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Article

Sustainable Use of Taveiro (Portugal) Red Clays for Structural Ceramic Applications: Mineralogical and Technological Assessment

by
Carla Candeias
*,
Helena 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(9), 910; https://doi.org/10.3390/min15090910
Submission received: 1 July 2025 / Revised: 18 August 2025 / Accepted: 25 August 2025 / Published: 27 August 2025
(This article belongs to the Special Issue From Clay Minerals to Ceramics: Progress and Challenges)
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Abstract

The technological potential and sustainability of red clays from the Taveiro region (Coimbra, Portugal) for structural ceramic applications have been investigated. Thirteen representative samples granulometric, mineralogical, chemical analysis, and technological characterization were conducted to determine the suitability for extrusion-based ceramics, aligned with circular economy and climate goals (e.g., PNEC2030, RNC2050). The samples exhibited a high fine fraction content (<0.002 mm up to 76%) and plasticity index (PI; up to 41%), associated with significant smectite, illite, and kaolinite content. Bulk mineralogy was dominated by Σ phyllosilicates (up to 77%) and quartz (12%–29%), while chemical analyses showed high SiO2 and Al2O3 content, moderate Fe2O3, and low CaO/MgO, typical of aluminosilicate clays for red ceramics. High cation exchange capacity (CEC; up to 49 meq/100 g) and specific surface area (SSA; up to 83 m2/g) reflected smectite-rich samples. Firing tests at 900 and 1000 °C demonstrated decreasing water absorption and shrinkage with increased temperature, with some samples yielding lower porosity and higher strength (~12 MPa), confirming suitability for bricks and tiles. Two samples showed higher plasticity but greater shrinkage and porosity, suggesting applicability in porous ceramics or blends. This work highlights the role of mineralogical and technological indicators in guiding the eco-efficient use of georesources for ceramic manufacturing.

1. Introduction

Clay, a versatile raw material, has been crucial in human history. Recent trends emphasize the sustainable valorization of these local georesources in ceramic applications, such as bricks, tiles, and earthenware. In many parts of the world, ceramic producers are seeking local clay deposits and industrial by-products to reduce environmental footprint, promoting sustainable valorization of local resources and strengthening regional economies. Vasić et al. [1] highlighted the potential of locally sourced clays, noting that integrating such resources within the ceramics value chain can reduce transportation-related CO2, minimize reliance on imported materials, and facilitate the reuse of clay-based waste in new products. In practice, local ceramic factories are often co-located with clay quarries to minimize hauling distances, thereby reducing fuel use and emissions. The characterization of local resources and the definition of the best practices to be used by the ceramic industry are essential to reduce environmental impact and reduce production costs. La Noce et al. [2] reported how 0.4% basalt fiber addition to locally available clays can reduce drying shrinkage by ~25% and boost compressive strength by ~30%, highlighting clay’s mechanical adaptability and potential for eco-friendly brick production without cement. Also, Rotondi et al. [3] explored muscovite-inorganic waste blends, reporting how clay bodies can be engineered for urban greening. Clay-based circular design can drive both resource efficiency and social value, bonding traditional knowledge with contemporary production technologies in harmony with environmental ethics [4]. Altogether, sustainable ceramics initiatives worldwide, from emerging economies to developed regions, showed that exploiting local clays can support both economic resilience and environmental goals [1]. Worldwide, ceramic industries are using regional clay deposits to both cut carbon and promote rural livelihoods. In Serbia, different clay deposits were found viable for structural ceramics, indicating savings in energy, transport, and infrastructure costs, supported by sustainable production models [5]. Additionally, circular economy frameworks emphasize reuse, local supply chains, and resource longevity.
Portugal has a significant clay-based industrial tradition, featuring prominent tile, brick, and earthenware sectors. According to Santos et al. [6], the country has over fifty recognized clay deposits targeted for eco-efficient calcined-clay cement, underscoring the national resource strategy and climate commitment. This aligns with national climate goals [7,8] demanding decarbonization of the cement and ceramics sectors, placing local clays at the center of Portugal green industrial strategy. Studies emphasized that a detailed characterization of local clays is key to the sustainable development of the national ceramics industry. Coroado et al. [9] studied clays from Vila Nova da Rainha (Portugal) and reported that locally sourced materials, when properly characterized, offer excellent performance for construction ceramics, supporting sustainable production through reduced energy requirements and minimized transport distances. Lisboa et al. [10] conducted a survey of clay deposits in the Tábua region (central Portugal) and demonstrated that locally sourced clays hold mineralogical and technological properties suitable for structural ceramics, enabling more sustainable production through lower energy consumption and reduced transport. Candeias et al. [11] investigated clays from Bustos (Portugal) and highlighted that understanding regional clay deposits directly aids cost-effective, low-energy production of ceramic products. Portugal Centre region economy is closely tied to clays and ceramic industry and has long promoted circular innovation and economies. This context underscores that Portuguese clay resources should be harnessed locally, not only to reduce carbon footprints but also to reinforce local economies and preserve traditional craft skills. The present study focuses on red clay deposits from the Coimbra-Taveiro area, in central Portugal, some of which have been historically quarried for nearby ceramic production. In synthesizing global insights with national trends and local geology, this study aims to demonstrate the multilevel value of locally sourced clay resources. From building resilient economies to reducing embodied carbon, circular clay use is both technically viable and environmentally critical. Samples were subjected to chemical analysis, mineral phases identification, and different assays to determine clays suitability for ceramics use, aligned with the sustainable use of local georesources and decarbonization goals, bridging global best practices with regional and community-oriented environmental stewardship.

2. Materials and Methods

2.1. Study Area Geological Context

The studied samples were collected in Taveiro area (central Portugal), on the left bank of the Mondego River, where a transition occurs between adjacent Mesozoic terrains and the alluvial plains formed by Cenozoic deposits of the lower Mondego. These deposits form a vast alluvial plain and occupy an area of ~15,000 ha, with a maximum width of ~4 km and an estimated volume of alluvium between 3 and 4 Mm3. There are significant finger-like extensions corresponding to tributary valleys, i.e., the Cernache stream and Ega, Arunca, and Pranto rivers [12].
The Lusitanian Basin is located on the western Iberian margin in Portuguese territory. The basin is ~180 km wide (E–W) and ~320 km long (N–S), with the continental area > 23,000 km2 being bounded to the south by a branch of the Porto-Coimbra-Tomar fault and to the north, although uncertain, connected to Mesozoic depressions located between the platform and the alignment of Monte do Porto-Monte de Vigo-Galician Bank [13]. The basin is generally defined by a Mesozoic and Cenozoic alignment trending northward, with a maximum estimated sediment thickness of ~4000 m [14]. From the Triassic to the Cretaceous periods, the basin experienced subsidence due to E–W extension associated with the subsequent Atlantic expansion [14]. The low rate of subsidence during the Cretaceous and Tertiary is reflected in a thin sedimentary record with numerous discontinuities [15]. The oldest sedimentary record corresponds to the initial differentiation of an intracratonic sedimentary basin, marking a phase of rifting prior to the later opening of the Atlantic, beginning in the Late Triassic [15]. The Mesozoic is characterized by sedimentation in depressions by thresholds controlled by subsidence and tilting of blocks along normal faults. South of the Mondego River in a small triangular coastal basin, two main continental sedimentary formations crop out: the Taveiro sandstones and clays, and the clayey-sandy and conglomeratic formation of Senhora do Bom Sucesso. The Taveiro sandstones and clays formation reaches up to 170 m in thickness, upon Upper Cretaceous sediments with coarse sandstones to the E and a carbonate formation to the W, where the angular unconformity is particularly clear [16]. These clays and sands constitute a heterochronous formation, with younger ages in the more western outcrops of Silveirinha-Carvalhais (Lower Eocene–Paleocene) and older ages in the easternmost ones, such as Taveiro (Upper Campanian-Maastrichtian). According to Pena dos Reis [16], the sedimentological history of the Taveiro clays area is marked by deposition in a continental environment. It is a setting that existed from the Late Senonian (Campanian-Maastrichtian) to the Early Paleocene-Eocene, is marginal, low-lying, and possibly coastal, near base level, and forms an alluvial plain with highly sinuous channels and a dominant flow from E to W–NW. The climate was likely generally warm and subtropical with well-defined seasons. The deposition of Taveiro sandstones and clays likely occurred under the influence of a stable eastern block with reduced relief and weak extensional tectonics and a reactivated diapiric structure to the west. The subsidence was likely weak, as inferred from the thinness of the formation and the absence of major internal erosion phases. Taveiro Clays are composed of brick-red silty-sandy layers, sometimes grayish, with more clayey intercalations and traces of carbonates [16], as well as conglomeratic transitions that may be rich in intraclasts, i.e., relict carbonate encrustations, Fe–Mn pisolites, and peat.

2.2. Samples Collection

A total of 13 samples were collected for this study in 4 locations (Table 1) where raw materials are collected for the ceramic industry, with well-defined and easily accessible outcrops and vertical sections several meters high (Figure 1). Samples were collected from the most representative and distinct materials in each pit, with different clayey levels, showing continuity at the scale of the pit. Ribeira de Frade pit samples (RF1, RF2, RF3, and RF4) were collected from an abandoned clay pit that is presently almost entirely overgrown with riparian vegetation but is nevertheless a protected area for its potential interest. In this medium-sized clay pit, clayey materials with reddish and grayish hues can be observed. Taveiro pit samples (TV1, TV2, and TV3) were collected from a larger pit, revealing its former exploration importance. At the edge of the pit, there were reddish clayey levels interbedded with whitish silt that was clayey and interspersed with sands containing abundant rounded pebbles and a very inconsistent thickness. The Reveles pit samples (RV1, RV2, and RV3) were collected from an active pit, with slopes < 20 m high and generally exhibiting 2 to 3 benches. The materials observed on these slopes showed pale reddish-brown to grayish coloration, with grain sizes mainly varying between clay and silty clay dimensions. At the top of the active São Pedro pit is the Bom Sucesso Formation, while below lies the “Argilas de Taveiro” Formation [16]. In this pit, were collected samples SP1, SP2, and SP3.

2.3. Samples Preparation and Analysis

Samples were wet sieved to achieve the <0.063 mm fraction and dried. The <0.002 mm fraction was obtained by sedimentation based on the Stokes’ law. The prepared samples were then subjected to various analytical techniques for characterization purposes. To identify the mineral phases present, samples were analyzed using a X-ray diffraction (XRD) Philips/Panalytical powder diffractometer (Malvern Panalytical, Malvern, Worcestershire, UK), model X’ Pert Pro MPD, with 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. Minerals were identified in the diffractograms using the procedures described in Brown and Brindley [18] and the International Centre for Diffraction Data (ICDD) database in the High Score Plus v4.9® software. The relative abundance of each identified mineral phase was estimated semi-quantitatively using the reflection intensities of the peak area method [19,20,21]. Qualitative and semi-quantitative analyses were performed following the methodologies described in Candeias et al. [11]. To assess oxides Al2O3, CaO, Fe2O3, K2O, MgO, MnO, Na2O, P2O5, SiO2, and TiO2 and trace elements As, Ba, Cr, Cu, Nb, Ni, Pb, Rb, Sr, V, Zn, and Zr, samples were analyzed using a X-ray fluorescence (XRF) Panalytical Axios PW4400/40 (Malvern Panalytical, Malvern, Worcestershire, UK), with Rh radiation, and flame photometer Corning 400 (Na and K content). Loss of ignition (LOI) was assessed by calcination on a muffle at 1000 °C for 3 h.
Cation exchange capacity (CEC) is defined as the maximum quantity of any cation that the clay can adsorb [22]. CEC and the concentration of the main exchangeable cations (Ca2+, Mg2+, Na+, and K+), were determined using a saturating solution of NH4CH3COO (1 M, pH 7), followed by displacement of the adsorbed ammonium ions with KCl and quantification by atomic absorption spectroscopy (AAS) using GBC 906 AA equipment (GBC Scientific Equipment Pty Ltd., Keysborough, Australia). Specific surface area (SSA) was evaluated using the BET (Brunauer–Emmett–Teller) method, using Gemini II 2370 equipment (Micromeritics® Instrument Corporation, Norcross, GA, USA) [17,23]. Samples expandability was analyzed following standard LNEC E200-67 [24]. The samples pH was measured using a HI 8014 calibrated apparatus. Density was assessed using standard LNEC E202-67 [25], which determines the bulk density of ceramic bodies by relating the dry mass to the corresponding volume. In ceramic materials, density is a parameter used to evaluate the degree of compaction and vitrification achieved during firing, directly influencing mechanical strength, thermal conductivity, and overall durability. The plasticity index (PI) and plastic (PL) and liquid (LL) limits were assessed following the Atterberg limits [26] and using the expression PI = LL − PL [11], which define the range of water content over which a clay remains plastic and workable, essential for shaping processes such as extrusion or molding. The Atterberg limits allow to predict the clay’s forming behavior, drying sensitivity, and potential for shrinkage or cracking, thereby guiding raw material selection and processing adjustments.
Extrusion was assessed using a Netzsch 250/05 extruder (Netzsch Group, Selb, Germany), equipped with a vacuum capacity of 0.85 Pa. This process simulates industrial forming conditions by forcing the plastic clay body through a die under controlled vacuum, removing entrapped air, and improving particle packing. In ceramic production, extrusion testing evaluates the material’s workability, cohesion, and response to shaping, ensuring that the clay mixture can produce defect-free green bodies suitable for drying and firing. Drying process was performed to evaluate ceramic properties before firing, critical for preventing defects such as warping or cracking. Linear shrinkage was measured on dried specimens using a caliper [27] to quantify dimensional changes caused by moisture loss, which reflect clay plasticity and particle packing. The flexural strength (FS) of the dried specimens was determined in accordance with standard ASTM C689-97 [28], providing an assessment of the mechanical integrity of the green bodies. In ceramic manufacturing, these parameters are essential for predicting how a material will withstand handling, transportation, and early thermal treatment without failure.
Firing tests on extruded and dried specimens were conducted at 900 and 1000 °C using a Termolab electric chamber furnace, with a heating rate of 5 °C/min and a soaking time of 60 min at the target temperature [17]. These firing temperatures correspond to the most representative industrial firing conditions for the structural red-ceramic industry (e.g., bricks and tiles) in Portugal and southern Europe. Tunnel kilns and intermittent furnaces typically operate between 900 and 1020 °C for bricks and roofing tiles, balancing mechanical performance, energy consumption, and fuel costs, being 900 °C the lower limit before significant vitrification and 1000 °C the upper limit before potential deformation. Water absorption (WA) capacity was determined in the fired specimens by immersion in water, and boiled for two hours, and cooled for four hours [17]. After removing excess of surface water, specimens were weighed and dried in an oven at 110 °C for 24 h and weighed again, with WA being the relative percentage of the different weights. Water absorption assesses materials porosity, directly related to the degree of vitrification, density, and durability. In structural ceramics, WA is a key indicator of performance, influencing mechanical strength, frost resistance, and compliance with product standards.

3. Results and Discussion

The granulometric composition of the Taveiro samples revealed the prevalence of the clay size fraction (<0.002 mm), suggesting potential plasticity (Figure 2). Most samples exhibited a well-developed clay size fraction, ranging from 46 (SP3) to 76% (RV3), and silt size fraction (0.063–0.002 mm) varying between 21 and 43%. The sand size fraction (>0.063 mm) was generally low (3%–15%), confirming the fine-grained nature of these materials. Samples from Ribeira de Frades (RF) showed the highest clay size content in RF1 (71%) and RF3 (64%), corresponding to grayish and silty reddish clays with gray nodules. The reddish tone in RF2 (49% clay size) correlated with the highest sand content (15%), possibly reflecting a more detrital character or proximity to transitional facies. These results were consistent with grain size distributions reported for clays from the Tábua region (Portugal), where samples showed high clay size fraction (>60%) and distinct reddish to grayish hues linked to depositional and weathering conditions [10]. In Taveiro pit (TV), samples TV2 and TV3 showed the highest clay size fraction (70% and 60%, respectively), consistent with the reddish to reddish-silty color and the intercalation with siltier units. The TV1 sample, slightly more heterogeneous in color (grayish to reddish), showed a slightly lower clay size content (53%) and a higher sandy fraction (12%). This texture may reflect localized sedimentological variability, comparable to those reported from the Tagus Basin (“Argilas de Tomar”), where mixed clay–silt textures with sand inputs often result from fluctuating depositional energies in fluvio-lacustrine settings [9]. Reveles (RV) samples granulometry showed good potential for ceramic use, with RV3 reaching the highest clay size content of all samples (76%). This dark reddish sample contrasted with the slightly lighter tones of RV1 and RV2, both of which had a prevalence of the <0.002 mm fraction (57% and 60%, respectively). These findings align with Lisboa et al. [10], who documented similarly colored clays in the Tábua region, often containing > 65% clay size content, particularly in slopes and erosion-influenced sites where fine sediment deposition is prevalent. São Pedro (SP) samples SP1 and SP2 exhibited similar values (61%–62% clay size), while SP3 showed 46%, with the highest silt content (43%) of all samples. This variability may reflect compositional heterogeneity at the transition between the “Argilas de Taveiro” and the overlying Bom Sucesso Formation [16]. Sample SP3 darker reddish coloration and lower clay size fraction suggested a different provenance or early diagenetic alteration, indicating the intra-unit heterogeneity documented in clay units (e.g., Trindade et al. [29]). Overall, the granulometric profile of the Taveiro samples confirmed their suitability for ceramic applications, particularly where a high plasticity index and fine texture are required, typical in red earthenware production. These findings were in line with general standards for ceramic raw materials, which typically require >50% clay size fraction to ensure workability and sintering performance [30].
The minerals semi-quantitative content was estimated after XRD analysis of the <0.063 mm fraction showed a dominance of Σ phyllosilicates (58%–77%), reflecting the dominance of the <0.002 mm size particles, highlighting high plasticity and potential for ceramic applications (Figure 3). The Σ phyllosilicates include clay minerals (e.g., smectite group, illite, kaolinite group, chlorite group, sepiolite, palygorskite), the mica group (e.g., muscovite, biotite, sericite), and other sheet silicates such as talc and vermiculite. Quartz showed an inverse trend, varying from 29 (RF1) to 12% (TV1), in line with clay size content. Sample TV1 displayed the highest content of Σ phyllosilicates (77%) and the lowest of quartz (12%), linked to the clay size content. The TV2 and TV3 samples, with clay size content of 70 and 60%, respectively, showed high Σ phyllosilicates content (~72%), reinforcing their suitability for ceramics. Sample RV3 showed the highest clay size fraction of 76%, with only 22% quartz. Samples RV1 and RV2 mineral composition (64%–65% Σ phyllosilicates, 19%–23% quartz) may be influenced by depositional heterogeneity in the pit. The São Pedro samples showed more variability, with SP1 and SP2 showing 61 and 67% Σ phyllosilicates and 26 and 28% quartz, respectively, while SP3 displayed 58% Σ phyllosilicates and 29% quartz. These results aligned with its darker reddish color, possibly stemming from minor provenance or early weathering changes. Comparable observations from Trindade et al. [29] observed heterogeneity in Algarve clay units, where samples with higher Fe2O3 content (up to ~9%) and smaller mean particle sizes were associated with more intense reddish hues. Similarly, the present work sample showed Fe2O3 content within the upper range of the dataset and small mean grain size particles, suggesting that both compositional and textural factors contributed to the color differences.
High Σ phyllosilicates content (>58%) can be valuable for ceramics, ensuring plasticity, green strength, and good sintering. Lisboa et al. [10] reported similar mineralogical dominance (>60% clay minerals) in Tábua clays, reinforcing the potential for production of red earthenware. The range of Σ phyllosilicate concentration suggested variable shrinkage behavior and workability, useful for tailoring clay mineral blends for specific ceramic products. Quartz (SiO2) content (12%–29%) can be beneficial for thermal behavior and mechanical strength in fired ceramics, though excessive amounts can reduce plasticity [9]. The low quartz in TV1 and the high quartz in RF1 provide flexibility for differing degrees of vitrification and thermal shock resistance. Small fractions of anatase (TiO2) and anhydrite (CaSO4) were present (2%–3%). Anatase, a Ti dioxide polymorph, may influence burn coloration but is unlikely to affect mechanical strength at these concentrations. The occurrence of Ca-sulphate was initially identified as anhydrite; however, it likely corresponds to gypsum (CaSO4·2H2O) as the stable phase under ambient conditions in clay-rich environments. The detection of anhydrite may be an artifact of partial dehydration of gypsum during sample drying or XRD analysis, a phenomenon noted in other studies [31,32]. Trace K-feldspars (2%–6%) contribute to fluxing, aiding vitrification at lower temperatures. Dolomite (CaMg(CO3)2) and siderite (FeCO3) were scarce and not expected to influence performance significantly, with a presence close to but not below the detection limit.
The mineralogical semi-quantitative composition of the <0.002 mm fraction was dominated by smectite, followed by illite and kaolinite (Figure 4). The predominance of smectite in most samples ranged from 2.5% (SP1) to 87% (RV2), suggesting a strong potential for applications requiring high plasticity and water retention. These characteristics are especially relevant for extrusion-based ceramic processes, such as brick or tile production. Smectite-rich materials are highly plastic, displaying strong shrinkage upon drying, and tend to vitrify at lower temperatures due to the fine particle size and high surface area. Samples TV1 (74%), RF4 (69%), RV2 (87%), and RV3 (68%) fall into this category, being suitable for plastic ceramic bodies. These values are comparable to those found by Trindade et al. [29] in smectitic units of the Algarve Basin, often exceeding 60% smectite and being used in the local ceramic industry, with favorable forming properties. Smectite content above 60% is generally associated with expansive behavior, making drying control essential. However, the expansive nature also enhances sintering and final product strength when blended with tempering agents (e.g., quartz). This behavior has also been reported in the Tábua region, where Lisboa et al. [10] concluded that materials with 55%–65% smectite were highly suited for structural ceramics such as bricks and roof tiles.
The role of illite in ceramic matrices is critical, contributing to moderate plasticity and improving the sintering process due to the presence of interlayer K+, acting as a flux [33]. Illite content in samples SP3 (38.5%), SP1 (51.5%), and TV2 (32%) revealed balanced clay mineralogy with significant illite proportion. These samples are suitable for blending with smectitic clays to mitigate shrinkage and improve dimensional stability. Similar mineral content was identified by Coroado et al. [9] in clays from Vila Nova da Rainha, where illite-rich samples exhibited low shrinkage and stable color after firing, suiting them for rustic ceramics and decorative applications. Kaolinite is present in all samples but predominant in SP2 (77%) and high in SP1 (46%). These samples present low smectite content (5% and 2.5%, respectively), indicating a high refractory character and low plasticity. Kaolinitic clays are desirable for applications requiring dimensional stability. Though high kaolinite contents are advantageous in high-temperature ceramics, São Pedro samples (particularly SP2) may require plasticizers in the formulation due to the limited forming capability. These findings align with Dias et al. [34], concluding that kaolinite-rich clays required mixing with plastic clays to produce dense ceramics with minimal warping. The co-occurrence of kaolinite and illite in the São Pedro samples also suggested partial weathering of feldspathic sources, while the high smectite content in Taveiro and Reveles reflected more intense hydrolysis of mafic or tuffaceous parent material [29].
The chemical composition of the samples (Table 2) is characteristic of structural ceramic raw materials, with SiO2 (62%–73%) and Al2O3 (14%–31%) dominating, consistent with aluminosilicate-rich clays. Higher SiO2/Al2O3 ratios, particularly in SP1 and RV1, reflected higher quartz content and supported the mineralogical data, showing 23%–29% quartz (Table 3). The Fe2O3 varied between 2.65% (RF3) and 6.29% (TV1/TV2), linked to the reddish tones in most of the samples and indicating favorable Fe-oxide content for red firing ceramics [9]. These values fall within the 3%–9% Fe2O3 range typical of ball clays used globally (e.g., Kagonbé et al. [35]). The MgO (1.0%–3.8%) and CaO (0.5%–1.1%) content were low but not negligible. While MgO occurs from smectite, illite, and minor dolomite, matching higher smectite contents (e.g., RV2 with 1.74% MgO vs. 87% smectite), the CaO sources include minor anhydrite and dolomite. The low lime content (<1.1%) ensures minimal decarbonation and avoids thermal expansion issues upon firing [36]. The TiO2 content (0.32%–1.17%), with origin from anatase and possibly rutile, while not technologically critical, can influence coloring. The Na2O and K2O (0.07%–0.42% and 1.78%–5.12%, respectively) derived from feldspars and illite that act as natural fluxes during vitrification, while the higher K2O in RF2 (5.12%) suggested higher feldspathic content, improving firing temperatures. Loss of ignition (3.9%–9.9%) reflected organic matter and structurally bound water, corresponding to high smectite content (e.g., RV2, LOI 5.21% vs. smectite 87%). The LOI values were within expected ranges for smectitic clays used in red ceramics [37].
Similar mineral and chemical content results were found in clays from Fez’s Saïss Basin (Morocco) [36] with a chemical composition of >60% SiO2, 20%–24% Al2O3, and 3%–6% Fe2O3, and mineralogical content included smectite, illite, kaolinite, quartz, and feldspars. These clays were considered suitable for extrusion-based ceramic production due to plasticity and fluxing behavior. A study by Ramos et al. [38] focusing on clays from Parelhas (Brazil) showed that two of the samples used for ceramic production presented a content of SiO2 54%–59%, Al2O3~17%, and Fe2O3 9%–11%. The present study samples RV2, TV1, and TV2 revealed a content slightly more siliceous and less ferruginous than the Brazilian clays, falling within the comparable compositional range typical of red ceramic materials. Parelhas clays were dominated by smectite, with quartz and minor feldspar phases, while kaolinite was notably more abundant in one sample. Samples TV1, TV2, and RV2 contained high smectite contents (up to 87%), quartz (12%–29%), and feldspars (2%–6%), with kaolinite-rich samples SP2 and SP1, aligned with one Parelhas sample. The high smectite content in both datasets contributes to elevated plasticity indices and suitable workability for extrusion-based ceramic processes. Loss on ignition (LOI) results from Taveiro clays were consistent with those reported for the Brazilian clays (8.26%–8.63%), reflecting similar volatile content, including structural water from clay minerals. In another study [39] on red clays from Barind, the authors revealed a composition dominated by illite, kaolinite, smectite, mica, quartz, and feldspars, similar to the present study clays, suggesting that both clays hold high fluxing potential and favorable plastic behavior for ceramic applications. Chemically, the Barind clays showed SiO2 (55%–58%), Al2O3 (17%–19%), Fe2O3 (8%–10%), K2O (3%), and LOI (8%–10%). Taveiro clays, particularly TV1, RV2, and SP2, showed comparable SiO2 and Al2O3, with slightly lower Fe2O3 content. The chemical composition of Taveiro clays, rich in SiO2 and Al2O3 with moderate Fe2O3 and essential fluxing oxides, supports the suitability for red structural ceramics. Comparisons with other studies confirmed that these compositions emphasize the suitability of these clays for both ceramic bodies.
The analysis of major oxide ratios provides valuable insights into the genesis, alteration processes, and technological behavior of clay materials. Among the most informative indices is the SiO2/Al2O3 ratio, used as a proxy for the silicate mineralogy and the relative abundance of quartz and clay minerals. In this study, this ratio ranged from 1.79 (SP2) to 5.35 (RV3), suggesting a broad variability in mineral composition across the samples (Table 3). Lower values, particularly in SP1 (2.3) and SP2 (1.79), were consistent with the dominance of kaolinite and illite. In contrast, higher ratios in RV1 and RV3 suggested increased quartz content, confirmed by mineralogical content. Ratios of MgO/Al2O3 and K2O/Al2O3 are frequently used to understand the presence of smectites, illites, and associated exchangeable cations [40]. The MgO/Al2O3 values ranged from 0.00 (SP1 and SP2) to 0.24 (TV2), with smectite-rich samples like TV1 and TV2 exhibiting higher Mg enrichment. These elevated Mg levels likely reflected interlayer cation content in trioctahedral smectites, reinforcing the mineralogical interpretations. The K2O/Al2O3 ratios, ranging between 0.06 and 0.31, were indicative of illitic content, as K is a major interlayer component of illite. RF2 and RF3 reflected a stronger illite contribution. Lower K2O/Al2O3 values (e.g., 0.06 in SP2) aligned with kaolinite-rich compositions, where K is typically absent or minimal. The CaO/Al2O3 ratios remained low across all samples (<0.05), confirming the minimal presence of carbonate or lime-rich phases. Similarly, Fe2O3/Al2O3 ratios can be linked to ferruginous clay minerals or discrete Fe-oxide phases such as goethite and hematite. Higher values in RV3 and SP3 suggested a more significant Fe-bearing component, possibly due to post-depositional Fe mobility or original depositional enrichment. The molar oxide ratios corroborate the mineralogical interpretations and align with observations from other ceramic clay studies. For example, M’barek-Jemai et al. [41] studied clays from Bargou–Bou Arada (Tunisia) and suggested that higher MgO/Al2O3 and K2O/Al2O3 ratios were characteristic of samples with high illite and smectite content, improving plasticity and altering firing color. Studies of clays from the Sidi Khalif formation (Tunisia) revealed that variations in the Fe2O3/Al2O3 ratio correlate with red hues in fired ceramics, with accessory illite, kaolinite, and smectite phases playing contributory roles [42].
Recent studies have emphasized mineralogical composition and physicochemical properties as the handlers of clay workability and firing performance [43]. High proportions of layered clay minerals like smectite or illite impart higher plasticity and cation exchange capacity (CEC) due to large specific surface areas (SSA), while kaolinite-rich clays are less plastic but more refractory [11]. Most red clays contain non-clay inclusions (e.g., quartz, feldspars, carbonates, Fe-oxides), which act as natural fillers and fluxes. The key parameters for technological characterization include CEC and exchangeable cations (Ca2+, Mg2+, Na+, K+), reflecting clay mineralogy (e.g., smectite-rich clays show higher CEC) and affecting plasticity and interaction with additives; higher SSA (e.g., smectite or mixed-layer clays) correlates with higher water adsorption and plasticity; expandability, which indicates the fraction of expansible smectitic layers in mixed-layer clays (illite/smectite), influencing shrink–swell behavior (e.g., smectite-rich clays have higher expandability and must be managed to prevent excessive firing shrinkage); pH, which can influence dispersion and rheology (e.g., alkaline clays tend to deflocculate more easily, aiding molding, while acidic clays may require deflocculants); density, once denser clays or those that densify well upon firing produce stronger ceramics [35]; and plasticity indices (Atterberg Limits—Liquid Limit (LL), Plastic Limit (PL), and Plasticity Index (PI)), which allow to estimate workability, e.g., high-PI clays (>20%) have more plasticity and molding capacity, but too-high plasticity can cause drying shrinkage issues, being estimated that optimal brick clays often show PI in a moderate range (e.g., 10%–30%) [44].
Samples cation exchange capacity (CEC) ranged widely, from ~6–10 meq/100 g in SP1 and SP2 to ~49 meq/100 g in sample RV2 (Table 4). This variation correlated strongly with clay mineral content, with samples rich in smectite (e.g., RV2 ~ 87% smectite) showing the highest CEC, whereas those dominated by illite–kaolinite (SP1, SP2 < 5% smectite) showed the lowest CEC, with typical ranges for kaolinite of ~5–15 meq/100 g and illite of ~10–40 meq/100 g, while smectites can reach 80–150 meq/100 g [45]. Smectitic clays (Table 3) approach the lower end of montmorillonite capacity, and the predominantly kaolinitic SP2 and illitic SP1 clays showed the expected low exchange capacity. Samples exchangeable cations were predominantly Ca2+ and Mg2+, indicating that clays were Ca/Mg-saturated. In the high-CEC RV2, Ca2+ and Mg2+ together comprised ~74% of its exchange, like other smectitic samples (RF and TV series). Exchangeable K+ was generally low in these clays, which was expected as much of the K in illitic clays is held in non-exchangeable interlayers [46]. The prevalence of Ca/Mg likely reflected the geochemical environment, e.g., Na leaching and fixation of Ca/Mg from groundwater, explaining the moderate swelling behavior as Ca-smectites swell less than Na-smectites [47]. The specific surface area (SSA) of the samples also ranged from ~18 m2/g (RF3) to ~76 m2/g (RV2), with higher values corresponding to smectite-rich clays, which have extremely fine particle sizes and internal layer surfaces that contribute to the large SSA. For instance, RV2, with 87% smectite, presented ~75.9 m2/g SSA, and RF4 and TV3 also revealed high SSA~70–75 m2/g, while SP1 and SP2, mostly kaolinite/illite (higher Al2O3 content) and negligible smectite, presented the lowest SSA and expandability. The expandability measurements further reinforce mineralogical interpretation, with smectite-rich samples showing significant results. Higher expandability suggests greater shrink–swell potential, which, in ceramic processing, can lead to higher drying shrinkage and risk of cracking if not managed, with the presence of smectite expected to increase the plasticity and impact drying behavior (more swelling when moist, more shrinkage upon drying). Samples density ranged from ~2.4–2.8 g/cm3, aligning with mineral composition, with smectite-rich samples showing slightly lower values, as smectite has high internal water and low-atomic-weight cations in its structure, yielding a lower specific gravity [48]. In contrast, samples with more kaolinite showed higher densities, such as SP1, dominated by illite and kaolinite with negligible smectite, showed the highest densities. Clays with lower particle density, such as smectite-rich materials, tend to exhibit higher porosity in the green state and require greater compaction to achieve adequate forming density in ceramic bodies [49]. However, all the studied samples densities fall in the typical range for clay materials, and the presence of minor heavy minerals (like Fe-oxides or dolomite/siderite) is too small to drastically affect density. The pH of the clays suspensions was neutral to slightly alkaline for most samples, falling between about 7 and 8. In ceramic processing, a near-neutral pH is usually beneficial, once extremely acidic or basic clays can cause deflocculation or flocculation issues [50], being not expected in the studied samples dispersibility problems.
Based on the above mineralogical and CEC results, three high-plasticity clay samples (one from each regional group—RF, TV, RV, and SP) were selected for detailed consistency (Atterberg limits) tests, namely RF1, TV3, RV2, and SP3. These samples were chosen because they presented relatively high clay fractions and enrichment in smectitic clay minerals, and the highest CEC. Samples SP1 and SP2, dominated by illite and kaolinite and exhibiting the lowest CEC, were not selected for plasticity tests because of the low smectite content results leading to reduced plasticity, making them unsuitable for extrusion-based ceramic applications like bricks or tiles [11]. It is also a common approach in ceramic raw material studies to focus on the more plastic (often red burning) clays, since those govern the processing parameters. The Atterberg limit classifies these clays as highly plastic, with liquid limits (LL) of ~42%–59% and plasticity indices (PI) of ~29%–41% (Table 4). The presence of smectite (expandable clay) explained the PI values, since the large SSA allows to absorb and hold more water [51]. In the Casagrande plasticity chart (Figure 5), samples were positioned well above the A-line, within the domain typically associated with inorganic clays of medium to high plasticity, and below the U-line, suggesting significant plasticity and workability, essential for ceramic shaping, especially in extrusion and molding processes. The results were indicative of a significant proportion of active clay minerals, in agreement with mineralogical data.
Sample RF1 was classified with very high plasticity clay, and RV2 showed a high value; however, interestingly, RV2 had a slightly lower result than RF1 despite the higher smectite content, which may be due to RF1 higher clay fraction content, contributing to a higher water-holding capacity and consequently LL [51]. Samples TV3 and SP3 showed intermediate plasticity, with all samples showing LL within the 30%–60% range, defined as being suitable for ceramics [52]. SP3 presented the lower clay content, with smectite and illite, revealing the lowest PI of the group; nevertheless, it was classified in the plastic clay domain and not silt, similarly to RF1, TV3, and RV2 [53]. RF1, TV3, and RV2, with >55% smectite, showed low PL of ~ 14%–18% considering the clay fraction content (≥60%), suggesting an adhesive consistency even at low moisture. Illite and kaolinite, in contrast, tend to increase PL, contributing less to LL. Sample SP3, with 39% illite, PL was lower than in other samples, what might be explained by the lower clay fraction [51,54]. Samples activity (PI divided by clay fraction) [55] of the samples ranged from ~0.5–0.7, with typical values of Ca-smectite clays, while Na-smectites activity was >1 or even ~4 in extreme cases. The high plasticity is considered beneficial for shaping processes like extrusion or molding, but also presents a tendency for higher drying shrinkage and potential deformation [11]. Clays with PI ≥ 40 (high plasticity) improve workability [56], being quite adhesive and likely to shrink considerably upon drying, but if mixed with a lean clay (or non-plastic additives) can be essential to produce bricks that dry without cracking [57]. In comparison to other structural red clays studies, these samples exhibited similar or slightly higher plasticity, such as clays from northern Cameroon [33] with a high smectite content and PI reaching up to 30%, requiring careful drying management due to their substantial shrinkage potential. A study by Ben Salah et al. [58] reported that adding 10 wt% smectite-rich clay to an illite-kaolinite base improved tile–body consistency and acted as a natural plasticizer in tile mixtures, indicating that these clays can serve as effective plasticizer components in ceramic blends.
All four selected samples showed relatively low green shrinkage (Figure 6). The extrusion moisture required for RF1 and TV3 (33 and 31%, respectively) was higher than for RV2 and SP3 (24 and 25%, respectively), reflecting the higher water-holding capacity. High-plasticity clays demand more water yet do not produce stronger bodies, as excess moisture tends to reduce green strength [11]. Accordingly, RF1 and TV3 (weaker green bodies, ~5–6 MPa) showed lower results in bending strength after drying than RV2 and SP3 (~8–9 MPa), needing less water for workable consistency. All green linear shrinkage values were <10%, a range generally believed acceptable for dimensional stability. Nevertheless, the high RF1/TV3 PI suggested the need for controlled drying to prevent warping or microcracks [59]. The SP3 and RV2 combined lower plasticity and higher solid content in the plastic paste, yielding stronger, more dimensionally stable green bodies, while RF1/TV3 were more prone to drying issues if not managed properly.
After firing at 900 °C (Figure 7), all samples showed the expected sintering trend, with high values of water absorption and porosity. Nevertheless, RF1 and TV3 resulted in porous bodies, with high linear shrinkage and moderate flexural strength. In contrast, RV2 and SP3 resulted in denser products. The high absorption and low density of RF1/TV3 reflected the dehydroxylation and mineral decomposition of smectite, creating fine pores and hinders full densification at 900 °C [33]. In contrast, possibly due to the quartz and feldspar content acting as non-shrink fillers, RV2/SP3 showed reduced overall shrinkage and yielded a smaller pore volume. The RV2 high Ca-Mg content may favor formation of crystalline refractory phases, such as spinel and gehlenite [60], during firing, which can delay densification compared to a more homogeneous glassy phase, while TV3 higher alkali content (K2O + Na2O) may promote early vitrification [61]. Experimental studies showed that clays richer in Fe2O3 and alkalis exhibited enhanced sintering behavior [60], with TV3 having slightly higher flux content (and Fe2O3) and marginally lower absorption than RF1, despite similar smectite content. As temperature increased, water absorption and porosity decreased while strength increased, similar to Kagonbé et al. [35] studies with fired pottery, where absorption declined with temperature as the body densified.
At 1000 °C, the densification differences persisted, but all samples results had improved. Samples RF1 and TV3 absorption dropped further (Figure 7), nonetheless still higher than RV2 and SP3. Correspondingly, RF1 and TV3 decreased more (~24%–28% vs. ~13%–15%) and strengthened slightly, while RV2 and SP3 reached ~12.2–12.5 MPa. Results suggested that higher firing temperature can promote partial vitrification of alkali and Fe-bearing phases, but porosity in RF1 and TV3 remained higher than in RV2 and SP3. The results underscore the role of composition, as the extensive glass formed in smectite-dominated mixes yielded high shrinkage (and risk of warping) but still retained many micropores [62]. Overall, both shrinkage and porosity decreased with firing temperature, and strength slightly increased. High CEC (especially Ca2+/Mg2+ in RV2) reduces swelling and favors the formation of some crystalline phases rather than extensive glass. Alkali oxides and Fe2O3 in TV3 can aid sintering, while RV2 low Na2O (~0.02%) contribute to a more refractory product (lower shrinkage and absorption). SP3 higher coarser fraction and moderate feldspar content tended to result in a rigid skeleton, leading to minimal shrinkage and water uptake. The results were similar to other studies, with clays rich in expansive smectite needing blending for strength, as excessive smectite leads to porosity and defects [62]. All four clays can be used in ceramics at these firing temperatures, with RV2 and SP3 being more suitable for structural applications like floor/wall tiles or bricks, and RF1 and TV3 for porous ceramics, e.g., insulating supports or low-strength bricks. Blending RF1 and TV3 with fluxing or non-clay material would be desirable to meet conventional standards, as technical firing parameters overlap reported ones for earthenware clays (e.g., [35,59]).
To complement the assessment of clays suitability for ceramics, the Fiori diagram was applied to evaluate samples expected behavior during ceramic firing [63,64]. The ternary diagram is widely used in ceramic science, enabling the classification of clayey raw materials based on the normalized proportions of SiO2, Al2O3, and the sum of TiO2 + Fe2O3 + MgO + CaO + Na2O + K2O. These oxides strongly influence sintering, shrinkage, and vitrification behavior [42,44]. The Fiori diagram defines four zones representing distinct technological behaviors, i.e., Zone A, associated with porous ceramics or refractory bodies; Zone B, representing clays suited for common structural ceramics (e.g., bricks, tiles), balancing plasticity and fluxing components, allowing efficient sintering with acceptable shrinkage and strength; Zone C, corresponding to well-vitrifying clays that have higher content of fluxing oxides and promote early vitrification; and Zone D, that includes highly fluxed clays, often with excessive alkali and alkaline earth contents, not suitable alone for ceramic bodies due to potential melting or deformation during firing, but can be used in small proportions in formulations. The chemical composition of the samples in the Fiori diagram showed that samples SP2 and SP1, with high Al2O3 and low fluxing oxides, were classified as Zone A (Figure 8). These clays were kaolinite- and illite-dominated, had lower CEC and SSA, confirming the refractory or porous ceramic behavior and being more suitable for blending with more plastic clays to adjust drying and firing shrinkage. Samples RV1 and RV3 were projected on the border of Zones A and B, reflecting moderate plasticity and a balance between Σ phyllosilicates and fluxing oxides. The high SiO2/Al2O3 ratios (Table 3) and moderate Fe2O3 content support the classification as raw materials for common red ceramics, such as bricks or hollow blocks. The remaining samples were classified as Zone B, suitable for structural ceramic applications. These clays showed balanced alumina and flux contents, moderate to high plasticity, and acceptable technological performance at 900 and 1000 °C. Samples TV2 and TV3, positioned on the boundary between Zones B and C, exhibited higher flux content, supporting enhanced vitrification behavior. No samples were classified as Zone D, indicating that none were inherently overfluxed or prone to melting during firing, confirming that all samples showed the potential to be used either alone or in blended formulations for red ceramic production.

4. Conclusions

The studied clays exhibited promising technological properties for structural ceramic applications. Granulometric analysis confirmed a predominance of fine clay fractions (>50%), essential for plasticity and shaping. Mineralogically, most samples were rich in smectite and illite, contributing to high CEC (up to 49 meq/100 g) and SSA (up to 83 m2/g). The exchangeable cation dominance of Ca2+ and Mg2+ suggested the prevalence of divalent interlayers with moderate swelling, supporting plasticity control. The Atterberg limits confirmed high to very high plasticity (PI 29%–41%), particularly in two samples, allowing good moldability with manageable drying shrinkage.
Technological assays after drying revealed extrusion moisture content ranging from 24%–33%, with green linear shrinkage < 10% and flexural strength up to 9 MPa. These values indicate acceptable mechanical behavior for structural clay products, as they fall within ranges that ensure good workability, resistance to handling, and reduced risk of cracking before firing. Firing tests at 900 °C and 1000 °C showed densification trends, with water absorption decreasing and mechanical strength increasing with temperature. At 1000 °C, RV2 and SP3 showed lower water absorption and higher strength (~12 MPa), meeting typical industrial requirements for bricks and tiles. In contrast, RF1 and TV3 retained higher porosity, suggesting applications in porous ceramics or blends.
The mineralogical and chemical composition, particularly the fluxing oxides (K2O, Na2O, Fe2O3), enhanced vitrification, with one showing improved sintering. The mineral phases of samples after firing of these and other samples will be analyzed and interpreted. The Fe2O3 content influencing the red coloration in these clays also contributes to enhanced ceramic performance by promoting fluxing behavior during firing, reducing water absorption, and improving mechanical strength. Samples RV2 and SP3 combined favorable plasticity, mineralogy, and firing performance, confirming the viability for red ceramic production. Samples may require blending with lean clays or fluxes to optimize performance. Overall, these clays showed versatile potential for ceramics, with processing routes adjustable by mineralogical and technological parameters.
In addition to the technological suitability, the use of these local clay resources aligns with broader sustainability and decarbonization goals. By valorizing nearby georesources, transportation-related emissions are reduced, supporting circular economy principles and promoting regional ceramic industries. Samples analysis and performance confirmed that these materials can contribute to lowering the embodied carbon of ceramic production, while fostering environmentally responsible and community-oriented resource management in Portugal and elsewhere.

Author Contributions

Methodology, C.C. and F.R.; validation, C.C., H.S. and F.R.; formal analysis, H.S. and C.C.; investigation, H.S., C.C. and F.R.; writing—original draft preparation, C.C.; writing—review and editing, C.C. and F.R.; supervision, C.C. and 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) 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 00910 g001
Figure 1. Outcrops of sample locations: (a) Ribeira de Frades (RF); (b) Taveiro (TV); (c) Reveles (RV); and (d) São Pedro (SP). Adapt. Santos [17].
Figure 1. Outcrops of sample locations: (a) Ribeira de Frades (RF); (b) Taveiro (TV); (c) Reveles (RV); and (d) São Pedro (SP). Adapt. Santos [17].
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Minerals 15 00910 g002
Figure 2. Samples granulometric distribution.
Figure 2. Samples granulometric distribution.
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Figure 3. Mineral phases identified in the samples < 0.063 mm fraction.
Figure 3. Mineral phases identified in the samples < 0.063 mm fraction.
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Figure 4. Clay mineral phases identified in the samples < 0.002 mm fraction.
Figure 4. Clay mineral phases identified in the samples < 0.002 mm fraction.
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Figure 5. Samples projected in the Casagrande diagram.
Figure 5. Samples projected in the Casagrande diagram.
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Figure 6. Samples after drying: results of shrinkage, flexural strength, and extrusion moisture.
Figure 6. Samples after drying: results of shrinkage, flexural strength, and extrusion moisture.
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Figure 7. Samples after firing at 900 and 1000 °C: results of water absorption (in %), shrinkage (in %), and porosity (in %) on the left axis, and flexural strength (in MPa) on the right axis.
Figure 7. Samples after firing at 900 and 1000 °C: results of water absorption (in %), shrinkage (in %), and porosity (in %) on the left axis, and flexural strength (in MPa) on the right axis.
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Minerals 15 00910 g008
Figure 8. Studied clays projected in Fiori diagram, defining the potential ceramic behavior: A, porous/refractory; B, structural ceramics; C, vitrifying clays; and D, overfluxed compositions.
Figure 8. Studied clays projected in Fiori diagram, defining the potential ceramic behavior: A, porous/refractory; B, structural ceramics; C, vitrifying clays; and D, overfluxed compositions.
Minerals 15 00910 g008
Table 1. Sample locations, colors, and pit exploration purposes.
Table 1. Sample locations, colors, and pit exploration purposes.
IDCoordinatesColorAssociated Activity
MP
RF1169,250359,250grayishCeramic industry
(not active nowadays)
RF2reddish
RF3silty reddish with gray nodules
RF4light silty reddish
TV1165,000358,850grayish to reddishCeramic industry
(not active nowadays)
TV2reddish with silt
TV3reddish
RV1168,050357,950grayishCeramic industry
RV2reddish
RV3dark reddish (darker tone)
SP1168,125356,500reddishCeramic industry
SP2reddish
SP3dark reddish
Table 2. Major oxides content of the samples < 0.063 mm fraction (in %).
Table 2. Major oxides content of the samples < 0.063 mm fraction (in %).
IDMgOAl2O3SiO2CaOTiO2Fe2O3MnONa2OK2OLOI
RF12.4515.6267.920.510.925.120.010.272.905.01
RF22.2316.5767.020.490.914.820.040.295.125.00
RF33.4221.9161.930.500.662.650.040.352.955.33
RF43.7619.1662.430.660.853.040.040.422.555.30
TV13.4717.7962.900.690.696.290.050.192.695.27
TV23.4818.1063.101.080.936.140.152.122.535.83
TV33.4818.1063.101.080.936.140.152.122.535.83
RV11.0314.4873.220.651.083.630.010.072.314.09
RV21.7415.5966.961.030.873.050.010.021.785.21
RV31.1013.5472.480.611.175.550.050.102.203.91
SP10.1031.0671.320.610.895.970.020.134.449.44
SP20.1030.4854.480.220.322.240.030.071.909.96
SP30.6514.5969.950.521.165.330.050.232.984.22
Table 3. Samples molar oxide ratios.
Table 3. Samples molar oxide ratios.
IDSiO2/Al2O3MgO/Al2O3Na2O/Al2O3K2O/Al2O3CaO/Al2O3Fe2O3/Al2O3
RF14.340.160.020.210.020.33
RF24.050.130.020.310.030.29
RF32.900.160.020.220.020.24
RF43.360.200.020.140.040.16
TV13.240.210.010.160.040.19
TV23.560.240.010.180.050.35
TV33.490.210.010.160.040.34
RV15.040.070.010.160.040.25
RV24.300.060.000.140.040.20
RV35.350.080.010.160.050.41
SP12.300.000.000.140.000.19
SP21.790.000.000.060.010.07
SP34.790.040.020.200.040.37
Table 4. Technological properties of the clay samples studied.
Table 4. Technological properties of the clay samples studied.
IDCECExchangeable CationsSSAExpDenpHPILLPL
Ca2+Mg2+Na+K+
RF144.222.213.81.92.271.125.02.57.241.159.218.1
RF236.019.013.01.51.849.639.22.57.8---
RF337.019.314.11.31.747.033.02.67.7---
RF435.018.627.83.80.874.720.22.46.8---
TV142.016.624.11.91.669.727.42.46.9---
TV239.014.621.01.91.477.540.02.57.2---
TV346.016.624.11.91.682.753.02.47.939.656.817.2
RV128.413.310.91.01.159.222.52.68.2---
RV249.227.221.21.51.975.938.02.58.030.243.913.7
RV328.813.612.41.01.258.020.02.68.1---
SP16.45.21.20.10.425.311.02.87.0---
SP210.05.62.10.00.326.015.02.76.9---
SP322.410.58.31.51.342.225.02.78.029.142.213.1
CEC—Cation exchange capacity (meq/100 g); Exchangeable cations in meq/100 g; SSA—Specific Surface Area (m2/g); Exp—Expandability (%); Den—Density (g/cm3); PI—Plastic index; LL—Liquid limit; PL—Plastic limit.
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Candeias, C.; Santos, H.; Rocha, F. Sustainable Use of Taveiro (Portugal) Red Clays for Structural Ceramic Applications: Mineralogical and Technological Assessment. Minerals 2025, 15, 910. https://doi.org/10.3390/min15090910

AMA Style

Candeias C, Santos H, Rocha F. Sustainable Use of Taveiro (Portugal) Red Clays for Structural Ceramic Applications: Mineralogical and Technological Assessment. Minerals. 2025; 15(9):910. https://doi.org/10.3390/min15090910

Chicago/Turabian Style

Candeias, Carla, Helena Santos, and Fernando Rocha. 2025. "Sustainable Use of Taveiro (Portugal) Red Clays for Structural Ceramic Applications: Mineralogical and Technological Assessment" Minerals 15, no. 9: 910. https://doi.org/10.3390/min15090910

APA Style

Candeias, C., Santos, H., & Rocha, F. (2025). Sustainable Use of Taveiro (Portugal) Red Clays for Structural Ceramic Applications: Mineralogical and Technological Assessment. Minerals, 15(9), 910. https://doi.org/10.3390/min15090910

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