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Article

Characterization and Suitability for Ceramics Production of Clays from Bustos, Portugal

by
Carla Candeias
*,
Isaac Santos
and
Fernando Rocha
GeoBioTec Research Unit, Geosciences Department, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(5), 503; https://doi.org/10.3390/min15050503
Submission received: 27 April 2025 / Revised: 2 May 2025 / Accepted: 7 May 2025 / Published: 9 May 2025
(This article belongs to the Section Clays and Engineered Mineral Materials)
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Abstract

:
Clays are fundamental raw materials in the ceramics industry due to their plasticity, mineralogical composition, and thermal behavior. This study characterizes four clay samples from Bustos (Portugal), aiming to assess their suitability for ceramic applications through granulometric, geochemical, mineralogical, and technological assays, looking at aspects such as their plasticity and sintering behavior. A textural analysis of the samples revealed distinct granulometric profiles, being dominated by silty–clayey fractions and low amounts of coarse particles, indicating high plasticity potential. Three samples showed an alkaline pH (8.17–8.63), and one an acidic pH (5.11), which can significantly influence the rheology and firing behavior of the ceramic body. Samples had a predominance of phyllosilicate minerals, followed by quartz and magnetite–maghemite, and trace amounts of feldspars, anatase, bassanite, and siderite. In the clay fraction, smectite, illite, and kaolinite were identified. By combining classical analysis techniques with ceramic technology principles, this study contributes to the sustainable development of local ceramic industries, emphasizing the importance of characterizing natural raw materials for industrial applications. The plasticity tests showed strong workability in two samples, which exhibited high values of plasticity and moldability, making them suitable for shaping processes in ceramic production. Also, sintering behavior tests revealed that the same clays exhibited good densification during firing, with relatively low shrinkage.

1. Introduction

Clays constitute the basis of traditional and industrial ceramics, owing to their inherent plasticity, workability, and transformation behavior under thermal treatment [1]. These naturally occurring fine-grained materials have enabled the creation of a wide range of ceramic products, from simple earthenware to advanced technical ceramics. Clay’s value lies in the unique interaction between its physical and chemical characteristics, which control their forming behavior, their firing response, and the properties of the final product. As the ceramics industry increasingly seeks to improve energy efficiency, reduce waste, and tailor materials to specific applications, a detailed characterization of clay materials has become an essential research priority.
The functionality of clays in ceramic bodies is largely dictated by their mineralogical composition, granulometric profile, and chemical characteristics. Clay minerals such as kaolinite, smectite, and illite exhibit a layered silicate structure, whose hydration properties and thermal transformations make them crucial in ceramic processing. Kaolinite, with its relatively low plasticity and high refractoriness, is especially valued for applications requiring high thermal resistance, such as porcelain and sanitaryware. Upon firing, it decomposes into metakaolinite and subsequently forms mullite, a stable aluminosilicate phase contributing to the mechanical strength and thermal stability of the ceramic matrix [2,3].
In contrast, smectitic clays, particularly montmorillonite, are characterized by a high plasticity and swelling capacity, attributed to their large surface area and high cation-exchange capacity (CEC). These properties facilitate molding and extrusion processes, making smectite-rich clays ideal for red ceramics and structural bricks [4,5]. However, their tendency to shrink and crack during drying and firing imposes constraints on their dimensional control and long-term durability. Illitic clays, with intermediate behavior, are valued for their plasticity and fluxing capacity due to the presence of potassium, which lowers the vitrification temperature and improves densification at moderate firing ranges [6].
The particle size distribution of clays also plays a fundamental role in their ceramic applications. It governs their rheological behavior during shaping, as well as their porosity and mechanical strength after firing. Finer particles generally enhance workability and packing efficiency, promoting better sintering behavior [7]. However, excessive fineness can increase water demand and cause drying-related issues. A well-graded mixture of clay, silt, and sand fractions is often preferred to balance plasticity, drying rate, and structural integrity. Amorós et al. [8] showed that optimizing the grain size of quartz in clay blends reduces thermal stress and firing deformation, improving the quality of tiles and bricks.
In addition to their mineralogy and texture, chemical characteristics such as pH and oxide composition strongly affect clay processing and ceramic quality. The pH influences the dispersion and flocculation of clay suspensions, which in turn affects slip casting, extrusion, and rheological stability. Different types of clays may naturally have different pH ranges depending on their mineralogical composition. Alkaline clays tend to disperse more readily in aqueous systems, facilitating homogeneous molding and reducing the need for chemical deflocculants [9]. Acidic clays may require pH adjustments to achieve similar processing behavior. Moreover, the presence of fluxing oxides (e.g., K2O, Na2O, and CaO) promotes vitrification, while coloring oxides such as Fe2O3 and TiO2 influence the esthetic and physical outcome of the fired product [10].
The present study conducts a comprehensive assessment of four clay samples, emphasizing the technological parameters relevant to ceramic production. It explores how differences in mineralogical composition, particle-size distribution, and chemical composition translate into variations in the workability, drying behavior, vitrification potential, and mechanical performance of ceramic bodies. This approach aligns with recent efforts to engineer clay formulations tailored for specific applications, from structural ceramics to decorative glazes. However, few studies have systematically evaluated how the inherent properties of locally sourced clay can be optimized to enhance its sustainability, reduce production costs, and improve its industrial efficiency, a key challenge in modern ceramic manufacturing. The study contributes to a deeper understanding of the functional behavior of clays in ceramics, highlighting the critical links between raw material characteristics and end-use performance. By addressing this gap, it provides insights into the sustainable utilization of regional clay resources, minimizing energy consumption, and tailoring compositions for cost-effective, high-performance ceramics. Such insights are vital for advancing sustainable ceramic manufacturing practices, optimizing the use of local resources, and reducing the energy required.

2. Materials and Methods

2.1. Study Area Context

The study area was located in the municipality of Oliveira do Bairro, in Aveiro (Portugal). Geologically, the clay pit (Figure 1) lies within the Lusitanian Basin, part of a Meso-Cenozoic sedimentary sequence occupying the western edge of continental Portugal, known as the Meso-Cenozoic Western Margin [11]. This margin lies west of the Iberian Massif, at the boundary between the Central Iberian and Ossa-Morena Zones. The Lusitanian Basin developed during the Mesozoic in the context of Pangea’s breakup and the opening of the North Atlantic. It is an extensional basin associated with a non-volcanic Atlantic-type continental rift margin [12]. Basin sediments were primarily deposited over units of the Ossa-Morena Zone (ZOM) and South Portuguese Zone (ZSP), part of the Iberian Massif, reaching a maximum thickness of ~5000 m in some areas [13]. The oldest deposits date to the Middle–Late Triassic (Ladinian–Carnian), while the youngest sediments, linked to lithospheric stretching, date to the Late Aptian age [14]. Geomorphologically, the area is low-lying and flat, dominated by ancient beach deposits, dunes, and aeolian sands. Drainage flows northward toward the Ria de Aveiro and northeast toward the Cértima River and Pateira de Fermentelos. Gentle valley slopes result in slow water flow and frequent retention zones.
The clay pit lies within the Vagos clays unit inserted in the Aveiro clays (“Argilas de Aveiro”) unit [15], representing the youngest Cretaceous sediments (Santonian–Maastrichtian, ~60 ± 5 Ma) [16]. The unit outcrops across Aveiro, Ílhavo, Vagos, Oliveira do Bairro, and Mira municipalities, aligned NNW-SSE with a NW dip. In Oliveira do Bairro, beds dip N60° E, 8° W, while near Bustos, the dips are N50° E, 3° NW. The thickest sections are covered by ancient beach sediments, dunes, and terrace sands [17]. The “Argilas de Aveiro” formed in a low-energy, shallow, freshwater-to-brackish wetland under warm, humid conditions, with complex channel networks. The unit comprises alternating reddish and greenish clays (0.30–1.50 m thick) interbedded with yellowish or light-gray dolomitic marly limestones [15]. The clay fraction is dominated by illite–smectite assemblages. Illite, with low crystallinity [14], is the most abundant clay mineral, though smectite (often calcareous) dominates in upper layers, reaching ~70% in Bustos area [18]. Kaolinite and irregular illite–vermiculite/smectite interstratifications are minor components. Accessory minerals include quartz, feldspars, dolomite, calcite, opal, zeolites, gypsum, anhydrite, sulfates, and pyrite [14].

2.2. Samples Description and Analysis

The four raw materials samples used for this study were collected from the clay pit (deposit) in Bustos (Figure 1), selected according to their visual differences: B1—red clay; B2—blue clay; B3—paste; and B4—light blue clay (Figure 2). Samples were dried at 40 °C and then manually disaggregated (through granulometry, technological assays, etc.), and a portion of <0.063 mm was pulverized in agata mill (for chemical and mineralogical analyses, etc.). Fraction < 2 µm was obtained via sedimentation. To determine granulometry, 100 g of each clay sample was subject to wet sieving through 1, 0.5 and 0.063 mm mesh sieves. To determine the relative grain size distribution of a <0.063 mm fraction, a portion was analyzed using a Sedigraph® III Plus V1.01 (Micromeritics Instruments, Atlanta, GA, USA), based on the sedimentation theory (Stokes’ law) and the absorption of X-radiation (Beer-Lambert law) [19]. The samples’ pH was assessed in a 1:2.5 clay/water solution and determined using a Hanna® pH meter, model HI 9025 (Hanna Instruments, Cluj Napoca, Romania).
The chemical composition of fraction < 0.063 mm was assessed via XRF, with a Panalytical Axios PW4400/40, with Rh radiation. The content of trace elements was determined via ICP-MS, with an Agilent Technologies—7700 Series. Mineral phases of fractions < 0.063 mm and <2 µm were identified using XRD, with a Philips/Panalytical powder diffractometer, model X’ Pert Pro MPD. This equipment uses a Cu-X-ray tube operated at 50 kV and 30 mA, with data collected from 2 to 70° 2θ with a step size of 0.01° and a counting interval of 0.02 s. The identification and semi-quantification of the different mineral phases were based on measured peak areas of the basal reflections, considering the full width at half maximum, and then weighted using empirically estimated factors or reflection powers [21].
The specific surface area (SSA) was assessed using the BET (Brunauer–Emmett–Teller) method with a Micromeritics® Instrument Corporation Gemini II 2370 equipment [22]. The plasticity index (PI), as well as the plastic (PL) and liquid (LL) limits, were assessed according to the Atterberg limits [23] as the difference between the maximum water content above which the mass loses enough consistency to be moldable (LL) and the minimum water content above which the mass becomes moldable (PL), PI = LL − PL, in % [24,25]. Casagrande [26] established a diagram to identify the materials’ plastic properties and Gippini [27] defined the main domains of the diagram for clay raw materials [28], with four classes: no plasticity (0 < LL < 20), low plasticity (20 < LL < 35), medium plasticity (35 < LL < 50), and high plasticity (LL < 50).
The production of a ceramic object involves defining its shape, and thus requires the clay paste to be molded so that it acquires a defined three-dimensional form that is as close as possible to the final shape [29] through consolidation into a compact body by eliminating previously added water. To assess this parameter, the extrusion method was used, with a Netzsch 250/05 extruder with a vacuum capacity of 0.85 Pa [20]. A critical parameter in ceramic production is the water added to the clay, which must be sufficient to enable the shaping of the ceramic piece. To determine the amount of water required to shape the ceramic body, it is necessary to evaluate the quantity and type of clay used [30]. The extrusion moisture content was evaluated according to the NP 84:1965 standard [31].
Firing tests were conducted with the samples at different temperatures, with a heating cycle of 10 °C/min and a 60 min soak time at the maximum target temperature, using a Termolab® electric furnace (Nabertherm GmbH, Lilienthal, Germany) at two distinct temperatures, 900 and 1000 °C, using four test specimens per clay type for each temperature. Buller rings were used during firing to obtain an indicative measurement of the temperature reached by the samples inside the furnace. These rings, with temperature ranges of (750–1000 °C) and (960–1250 °C), were made of refractory ceramic material, and shrank during the firing process [32]. The green-dry shrinkage occurs when a clay body is dried after shaping, resulting in a reduction in its dimensions. Dry-fired shrinkage occurs due to the high temperatures induced during the firing process, while total shrinkage characterizes the linear (or volumetric) variation in the ceramic body as a finished product. Shrinkage depends on the mineralogical composition, the structural order of minerals, the particle size and shape, exchangeable cations, plasticity, and other factors. Linear shrinkage measurements were performed on the extruded samples, based on pre-marked reference points spaced 10 cm apart [33].
Color control is essential in the ceramic industry, although, in the red clay products, such as tiles, bricks, and roofing tiles, color is generally not considered a fundamental parameter. The fired color depends primarily on the content of chromophoric elements (e.g., Fe and Ti), the firing conditions, the kiln atmosphere (oxidizing or reducing), the type of generated phases (vitreous or crystalline), and the presence of soluble compounds [31]. For this study, the CIELAB method was used, according to ISO 11664-4:2019 [34]. Flexural strength (RMF) is more commonly evaluated than other mechanical properties, such as compressive properties, torsional properties, tensile properties, bending, toughness, fatigue, or creep resistance, as RMF can simulate the forces that occur during the handling of ceramic objects. Additionally, it can be correlated with plasticity and porosity: <30 kgf/cm2, has low plasticity and high porosity; 30–70 kgf/cm2, has normal plasticity and porosity; and >70 kgf/cm2 has high plasticity (“fat” clay) and low porosity, which may lead to drying defects. The flexural strength was determined according to ASTM C689-03a [35] at different temperatures, and RMF was assessed using ASTM C674-88 [36]. Tests were conducted using a Shimadzu AG-25TA (Agilent, Santa Clara, CA, USA) press with a 1 kN load cell. Operational conditions included an 8.0 cm span between supports and a loading speed of 1 mm/min.
The water absorption of fired ceramic bodies is directly related to their porosity. The higher the porosity, the greater the water absorption. Moisture expansion can be influenced by factors such as firing conditions, thermal cycle, maximum firing temperature, and dwell time at peak temperature [31]. Water absorption capacity was evaluated for four halves (when possible) of fired specimens per firing temperature, following the RMF test, in accordance with the ASTM C373-88 [37]. Following the same standard, apparent porosity was determined, which was defined as the ratio between the volume of open pores and the total volume of the sample. Efflorescence occurs as superficial or sub-superficial crystalline deposits, and their effects can be purely esthetic or can lead to material degradation via flaking or powdering [17] depending on the presence of soluble elements in the raw materials, on the structural characteristics of the ceramics produced, and on the contaminants introduced during manufacturing. To assess these manifestations, test bars fired at different temperatures were used, and efflorescence was evaluated through examining the intensity and extent of the resulting salt stains on the bars [38].

3. Results and Discussion

The samples texture revealed similarities in the particle-size distribution between samples B1, B2, and B4 (Figure 3). In these samples, the particles were mostly <0.063 mm in size, having a silty–clayey character, with a very reduced fraction of sandy-sized particles (>1 mm with 0.00 to 0.14%, and >0.5 mm with 0.00 to 0.007%). Sample B3 showed a different granulometry, containing more sand, particularly fine sand (<1 mm with 0.35%, and <0.5 mm with 3.34%). The particle-size curves, analyzed via Sedigraph (Figure 3), revealed that particles < 2 µm represented 96.4 (B1), 73.1 (B2), 79.5 (B3), and 75.9% (B4) of the <0.063 mm fraction.
The observed textural variations correlated with the pH measurements. Samples B1, B2, and B4 showed alkaline pH values (8.17–8.63), whereas B3 was distinctly acidic (5.11). Acidic clays (pH 4.5–6.5) may require adjustments to achieve the proper rheological properties, whereas clays with a higher pH (8–9), might behave differently during firing, affecting vitrification and the final physical properties [39]. This acidic environment in B3 may be linked the dominance of smectite in the <0.002 µm fraction, with a high cation-exchange capacity promoting H+ retention, while the absence of carbonate minerals eliminates the natural buffering capacity.
The identified mineral phases in samples < 0.063 mm fraction, showed a high ∑phyllosilicates content, particularly in sample B1 (Table 1). Smaller amounts of quartz (SiO2), and, to a significant extent, magnetite–maghemite (Fe3O4-γFe2O3) were also found. Secondary minerals that were present included K-feldspars, plagioclase, anatase (TiO2), bassanite (CaSO4·0.5H2O), and siderite (FeCO3). In the <2 μm fraction (Table 2), smectite (B3 81%; B4 61%), illite (K0.65Al2[Al0.65Si3.35O10](OH)2; B1 73%; B2 53%), and kaolinite (Al2(Si2O5)(OH)4; ranging from 5 to 25%) were identified.
In ceramics applications, ∑phyllosilicates, which include kaolinite, illite, smectite, talc, micas, and chlorite, impart plasticity to clay bodies, facilitating shaping and forming processes. The layered structure, when hydrated, enhances moldability. Upon firing, phyllosilicates undergo transformations that can affect the sintering process. For instance, smectite has significant plasticity and swelling capacity when hydrated, enhancing the workability of clay bodies, and allowing for easier shaping and molding during the forming process. Upon firing, smectite undergoes dehydroxylation and structural collapse, which can influence the densification and mechanical strength of the ceramic product [40]. Illite (a variety of muscovite) can contribute to the plasticity of clay bodies, although to a lesser extent than smectite. Its presence aids in shaping processes while maintaining dimensional stability. As a fluxing agent, illite contains alkali cations, like K, which can act as a fluxing agent during firing, lowering the melting point of the clay body, promoting vitrification, and enhancing the strength and durability of the final ceramic product [41]. Kaolinite, in ceramic products, imparts low plasticity to clay bodies, which can be advantageous for certain forming methods. It is highly refractory, making it suitable for high-temperature applications. During firing, kaolinite transforms into mullite, a phase that significantly enhances the mechanical strength and thermal stability of ceramics [42]. The presence of phyllosilicates also influences the porosity and permeability of ceramic materials, affecting properties like their insulation and filtration capabilities.
Quartz acts as a skeletal framework within the ceramic body, enhancing mechanical strength and reducing deformation during firing. Its low thermal expansion aids in controlling shrinkage and preventing cracking during cooling [43]. In glazes, quartz contributes to viscosity control, transparency, and surface hardness, improving the esthetic and functional qualities of the ceramic product [44]. The Fe-oxides magnetite–maghemite can be present as impurities in ceramic raw materials, which may lead to discoloration, black specks, or structural weaknesses in the final product due to reactions during firing. It is crucial to monitor and control their content to ensure ceramic items have the desired quality and appearance. K-feldspars and plagioclase are essential fluxes in ceramic bodies, in balanced proportions [45]. They lower the melting temperature of the ceramic mixture, facilitating vitrification and densification during firing, and enhance the mechanical properties and durability of the final product. Anatase, a polymorph of Ti-dioxide, can influence the color and opacity of ceramic glazes and, in some advanced ceramics, imparts photocatalytic activity, leading to self-cleaning surfaces [46]. Bassanite, a form of Ca-sulfate, is a primary component in plaster molds used for slip-casting due to its quick-setting properties, and when used in small amounts, can modify the setting time and workability of ceramic slips [47]. Siderite, a Fe-carbonate mineral, upon decomposition during firing, releases Fe-oxides, which can alter the color of the ceramic body or glaze, and its decomposition releases CO2, which may affect the porosity and texture of the ceramic product [48].
The content of major elements in the samples (Table 3) affect the mineral phases that were identified, showing high SiO2 and Al2O3, with moderate fluxing oxides (K2O, Na2O), being essential for vitrification during ceramic firing. This content could be more suitable for ceramic tile and brick manufacturing. The alumina, fluxing agents, and LOI reflected the dominance of hydrated phyllosilicates, such as kaolinite, illite, and smectite, and can be associated with the advanced alteration (hydrolysis) of K-feldspars. The Fe2O3 levels in all samples, related to the presence of magnetite–maghemite, were higher than the maximum allowed for white ceramics (4%), which was higher in samples B1 and B3, and can impart visible reddish tones to the ceramic bodies. Although present in low amounts, the MgO may be related to smectite. The relatively low TiO2 content indicated the weathering state of the clays and could be linked to the minor presence of anatase. The CaO, although detected in trace percentages, is related to minerals such as bassanite and siderite. The low Na2O in samples B2 and B3 suggested that Na-feldspar is a residual mineralogical constituent of the samples, consistent with the mineralogical data. The chemical composition also revealed that K2O levels were relatively higher than those of Na2O and CaO. Fluxing oxides (K2O + Na2O) ranged from 3.76% (B4) to 4.95% (B2), suggesting moderate vitrification potential. Combined with the high percentages of Fe2O3 and MgO, this suggested the low refractoriness of the analyzed clays. The high Fe2O3 + K2O/Na2O content suggested good fusibility and plasticity, with these samples being more suitable for roof tiles, wall and floor tiles, and bricks. The addition of fluxing agents (K2O, Na2O, and CaO) can play a vital role in sintering behavior. According to Zhou et al. [49], blends with optimized flux compositions achieve water absorption < 0.5% and flexural strengths of up to 60 MPa at 1080–1120 °C. Bustos samples, particularly B2 and B4, could be further enhanced when blended with such fluxes or feldspathic materials to improve vitrification and reduce firing temperatures.
The trace elements content in the clay samples is presented in Table 4. The V (71–100 mg/kg) and Cr (51–72 mg/kg) have ionic radii and charges similar to those of Fe3+ or Ti4+, allowing for them to be used as a substitute in minerals such as magnetite, ilmenite, or biotite [50]. The Sr can be used as a substitute for Ca2+ in the structure of plagioclase and, to a lesser extent, for K+ in K-feldspars, although this is less easy due to size and charge differences [51]. The elements Co (8.3–11.7 mg/kg), Ni (15–37 mg/kg), Cu (15–23 mg/kg), Zn (16–24 mg/kg), and Pb (22–31 mg/kg) show an affinity for being incorporated into Al minerals. These transition metals can substitute Al3+ or occupy interstitial or defect sites in Al-rich minerals like clays (e.g., illite and kaolinite) or feldspathoids. In phyllosilicates, where Al is common in the octahedral layer, they are especially able to participate in ion exchange or isomorphic substitution. This behavior also affects the coloration and chemical reactivity of ceramic bodies [52]. In the studied samples, colorant metals (Co, Cr, Cu, V) were present in moderate amounts, especially in B1 and B2 (Table 4). The Zr content exceeded 200 mg/kg in all samples, which can contribute to enhance opacity and strength in glazed ceramics [53]. Sample B2 showed enrichment in rare earth elements (REE; Ce, La, Nd), suggesting potential for its use in high-performance or functional ceramics [54]. The chemical content of Bustos clays suggests that sample B1 is more suitable for red ceramics, providing a good colorant base (Fe2O3, Cr); sample B2 presented balanced oxides, flux content, and REE enrichment, suggesting high-tech ceramic potential; B3, with high silica, low LOI, and balanced coloring elements, is suitable for stoneware; and B4, with moderate fluxing and Fe2O3, is suitable for general ceramics.
The specific surface area (SSA) of the clay samples, assessed via BET assay, was 116.53 (B1), 57.83 (B2), 57.37 (B3), and 67.46 m2/g (B4). Clay B1 presented with a very high SSA, suggesting a high content of fine clay minerals, such as smectite or highly disordered illite [55]. Clays with high SSA contents were shown to have more surface interaction with water, increasing their plasticity and moldability. The B1 is extremely plastic, but also prone to cracking, warping, or drying defects if not managed with care. Due to this increased water retention, it will take longer to dry, leading to a higher risk of cracks unless properly controlled [54]. Also, the high SSA enhances sintering at lower temperatures, promoting densification and strength, and improves bonding and dispersion. Samples B2 and B3 showed a moderate SSA, revealing a balanced workability, making them suitable for general ceramic shaping and glazing, and a consistent forming and sintering behavior, suggesting potential use in tiles, bricks, or general red ceramics. Sample B4 had a slightly higher SSA than B2 and B3, being more plastic and reactive, and a good sintering potential, but needs to be monitored during drying and shrinkage.
The samples’ plastic index (PI) projection on the Casagrande diagram [27], showed that samples B1, and B3 displayed high plasticity (43.3% and 42.1%, respectively), B2 medium-high plasticity (32.2%), and sample B4 had medium plasticity (26.9%). Thus, B2 and B4 displayed a good extrusion performance with minimal drying defects, while B1 and B3 exhibited higher susceptibility to cracking and deformation during drying. The plasticity indices were consistent with the ∑phyllosilicates content once more clay minerals were present, offering the material higher plasticity. The plasticity results were similar to previous studies by Coroado [17] (18.7 < PI < 30.4) and Ferreira [56] (4.4 < PI < 39.1).
The extrusion moisture content, determined for clays B1 (38.7%), B2 (24.0%), B3 (35.0%), and B4 (23.8%), was relatively high (Figure 4). Notably, B1 and B3—characterized by lower plasticity—required a higher water content to achieve the necessary workability. However, this increase in moisture does not enhance the extrusion process; on the contrary, it increases the likelihood of defects during drying and firing, such as an extended drying time and a greater risk of warping or cracking [57]. Clays B2 and B4 require less of the water paste formulation, suggesting they may have a better performance in terms of extrusion and drying. The test specimens from these clays did not exhibit structural defects. In contrast, the specimens produced with clays B1 and B3 showed warping and cracking, which were particularly pronounced in B3 clay. Linear shrinkage from a green (wet) to a dry state is a critical parameter for assessing clay body stability. All clay samples exhibited green-to-dry linear shrinkage below 10%, being ranked B1 (9.3%) > B3 (8.7%) > B2 (8.5%) > B4 (8.0%). Excessive shrinkage can lead to the formation of defects, like warping or cracking, and must be controlled to ensure dimensional stability and structural integrity.
Flexural strength, which reflects the material’s resistance to deformation under load, was not assessed for specimens B1 and B3 due to the presence of significant structural defects. This omission underscores the critical role of optimized moisture content and drying protocols. Clay B2 presented higher values (11.7 N/mm2) than B4 (5.6 N/mm2), indicating reasonable mechanical strength for unfired ceramic bodies (Figure 4). Based on the flexural strength classification for dried clay specimens, sample B4 falls within the range of typical plastic clays with moderate porosity (30–70 kgf/cm2 or 2.9–6.9 N/mm2), while B2 exhibits characteristics of higher plasticity and lower porosity, exceeding 70 kgf/cm2 or 6.9 N/mm2 [31]. This makes B2 particularly suitable for applications requiring dense and mechanically robust ceramic bodies [58].
The color assessment after drying revealed that sample B1 presented a reddish hue, indicative of higher Fe-oxide contents, while samples B2, B3, and B4 exhibited similar gray-brown shades, with B3 displaying a more intense coloration. The results obtained in this study were consistent with those reported by Coroado [17], who found extrusion moisture ranging from 26% to 29%, linear shrinkage between 8% and 10%, and flexural strength values between 11 and 15.8 N/mm2 (112 to 161 kgf/cm2). Conversely, Ferreira [56] reported significantly lower extrusion moisture (7.9%) and shrinkage (1.9%), likely due to methodological differences such as the use of pressed pellets rather than extruded specimens.
The firing of the test specimens, conducted at heating rates of 10 °C/min and a 1 h plateau at the maximum temperature, revealed distinct behaviors for each of the two clays, preventing the determination of certain characteristics. The specimens produced with clays B2 and B4 allowed for the determination of all ceramic properties, while those made from clays B1 and B3 experienced warping and cracking during firing (Figure 5), restricting the evaluation to only total shrinkage, color changes, and the manifestation of efflorescence. These issues, which were evident during processing and after drying, were exacerbated during the firing process.
For clay B1, the total shrinkage decreased with temperature, from 14% at 900 °C to 11.1% at 1000 °C, suggesting overfiring. This phenomenon resulted in material swelling due to gas expansion. The semi-plastic state and the formation of an impermeable vitreous phase on the surface, combined with the pressure from expanding gases, lead to the subsequent swelling of the pieces [17]. According to the author, the formation of the “black heart” occurs when iron in its reduced form (Fe3O4 and/or FeO) appears in the core area, stemming from the partial reduction in Fe2O3 due to the carbonization environment and lack of air circulation caused by premature vitreous phase formation. In this study, clay B1 did not present efflorescence, and the color changed from brownish at 900 °C to reddish at 1000 °C, suggesting some degree of vitrification and highlighting the issue of overfiring at the higher temperature. Clay B3 showed increasing shrinkage values from 10.3% at 900 °C to 11.6% at 1000 °C, and the color of the fired sample maintained a brownish-orange hue at both firing temperatures. However, this clay showed a stable firing behavior but did not perform as well in terms of mechanical strength and porosity.
Clay B2 exhibited increasing total shrinkage values, from 10.5% at 900 °C to 12.4% at 1000 °C (Figure 5 and Figure 6). Along with this, the flexural strength increased significantly, from 48.7 N/mm2 to 50 N/mm2, which are considerably good values for a raw clay material. This increase in strength was accompanied by a significant reduction in water absorption, from 8.9% at 900 °C to 3.9% at 1000 °C, correlating with a decrease in apparent porosity, from 17.7% to 8.6%. These results suggest that clay B2 undergoes effective densification and vitrification during firing, making it a good candidate for applications requiring high mechanical strength and low porosity. The color of the fired specimens also remained a more intense brownish-orange hue at 1000 °C, and no efflorescence was observed, confirming good control over the firing conditions.
For clay B4, the total shrinkage increased from 10% at 900 °C to 12.3% at 1000 °C (Figure 5 and Figure 6). This increase in shrinkage was accompanied by a marked rise in flexural strength, from 22.9 N/mm2 to 51.9 N/mm2, which was the highest among all the tested clays. The reduction in apparent porosity, from 18.2% at 900 °C to 7.4% at 1000 °C, along with the corresponding decrease in water absorption (from 8.7% to 3.8%), suggest that clay B4 undergoes significant densification at higher temperatures. The color of the fired specimens, similar to B2 and B3, also displayed a more intense brownish-orange hue at the higher temperature. Like the other clays, B4 did not exhibit efflorescence, further confirming the effective control over firing conditions.
In summary, clay B2 displayed the most favorable properties for high-performance applications, with a combination of high flexural strength, low porosity, and minimal water absorption. Clay B4 also showed good results, particularly in terms of mechanical strength, while clays B1 and B3 exhibited significant issues with warping and cracking, which limit their suitability for ceramic applications requiring high structural integrity. None of the samples exhibited efflorescence at either temperature, indicating a minimal presence of soluble salts that might otherwise migrate to the surface during drying. This absence is favorable for esthetic and structural integrity in architectural ceramics [59]. The behavior of the clays in this study can be compared to the ceramic properties of Bustos red clay (BSTV), studied by Coroado [17]. At 900 °C, BSTV showed 13.3% total shrinkage, 37.8 N/mm2 of flexural strength, and 2.9% water absorption, which are comparable with the results obtained for clays B2 and B4. However, at 1000 °C, no data were provided on BSTV properties due to the significant pyroclastic deformations and the formation of the “black heart”. The color of the BSTV test specimens ranged from red to dark red, and no efflorescence was observed, aligning with the results of clay B1, which also exhibited a reddish hue after firing.

Results Integration

This study provides a comprehensive characterization of clay samples from a Bustos deposit, focusing on four clay samples’ mineralogical, chemical, textural, and technological properties, with particular emphasis on their applicability in the ceramic industry. All samples exhibited a fine average grain size, dominated by the clay fraction. The identified mineral phases revealed a high abundance of phyllosilicates, primarily smectite, illite, and kaolinite, along with quartz and magnetite–maghemite. Accessory minerals included K-feldspars, plagioclase, anatase, bassanite, and siderite. The samples were predominantly composed of silica, alumina, Fe-oxide, K-oxide, and Mg-dioxide. The pH values were typically alkaline, making them compatible with most ceramic formulations. Specific surface area (BET) values ranged from moderate to high (57.4 to 116.5 m2/g), correlating well with the fine grain size and high phyllosilicate content. Consistency limit values indicated high plasticity in samples B1, B2, and B3, and medium plasticity in B4. The plasticity index ranged from 26.9% (B4) to 43.3% (B1), confirming the materials’ suitability for extrusion and plastic formation, especially sample B4, which was plotted within field B of the Casagrande chart. The clay-rich nature requires high water content for forming, and samples should ideally be blended with less plastic materials to achieve optimal working ranges. High linear shrinkage and total shrinkage were observed, particularly in B1, which also showed signs of overfiring. Samples B2 and B4 displayed good flexural strength and reduced porosity at high temperatures (900 °C and 1000 °C), indicating efficient sintering. B1 and B3 showed the development of microcracks, which developed during firing, likely due to their fine particle size, high plasticity, and uneven drying. No efflorescence was observed, and final fired colors ranged from red to brown-orange. The extrusion process may lead to the laminar orientation of phyllosilicates, leading to structural weaknesses such as exfoliation and delamination. Cracking during drying was attributed to differential shrinkage between the surface and core, exacerbated by moisture gradients and reabsorption. A gradual temperature increase during drying and controlled relative humidity are recommended to mitigate structural defects.

4. Conclusions

Bustos clays revealed strong potential for use in structural ceramic materials such as bricks, tiles, and extruded products. Their mineralogical and plastic properties were well-suited to plastic-forming methods. However, due to their high plasticity and shrinkage behavior, the clays must be carefully managed during processing and should ideally be blended with leaner clays to prevent structural flaws. Among the samples, B2 and B4 stand out due to their mechanical performance and dimensional stability after firing, while B1 and B3 may require compositional adjustments or process optimization due to issues related to overfiring and cracking. This work highlights the importance of integrated mineralogical, chemical, and technological assessments in guiding the sustainable valorization of local clays for ceramic applications.

Author Contributions

Methodology, F.R.; validation, I.S. and F.R.; formal analysis, I.S.; investigation, I.S. and F.R.; writing—original draft preparation, C.C.; writing—review and editing, C.C. and F.R.; supervision, F.R.; funding, F.R. All authors have read and agreed to the published version of the manuscript.

Funding

This work was partially supported by GeoBioTec (UIDB/04035/2020) Research Centre, funded by FEDER funds through the Operational Program Competitiveness Factors COMPETE and by National funds through FCT.

Data Availability Statement

Data will be made available if required.

Conflicts of Interest

The authors declare no conflicts of interest.

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Minerals 15 00503 g001
Figure 1. Location of the study area in Portugal (red box) and the clay pit deposit. Adap. QGis® and Google Earth®, April 2025.
Figure 1. Location of the study area in Portugal (red box) and the clay pit deposit. Adap. QGis® and Google Earth®, April 2025.
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Figure 2. Collected samples: (a) B1; (b) B2; (c) B3; and (d) B4; adapted from [20].
Figure 2. Collected samples: (a) B1; (b) B2; (c) B3; and (d) B4; adapted from [20].
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Figure 3. Texture of the analyzed samples of the sand, silt, and clay fractions obtained via wet sieving (left); fraction < 0.063 mm analyzed via Sedigraph (right). Adapted from [20].
Figure 3. Texture of the analyzed samples of the sand, silt, and clay fractions obtained via wet sieving (left); fraction < 0.063 mm analyzed via Sedigraph (right). Adapted from [20].
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Figure 4. Forming properties and post-drying characteristics of the test specimens. Flexural strength after drying (N/mm2) marked as green dots. Adapted from [20].
Figure 4. Forming properties and post-drying characteristics of the test specimens. Flexural strength after drying (N/mm2) marked as green dots. Adapted from [20].
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Figure 5. Test specimens structurally represented after drying at 110 °C and firing at 900 °C and 1000 °C. Adapted from [20].
Figure 5. Test specimens structurally represented after drying at 110 °C and firing at 900 °C and 1000 °C. Adapted from [20].
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Figure 6. Ceramic properties of clays B2 and B4 at 900 °C and 1000 °C, with flexural strength (FS; N/mm2), shrinkage (S; %), water absorption (WA; %) and porosity (P; %). Adapted from [20].
Figure 6. Ceramic properties of clays B2 and B4 at 900 °C and 1000 °C, with flexural strength (FS; N/mm2), shrinkage (S; %), water absorption (WA; %) and porosity (P; %). Adapted from [20].
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Table 1. Mineral phases identified in fraction < 0.063 mm (in %). Adapted from [20].
Table 1. Mineral phases identified in fraction < 0.063 mm (in %). Adapted from [20].
MineralB1B2B3B4
∑ phyllosilicates76727069
quartz14212020
K-feldspar2125
plagioclase00.510
siderite0001
bassanitetrtrtrtr
anatase0.50.511
m-m7.5564
Table 2. Mineral phases identified in fraction < 2 µm (in %). Adapted from [20].
Table 2. Mineral phases identified in fraction < 2 µm (in %). Adapted from [20].
MineralB1B2B3B4
smectite16228161
illite73531421
kaolinite1125518
Table 3. Chemical contents of major elements in studied samples (in %). LOI—loss on ignition.
Table 3. Chemical contents of major elements in studied samples (in %). LOI—loss on ignition.
VarB1B2B3B4
Al2O321.2821.1820.4621.03
CaO1.110.580.550.68
Cl0.050.040.050.08
Fe2O39.165.796.545.77
K2O4.454.653.513.78
MgO2.912.072.051.85
MnO0.030.020.030.02
Na2O0.270.240.300.25
P2O50.060.050.030.05
SiO252.3958.1660.5759.21
SO30.020.020.070.03
TiO20.880.880.960.92
LOI6.915.984.736.12
Table 4. Trace elements content of the analyzed samples (in mg/kg).
Table 4. Trace elements content of the analyzed samples (in mg/kg).
VarB1B2B3B4
Ba200300300200
Ce682005565
Co11.79.28.38.6
Cr72517054
Cs23281723
Cu23171815
Ga24221520
La40623044
Nd33582429
Ni37181615
Pb28312226
Rb300300200200
Sn15181316
Sr86955387
Th14191217
V8610071100
Y20341824
Zn20241621
Zr200300300300
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Candeias, C.; Santos, I.; Rocha, F. Characterization and Suitability for Ceramics Production of Clays from Bustos, Portugal. Minerals 2025, 15, 503. https://doi.org/10.3390/min15050503

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Candeias C, Santos I, Rocha F. Characterization and Suitability for Ceramics Production of Clays from Bustos, Portugal. Minerals. 2025; 15(5):503. https://doi.org/10.3390/min15050503

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Candeias, Carla, Isaac Santos, and Fernando Rocha. 2025. "Characterization and Suitability for Ceramics Production of Clays from Bustos, Portugal" Minerals 15, no. 5: 503. https://doi.org/10.3390/min15050503

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Candeias, C., Santos, I., & Rocha, F. (2025). Characterization and Suitability for Ceramics Production of Clays from Bustos, Portugal. Minerals, 15(5), 503. https://doi.org/10.3390/min15050503

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