite	X
Kaolinite	X	X
Quartz	X	X	X	X	X	X	X	X	X	X	X	X	X	X
Calcite	X	X	X	X		X	X		X	X		X	X
Microcline	X
Dolomite	X	X
Clinochlore		X
Muscovite		X
Magnetite		X
Anorthite					X			X		X	X		X	X
Sanidine				X
Diopside					X			X			X			X
Hematite			X	X	X	X	X	X	X	X	X	X	X	X
Orthoclase						X	X		X	X		X
Mullite											X			X

summarizes that when only clay-based materials are used in the so-called control ceramics, the technological variations observed are primarily controlled by dehydroxylation, restructuring of phyllosilicate groups, and the decarbonation of Ca- and Mg-bearing compounds. With this mineralogical composition, no fully developed ceramic material is obtained at 800 °C; ceramic characteristics are only developed at higher temperatures, between 950 and 1100 °C.

The incorporation of tungsten-mining residues at 10, 25 and 50% enhances the reactivity of the mixtures, promoting the early formation of phases such as anorthite, which progressively replaces orthoclase. At 1100 °C, the resulting mineral assemblages converge toward a similar set of stable ceramic phases, namely quartz, anorthite, diopside, hematite and, occasionally, mullite [114].

3.3. Particle Size Distribution

The particle size distribution of the raw materials is shown in Figure 6.

The particle size distribution of the raw materials reveals marked differences between the Víznar clay and the mining sludge. The clay exhibits a narrow unimodal distribution, predominantly within the fine fraction (≈1–80 µm), with a modal size around 10–15 µm, indicating a relatively homogeneous material. In contrast, the mining sludge shows a significantly broader distribution, ranging from approximately 5 µm to over 1000 µm, with a dominant particle size between 150 and 300 µm, reflecting a highly heterogeneous granular system. This granulometric contrast results in a composite system combining fine and coarse fractions, which is expected to influence particle packing and pore structure development during thermal treatment, as finer particles enhance packing and sintering reactivity, while coarser particles act as temper and promote pore formation [35,115]. Consequently, the resulting microstructure and porosity evolution are consistent with the physicomechanical behaviour observed in clay-based ceramic systems [116]. The implications of this granulometric contrast on porosity and bulk density are discussed in the subsequent sections.

3.4. Hydric Tests

The results of the water absorption and desorption tests performed on the bricks are presented in Figure 7, which shows the mass variations as a function of time. According to the absorption curves, all tested ceramic samples exhibit a free water absorption capacity ranging from 13.77% to 22.02%, representing differences of nearly 50% between the lowest and highest values. Overall, the general trend observed in all ceramic samples indicates that higher firing temperatures lead to slightly lower water absorption values [116,117,118].

The test lasted approximately 60 days, until the samples became fully saturated under atmospheric pressure. It should be noted that the highest absorption rate in all samples occurred within the first 10–20 min from the beginning of the test.

In the specific case of the control samples (CCL, CCM, and CCH), which do not incorporate tungsten-mining additives, it was observed that their water absorption increases with rising firing temperature, from 19.51% at 800 °C to 20.96% at 950 °C, and 21.53% at 1100 °C. Although these values are very similar across the three temperatures, they represent a slight overall increase of just over 9% between 800 °C and 1100 °C. This behaviour is likely associated with a somewhat greater formation of liquid phase in the more highly sintered ceramics (i.e., those fired at higher temperatures), which promotes the development of pores with larger diameters [119,120], particularly vacuolar-type pores, potentially originating from gas release at elevated thermal ranges [121,122,123].

Regarding the group of sustainable bricks, those incorporating 10% tungsten-mining waste (MC10L, MC10M, and MC10H) exhibit water absorption values of 20.45% at 800 °C, 22.00% at 950 °C, and 22.34% at 1100 °C. The maximum percentage differences are slightly above 8%, indicating relatively modest variations in absorption across the firing temperatures. This behaviour may be attributed to the tendency of fluxing additives to become increasingly reactive at elevated temperatures, which, at 1100 °C, may promote the development of additional meso- and macroporosity [124,125,126,127].

The group of ceramics incorporating 25% tungsten-mining waste (MC25L, MC25M, and MC25H) exhibits maximum water absorption values of 18.39% at 800 °C, 19.15% at 950 °C, and 15.31% at 1100 °C. In this case, the values at 800 °C are lower than those of the previously discussed samples, increasing slightly at 950 °C, but decreasing at 1100 °C relative to both 800 °C and 950 °C. The difference between the minimum and maximum values represents a moderate variation, amounting to approximately 17% between 1100 °C and 800 °C, which is a considerable change that may be attributed to the higher flux content in the chemical composition of the mining sludges, particularly Fe, Na, and K (see Table 2) [84,128,129,130,131].

Finally, for the sustainable bricks incorporating 50% tungsten-mining waste at different firing temperatures (MC50L, MC50M, and MC50H), the water absorption values are 16.16% at 800 °C, 16.35% at 950 °C, and 13.73% at 1100 °C, with a difference of nearly 4% between the maximum and minimum values. These results indicate, first, that at higher temperatures (1100 °C) the extent of liquid-phase formation in the sustainable bricks becomes significant, leading to a microstructure dominated by micropores [76,132]. As a consequence, the amount of absorbed water is much lower than that observed in the rest of the ceramic production. Second, these findings support the conclusion that increasing the proportion of tungsten-mining waste in the bricks reduces their water uptake, thereby enhancing their potential long-term durability [133,134].

The desorption index values of the bricks range from 0.69% to 0.81%, corresponding to the sustainable bricks MC50M and MC10H, respectively. In all cases, the bricks studied exhibit medium-to-low drying indices, as complete drying generally occurs within approximately five days.

3.5. Mercury Intrusion Porosimetry (MIP)

The results of the mercury intrusion porosimetry analysis are summarized in

Table 4. Mercury intrusion porosimetry (MIP) results for control bricks and sustainable bricks incorporating tungsten mining waste. Po (%) represents the total accessible porosity, while pore size distribution is classified into mesopores (0.002–0.05 µm), macropores (0.05–10 µm) and air voids/cracks (>10 µm). Bulk density (Dt, g/cm³) was determined from mass-to-volume ratios.

		Pores Typology µm (%)			Bulk Density
	Po (%)	Mesopores (0.002–0.05)	Macropores (0.05–10)	Air Voids/Cracks (˃10)	Dt (g/cm3)
CCL	38.60	16.03	81.32	2.65	1.67
CCM	37.61	8.51	89.33	2.16	1.63
CCH	37.40	56.61	40.35	3.03	1.62
MC10L	38.40	16.15	78.05	5.80	1.65
MC10M	35.80	4.64	92.97	2.39	1.65
MC10H	34.90	3.49	92.54	3.97	1.62
MC25L	38.57	20.22	76.16	3.62	1.88
MC25M	34.92	6.12	89.34	4.54	1.74
MC25H	33.81	4.86	90.57	4.57	1.72
MC50L	37.59	8.26	86.38	5.36	1.91
MC50M	31.77	6.69	89.37	3.94	1.84
MC50H	24.80	6.57	85.70	7.73	1.83

. The total porosity values of all sustainable brick samples, including ceramic materials without additives and those incorporating tungsten-mining by-products, exhibit substantial variability as a function of the formulation, the additive proportions, and the firing temperature [135,136]. These results reveal a consistent trend toward lower total porosity with increasing firing temperature, which is partially aligned with the proportion of mining-derived additive incorporated into each specimen [137,138,139].

These maximum and minimum values, 38.60% for CCL and 24.80% for MC50, represent a difference of nearly 56% between both extremes. This indicates that substantial changes occur between the samples fired at low and high temperatures (from 800 °C to 1100 °C). In general, the incorporation of tungsten-mining residual sludges tends to reduce total porosity, an effect that is reinforced at higher firing temperatures and is particularly evident at a 50% additive proportion at 1100 °C [140,141].

These variations in pore structure directly influence the mechanical behaviour of the materials, as higher porosity and irregular pore distribution can reduce structural integrity, while densification and pore refinement enhance mechanical strength and durability.

The bulk density values range from 1.62 to 1.91 g/cm³, showing a general increasing trend with the incorporation of tungsten mining sludge and higher firing temperatures. The lowest values are observed in control samples (e.g., 1.62 g/cm³ for CCH), whereas mixtures with higher sludge content, particularly MC50, reach the highest densities (up to 1.91 g/cm³). This behaviour is consistent with the reduction in total porosity and the enhanced densification associated with fluxing phases present in the mining waste [140,141], which promote vitrification and particle bonding during firing. These results are also in agreement with the granulometric characteristics discussed in Section 3.3, where the combination of fine and coarse fractions is expected to influence packing efficiency and pore structure development [35,115]. However, the relationship is not strictly linear, reflecting the complex interplay between pore structure, gas evolution, and the heterogeneous nature of the added sludge.

The pore size distribution is predominantly unimodal, with average diameters ranging from 0.05 to 10 µm. However, notable variations in the proportions of mesopores, macropores, and air voids/cracks are observed depending on the sample type and firing temperature (Figure 8). Quantitatively, the highest proportion of pore sizes across all samples corresponds to macropores, with maximum and minimum values ranging from 92.97% to 40.35%, representing differences greater than 57%. The mesopore fraction is highly heterogeneous (56.61% to 3.49%), whereas, interestingly, the air void/crack category exhibits relatively consistent values (7.73% to 2.16%).

A particular case is represented by the so-called control ceramics (CC). It can be inferred that increasing the firing temperature leads to a marked reduction in the macropore percentage, decreasing from 81% in CCL to 40% in CCH. This behaviour suggests a densification effect induced solely by the thermal treatment, without the contribution of additional fluxing agents such as those present in the tungsten-mining sludges.

In contrast, the sustainable ceramic bricks incorporating tungsten-mining sludges exhibit a relatively opposite trend, as increasing the firing temperature promotes the formation of macropores, reaching a maximum value of 92.97% in MC10M. This behaviour is undoubtedly attributable to the presence of fluxing agents in the tungsten-mining waste sludges, as well as to their inherent compositional heterogeneity (see Table 2), and to a vacuole formation process within the vitreous phase, directly associated with gas release caused by the excess of fluxing components [142,143]. Finally, although the air void/crack fraction is minor in all bricks, its presence is variable and may be conditioned by internal contraction stresses generated during the high-temperature firing of the liquid phase. However, this interpretation is debatable, given the high residual content (50%), which not only acts as a fluxing component but also functions as a temper and as a structural framework former [115,129,144].

3.6. Ultrasonic Pulse Velocity

The ultrasonic pulse velocity (Vp) values obtained for the different samples under study, both without additives and with tungsten mining additives, together with the required unfired reference samples used to assess the potential influence of the additive on the clay-based material, as well as the firing temperature, are presented in

Table 5. Mean values and standard deviations of the ultrasonic pulse velocity test (m/s) for the samples analyzed, including NF (unfired), CC (ceramic material without tungsten-mining additive), MC10, MC25, and MC50 (ceramics incorporating 10, 25, and 50% additive), and L, M, and H (firing temperatures of 800, 950, and 1100 °C).

	Sample	$\overline{\mathrm{x}}$	σ	Sample	$\overline{\mathrm{x}}$	σ	Sample	$\overline{\mathrm{x}}$	σ	Sample	$\overline{\mathrm{x}}$	σ
NF	CC	1492	53	MC10	1515	58	MC25	1549	48	MC50	1680	33
800	CCL	2019	52	MC10L	1490	54	MC25L	1991	47	MC50L	2010	52
950	CCM	2283	66	MC10M	2200	82	MC25M	2256	35	MC50M	2389	37
1100	CCH	2592	52	MC10H	2600	49	MC25H	2840	57	MC50H	2963	41

. In general, the unfired samples exhibit low ultrasonic transmission values, with a slight increase depending on the absence or presence of sludge at 10, 25, and 50%. The values range from 1492 m/s to 1680 m/s, representing small differences of slightly more than 10%. This behaviour is consistent with the expected increase in mechanical strength as a function of the amount of sludge incorporated [145,146,147].

With respect to the fired samples without additives, referred to as the control samples (CC at 800, 950, and 1100 °C), consistent increases in ultrasonic velocity are observed, ranging from 2019 to 2592 m/s. This represents nearly a 30% improvement in the compaction of the samples made entirely of clay-based material. This behaviour is consistent with established ceramic science, as temperatures close to 800 °C promote dehydroxylation and particle packing, followed by the development of phases exhibiting optimal levels of vitrification between 950 and 1100 °C, where the formation and growth of an amorphous liquid phase occur. These processes enhance the sintering of the constituent materials and promote the closure of a significant proportion of pores, although small shrinkage-induced fissures may appear in the ceramic matrix [148,149].

In the remaining sustainable ceramic bricks incorporating 10, 25, and 50% sludge, there is a general trend of increasing ultrasonic velocity as the proportion of tungsten-mining additive increases (10, 25, and 50%). As expected, ultrasonic pulse velocity also increases with the rise in firing temperature from 800 to 1100 °C. At 800 °C, the values exhibit an increase of approximately 35% as the additive content rises from 10% to 50%, although it is noteworthy that the 10% mixture shows very low values, around 1490 m/s, which are similar to those of the unfired samples. This is attributed to the mere dehydroxylation of the clays and the absence of vitrification–sintering at this temperature. At 950 °C, the increases do not reach 10% when comparing ceramics with the lowest additive content to those containing 50%. In particular, the ultrasonic velocity of the 10% sludge mixture at this temperature is nearly 4% lower than that of the sample without mining additives, likely due to the limited action of the fluxing elements (K, Na, Ca) at this temperature, combined with the low additive proportion. At 1100 °C, the sustainable bricks show increases of approximately 14%, with velocities ranging from about 2600 m/s (10% sludge) to nearly 3000 m/s (50% sludge). This indicates that increasing the additive content in the mixtures promotes effective ceramic densification and the formation of an amorphous glassy phase at higher sludge incorporations [81,150,151]. This behaviour is consistent with the microstructural observations described in Section 3.1, where progressive densification and reduction in pore connectivity are observed with increasing temperature and sludge incorporation.

Figure 9 clearly illustrates that higher contents of mining residue and higher firing temperatures lead to improved compaction and enhanced mechanical reinforcement in the sustainable bricks, as well as to superior ceramic properties. These improvements are attributed to the action of the mining residue, which is rich in SiO₂–Al₂O₃ and contributes to the formation of solid structures that act as genuine load-bearing networks, together with the presence of crucial fluxing elements and minerals found in the tungsten-mining residues used in this study (Ca, Na, K, Sn, Ba). This enhancement undoubtedly represents a relevant advantage in terms of recyclability, sustainability, and the reuse of waste materials [15].

3.7. Compressive Strength

The compressive failure strength values for the different ceramic bricks, including the control samples without tungsten mining additives and those incorporating 10%, 25%, and 50% additives, are presented in

Table 6. Mean values and standard deviations of the modulus of rupture for the bricks without mining additives (CC), and for the sustainable ceramic bricks incorporating tungsten mining additives at different proportions (10%, 25%, and 50%), with respect to the studied firing temperatures. Mean values and standard deviations are expressed in MPa.

	Sample	$\overline{\mathrm{x}}$	σ	Sample	$\overline{\mathrm{x}}$	σ	Sample	$\overline{\mathrm{x}}$	σ	Sample	$\overline{\mathrm{x}}$	σ
800	CCL	11.97	0.1	MC10L	14.23	3.2	MC25L	16.95	3.2	MC50L	16.79	0.9
950	CCM	20.25	4.9	MC10M	17.55	2.4	MC25M	17.70	4.6	MC50M	18.26	1.3
1100	CCH	13.32	0.2	MC10H	20.42	2.5	MC25H	26.19	6.6	MC50H	23.88	3.2

and illustrated in Figure 10. The results exhibit a range between 11.97 and 26.19 MPa, with clear variations as a function of composition and firing temperature across the different types of sustainable bricks.

In the control samples (CC), the strength increases markedly between 800 and 950 °C, rising from 11.97 MPa (CCL) to 20.25 MPa (CCM), which represents almost a 70% improvement in mechanical strength. This behaviour is coherent with the expected sintering processes occurring in this temperature range for bricks without additives, where reductions in open porosity and particle rearrangement take place, accompanied by interparticle bonding and the formation of sintering necks. However, after firing at 1100 °C, the strength decreases to 13.32 MPa in sample CCH, representing a reduction in mechanical strength of more than 50%. This decrease may be associated with structural weakening linked to decarbonation processes, which involve the release of significant amounts of CO₂ that can lead to the formation of secondary pores or microcracking. Likewise, the heating rate used may influence the sintering process by promoting too rapid a sintering stage, with accelerated formation of the liquid phase and the development of closed porosity trapped within the glassy matrix. In such conditions, microbubbles may form, ultimately leading to a loss in uniaxial compressive strength [152,153,154,155].

In the mixtures containing 10% sludge (MC10), the strength values increase progressively with temperature, from 14.23 MPa at 800 °C to 20.42 MPa at 1100 °C in sample MC10H, representing an improvement of nearly 45% in mechanical strength. This behaviour confirms a progressive densification effect influenced by the presence of waste in small proportions, together with the fluxing action of Na, K, and Ca, particularly at 1100 °C. The enhancement in strength observed in these sustainable 10% bricks can be attributed to several factors. First, the very fine particles present in the mining waste act as sintering nuclei due to their higher specific surface area, which also promotes solid-state diffusion because of the chemical composition of the residue. Likewise, the consolidation of pore necks associated with the sintering process is expected as the temperature increases, generating a reduction in open porosity and improving the internal cohesion of the ceramic matrix. Additionally, the fluxing components present in tungsten mining waste, which despite their heterogeneous nature contribute to lowering the vitrification temperature, favour densification at intermediate temperatures (around 950 °C) and promote the formation of a liquid phase capable of filling pores [156,157,158,159,160].

The ceramics containing 25% additive (MC25) exhibit a similar trend, with strength increases of approximately 55% as a function of temperature. Notably, the value of 26.19 MPa obtained for the sustainable brick MC25H at 1100 °C represents the highest strength among all typologies, indicating that this addition level (25%) optimizes matrix consolidation and enhances mechanical performance. This superior behaviour can be attributed to several factors, including improved densification during sintering due to enhanced particle coalescence, the development of more consolidated sintering necks, reduction in open and interconnected porosity, and a decrease in the viscosity of the liquid phase, which limits the formation of microbubbles. In addition, an improved balance between crystalline and glassy phases is achieved, with a sufficient proportion of the latter to promote adequate cohesion, prevent deformation, microbubble formation, or microcracking, minimize internal heterogeneities and defects, and ensure optimal stabilization of volumetric shrinkage in this typology of sustainable ceramic bricks (MC25H) [137,161,162,163].

Finally, the mixtures containing 50% sludge (MC50) also exhibit increasing strength with temperature, reaching a minimum–maximum range from 16.79 MPa at 800 °C to 23.88 MPa at 1100 °C in sample MC50H, which represents an approximate increase of 42% between both temperatures. However, this maximum value for the combination of 50% tungsten mining waste and the highest firing temperature of 1100 °C (23.88 MPa) remains lower in uniaxial compressive strength compared with the sample containing 25% additive fired at 1100 °C. On the other hand, this group of bricks incorporating 50% mining waste clearly outperforms the control samples (without tungsten mining residue), confirming the potential of the waste to enhance the mechanical performance of sustainable ceramic bricks. This slightly anomalous behaviour may be explained first by a certain restriction in the sintering process due to the excess of fluxing components (Na, Ca, and K), which increases the amount of liquid phase but can also lead to an imbalance between the glassy and crystalline phases. Similarly, a rise in closed porosity may occur, with microbubbles formed by trapped gases (CO₂, SO₂ or small amounts of water vapour), which in some cases become a critical factor in structural ceramics. This behaviour can be interpreted in light of the observed microstructural evolution. The increase in compressive strength with firing temperature and sludge content is associated with enhanced vitrification and the formation of a more continuous and compact ceramic matrix. The reduction in open porosity and the strengthening of interparticle bonds improve load transfer within the material. However, at higher temperatures, the formation of excessive glassy phases and closed porosity may generate internal stress concentrations, which can explain local reductions in mechanical strength.

Finally, it is plausible to infer that an excessively glassy matrix may reduce the toughness of the ceramic material, increasing its brittleness [164,165,166,167].

In any case, as part of the discussion, all the sustainable ceramic brick samples exceed the minimum mechanical strength threshold, which is 9.81 MPa for solid and vertically perforated bricks and 4.90 MPa for structural hollow bricks, according to the applicable standards [168,169].

3.8. Environmental Scanning Electron Microscope (ESEM)

The ESEM analysis of the ceramic products revealed a heterogeneous texture and elemental composition consistent with the raw materials used, residual natural clays and tungsten mining tailings, combined in the known proportions of 10, 25, and 50%. This significant heterogeneity is characteristic of the production of sustainable bricks and ceramic materials (Figure 11).

A detailed visual inspection of the twelve subfigures in Figure 11 highlights the following representative microstructural features. In the control samples (no sludge), subfigure 1 (800 °C) shows a representative pore (blue circle) associated with low-temperature firing, while the red dashed rectangle highlights clay lamellae with low sintering and high porosity. In subfigure 5 (950 °C), the pore morphology is maintained, and the highlighted area indicates the onset of densification. In subfigure 9 (1100 °C), the pore remains representative of the matrix, whereas the red dashed rectangle shows a sintered region together with a fractured temper grain, attributable to the elevated firing temperature.

For samples containing 10% sludge, subfigure 2 (800 °C) exhibits a large representative pore (≈200 µm) and a cluster of clay and temper grains with negligible sintering. In subfigure 6 (950 °C), the pore is associated with partially sintered clay regions showing contraction around temper grains. In subfigure 10 (1100 °C), the pore adopts a typical vacuolar morphology within a sintered matrix, while the highlighted area corresponds to a solidified liquid phase containing characteristic gas-escape nanopores.

In the samples with 25% sludge, subfigure 3 (800 °C) shows a pore typical of poorly sintered material and the presence of unmelted metallic particles (red arrows). In subfigure 7 (950 °C), moderate sintering is observed, with pores and persistent unmelted metallic inclusions. In subfigure 11 (1100 °C), a highly sintered matrix is evident, with vacuolar pores, residual unmelted metals, and mullite recrystallizations (green rectangles).

Finally, in samples containing 50% sludge, subfigure 4 (800 °C) presents a pore typical of non-sintered material and an elongated temper grain reflecting initial heterogeneity. In subfigure 8 (950 °C), progressively vitrified phyllosilicate grains (≈280 µm) coexist with partially melted and resolidified areas (≈60 µm), together with unmelted metallic particles. In subfigure 12 (1100 °C), nearly fully melted phyllosilicate phases dominate, accompanied by residual metallic inclusions and mullite recrystallizations.

In general, the ceramic matrix is composed mainly of phyllosilicates, which constitute the dominant materials and are particularly rich in calcium (>10%), potassium, sodium, and iron, as confirmed by the XRF and XRD results. These analyses also reveal the presence of other minerals such as muscovite, a potassium and aluminum silicate characterized by its platy habit and fluxing capacity at specific temperatures, as well as visually identifiable kaolinite and illite aggregates. Quartz crystals are also noteworthy due to their size and abundance, being dominant in the mining residues owing to the veins from which tungsten is extracted, and therefore prevailing in the starting sludge materials. Additionally, calcium and magnesium carbonates, calcium sulphates, and iron recrystallizations have been identified. Finally, in the ceramic pastes containing mining sludge, arsenic has been detected along with other metallic elements such as zinc, titanium, barium, tin, and tungsten [170]. In the aforementioned Figure 11, backscattered electron microscopy images of the studied samples are shown, where textural features and various mineral morphologies are identified.

The samples fired at 800 °C exhibit an irregular surface. The dehydrated clay lamellae appear arranged in compact aggregations, with pores of varied morphology and sizes ranging from 10 to 50 µm. In general, both the ceramic materials without residue and those containing tungsten mining residues (sustainable bricks with 10, 25, and 50% additions) display the characteristic texture of underfired ceramics, that is, poor densification and an absence of vitrification, where only the hydroxyl groups have been removed from the clay minerals and part of the CO₂ present in carbonates has volatilized, together with the combustion of organic matter [171]. Mineral grains that behave as temper within this thermal range appear well defined and without signs of thermal alteration. Clusters of unmelted metallic fragments are also observed, highlighted with red dashed rectangles accompanied by an arrow (Figure 11, images 1 to 4).

In the 950 °C range, particle contacts become more evident and a more densified matrix with signs of sintering can be distinguished (Figure 11, images 5 to 8). Dehydroxylation is complete in the clay materials and the pores continue to exhibit irregular morphology, with heterogeneous sizes ranging from 10 to 200 µm. Some mineral grains, such as carbonates and quartz, show slight thermal alterations and small shrinkage cracks. In the mixtures containing 25% sludge, the quartz grains are clearly identifiable and iron recrystallizations, together with associated metals such as zinc, are prominent, marked with red dashed lines and an arrow. Moreover, mica fragments are clearly visible, although they show evidence of partial melting (Figure 11, image 8). Overall, this group of ceramics without additives (Figure 11, image 5) as well as those with 10, 25, and 50% additions (Figure 11, images 6 to 8) display partially densified matrices with contact pores originating from the tungsten mining additives and the clays, together with clusters of partially vitrified materials [172].

At 1100 °C, the compactness and vitrification of the matrix are evident in all sample types, although they are more pronounced in the mixtures containing tungsten mining sludge than in the ceramics without additives, due to the lower flux content in the latter mixtures (Figure 11, images 9 to 12). Visually, all matrices within this thermal range show very good vitrification, and the densification of the sustainable bricks depends on the variability in the content of mining additives. The ceramics with 10, 25, and 50% mining residue exhibit a typical texture of semi complete vitrification, and the pores, which are irregular in size, adopt circular shapes along with the characteristic presence of vacuoles or bubbles formed by gas release and the coalescence of adjacent pores. Some phyllosilicates retain their laminar habit, although large vitrified areas are present between the particles [173]. The quartz grains display substantial thermal alteration with intergranular fractures, and remnants of unmelted metals derived from mining residues are also observed (Figure 11, images 9 to 12). The optimal mixture based on electron microscopy criteria is the one combined with 25% mining residue at 1100 °C, where mullite recrystallizations can be observed within green dashed rectangles (Figure 11, image 11). These microstructural features have direct implications for the mechanical behaviour of the studied materials. The development of partially vitrified matrices and enhanced interparticle bonding at higher firing temperatures promotes densification, which is consistent with the increase in compressive strength and ultrasonic pulse velocity observed in the samples. Conversely, the presence of pores, microcracks, and heterogeneous phase distributions can act as stress concentrators, locally weakening the ceramic matrix and influencing its mechanical performance.

Figure 12 presents the elemental maps and their associations for the sustainable brick containing 25% tungsten mining residues fired at 1100 °C (Figure 12, images 1 to 8). The distributions of silica alumina phases that form the ceramic matrix can be observed, together with several of the main fluxing elements derived from the clay and from the mining residues, such as potassium and calcium. In addition, nodules of hematite recrystallizations are visible (Figure 12, image 6), along with vitrified matrix nodules enriched in zinc, which has also acted as a fluxing agent (Figure 12, image 7).

3.9. Leaching Test

Table 7. Concentration, in parts per billion (ppb), of the chemical elements determined in the eluates obtained from the leaching test by ICP-MS, as well as their retention within the ceramic matrix as a function of the mixture composition and the temperature assessed. The elements are listed according to their toxicity level for human health, following [174]. The letter L denotes samples fired at 800 °C, and H those tested at 1100 °C.

	Sludges	MC10L	MC25L	MC50L	MC10H	MC25H	MC50H
As	9769.30	97.60	276.80	1481.20	88.70	202.80	826.80
Cd	77.10	0.04	0.00	1.40	1.10	0.90	4.10
Pb	1.20	0.00	0.20	0.20	0.70	0.70	0.70
Ni	367.60	0.80	0.90	1.20	0.80	2.70	5.80
Co	109.90	0.00	0.07	0.40	0.20	0.06	0.70
Cu	68.53	10.40	0.50	3.00	10.00	10.40	23.60
W	12,571.30	69.10	593.00	778.10	81.90	88.20	674.00
Mn	2168.93	0.03	0.30	0.10	1.30	2.10	2.70
Fe	4186.70	6.40	11.10	6.50	11.60	8.50	8.10
Zn	13,907.80	0.00	0.00	0.00	16.40	3.30	43.20

reports the concentration of chemical elements leached from the corresponding eluates of the mining sludge samples and the ceramic samples of sustainable bricks, expressed in parts per billion, and Figure 13 illustrates the relative retention of these chemical elements.

According to previous studies [58,175,176], the concentrations of the most prevalent and environmentally hazardous elements present in the sludge samples were evaluated. These elements were ranked from highest to lowest toxicity to human health and the environment, following the classifications reported by [177,178,179,180], namely: As, Cd, Pb, Ni, Cu, Co, Mn, Zn, Fe, and W.

In general, all ceramic materials inhibit the leaching capacity inherent to mining sludges, as all sustainable bricks, both those sintered at very low temperatures (800 °C) and those produced at optimal sintering temperatures (1100 °C), exhibit a strong retention of toxic elements. This represents a key characteristic of these sustainable bricks manufactured from tungsten mining tailings.

To contextualize the environmental performance of the developed materials, the leaching results were compared with commonly accepted regulatory criteria and standard reference frameworks for hazardous elements, such as those defined in European regulations (e.g., Council Decision 2003/33/EC; EN 12457-2) [181,182] and widely used leaching protocols such as the U.S. EPA Toxicity Characteristic Leaching Procedure (TCLP, Method 1311) [183]. Although direct comparison is limited due to differences in testing procedures and units, these frameworks provide a widely accepted basis for assessing the environmental behaviour of the materials.

The concentrations measured in all ceramic samples are significantly reduced compared to the raw sludge and fall within ranges typically considered acceptable for ceramic construction materials according to these regulatory and technical frameworks. This behaviour is consistent with previous studies demonstrating the effective immobilization of heavy metals within ceramic matrices during firing, which significantly limits their mobility and environmental risk [47,184].

Therefore, the obtained results suggest that the manufactured ceramic bricks present a low environmental risk and can be considered suitable for construction applications, although compliance with site-specific regulatory requirements should be further verified where necessary.

The results indicate that the sustainable bricks developed in this study effectively retain toxic elements, supporting their potential for environmentally responsible applications and contributing to the reuse of mining residues that would otherwise be difficult to manage within conventional recycling streams.

The higher amount of leachates observed in the sludges is explained by their lack of encapsulation within a ceramic matrix or any other encapsulating binder, as well as by the fine particle size and the high specific surface area characteristic of these sludges [185,186], factors that confer to these waste materials a high chemical reactivity when exposed to weathering and, in particular, to leaching processes.

Figure 13. Variability in the reduction in leaching of contaminant elements in ceramic samples of sustainable bricks incorporating tungsten mining tailings at 10, 25, and 50 percent, fired at two temperatures (800 and 1100 °C). The reduction in leaching retained by the ceramic matrix, relative to the tungsten mining residues, is expressed in ppb. The graphical order is based on toxicity to human health according to [187].

Figure 13. Variability in the reduction in leaching of contaminant elements in ceramic samples of sustainable bricks incorporating tungsten mining tailings at 10, 25, and 50 percent, fired at two temperatures (800 and 1100 °C). The reduction in leaching retained by the ceramic matrix, relative to the tungsten mining residues, is expressed in ppb. The graphical order is based on toxicity to human health according to [187].

In detail, for Mn, there is substantial encapsulation within the ceramic matrix of the sustainable bricks, with leachate concentrations approaching zero in the samples fired at 800 °C across all formulated proportions (10, 25, and 50 percent), as well as drastic inhibitions in the bricks fired at 1100 °C, which achieve even higher encapsulation efficiencies, specifically nearly five orders of magnitude. Similarly, Zn exhibits zero leachate values in the samples fired at 800 °C at all concentrations, with inhibitions exceeding 3.6 orders of magnitude, corresponding to more than a 4000-fold reduction in the metallic element. In the case of Cd, which is particularly important due to its toxicity [186], variations in leaching show differences of nearly 3.3 orders of magnitude, with inhibitions greater than 1900-fold. Co exhibits variations that in some cases reach complete inhibition, with reductions exceeding 3.25 orders of magnitude and inhibitions greater than 1800-fold. Fe shows variability above 2.8 orders of magnitude, with inhibitions exceeding 650-fold. Ni displays variations of 2.66 orders of magnitude, W of 2.26, Cu of 2.14, As of 2.04, and Pb of 0.78 orders of magnitude.

A greater degree of leaching is observed for tungsten and arsenic, which may indicate that thermally induced mineral decomposition can lead to the formation of mineral phases with heterogeneous stability, making certain elements more prone to leaching. The results of the test suggest that As, like W, can be more readily mobilized and extracted by the leaching agent in some of the designed mixtures after thermal treatments at 800 and 1100 °C. This behaviour is consistent with the pH values, which is of relative significance (Figure 14), since an increase in pH toward more basic eluates compared with those measured in the sludges may be associated with this enhanced mobility.

Similarly, according to [187], the mobility of tungsten in water with neutral pH is classified as intermediate to low. However, the analysis of the eluates indicates a release of W in all ceramic samples, despite the fact that the measured pH values are not excessively acidic or basic, but fall within the range of 6 to 8. In the case of W, this behaviour could be related to the thermal treatments applied during firing, which may enhance the mobility of this metal. In this regard, the samples fired at 1100 °C exhibit tungsten concentrations that are very similar to those of the same mixtures fired at 800 °C [58,184,185].

3.10. Colour

Table 8. Colour values of the raw materials and ceramic products according to the CIE Lab 1976 system [188]. λ (dominant wavelength); L* (lightness); a* (red tone +, green tone –); b* (yellow tone +, blue tone –); ΔE (total colour difference of the samples containing tungsten mining residues relative to the undoped samples at all firing temperatures).

	SAMPLE	λ		L*		a*		b*		ΔE
		$\overline{\mathrm{x}}$	σ	$\overline{\mathrm{x}}$	σ	$\overline{\mathrm{x}}$	σ	$\overline{\mathrm{x}}$	σ
	CLAY	574.27	0.03	59.09	0.20	2.82	0.06	16.90	0.17
	SLUDGE	572.29	0.05	53.79	0.19	0.36	0.02	5.98	0.04
800 °C	CCL	586.18	0.37	53.11	0.31	19.31	0.92	25.44	1.00	0.00
	MC10L	585.15	0.20	58.27	0.81	19.04	0.66	26.63	0.88	5.30
	MC25L	586.90	0.22	55.68	0.32	22.59	0.50	29.33	0.46	5.70
	MC50L	585.20	0.44	53.42	1.62	19.55	0.86	28.28	0.80	2.87
950 °C	CCM	587.32	0.07	54.43	0.56	21.14	0.13	25.95	0.43	0.00
	MC10M	586.63	0.07	56.66	0.53	20.17	0.48	25.48	0.75	2.48
	MC25M	587.53	0.35	51.22	0.69	20.59	0.41	25.01	0.25	3.39
	MC50M	586.97	0.33	53.28	1.41	21.03	0.41	26.71	0.48	1.38
1100 °C	CCH	586.09	0.30	57.54	0.73	19.62	0.47	25.61	0.46	0.00
	MC10H	583.95	0.30	54.96	0.87	16.28	0.22	24.91	0.32	4.28
	MC25H	585.33	0.96	52.54	1.04	14.64	0.63	19.12	0.45	9.59
	MC50H	588.32	0.09	49.15	1.17	19.64	0.32	22.20	0.45	9.06

presents the chromatic values of the raw materials and of the resulting ceramic products, including both the samples without tungsten mining residues and the sustainable bricks incorporating 10, 25, and 50% tungsten mining additives, fired at 800, 950, and 1100 °C. Figure 15 displays the corresponding chromatic coordinates (a* and b*) in the chromaticity plane and along the lightness axis (L*), whereas Figure 16 shows the total colour differences (ΔE) of the ceramic samples of sustainable bricks containing tungsten mining residues, relative to the control ceramics that do not include mining sludge.

All the analyzed materials, including the raw materials and the manufactured ceramic products containing 10, 25, and 50% tungsten mining sludge, and fired at 800, 950, and 1100 °C, are positioned in the yellow (b*+) and red (a*+) tonal quadrant. The lightness values of all samples range between 60% and 49% on the L* scale.

Regarding the unfired raw materials used in this study, both the clay and the sludge exhibit a predominant yellow tone. In the reconstructed colour chart, based on the mean L*, a*, and b* values, they may appear to include greenish hues, although this is not the case quantitatively. The clay is more saturated and contains a higher proportion of yellow relative to red, almost six times higher, which, together with its lightness component, results in a yellowish and slightly pinkish washed-out colour.

In contrast, the dominant tone of the additives is also yellow, with a much lower presence of the red component. In the raw state, the additives are almost seventeen times more yellow than red, and together with their very low lightness value, this produces a very pale yellow colour, almost greyish. The differences in the yellow component between the clay and the sludge are approximately threefold in favour of the clay, while for the red component, the clay contains nearly eight times more than the additives.

The amount of yellow and red in both the clay and the sludge mainly derives from iron oxide in different mineral phases, chemical valence states, and hydration states, whether Fe₂O₃ (ferric oxide, red-brown hematite), FeO(OH) (ferric hydroxide, yellow-ochre goethite), or FeO (ferrous oxide, dark grey to black). It is noteworthy that the highest chemical proportion of Fe is found in the mining residues, approximately 11%, rather than in the clay, which contains about 6%. It becomes evident that, due to the thermal transformation of these oxides and their incorporation into the ceramic materials, both with and without additives, the iron oxides present in the raw materials undergo oxidation during firing. This process leads them, at varying intensities, to shift toward red hues at the expense of yellow hues, in accordance with the characteristics of the firing process [89,189,190].

With respect to all the ceramic samples analyzed, all ceramics, both with and without additives, are located in the yellow–red quadrant in the a*–b* chromaticity plane. Regarding lightness values, all ceramic materials fall between 58 and 49 percent as maximum and minimum, respectively, which suggests an appearance close to grey. In general, all ceramic materials at all studied temperatures shift toward more orange hues compared to their unfired raw materials, whether they contain no additives, as in the case of the control ceramics, or are doped with mining residues, as in the case of the sustainable bricks at 10, 25, and 50 percent. Overall, the most saturated orange shifts occur at the highest firing temperature of 1100 °C.

Specifically, in the case of ceramics that do not contain mining residues, the chromatic variation from the clay raw material to the ceramic products at all firing temperatures results in a 100 percent increase in colour saturation. Chromatically, this also represents an increase in the red hue relative to the yellow hue compared to the unfired state, although the red and yellow components do not reach an equal balance. The resulting colour in the non-doped ceramics is an orange hue with a slight predominance of yellow. The lightness L* of all ceramics without additives decreases by almost 12 percent compared to the raw materials, and interestingly, the samples fired at the highest temperatures (950–1100 °C) display higher lightness values.

The effect of adding 10 percent mining residues, relative to the amount of clay in the recyclable ceramic products obtained (sustainable bricks), is analogous to that observed in the non-doped ceramics. The samples shift from yellow hues toward orange hues, although with a predominance of yellow over red. The increase in saturation, from the initial saturation of the raw mixture to the fired state, exceeds 300 percent. The lightness values observed at this proportion, compared to the non-doped ceramics, show higher lightness at lower temperatures, 800–950 °C, with values similar to those of the ceramics without additives.

For the ceramic mixtures with 25 percent mining residues, the effect is more pronounced, such that the samples appear more orange, particularly due to the influence of lightness L*, together with the higher additive content. In general, all samples are more luminous than the ceramics without additives. In the chromaticity plane, the ceramic samples show an increase in saturation of up to 300 percent compared with their unfired mixtures, except for the mixture fired at the highest temperature, 1100 °C, which exhibits lower saturation than the others, although still showing an increase above 140 percent. Their hues shift toward red from the original yellow hue, although the yellow component remains slightly higher. As a result, these samples display a highly saturated orange appearance.

Ceramic mixtures containing 50 percent mining residues experience increases in saturation above 200 percent, and, as occurs with the mixtures at other temperatures, they shift more intensely toward red hues. This effect is particularly evident in the sample with 50 percent residues fired at 1100 °C, which becomes the most orange of all the samples analyzed. This behaviour is a consequence of the oxidation of the iron-bearing components present in the residues at that temperature, largely due to the transformation from goethite to hematite [191,192,193,194,195,196]. The lightness values, however, are lower than those of the non-doped ceramics, with an approximate 10 percent decrease in the L* value.

Finally, regarding the total colour differences, ΔE*ab 1976 [188,197], the results show that all ceramic samples containing mining residues exhibit significant variations in the colorimetric parameters L*, a*, and b*, with greater or lesser magnitude. The observed changes range from slightly above 1 unit to nearly 10 units [198,199,200,201]. The largest differences generally occur in the ceramics fired at the highest temperature, 1100 °C, particularly in those doped with 25 and 50 percent residues, while notably smaller differences appear in the samples fired at intermediate temperature, 950 °C, compared with those fired at the lowest temperature, 800 °C. Almost all ceramic samples doped with mining residues present variations perceptible to the human eye according to [202], except for the samples with 50 percent tungsten mining residues fired at 800 °C, and those with 10 and 50 percent residues fired at 950 °C.

All the chromatic changes observed in these ceramic bricks produced from the recycling of clays and tungsten mining sludge are of fundamental importance for sustainable construction, architectural rehabilitation, and the restoration and conservation of architectural heritage. In particular, the ability to control the final colour of sustainable ceramic products through the greater or lesser addition of mining residues and the adjustment of firing temperature is especially valuable [203,204,205,206].

4. Discussion

The chemical composition of the raw materials shows that both the clay and the mining sludges consist predominantly of SiO₂ and Al₂O₃, a feature that ensures suitable ceramic behaviour. The fluxing oxides (CaO, K₂O, Na₂O, MgO, Fe₂O₃) promote a decrease in the vitrification temperature and enhance matrix densification. Although the mining sludges contain relevant concentrations of potentially hazardous elements, particularly As and F, their incorporation into ceramic mixtures enables their immobilization within appropriate firing temperature ranges. Overall, from a compositional standpoint, the chemical complementarity between clay and mining sludges supports their suitability for producing sustainable bricks across wide firing intervals [207,208,209].

The mineralogy of the clays and mining sludges includes phases suitable for ceramic development, notably phyllosilicates (illite, kaolinite, muscovite, paragonite), quartz, and minor proportions of carbonates, which control the main thermal transformations. From 950 °C up to 1100 °C, anorthite and diopside form [104,210], promoted by the Ca and Mg contributions derived from tungsten mining residues, indicating good ceramic reactivity, enhanced densification, and reduced porosity. The absence of mullite at low residue additions may be related to a deficit of aluminosilicates and fluxing phases in the mixtures, affecting “ceramic maturity.” Overall, the mineralogical assessment demonstrates that the controlled incorporation of residues, particularly at 25–50%, promotes stable and technologically favourable phases for sustainable brick production, improving densification, thermal stability, and mechanical performance.

Water absorption depends on both the firing temperature and the tungsten residue content. Bricks containing 50% residue exhibit the lowest absorption values (16.16–13.73%), with only a 4% variation across temperatures, revealing a well-sintered microstructure at 1100 °C dominated by nanopores. This more compact structure clearly reduces water uptake compared with the other formulations and suggests that higher residue contents generate more densified and stable matrices, resulting in improved durability [211,212].

Mercury intrusion porosimetry confirms that both total porosity and pore size distribution depend on the firing temperature and on the tungsten residue content. Increasing temperature reduces the porosity accessible to water and gases (from the perspective of hydraulic testing), particularly in formulations containing 50% additive, where a higher degree of matrix densification is observed due to the fluxing action of the tungsten mining residue. The pore size distribution is essentially unimodal and dominated by macropores, although its evolution varies among mixtures: in ceramics without residue, macropores decrease with temperature, whereas in residue-bearing formulations they increase due to the formation of vacuoles associated with gas release. Mesopores show high variability, and air voids or cracks are scarce, being more closely related to shrinkage-induced stresses than to residue incorporation. Overall, the mining sludges act as microstructural modifiers, adjusting porosity and promoting densification in sustainable ceramic materials during firing [212,213].

The marked granulometric contrast between the fine Víznar clay and the coarser Panasqueira mining sludge emerges as a key controlling factor in the microstructural evolution and performance of the ceramic system. This contrast governs particle packing, sintering behaviour and pore structure development, linking the initial raw material characteristics to the final physicomechanical properties. The optimal balance between densification and porosity is achieved in the mixtures containing 25 wt% sludge fired at 1100 °C, where the interaction between the fine clay matrix and the coarser fraction promotes an efficient packing arrangement and controlled vitrification. This synergistic behaviour enhances densification while preventing the excessive formation of closed porosity, leading to superior mechanical performance, as confirmed by compressive strength and ultrasonic velocity results. In contrast, higher sludge contents (50 wt%) increase granulometric heterogeneity and favour the development of entrapped pores within the vitrified matrix, which can limit further improvements in mechanical properties despite the higher availability of fluxing agents.

These findings are consistent with previous studies highlighting the strong influence of sintering conditions and pore structure on ceramic performance, [124,127] as well as the critical role of particle size distribution in controlling densification processes, phase evolution and microstructural homogeneity in clay-based ceramic systems [35,115]. This highlights the importance of optimizing both composition and granulometry when incorporating mining residues into sustainable ceramic materials.

Ultrasonic testing shows that both the firing temperature and the tungsten residue content have a clear influence on the ultrasonic pulse velocity (Vp). Unfired samples exhibit low Vp values, with slight increases attributed to the mechanical effect of the sludge. After firing, the control ceramics progressively increase their Vp as a result of densification and glass-phase formation. In the sustainable bricks, the simultaneous increase in temperature and residue content further enhances Vp, particularly at 1100 °C, where the formulations containing 50% residue display the highest compaction levels. This behaviour confirms the fluxing and structural role of the residue, which promotes denser microstructures and improved mechanical performance [214,215,216].

Compressive strength shows a clear dependence on both firing temperature and tungsten residue content. Ceramics without additives increase their strength at 800 and 950 °C but exhibit a decrease at 1100 °C, likely due to over-vitrification and microcracking associated with rapid firing. In contrast, the sustainable bricks display a continuous increase in strength with temperature, highlighting the densifying effect of the sludge through enhanced glass-phase formation and effective sintering. The maximum value is obtained in MC25H (26.19 MPa), indicating that a 25% residue content provides an optimal balance between vitrification, stability, and cohesion. This result is further supported by the combined analysis of porosity and microstructural characteristics [31,34,35], which confirms that the optimal performance is achieved for samples containing 25 wt% sludge fired at 1100 °C. Although the mixtures with 50% residue do not surpass this value, they maintain strengths higher than those of the residue-free ceramics. Moreover, all formulations comfortably meet regulatory requirements (>9.81 MPa for solid bricks and >4.90 MPa for hollow units), confirming the potential of mining residue to improve the mechanical performance of sustainable ceramic bricks [217,218,219].

Electron microscopy images highlight a marked compositional heterogeneity consistent with the clay-based raw materials and the tungsten residues. At 800 °C, underfired textures predominate, characterized by dehydroxylated phyllosilicates, minimally altered grains, and irregular pores, with no evidence of vitrification. At 950 °C, a partially densified matrix is observed, with narrower interparticle contacts and incipient thermal alterations in quartz and carbonates, together with metallic aggregates derived from the residue (Fe, Zn). At 1100 °C, vitrification becomes dominant, especially in the residue-bearing mixtures, where vacuoles resulting from gas release and mullite recrystallization appear, indicating advanced sintering and greater “ceramic maturity.” Overall, the observations confirm that the residue acts as an effective flux, accelerating glass-phase formation and modifying microstructural evolution, particularly at 25–50% additions and at high firing temperatures [124].

Leaching tests demonstrate that the incorporation of mining sludges into ceramic matrices significantly reduces the mobility of heavy metals compared with the untreated residue, at both 800 °C and 1100 °C. The pronounced decrease in As, Cd, Zn, Mn, Co, and Ni, with reductions ranging from two to more than four orders of magnitude, confirms highly effective encapsulation associated with vitrification and pore closure within the ceramic matrix. Although tungsten and arsenic exhibit higher leaching values than the rest, their concentrations remain far below those of the non-encapsulated residue, likely due to the formation of phases with variable stability during firing and to slight variations in eluate pH. Overall, the results indicate that these sustainable bricks immobilize toxic elements with high efficiency, supporting their potential environmental suitability and their capacity to stabilize mining wastes in durable ceramic products [220].

From an application standpoint, the developed ceramic materials can be considered suitable for use in both new construction and architectural rehabilitation. Their physical and mechanical properties, including compressive strength values well above regulatory thresholds, adequate ultrasonic velocity, and controlled porosity, ensure sufficient structural performance for masonry elements such as solid bricks, facing bricks, and non-load-bearing components. In the context of architectural rehabilitation, several factors support their suitability. First, the controlled chromatic response achieved through varying firing temperature and sludge content allows the reproduction of traditional reddish–orange tonalities commonly found in historical ceramic materials, facilitating visual integration in heritage contexts. Second, the observed microstructural stability and reduced water absorption at higher firing temperatures contribute to improved durability, which is essential for long-term conservation interventions [116,117,118,119]. Third, the effective immobilization of heavy metals demonstrated by the leaching tests supports environmental safety, a key requirement for the reuse of waste-derived materials in built environments [47,220].

Additionally, the incorporation of mining residues aligns with circular economy strategies and sustainable restoration practices, where the use of secondary raw materials is increasingly encouraged [213]. Overall, the combination of mechanical performance, durability, esthetic compatibility, and Environmental performance supports the potential application of these sustainable ceramic bricks in restoration works, façade replacement, and architectural rehabilitation projects.

CIELab analysis shows that all ceramic formulations fall within the yellow-red quadrant, controlled by the iron content and its thermal transformations. Firing induces a progressive shift toward more orange hues, with clear increases in a* and decreases in L*, an effect that is more pronounced at 1100 °C and in mixtures with higher sludge contents, where oxidation during firing and vitrification enhance colour saturation with the appearance of hematite. Additions of 25 to 50 percent reinforce this behaviour due to their higher iron contribution and the initial reduction in luminosity, resulting in redder, more saturated, and darker materials. The ΔE variations are, in most cases, perceptible to the human eye when compared with the control ceramics, especially at elevated temperatures. Overall, the proportion of residue and the firing temperature allow the final colour to be modulated in a reproducible manner, which adds value for architectural and conservation applications where chromatic response is a highly relevant parameter [221,222].

It should be noted that the selected ranges of sludge incorporation (10–50 wt%) and firing temperatures (800–1100 °C) were defined based on previous studies and on the expected behaviour of flux-rich ceramic systems. Although a finer resolution of intermediate values could further refine the identification of optimal conditions, the results obtained allow the main trends governing densification, vitrification, and mechanical performance to be clearly established.

Furthermore, higher sludge contents (>50 wt%) or firing temperatures (>1100 °C) are expected to promote excessive liquid phase formation, leading to over-vitrification, increased closed porosity, and potential deterioration of mechanical properties. Therefore, the selected experimental window represents a balanced and appropriate range for evaluating the feasibility and performance of these sustainable ceramic materials.

The transportation of raw materials between the source locations may contribute to the overall environmental impact of the developed ceramic materials. However, this factor must be considered within the broader framework of resource efficiency and waste management. The reuse of abundant mining sludge and industrial clay residues reduces the demand for primary raw materials and minimizes waste disposal, resulting in an overall environmental benefit. Furthermore, this approach is inherently adaptable and can be implemented using locally available residues in different geographical contexts, thereby reducing transport-related impacts in practical applications.

The mechanical performance of the sustainable ceramic materials is strongly governed by their microstructural characteristics. The interplay between pore structure, phase transformations, and vitrification processes controls the degree of densification and, consequently, the load-bearing capacity of the material. Microstructural observations indicate that increased sludge incorporation and higher firing temperatures promote the formation of more compact matrices; however, excessive vitrification may also induce heterogeneities and trapped porosity, which can negatively affect mechanical stability. Therefore, an optimal balance between densification and microstructural homogeneity is essential to achieve improved mechanical performance.

5. Conclusions

As a general observation, both the firing temperature and the proportion of tungsten mining sludge govern the overall ceramic behaviour of the sustainable bricks developed in this study, since they determine fundamental factors such as mineralogy, densification variations, pore structure, and mechanical performance, while also potentially contributing to improved environmental performance and colour development. These two variables, particularly at waste incorporation levels of 25–50% and under high-temperature firing conditions, promote the formation of stable compounds with optimal sintering, thereby strengthening the newly produced materials and ensuring the immobilization of toxic elements, resulting in improved technical, esthetic, and environmental properties of the sustainable ceramic bricks generated in this work. Accordingly, with regard to the technical ceramic performance, the following specific conclusions can be drawn:

1. There is a high chemical compatibility between the raw materials and favourable resource efficiency in the ceramic transformation process, since both the clays and the mining residues exhibit high SiO₂ and Al₂O₃ contents, balanced by fluxing oxides. This indicates that an appropriate stoichiometric balance is achieved to enable an efficient ceramic reaction.

2. The formation of stable mineral phases is promoted by the composition of the raw materials, since anorthite and diopside develop between 950 and 1100 °C when incorporating 25–50% of mining residue, thereby enhancing the densification of the ceramic matrix, even though mullite recrystallization does not occur at low residue contents.

3. Porosity and water absorption are directly associated with microstructural densification, as evidenced by porosimetry, water absorption tests, and ESEM analyses. Mixtures containing 50% sludge develop highly dense matrices with nanopores at 1100 °C, which results in very low water absorption. Vacuoles appear as a consequence of gas release, attributable to the effectiveness of the residue acting as an efficient flux.

4. The sustainable bricks exhibit good mechanical properties and effective structural consolidation, as demonstrated by the increases in ultrasonic pulse velocity and compressive strength, both of which reflect progressive densification. The control ceramics improve up to 950 °C but weaken at 1100 °C due to the appearance of microcracks caused by internal shrinkage, whereas the sustainable bricks show exponential improvement, reaching their optimal performance at a 25% additive content. In all cases, the formulations far exceed the regulatory requirements for mechanical performance, confirming their suitability for sustainable construction applications. Furthermore, the combination of adequate mechanical performance, controlled porosity, chromatic adaptability, and environmental performance supports the application of these materials not only in conventional construction but also in architectural rehabilitation, where compatibility with existing materials, durability, and environmental performance are important requirements.

5. Finally, it can be stated that the sustainable bricks exhibit good environmental performance, since the excellent vitrification and pore closure reduce the mobility of heavy metals by two to four orders of magnitude, supporting the feasibility of these ceramic materials. Additionally, through appropriate adjustment of additive proportions and firing temperature, it is possible to induce controlled chromatic modulation based on ΔE monitoring and its alignment with human perceptibility thresholds.