Exploring Residual Clays for Low-Impact Ceramics: Insights from a Portuguese Ceramic Region

Abstract

This study investigates the potential of residual clays from a traditional ceramic-producing region in southern Portugal as raw materials for red ceramic applications. This work aims to support more sustainable ceramic practices through the local valorization of naturally available, underutilized clay resources. A multidisciplinary approach was employed to characterize clays, integrating mineralogical (XRD), chemical (XRF), granulometric, and thermal analyses (TGA/DTA/TD), as well as technological tests on plasticity, extrusion moisture, shrinkage, and flexural strength. These assessments were designed to capture both the intrinsic properties of the clays and their behavior across key ceramic processing stages, such as shaping, drying, and firing. The results revealed a broad diversity in mineral composition, particularly in the proportions of kaolinite, smectite, and illite, which strongly influenced plasticity, water demand, and thermal stability. Clays with higher fine fractions and smectitic content exhibited excellent plasticity and workability, though with increased sensitivity to drying and firing conditions. Others, with coarser textures and illitic or feldspathic composition, demonstrated improved dimensional stability and lower shrinkage. Thermal analyses confirmed expected dehydroxylation and sintering behavior, with the formation of mullite and spinel-type phases contributing to densification and strength in fired bodies. This study highlights that residual clays from varied geological settings can offer distinct advantages when matched appropriately to ceramic product requirements. Some materials showed strong potential for direct application in structural ceramics, while others may serve as additives or tempering agents in formulations. These findings reinforce the value of integrated characterization for optimizing raw material use and support a more circular, resource-conscious approach to ceramic production.

1. Introduction

Residual clays, also known as primary clays, are formed in situ through the prolonged chemical weathering of silicate-rich rocks, a process involving leaching of soluble cations such as Ca²⁺, Na⁺, and K⁺, leading to the concentration of more stable minerals such as kaolinite, illite, and smectite in the soil profile [1]. The mineralogical composition and physical properties are heavily influenced by the parent rock type and the prevailing climatic conditions, leading to significant variability even within localized areas. Tropical and subtropical regions, where intense weathering conditions prevail, are where these clays are more common. These clays have been extensively used in the production of red ceramics, including bricks, tiles, and structural components. Their suitability for ceramic applications is largely determined by mineralogical composition, particle size distribution, and plasticity. For instance, kaolinite-rich clays are favored for their low shrinkage and good firing properties, while smectite-rich clays offer high plasticity but may pose challenges during drying and firing due to their expansive nature [2]. Several studies have highlighted the potential of residual clays in ceramic production. In Brazil, a study on clayey soils highlighted their suitability for red ceramic fabrication, with certain samples meeting the strength requirements for construction materials [3]. In China, red clays were characterized for use in ceramic manufacturing, emphasizing the importance of mineralogical and chemical analyses in determining their applicability [2].

The use of locally sourced residual clays in ceramic production offers significant economic and environmental benefits. Economically, it reduces dependence on imported raw materials, lowers transportation costs, and supports local industries and employment. Environmentally, it promotes sustainable georesources use by minimizing the carbon footprint associated with material transport and processing [4]. The incorporation of residual clays into ceramic products aligns with circular economy principles, encouraging the efficient use of natural resources and the reduction in waste. The comprehensive characterization of residual clays is crucial for optimizing their use in ceramic applications. Understanding their mineralogical, chemical, and physical properties enables manufacturers to predict their behavior during processing and firing, ensuring product quality and performance. Such studies also facilitate the identification of suitable clay sources, inform blending strategies to achieve desired properties, and contribute to the development of standards and guidelines for raw material selection [5].

Portugal has a rich tradition in ceramic production, with numerous regions exploiting local clay deposits for different applications. Studies in areas like Bustos [6] and Vila Nova da Rainha [7] have characterized local clays, assessing their suitability for ceramics, and have highlighted the importance of mineralogical and chemical analyses in determining their applicability. These investigations underscore the potential of Portuguese residual clays in supporting sustainable ceramic industries and preserving cultural heritage. The study area, São Pedro do Corval, in southern Portugal, is renowned for its pottery and ceramic craftsmanship, being one of the biggest centers of traditional pottery in the Iberian Peninsula, with a long-standing artisanal heritage that dates back centuries. The area is characterized by diverse geological formations, including metasedimentary rocks, which give rise to various residual clay deposits. These clays have been the raw material for handmade pottery production across generations in the region. The proximity of the clay sources to the village not only enabled the sustainable development of this craft but also shaped the socio-economic identity of the community. Despite the region’s significance in ceramic production, systematic studies on the characterization and suitability of its residual clays remain limited.

This study aims to characterize residual clays from São Pedro do Corval, one of the most relevant traditional ceramics (pottery) production regions in Portugal, by evaluating their mineralogical, chemical, and physical properties to assess their suitability for red ceramic applications. By focusing on underutilized local raw materials, this research promotes sustainable ceramic production aligned with circular economy principles and the valorization of regional resources. Using indices such as the chemical index of alteration (CIA) and the chemical index of weathering (CIW), it further explores the degree of weathering and its influence on clay quality. The findings are intended to support local ceramic industries, encourage sustainable resource management, and expand scientific knowledge on the potential of residual clays.

2. Materials and Methods

2.1. Sampling and Study Area Context

The village of São Pedro do Corval (southern Portugal; Figure 1) has a long tradition of handmade pottery. This artisanal activity is closely linked to the geological characteristics of the region, particularly the presence of residual clays derived from the intense weathering of local metamorphic rocks. Corval is located within the Ossa-Morena Zone (OMZ) of the Iberian Massif, a complex geotectonic unit characterized by ancient metamorphic and igneous rocks affected by multiple tectonothermal events [8]. The regional bedrock is mainly composed of phyllites, schists, quartzites, and minor carbonate units, which belong to Precambrian to Paleozoic sequences deformed during the Variscan orogeny [9]. Over time, the chemical weathering of these fine-grained metasedimentary rocks led to the formation of residual clay deposits, particularly in topographically stable areas. These residual clays are in situ weathering products, rich in illite, kaolinite, and some smectitic phases, depending on the original rock composition and local hydrothermal influences [10].

The residual clays of Corval are derived from the weathering of pelitic rocks (schists and hornfels). Field observations showed that these residual clays are directly influenced by the outcrops of metabasites, occurring in association with the schists of the Barrancos Formation, which crop out in the vicinity of the Reguengos Eruptive Massif [11]. Lamberto et al. [12] identified five distinct groups in Corval based on the regional geology and the types of rocks associated with these residual clay deposits.

For this study, 9 representative residual clay samples were collected at a depth of 40–50 cm to minimize the influence of surface alterations caused by weathering, biological activity, and atmospheric contamination. This sampling depth ensured that the materials reflected in situ characteristics of the clay deposit, avoiding the effects of prolonged exposure to air, moisture fluctuations, and organic matter accumulation. The collected samples were the following (Figure 1): A1 and A2 near the schists of the Barrancos Formation contact, B1 and B2 at the metabasite outcrops, C1 and C2 near a microgranite intrusion within the metabasites, D1 and D2 near the contact with Reguengos Eruptive Massif granites, and one sample (E1) from a small crop within the granites of the Reguengos Massif.

Figure 1. Study area on the map of Portugal (red square), sample locations (blue dots), and a detail of one sampling site (bottom right). Adapt. IGE [13] and Novo [14].

Figure 1. Study area on the map of Portugal (red square), sample locations (blue dots), and a detail of one sampling site (bottom right). Adapt. IGE [13] and Novo [14].

2.2. Samples Analysis

Samples were dried at ~40 °C for a period of 48 h and manually disaggregated. Samples were wet sieved (<0.063 mm), and a portion was pulverized in an agate mill, e.g., for chemical and mineralogical analyses. A <0.002 mm fraction was obtained via sedimentation of non-grounded samples. To determine the relative grain size distribution of the <0.063 mm fraction, samples were analyzed using a Sedigraph^® III Plus V1.01 based on the sedimentation theory (Stokes’ law) and the absorption of X-radiation (Beer–Lambert law) [6]. pH was determined in a 1:2.5 soil/water solution using a multiparametric meter Hanna, HI 98494 model (Hanna, Woonsocket, RI, USA). Chemical composition of the <0.063 mm fraction was assessed by X-ray Fluorescence (XRF), with a Panalytical Axios PW4400/40, and with Rh radiation. Raw material mineral phases of <0.063 and <0.002 mm fractions were identified using X-ray diffraction (XRD), with a Philips/Panalytical (Malvern, UK) 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. To identify minerals in diffractograms, we used a combination of procedures proposed by Brown and Brindley [15] and Heigh Score Plus v4.9^® with the International Centre for Diffraction Data (ICDD) database. After diffractogram interpretation, the relative abundance of each mineral phase was semi-quantitatively estimated based on the reflection intensities using the peak area method [16].

Raw materials undergo significant transformations during the firing process, resulting in mineralogical and structural changes. Clay minerals are among the first to decompose, while quartz and hematite tend to remain stable throughout the firing range. The most characteristic neoformed mineral phase is mullite, whose formation is directly associated with the breakdown of phyllosilicates and is therefore dependent on the heating rate. The appearance of mullite implies recrystallization, with grain size being primarily influenced by the maximum temperature reached. For this study, the mineralogical changes that occurred at high temperatures were investigated by XRD on representative portions of each specimen fired at different temperatures, in both the outer and inner parts. Cation exchange capacity (CEC) refers to the maximum number of exchangeable cations that a clay mineral can retain on its surface and within its interlayer spaces [17]. This property plays a key role in defining the physicochemical behavior of clays, particularly in ceramic applications where it can influence plasticity, ion diffusion, and sintering characteristics, with CEC values varying widely depending on mineralogy, particle size, crystallinity, and surface area [18]. In ceramic materials, assessing CEC helps to evaluate the interaction potential of clays with additives and to predict behavior during processing. In addition, the concentrations of the main exchangeable cations, including Ca²⁺, Mg²⁺, Na⁺, and K⁺, and total Fe, were determined. The CEC was determined on the <0.063 mm fraction using a saturating solution of ammonium acetate (1 M, pH 7), followed by displacement of the adsorbed ammonium ions with KCl. The concentration of the displaced cations was then quantified by atomic absorption spectroscopy (AAS) [14]. Expandability tests evaluate the volumetric variation that a raw material undergoes when it absorbs water by capillarity under well-defined conditions of compaction, moisture content, and confinement, being primarily governed by the nature and proportion of clay minerals present. Samples of the < 0.063 mm fraction were analyzed following standard [19], with tests performed on compacted cylindrical specimens (Ø ~50 mm; height ~20 mm) prepared from homogenized material. Each specimen was subjected to vertical confinement within a rigid mold and placed in contact with water from the base for a fixed period of 24 h, conducted in triplicate at a controlled room temperature (20 ± 2 °C) and relative humidity (50–60%). Vertical expansion was measured as percentage increase in specimen height using a micrometer gauge. Reported expandability values corresponded to the mean of three replicates, ensuring statistical reliability.

To assess the degree of chemical weathering of the residual clays, two indices were applied: the chemical index of alteration (CIA) [20] and the chemical index of weathering (CIW) [21]. These indices quantify the extent of depletion of mobile cations, primarily Ca²⁺, Na⁺, and K⁺, relative to Al₂O₃, which is considered immobile during weathering processes. The CIA was calculated using molar proportions of the major oxides according to the formula CIA = (Al₂O₃/(Al₂O₃ + CaO + Na₂O + K₂O)) × 100, where CaO represents the Ca oxide content associated with silicate minerals, excluding contributions from carbonates and phosphates. In this study, due to the predominantly silicate nature of the samples and low carbonate content, total CaO was used as a first-order approximation. The CIW modifies the CIA by excluding K₂O, which may behave conservatively in certain environments or become fixed in secondary minerals like illite. The CIW was calculated as CIW = [(Al₂O₃/(Al₂O₃ + CaO + Na₂O)] × 100. Both indices were determined using major oxide data obtained by XRF of the <0.063 mm fraction. Oxide values were converted to molar proportions prior to calculation. To further explore the consistency and geochemical meaning of both indices, the CIA/CIW ratio was considered as an indicator of post-alteration K mobility, with values close to 1 suggesting minimal K addition or remobilization, while lower values may indicate diagenetic processes or incomplete leaching. These indices were interpreted in combination with bulk chemical composition, mineralogical data (XRD), granulometric distribution, and exchangeable cation content to provide a comprehensive assessment of weathering intensity and its implications for ceramic suitability.

Ceramic assays aimed to evaluate the behavior of ceramic bodies throughout the various stages of the ceramic processes. Plasticity is a property that defines the rheological behavior in the presence of water, varying with the crystalline nature of the clay minerals [22]. The plasticity index (PI), as well as the plastic (PL) and liquid (LL) limits, were evaluated following the Atterberg limits [23] by the difference between the maximum water content above which the mass loses consistency to be moldable (LL) and the minimum water content above which the mass becomes moldable (PL), PI = LL − PL [6]. Casagrande [24] defined a diagram to identify the plastic properties of materials, while Gippini [25] identified the main domains in the diagram for clay materials [26] using four classes: no plasticity (0 < LL< 20), low plasticity (20 < LL < 35), medium plasticity (35 < LL < 50), and high plasticity (LL > 50). Specific surface area (SSA) was assessed using the BET (Brunauer–Emmett–Teller) method with an Gemini II 2370 equipment (Micromeritics^® Instrument Corporation, Atlanta, GA, USA) [27].

Thermal analyses allowed the assessment of raw material behavior under a uniform heating rate, enabling the design and control of the firing process. Operating conditions for thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were identical, as the thermograms were recorded simultaneously by thermal analysis (STA) [28,29], using a Netzsch STA 449 C thermobalance (NETZSCH-Gerätebau GmbH, Selb, Germany) equipped with a TASC 414/3A temperature control system. The heating rate was set to 10 °C/min up to 1000 °C using a sample mass of ~20 mg. TGA measurements were conducted under a nitrogen atmosphere (flow rate: 50 mL/min) to ensure an inert environment and accurately assess thermal decomposition behavior of the ceramic clay samples. Tests were conducted using ~20 mg of the powdered sample (<0.063 mm), with three replicates per sample to ensure repeatability, with nitrogen purge maintained throughout the analysis to prevent oxidation. For DSC, heat flow data were simultaneously acquired using the same sample and conditions as specified in [29], enabling the detection of endothermic and exothermic transitions (e.g., dihydroxylation and crystallization). Thermodilatometric (TD) analysis allowed the evaluation of volume variations resulting from contractions and expansions during a heating cycle of test specimens, caused by physical and chemical transformations [30]. The TD was carried out on cylindrical test specimens (Ø ~6 mm × length 25 mm), prepared from extruded and dried material, with a constant heating rate of 5 °C/min up to 1000 °C in the air atmosphere. Length changes were recorded continuously, and dilatometric curves were used to identify sintering onset, maximum shrinkage, and expansion events, with a minimum of two specimens per sample analyzed to ensure result reproducibility.

The forming process consisted of shaping ceramic pastes according to the desired geometry of the final object. The forming method used was extrusion [31] on samples A2, B1, and D2, selected based on other determined characteristics, and the representativeness of each sample was based on different lithologies identified during sampling. Extrusion was carried out using a Netzsch 250/05 extruder, equipped with a vacuum capacity of 0.85 Pa. The drying process was carried out to determine ceramic properties before firing. During drying, test specimens were subjected to mechanical and thermal stress, along with deformations that could lead to the development of tensile and/or shear cracks. Linear shrinkage was measured on dried specimens using a caliper [32], and flexural strength (FS) of the dried specimens (dimensions 3 × 4 × 45 mm) was determined in accordance with standard [33] (green), using a LLOYD Instruments testing machine, with a loading rate of 1 N/s at 20 ± 2 °C under ambient atmospheric conditions. Each specimen was placed on two support rollers 40 mm apart and loaded at the midpoint using a third roller (three-point bending configuration), with a minimum of three replicates per sample tested to ensure reproducibility. The FS values were calculated from the maximum load recorded at failure using the standard formula provided in the ASTM method.

Firing tests, on extruded and dried specimens, were conducted at 900 °C, 1000 °C, and 1100 °C using a Termolab electric chamber furnace (Nabertherm GmbH, Lilienthal, Germany) with a heating rate of 5 °C/min and a soaking time of 60 min at the target temperature. For water absorption capacity, the fired specimens were immersed in water, boiled for two hours, and then cooled for four hours. After removing excess surface water with a damp cloth, the specimens were weighed. Subsequently, they were dried in an oven at 110 °C for 24 h, cooled in a desiccator, and weighed again. Water absorption was calculated as a relative percentage to the dry mass of the specimen. Efflorescence tests were conducted on two halves of the fired specimens after flexural strength. The specimens were placed in a tray with deionized water reaching halfway up their height, left to rest for 24 h, and then dried in an oven at 110 °C for 24 h and assessed semi-quantitatively for the presence of efflorescence. Color of the fired bodies at different temperatures was also visually assessed [14]. All assays were performed under scientific defined criteria following international defined standards with protocols for ceramics.

3. Results and Discussion

The granulometric distribution of ceramic materials is essential to assess their technological behavior during shaping, drying, and firing processes. A balance between clay (<0.002 mm), silt (0.002–0.063 mm), and coarser fractions (>0.063 mm) is essential to ensure a desirable plasticity, mechanical strength, and dimensional stability. The studied samples (Figure 2) revealed that their clay fraction varied from 28.6% (E1) to 57.7% (B1). According to Bálint et al. [34], clayey materials with >40% clay-sized particles are generally considered highly plastic, being suitable for forming processes like extrusion and pressing [35]. Samples A1-2, B1-2, and C1-2 were classified as highly plastic, favorable for shaping but may require careful drying to avoid cracking due to high shrinkage [2]. Samples D2 and E1 presented lower clay content (<30%), with higher sand fraction content (>0.063 mm: 58.0% and 59.0%, respectively). These samples are likely to exhibit lower plasticity and greater dimensional stability and may require blending with more plastic clays to enhance workability [36]. Their coarser fraction content also suggests good drying behavior and reduced risk of deformation during firing, making them potentially suitable for grog or filler materials in ceramic bodies. The silt fraction contributes to the packing density of the ceramic body, with samples showing a 12–23% content, contributing to a balanced particle size distribution that can promote good green strength and sintering behavior [37].

The Winkler diagram (Figure 3), based on the relative proportions of <0.002, 0.002–0.020, and >0.020 mm fractions, further refines the suitability of the clays. Samples A1-2, C1-2, and D1 exhibited a well-balanced distribution, with 48–51% of the clay fraction and >33% sand-sized material classified within Winkler’s zone 3, well suited for extruded red ceramic products, such as roofing tiles, with adequate plasticity and dimensional stability [38]. Samples B1-2, due to their thinner particle content (>54% < 0.002 mm), were projected out of Winkler’s zones, and would benefit from blending with other materials. Samples D2 and E1, both with <30% clay and >60% coarser particles, were classified in Winkler’s zone 1, classified as suitable for solid bricks. To improve the suitability of these raw materials for ceramic applications, an increase in the 0.002–0.020 mm fraction would enhance the projection of the samples within domains defined in the Winkler diagram.

Mineral phases identified in the <0.063 mm fraction showed that phyllosilicates are dominant, ranging from 38% (C1) to 81% (B2), with variable amounts of quartz (SiO₂), plagioclase ((Na,Ca)[(Si,Al)AlSi₂]O₈), and K-feldspar (K(AlSi₃O₈)) (

Table 1. Mineral phases identified in <0.063 mm fraction (in %; tr—trace).

Mineral	A1	A2	B1	B2	C1	C2	D1	D2	E1
quartz	17	17	10	8	20	20	6	10	8
phyllosilicates	61	60	77	81	38	39	46	41	52
K-feldspar	4	2	1	2	3	7	4	6	3
plagioclase	2	7	2	2	21	15	16	14	13
calcite	-	2	-	1	1	tr	9	tr	2
dolomite	2	3	1	3	3	4	2	tr	2
opal C/CT	2	5	2	2	10	8	12	10	11
zeolite	3	-	-	-	1	2	-	-	2
goethite	tr	tr	tr	tr	-	-	-	-	-
anhydrite	2	1	1	tr	2	2	1	1	1
siderite	2	1	5	tr	tr	1	1	3	2
pyrite	1	1	tr	tr	tr	1	1	7	tr
anatase	3	-	tr	tr	1	-	-	-
hematite	1	1	1	tr	-	tr	-	-	1
amphibole	-	-	-	1	-	1	2	8	3

; Figures S1 and S2). In ceramics, quartz acts as a skeletal filler, enhancing mechanical strength and reducing drying shrinkage [42]. Feldspars contribute to alkali fluxes, promoting vitrification during firing, particularly in an intermediate temperature range (900–1100 °C). Carbonate phases, such as calcite (CaCO₃) and dolomite (CaMg(CO₃)₂), release CO₂ upon decomposition (600–800 °C), influencing porosity and acting as fluxing agents, and were found in low amounts, except in sample D1. The samples’ pH ranged 5.5 to 7.9, reflecting differences in mineralogical composition, particularly in the presence or absence of carbonates. A near-neutral pH (6.3–7.0) is typical of kaolinitic and illitic clays with low carbonate content. Clay sample color varied from light beige (C1) to bright orange (B1), reflecting differences in mineralogical composition, grain size, and possibly weathering processes. Although the bright orange hues in samples B1 and B2 may suggest elevated Fe content, the mineralogical analysis revealed only trace amounts of hematite (Fe₂O₃) and goethite (Fe³⁺O(OH)), and a low presence of siderite (FeCO₃) and pyrite (FeS₂) in B1, suggesting that the coloration may be influenced by finely dispersed or amorphous Fe phases not detected by XRD [43]. The beige tone of sample C1, despite a moderate phyllosilicate content and no detectable hematite or goethite, may be attributed to the higher presence of quartz and plagioclase, along with low concentrations of Fe-bearing minerals. Samples D2 and E1, darker in color, contained low Fe-bearing phases (siderite, pyrite in D2) and may reflect post-depositional transformation.

The clay minerals identified in the <0.002 mm fraction were kaolinite (26–31%; Al₂Si₂O₅(OH)₄), illite (K_0.65Al_2.0(Al_0.65Si_3.35O₁₀)(OH)₂), and smectite (e.g., montmorillonite (Na,Ca)_0.33(Al,Mg)₂(Si₄O₁₀)(OH)₂·nH₂O) (

Table 2. Mineral phases identified in <0.002 mm fraction (in %).

Mineral	A1	A2	B1	B2	C1	C2	D1	D2	E1
kaolinite	31	26	55	50	40	43	7	15	8
illite	10	3	30	39	15	2	5	2	61
smectite	59	71	15	11	45	55	88	83	31

). Samples A1-2 presented high smectite (59–71%), and kaolinite (26–31%) content, suggesting high plasticity and water retention capacity. These results support their classification within Winkler’s zone 3, although the smectite content may require careful drying to prevent cracking. Samples B1-2 exhibited a mixed assemblage with high kaolinite (50–55%) and illite (30–39%) contents and low smectite (11–15%) content, suggesting good workability with some reduced swelling potential. The illite content in B2 can enhance dry strength but may reduce plasticity compared to pure kaolinitic clays [44]. Samples C1-2 displayed balanced proportions of kaolinite (40–43%) and smectite (45–55%), with low illite (1–15%) content, and is thus expected to have good results in ceramic processing, combining good plasticity with moderate shrinkage and a good firing response. Samples D1-2 contained significantly more smectite (83–88%) and less kaolinite (7–15%), while E1 presented 61% illite, which enhances mechanical strength and thermal resistance but results in lower plasticity. The abundance of smectite in D1-D2 can contribute to plasticity but also raises concerns about volumetric instability unless managed via tempering. The crystallinity of illite was evaluated using the Kübler Index (KI), with results suggesting that samples from groups C and D exhibited the lowest KI values, indicative of a higher structural order [45]. These samples can be associated with the epizone, a low-grade metamorphic zone where illite recrystallizes toward muscovite. Samples B1-2 and E1 showed higher KI values, possibly related to strong diagenesis and poor crystallinity, typical of the diagenetic zone, with sample B1 exhibiting the most advanced diagenesis. Group A falls within the anchizone, transitional between diagenesis and metamorphism, with the highest Al/(Fe + Mg) ratios. Chemical differentiation based on the I(002)/I(001) reflection intensity ratios revealed that illites from the samples of groups A and D, as well as B1 and C1, were aluminous (ratios > 0.3) [46]. In contrast, samples B2, C2, and E1 contained ferromagnesian (biotitic) illite (ratios < 0.3), which is not common and was probably inherited from the parent rock. The chemical composition of these illites also indicates distinct mica–group mineral affinities: group A and sample D1 exhibited compositions close to muscovite (KAl₂(AlSi₃O₁₀)(OH)₂); B1, C1, and D2 were compositionally closer to phengite (KAl_1.5(Mg,Fe)_0.5(Al_0.5Si_3.5O₁₀)(OH)₂); and B2, C2, and E1 resembled a mixture of biotite (K(Fe²⁺/Mg)₂(Al/Fe³⁺/Mg/Ti)([Si/Al/Fe]₂Si₂O₁₀)(OH/F)₂) and muscovite [47].

The samples’ chemical compositions (

Table 3. Chemical composition of samples’ major elements (in %; LOI—loss of ignition).

ID	Al2O3	CaO	Fe2O3	K2O	MgO	MnO	Na2O	P2O5	SiO2	TiO2	LOI
A1	22.59	0.57	9.71	1.55	2.24	0.10	0.44	0.06	52.13	0.89	9.04
A2	19.73	1.30	9.25	1.10	2.65	0.08	0.65	0.05	55.41	1.02	8.62
B1	27.61	0.30	12.17	2.26	0.91	0.06	0.48	0.11	44.95	0.90	9.93
B2	27.68	0.74	11.04	1.49	2.14	0.07	0.38	0.05	44.01	0.85	11.18
C1	21.59	1.07	8.38	2.17	1.40	0.03	1.63	0.05	55.54	0.91	7.06
C2	21.80	1.34	8.65	2.17	1.41	0.04	1.05	0.04	54.65	0.92	7.69
D1	17.72	2.91	14.70	0.73	4.53	0.21	2.60	0.20	46.86	1.45	8.06
D2	19.89	3.20	11.60	1.70	5.31	0.15	1.07	0.12	46.78	1.26	8.82
E1	20.83	1.88	10.73	3.24	3.75	0.15	1.30	0.13	49.12	1.23	7.42

) showed that alumina (Al₂O₃), closely associated with clay minerals such as kaolinite and illite, ranged from 17.72 (D1) to 27.68% (B2). Samples B1-2, with the highest alumina content (>27%), can be related with the kaolinite and illite minerals present (Table 2), confirming the classification as aluminous clays with high plasticity and ceramic reactivity. Samples A1-2 presented relatively high alumina (20–23%) and moderate Fe₂O₃ (9–10%), supporting their kaolinitic–smectitic nature and reddish coloration. Samples C1-2 showed intermediate alumina (22%) and relatively low Fe₂O₃ (8–9%), consistent with their beige-to-yellowish tone and balanced clay mineralogy. Samples D1-2 and E1 displayed lower alumina content (18–21%), higher Fe₂O₃ (up to 14.7% in D1), and MgO (up to 5.3% in D2), with higher Na₂O (up to 2.6% in D1), which may reflect the presence of Na-rich smectites (e.g., montmorillonite, (Na,Ca)_0.33(Al,Mg)₂(Si₄O₁₀)(OH)₂·nH₂O) or Na–feldspars. The chemical composition supported the identified mineral phases, with higher smectite content and greater plagioclase abundance in these samples. The Fe₂O₃ content in D1 and D2 suggested the influence of Fe-bearing phases such as siderite and pyrite, even if in low concentrations. These results, along with higher loss of ignition (LOI) values, indicative of carbonates and volatile-bearing clays, were consistent with the less plastic, carbonate-influenced materials, often used as structural fillers or in low-shrinkage ceramic formulations. The K₂O present was related to a higher illite content or K-feldspar, which can contribute to early-stage fluxing behavior in ceramics [48,49]. Similarly, the relatively high MgO and Fe₂O₃ levels in D1-2 and E1 were related to the presence of ferromagnesian minerals, identified by I(002)/I(001) < 0.3, possibly inherited from the parent material. The higher LOI values in samples B1-2 were consistent with the clay mineral content, particularly kaolinite and smectite, which release structurally bound water during dehydroxylation >500 °C [6], with the contribution of organic matter expected to be minimal based on the sample color and mineralogy. Samples D1-2 contained carbonates, which decompose between 600 and 800 °C. The lower LOI values in samples C1-2 (7–8%) were consistent with lower clay mineral and carbonate contents, with higher proportions of non-volatile phases, such as quartz.

Samples B1 and E1 showed higher Ba content, which can be beneficial in ceramic production by acting as a mild flux, and may enhance glaze adherence in glazed products (

Table 4. Chemical composition of samples’ trace elements (in mg/kg).

ID	As	Ba	Cr	Cu	Nb	Ni	Pb	Rb	Sr	V	Y	Zn	Zr
A1	5	415	214	54	12	42	6	85	50	122	16	87	132
A2	6	357	286	50	13	45	8	63	74	118	16	78	189
B1	5	572	172	47	14	60	26	108	65	112	18	121	87
B2	5	417	177	38	15	52	32	86	83	116	16	107	77
C1	7	407	139	55	16	46	10	77	107	101	17	99	194
C2	14	450	138	44	15	37	19	70	114	96	16	97	164
D1	11	277	41	42	5	8	17	35	131	168	14	153	70
D2	5	365	141	50	12	89	27	74	116	163	16	126	86
E1	5	556	171	26	20	39	12	115	104	135	24	154	170

) [50]. Strontium, which may substitute for Ca in plagioclase or carbonates, showed higher values in samples with calcite or dolomite, which can influence color and melt behavior in glaze formulations. The elements Cr, Ni, and V, likely linked to detrital or residual Fe–Mg phases (e.g., smectites), showed their highest values in A1 and A2, reflecting the smectite and Fe-bearing clay minerals [51]. These are typically inert in red ceramics but may influence fired color, particularly under reducing conditions. Zinc, which can be linked to clay minerals or carbonates, was most enriched in samples D1-2 and E1, potentially indicating weathering or a contribution from minor hydrothermal activity [52]. The Rb and Y concentrations correlate with feldspar and mica contents, e.g., high Rb in E1 and B1, corresponding with increased illite and K-feldspar content, suggesting residual K-mica weathering [53]. The Pb levels were low to moderate, with slightly higher values in samples B2 and D2, but below thresholds for ceramic materials.

The Atterberg limits LL, PL, and PI allow the evaluation of the workability and water affinity of clayey materials, with a higher PI indicating greater plasticity and shrink–swell potential [54]. The samples with the highest plasticity indices were A1-2, B2, and C2 (

Table 5. Samples’ liquid limit (LL), plastic limit (PL), plasticity index (PI) (in %), and plasticity classification based on Gippini chart [25].

ID	LL	PL	PI	Plasticity Classification
A1	71.3	33.6	37.7	High plasticity clay
A2	87.2	38.6	48.6	Very high plasticity
B1	65.1	37.6	27.5	Medium–high plasticity
B2	71.2	32.6	38.6	High plasticity clay
C1	50.1	27.3	22.8	Medium plasticity
C2	59.4	22.4	37.0	High plasticity clay
D1	60.6	31.5	29.2	Medium–high plasticity
D2	62.5	31.7	30.8	Medium–high plasticity
E1	49.7	26.5	23.2	Medium plasticity

), with A2 exhibiting the highest LL and PI, reflecting its dominant smectite content (71%, <0.002 mm) and fine granulometry (>50%, <0.002 mm). These materials require more water for workability and are prone to significant drying shrinkage, requiring slow and controlled drying [51,55]. Samples A1, B2, and C2 also showed high PIs, consistent with their significant clay fractions (48–58%, <0.002 mm) and mixed kaolinite–smectite minerals, suggesting good plasticity for extrusion but a need for tempering to prevent deformation. Samples B1 and D1-2 revealed medium plasticity, with a moderate clay fraction (29–58%, <0.002 mm) and more coarse particles (>40%, >0.063 mm). Their lower PIs can be associated with reduced water demand and shrinkage, improving dimensional stability during drying and firing. Samples C1 and E1 showed the lowest PIs, which can be linked to their lower clay content (<30%, <0.002 mm) and higher coarser fraction. These samples usually have fewer difficulties during drying with minimal shrinkage but may require added plasticizers to form processes [56]. Sample projection in Gippini plasticity chart domains [25] revealed that A1-2, B2, and C2 were above the Gippini A-B boundaries, within the high-to-very-high plasticity domain, indicating excellent workability but significant shrinkage risk. A2 was projected near the top of the chart, reflecting its dominant smectite content, requiring slow and controlled drying. Samples B1 and D1-2 were projected in the medium–high plasticity field, associated with their mixed kaolinite–illite content and moderate clay fraction, balancing plasticity and dimensional stability. In contrast, C1 and E1 were classified in the medium plasticity region of the chart, consistent with their lower clay content (<30%, <0.002 mm) and higher coarser fraction, linked to easier drying and minimal shrink–swell behavior but requiring added plasticizers for formation [6].

The specific surface area (SSA) and further ceramic assays were only performed in three samples: A2 and D2, with higher smectite contents and plasticity indices; and sample B1, with its thinner particle size distribution (D50 = 0.006 mm). The SSA and cation exchange capacity (CEC) are useful tools that reflect the reactivity of clay minerals, particularly their interaction with water and other charged species, being closely linked to the mineral composition, having a direct influence on shaping behavior, drying response, and sintering dynamics in ceramic processes [51]. Sample B1 exhibited the highest SSA (

Table 6. Samples’ specific surface area (SSA; in m²/g), cation exchange capacity (CEC; in meq/100 g), expansibility (Exp; in %), and exchangeable cations K⁺, Na⁺, Ca²⁺, and Mg²⁺ (in mg/kg).

ID	SSA	Exp	CEC	K+	Na+	Ca2+	Mg2+
A1	-	43.5	42.1	33	19	421	432
A2	22.2	52.4	38.1	23	39	536	505
B1	47.9	15.0	20.2	27	21	286	158
B2	-	22.0	24.1	25	63	654	260
C1	-	35.4	28.0	26	49	569	308
C2	-	40.3	30.6	21	57	618	309
D1	-	44.0	36.0	21	48	978	408
D2	14.1	42.6	31.2	42	55	891	323
E1	-	41.7	31.7	68	43	726	242

), consistent with its high smectite content and mixed-layer clays, with high Al₂O₃ and Fe₂O₃. This high SSA suggests a strong water retention capacity, contributing to plasticity but also a high shrinkage risk and the need for controlled drying. This sample also showed low swelling, possibly due to its lower exchangeable Ca²⁺ and Mg²⁺ and a lower total CEC, suggesting partial collapse of smectite interlayers or surface blockage by Fe oxides. Sample A2 presented moderate SSA values, but a high swelling capacity and CEC, characteristic of smectite-rich and well-ordered clay assemblages, with highly reactive interlayer surfaces. The high Ca²⁺ and Mg²⁺ promote interlayer expansion, linked to elevated expansibility. The A-group clays were highly plastic and reactive, beneficial for forming but requiring care during drying and firing to manage shrinkage and structural integrity. Samples D1-2 displayed high swelling and a relatively high CEC, with a moderate SSA in D2, coupled with significant Ca²⁺ and Mg²⁺ content, suggesting the presence of expansive smectite phases. However, their high carbonate and plagioclase content reduce plasticity and increase the risk of lime popping or bloating unless firing profiles are adjusted. Group C showed a moderate-to-high CEC, expansibility, and intermediate exchangeable cation levels, aligning with the kaolinite and smectite content and granulometric profiles (zone 3 in Winkler’s chart) and suggesting suitability for extrusion-based red ceramics like roofing tiles and light blocks, with good forming properties and manageable shrinkage. Sample E1, with a moderate CEC and high swelling, with 61% illite, suggested that its exchange capacity may be mostly attributed to minor smectite or weathered illite. Its high K⁺ content is consistent with its illitic character and may be beneficial for early densification during firing. However, its coarser granulometry and lower clay fraction suggest low plasticity, being more suitable as a structural filler or grog material to control shrinkage in plastic-rich formulations.

The chemical weathering degree of the studied residual clays was evaluated using the chemical index of alteration (CIA) [20] and the chemical index of weathering (CIW) [21], based on the molar ratios of major oxides, providing insights into the extent of leaching of labile cations (Ca²⁺, Na⁺, K⁺) and the relative enrichment in alumina-rich minerals [57]. The CIA and CIW results suggested varying degrees of weathering among the analyzed clays, from poorly altered to highly leached profiles. The highest CIA and CIW values were observed in the B1-2 samples, associated with higher Al₂O₃ contents (>27%) and lower CaO and Na₂O (Figure 4), with low concentrations of exchangeable bases. These samples are rich in kaolinite and low in smectite, with minimal carbonates and feldspars. Their fine granulometry and acidic pH (5.5–6.7) were also consistent with intense leaching and advanced weathering. Samples D1, D2, and E1 exhibited the lowest CIA (71.4, 67.9, 69.7, respectively) and CIW (73.7, 72.4, 78.9, respectively) values, suggesting low-to-moderate weathering intensity. These samples contained higher amounts of exchangeable Ca²⁺ and Mg²⁺, as well as a higher content of coarse material (>0.063 mm, 46.8–59%). Intermediate behavior was found in samples A1-2 and C1-2, with CIA between 77.0 (C2) and 86.8 (A1) and CIW between 84.0 (C2) and 92.8 (A1). These samples showed mixed mineralogical compositions with varying proportions of kaolinite, smectite, and illite, and low-to-moderate contents of feldspars and exchangeable bases. Their granulometric profiles (<2 µm = 48.7–50.8%) suggested intermediate alteration stages. The samples’ CIA/CIW ratios ranged from 0.88 (E1) to 0.95 (A2, B2), showing moderate variability in K behavior post-weathering. All samples, except E1, presented CIA/CIW ratios from 0.91 (C1) to 0.95 (A2, B2), showing relatively stable K behavior after initial weathering, with little evidence of diagenetic enrichment. Sample E1 revealed a CIA/CIW ratio of 0.88, the lowest in the dataset, suggesting potential K-retention or K-fixation, consistent with illite dominance (61%) and the highest K₂O content among all samples (3.24%). Overall, the results suggest that most samples underwent moderate-to-advanced primary alteration, with only minor post-weathering K mobility.

Thermogravimetric analysis (TGA) allows the assessment of mass changes occurring during controlled heating of raw materials as a result of thermal decomposition or oxidation reactions. The thermogravimetric curves of samples A2, B1, and D2 (Figures S3–S5) were obtained at a heating rate of 10 °C/min up to 1000 °C. The samples revealed three main stages of mass loss, usual for natural clayey materials [43]. The first weight loss stage occurred below 150 °C, corresponding to the release of adsorbed and interlayer water from phyllosilicates, especially smectite in A2 and D2, known for high hygroscopicity [58]. The dehydration step accounted for ~4–6% of mass loss, consistent with their expandable clay mineral content. The second mass loss occurred between 200 and 600 °C, associated with dehydroxylation of structural hydroxyl groups in kaolinite and illite. Sample B1, with a higher kaolinite proportion in the <0.002 mm fraction, was more intensive. This stage overlaps with the oxidation of organic matter and the decomposition of minor sulfides (e.g., pyrite), especially in D2, with 7% pyrite. The third and most intense weight loss occurred between 600 and 800 °C and can be attributed to the decomposition of carbonate minerals such as calcite and dolomite. This stage released CO₂ and resulted in significant mass loss, exceeding 8–10%, depending on carbonate abundance, being more pronounced in D2, with its high pH and carbonate content. Beyond 800 °C, the TGA curves stabilized, indicating the formation of new mineral phases and the start of sintering processes. Sample A2 exhibited moderate overall mass loss and gradual transitions, suggesting good thermal stability for ceramic firing, while B1 showed sharper transitions due to concentrated kaolinite dehydroxylation, and D2 presented a more complex pattern due to volatile-rich phases (carbonates and sulfides), implying a need for controlled firing conditions to avoid bloating or pore development.

The differential thermal analysis (DTA), obtained under the same heating conditions as the TGA, identified typical endothermic and exothermic events corresponding to dehydration, dehydroxylation, and phase transitions in clay and accessory minerals (Figures S6–S8). Sample A2 showed a clear endothermic peak around 100–150 °C, associated with the loss of adsorbed water from smectite and kaolinite surfaces. A more prominent endothermic peak near 520–550 °C can be attributed to dehydroxylation of kaolinite, a transformation where structural hydroxyls are released to form metakaolinite, consistent with the kaolinite content in the clay fraction. The weaker curve ~600 °C may correspond to additional hydroxyl loss from illite or partial dehydroxylation of smectite. The curve does not present significant exothermic activity, indicating minimal organic matter or pyrite oxidation. Sample B1 showed a more intense endothermic peak at ~500 °C, characteristic of kaolinite dehydroxylation, in agreement with the kaolinite content. A minor endothermic peak around ~100 °C reflected surface-bound water. No significant exothermic events were detected, suggesting low organic or sulfide content, despite the presence of 5% siderite and 1% pyrite. The thermal behavior confirms a kaolinitic clay with stable firing behavior and relatively simple thermal decomposition. Sample D2 showed a curve with an initial endothermic event below 200 °C, corresponding to water loss, consistent with its high smectite content. A low-intensity endothermic peak at ~600 °C can be attributed to the dehydroxylation of smectite and minor kaolinite. A significant exothermic peak near ~400–450 °C likely corresponds to oxidation of pyrite, producing exothermic energy and sulfur gases. The thermal result suggests a chemically reactive material that may require controlled firing.

Thermodilatometric (TD) curves were obtained for the raw (unfired) samples A2 and B1, heated to 1000 °C, and D2, heated to 1100 °C (Figures S9–S11), providing a perception of linear dimensional changes during heating, critical to predict firing shrinkage and potential deformation. Sample A2 showed a progressive linear contraction starting ~200 °C, being more pronounced between 550 and 700 °C, corresponding to the dehydroxylation of kaolinite and sintering. A smooth and continuous shrinkage up to 950–1000 °C suggests good thermal behavior with limited structural instability. The total shrinkage was moderate (~3–5%), usual in typical red ceramic raw materials [59]. In sample B1, shrinkage started earlier and was more pronounced, with contraction ~450–600 °C, corresponding to the intense kaolinite dehydroxylation peak in DTA, suggesting a structural collapse followed by initial sintering. The overall shrinkage was slightly higher than A2 (~5–7%), consistent with its higher kaolinite content and finer granulometry. No evidence of bloating or expansion was observed, indicating stable densification. Sample D2 showed an initial minor expansion up to ~350 °C, possibly related to pyrite oxidation and gas release. From ~600 to 800 °C, moderate shrinkage was observed, linked to smectite dehydroxylation and carbonate decomposition. The shrinkage was irregular and less linear, suggesting uneven sintering due to the presence of volatile-rich phases. The presence of pyrite and carbonates increases the risk of bloating or porosity if not fired under controlled conditions.

Overall, the thermal analysis of the clay samples showed that A2 exhibited a typical kaolinitic–smectitic thermal profile with moderate water loss, well-defined dehydroxylation, and continuous shrinkage. Its thermal stability, combined with a balanced mineralogy and moderate carbonate content, makes it suitable for red ceramics requiring stable firing behavior and low porosity. B1, dominated by kaolinite (55%) and with lower smectite content, showed a sharper dehydroxylation event and more intense linear shrinkage. The low presence of volatile phases ensures predictable thermal behavior. These features make B1 ideal for applications requiring high plasticity and structural densification without risk of deformation. Sample D2, with high smectite content and volatile-rich accessory phases (e.g., pyrite, carbonates), presented the most complex thermal behavior. Early expansion, exothermic oxidation, and irregular shrinkage reflected a thermally reactive profile. While rich in plasticity, D2’s composition suggests a need for controlled firing (slow heating and oxidation stages) or blending with more thermally stable clays to mitigate gas release and sintering irregularities.

The forming and drying characteristics of clay bodies vary with water demand, plasticity, and interparticle cohesion [52]. Extrusion moisture (EM), green-to-dry shrinkage (GDS), and green flexural strength (FS) were evaluated for samples A2, B1, and D2 to assess the suitability for extrusion forming and subsequent drying (

Table 7. Samples’ extrusion moisture (EM; in %), green-to-dry shrinkage (GDS; in %), flexural strength (FS; in kgf/cm² and MPa), and color.

ID	EM	GDS	FS (kgf/cm2)	FS (MPa)	Color
A2	26.14	9.63 ± 0.15	133.93 ± 4.93	13.13 ± 0.48	Brown
B1	23.60	7.08 ± 0.10	82.76 ± 3.15	8.12 ± 0.31	Dark orange
D2	18.26	5.32 ± 0.43	69.76 ± 5.24	6.84 ± 0.51	Brown

). Sample A2 exhibited the highest extrusion moisture, reflecting its high fine fraction and smectitic content, associated with good plasticity, but requires higher moisture levels for workable extrusion [7]. Sample A2 showed high green-to-dry shrinkage, consistent with its great clay-bound water content and propensity for shrink–swell movement. Its green flexural strength was the highest, suggesting the strong interparticle cohesion typical of highly plastic clays [51]. Sample B1 required less water for extrusion and moderate shrinkage, reflecting a more balanced kaolinite–illite composition. The green flexural strength was lower than A2 but sufficient for handling and rapid processing. Sample D2, with the lowest EM and GDS, exhibited the greatest dimensional stability during drying, which can be associated with its high sand and carbonate content. However, its green FS was the lowest, suggesting reduced cohesion and potential handling constraints.

The ceramic suitability properties of clay samples A2, B1, and D2 after firing at a heating rate of 5 °C/min with a 60 min soak at the maximum temperature are presented in

Table 8. Samples’ firing temperature (FT; in °C), dry-to-fired shrinkage (DFS; in %), total shrinkage (green-to-fired) (TS; in %), flexural strength (FS; in kgf/cm² and MPa), water absorption (WA; in %), efflorescence (Ef), and color.

FT	DFS	TS	FS (kgf/cm2)	FS (MPa)	WA	Ef	Color
sample A2
900 °C	0.66 ± 0.12	10.17 ± 0.17	119.76 ± 22.19	11.74 ± 2.18	10.37 ± 0.20	greenish *	orange
1000 °C	1.21 ± 0.09	10.40 ± 0.18	65.90 ± 8.27	6.46 ± 0.81	10.06 ± 0.10	-	orange
1100 °C	0.93 ± 0.15	9.77 ± 0.20	34.70 ± 8.11	3.40 ± 0.79	19.77 ± 1.44	-	dark red
sample B1
900 °C	0.70 ± 0.13	7.50 ± 0.10	180.72 ± 12.76	17.73 ± 1.25	16.17 ± 0.08	-	orange
1000 °C	3.39 ± 0.13	9.92 ± 0.12	230.14 ± 18.20	22.57 ± 1.79	11.13 ± 0.22	greenish **	orange
1100 °C	5.78 ± 0.30	11.96 ± 0.48	261.05 ± 30.39	25.59 ± 2.98	6.49 ± 0.42	-	dark red
sample D2
900 °C	0.082 ± 0.079	5.56 ± 0.52	62.46 ± 5.36	6.12 ± 0.53	15.30 ± 0.16	greenish *	orange
1000 °C	0.46 ± 0.14	5.68 ± 0.62	61.32 ± 8.72	6.02 ± 0.86	15.05 ± 0.25	-	dark orange
1100 °C	1.38 ± 0.28	6.62 ± 0.83	79.34 ± 8.79	7.78 ± 0.86	13.84 ± 0.25	-	dark red

* trace; ** clearly visible.

. A progressive decrease in flexural strength was observed in sample A2, with increasing firing temperature from 900 to 1100 °C, despite the formation of higher-temperature crystalline phases. This decline in mechanical strength may be linked to the onset of over-firing effects, identified at 1100 °C through bloating and fractures, suggesting pyrometric deformation. Total shrinkage stayed relatively constant over the three firing temperatures (Figure S12). Water absorption showed a significant increase, peaking at 1100 °C, suggesting a rise in porosity. Sample A2 fired specimens exhibited signs of black core formation, a characteristic of over-firing possibly linked to incomplete oxidation of organic matter such as humic acids. The presence of V, a potential contributor to greenish efflorescence, was observed only at 900 °C, becoming soluble between 800 and 1000 °C, and returning to being insoluble >1000 °C, explaining the presence of greenish efflorescence at the lower temperature, likely due to K-vanadates or Na-molybdates [60]. Identified mineral phases (Figure S13) showed the persistence of quartz and hematite phases throughout the firing process. Quartz, although residual at all firing temperatures, showed a decreasing intensity in reflections, suggesting a progressive incorporation into the vitreous phase with temperature increase. Hematite (Fe₂O₃) reflection intensity increased with firing temperature, confirming its stability and enhanced crystallinity. Spinel-type (MgAl₂O₄) phases were indicative of high-temperature crystalline transformations emerging as firing progressed. These were attributed to the release of Al₂O₃ from clay minerals. Mullite (3Al₂O₃·2SiO₂), formed by the reaction between free SiO₂ and Al₂O₃, was also present at all temperatures. Additionally, γ-alumina was identified at 900 °C and remained detectable at higher temperatures, while K-feldspar exhibited more intense reflections at 1100 °C, highlighting an increased crystallization at higher temperatures. Muscovite also became more prominent at this temperature. The samples’ dark brown color after firing was in agreement with Dondi et al. [49] when Fe₂O₃ was over 3% (9.25 to 12.27%). According to the classification of dark-firing clays, based on the coarse fraction, carbonate, and Fe content, sample B1 is classified as a red clay, with its predominant kaolinite being plastic and refractory; samples A2 and D2 classified as red loams, considered suitable for extruded products and being used in heavy-clay products [14]. In summary, sample A2 demonstrated certain ceramic strength at lower firing temperatures and showed a tendency toward over-firing with increased water absorption at 1100 °C, suggesting limitations for ceramics applications and requiring higher mechanical performance and low porosity. Proper optimization of firing conditions will be essential to mitigate pyrometric deformation and efflorescence.

Clay sample B1 displayed an increase in mechanical performance with rising firing temperature. Flexural strength increased significantly from 900 to 1100 °C (Table 8), consistent with the growth of a denser microstructure and the formation of thermally stable crystalline phases at higher temperatures. Water absorption, an indirect measure of open porosity, exhibited a strong inverse relationship with temperature, decreasing from 900 to 1100 °C, suggesting increased vitrification and pore closure during sintering (Figure S14). Total shrinkage increased with temperature, reflecting the higher degree of densification and sintering. Mineral phases showed the persistence of quartz and hematite throughout the firing range (Figure S15). Quartz remained residual, with peak intensity decreasing with rising temperature, suggesting progressive dissolution into the glassy matrix. Hematite showed increasing peak intensity with temperature, indicating enhanced crystallization or phase stabilization under thermal treatment. Muscovite was identified at 900 °C in a pattern, but not at 1000 °C, suggesting thermal decomposition or incorporation in newly formed phases at higher temperatures. Mullite was identified at 900 °C, and was slightly diminished at 1000 °C, being linked to the beginning of solid-state reactions between alumina and silica, a usual phase transformation in kaolinitic clays [61]. A new spinel-type phase, attributed to γ-alumina, was observed at 900 °C and intensified up to 1100 °C, a mineral commonly associated with the decomposition of clay minerals and release of Al₂O₃, contributing to improved thermal resistance and mechanical properties at elevated temperatures. K-feldspar was more pronounced at 1100 °C, indicating enhanced crystallization or phase separation during high-temperature sintering. The increasing intensity of feldspar-related peaks correlates with the observed improvements in mechanical strength and color change (from orange to dark red), which was also affected by Fe oxidation states and phase interactions. Greenish efflorescence was clearly observed at 1000 °C, more pronounced than in A2, being likely related to soluble vanadates or molybdates formed during intermediate temperature firing. In summary, clay B1 demonstrated higher thermal performance than A2, with higher mechanical strength and lower porosity at increased firing temperatures. However, at intermediate temperatures (~1000 °C), efflorescence formation may affect the surface aesthetics or durability of the ceramic products. The progressive evolution of mineralogical phases, especially mullite, feldspar, and spinel-type alumina, contributes to the technological robustness of this clay body.

Clay sample D2’s total shrinkage was relatively low, with a slight increase with rising firing temperatures, from 900 to 1100 °C, indicating progressive but limited densification (Table 8). Water absorption showed high values at all temperatures, reflecting a reduction in open porosity due to enhanced sintering and partial vitrification. Mechanical strength was not significant, with a nonsignificant increase from 900 to 1100 °C, suggesting a less consolidated microstructure, even at higher temperatures, possibly due to a lower content of fluxing agents or poor particle packing (Figure S16). Greenish efflorescence was observed at 900 °C, also attributed to soluble V and Mo compounds. Mineral phases showed a consistent presence of quartz and hematite across all firing temperatures (Figure S17). Mullite reflections were identified and showed a growth trend with rising temperature, suggesting progressive solid-state reactions between Al₂O₃ and SiO₂, enhancing thermal resistance and dimensional stability. The appearance of γ-alumina and a spinel-type phase at 900 °C suggested early onset of high-temperature phase transformations, resulting from the dehydroxylation and decomposition of alumino-silicate minerals, increasingly prominent at 1000 and 1100 °C. The muscovite phase, typically stable at lower temperatures, was detected at 900 °C but not at higher firing temperatures, consistent with thermal decomposition. K-feldspar was observed across all temperatures, with increasing crystallinity, supporting fluxing behavior contributing to the vitrification process. In summary, sample D2 showed typical sintering behavior with decreasing porosity and increasing structural consolidation at higher temperatures. However, its relatively low flexural strength and persistent water absorption suggested limited densification compared to other clays, such as B1. The observed mineralogical transformations were consistent with those reported in kaolinitic and illitic clays, confirming the presence of quartz, hematite, mullite, feldspar, and spinel-type phases as a key contributor to ceramic evolution upon firing.

4. Conclusions

This study characterized residual clays from the São Pedro do Corval area and evaluated their suitability for red ceramic applications. The samples showed varied mineralogical and granulometric profiles, reflecting different lithological origins, which significantly influenced their technological behavior. Sample A2, derived from metasedimentary rocks, exhibited a high smectite content and fine particle size, resulting in very high plasticity and high extrusion moisture. These features indicate good workability, but also a need for controlled drying and firing due to shrinkage and efflorescence risk. Sample B1, with a kaolinite–illite composition and balanced granulometry, showed good firing behavior, low porosity, and high mechanical strength, especially at 1100 °C, making it suitable for ceramics. The chemical alteration degree, assessed through the CIA and CIW indices, revealed a gradient of weathering intensity across the samples. B1 and B2 recorded the highest CIA and CIW values, consistent with their advanced weathering, leaching of base cations, and enrichment in Al₂O₃. Sample D2, despite its plasticity, revealed a complex thermal profile with higher volatile content and lower strength after firing, suggesting its best use as a plastic additive. This sample showed the lowest CIA and CIW, linked to a coarser texture, exchangeable Ca²⁺ and Mg²⁺, and Al₂O₃ content suggesting limited leaching and partial alteration. Samples E1 and D2, with lower clay fraction contents and coarser textures, had reduced plasticity and shrinkage, making them potential temper materials to improve drying behavior in ceramic blends. Intermediate weathering and technological behavior were observed in A1-2 and C1-2. Thermal and firing analyses supported previous findings, confirming the formation of mullite and vitrification phases, as well as demonstrating the importance of integrating mineralogical, chemical, and thermal data in determining the ceramic potential of raw clays. Overall, samples A2 and B1 emerged as the most promising for red ceramic applications, while others may complement formulations depending on the product requirements. Future works on Corval residual clays will include the interpretation of potential toxic elements sources and further ceramics tests, such as compression tests.