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Characterization and firing behaviour of Kouoptamo clays (West Cameroon) for ceramic applications

Published online by Cambridge University Press:  28 September 2026

Pountouenchi Amadou Open the ORCID record for Pountouenchi Amadou [Opens in a new window] ,
Mauro Bortolotti ,
Mountapbeme Ibrahim Cherif ,
Kouotou Mfomboum Yacouba ,
Njuhou Saliou Open the ORCID record for Njuhou Saliou [Opens in a new window] ,
Levent Karacasulu ,
Njoya Dayirou ,
Nathalie Fagel Open the ORCID record for Nathalie Fagel [Opens in a new window]  and
Mattia Biesuz Open the ORCID record for Mattia Biesuz [Opens in a new window]
Pountouenchi Amadou*
Affiliation:
Applied Inorganic Chemistry, University of Yaounde 1: Université de Yaoundé I, Cameroon
Mauro Bortolotti
Affiliation:
Industrial Engineering, University of Trento: Universita degli Studi di Trento, Italy
Mountapbeme Ibrahim Cherif
Affiliation:
Applied Inorganic Chemistry, University of Yaounde 1: Université de Yaoundé I, Cameroon
Kouotou Mfomboum Yacouba
Affiliation:
Applied Inorganic Chemistry, University of Yaounde 1: Université de Yaoundé I, Cameroon
Njuhou Saliou
Affiliation:
Applied Inorganic Chemistry, University of Yaounde 1: Université de Yaoundé I, Cameroon
Levent Karacasulu
Affiliation:
Industrial Engineering, University of Trento: Universita degli Studi di Trento, Italy
Njoya Dayirou
Affiliation:
Applied Inorganic Chemistry, University of Yaounde 1: Université de Yaoundé I, Cameroon
Nathalie Fagel
Affiliation:
AGEs, University of Liege: Université de Liège, Belgium
Mattia Biesuz
Affiliation:
Industrial Engineering, University of Trento: Universita degli Studi di Trento, Italy
*
Corresponding author: Amadou Pountouenchi; Email: apount@yahoo.fr
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Abstract

Clays from Kouoptamo in West Cameroon were investigated to evaluate their suitability for ceramic applications. The four representative samples (ABK, AJK, ARK and AVK) from the study area were characterized. The clays had remarkably distinct chemical, structural and particle-size characteristics, resulting in different densification behaviours and physical and mechanical properties. ABK (the most kaolinite-rich sample) demonstrated the lowest compressive strength (≈25 MPa) and relatively high porosity (38%) after firing at 1200°C, indicating potential for refractory applications, whereas AJK (the most goethite-rich sample) achieved the highest bulk density (2.11 g cm–3) and compressive strength (≈50 MPa) at the same temperature, rendering it potentially useful for roofing tiles and bricks. ARK and AVK showed moderate densification and mechanical properties. The Kouoptamo clays may be suitable for a range of ceramic applications, from whiteware products to roofing tiles, depending on their composition and firing behaviour.

Keywords

Cameroonian raw materialsceramic materialsKouoptamo claysmineralogical characterizationmullitethermal behaviourvitrification

Information

Type
Article
Information
Clay Minerals , First View , pp. 1 - 11
Copyright
© The Author(s), 2026. Published by Cambridge University Press on behalf of The Mineralogical Society of the United Kingdom and Ireland.

Clayey materials constitute one of the most widely used natural resources in the ceramic industry (Asante-Kyei et al., Reference Asante-Kyei, Addae and Aflo2023; Mesrar et al., Reference Mesrar, Laila and Raouf2023; Pountouenchi et al., Reference Pountouenchi, Njoya, Mbey, Mache, Njoya and Yongue2023; Almusawi et al., Reference Almusawi, Obaid, Al-Khazraji and Zemam2024). Their ceramic performance depends heavily on their mineralogical composition, overall chemistry and structural organization of their constituent phases (Biswas et al., Reference Biswas, Grathoff, Naidu and Warr2024). Variations in the arrangement of tetrahedral silicate sheets and octahedral aluminium sheets lead to different physicochemical properties, thereby determining their suitability for applications ranging from whitewares to refractory ceramics (Asante-Kyei et al., Reference Asante-Kyei, Addae and Aflo2023; Mesrar et al., Reference Mesrar, Laila and Raouf2023; Mountapbeme et al., Reference Mountapbeme, Mbamyah, Pountouenchi, Njuhou, Mouafon and Ndzana2024, Reference Mountapbeme, Pountouenchi, Ndzana, Mouafon, Kouotou and Njuhou2026; Barakat et al., Reference Barakat, Eltohamy, Abdelrazek, Ammar, Elhawam and Fouad2025; Njuhou et al., Reference Njuhou, Achiwoue, Njindam, Mountapbeme, Kouotou, Pountouenchi and Njoya2025). Therefore, natural clays continue to attract considerable scientific and industrial interest due to their abundance, low cost and suitability for thermal processing (Candeias et al., Reference Candeias, Novo and Rocha2025; García-Romero et al., Reference García-Romero, Manchado and Suárez2025; Lazorenko & Kasprzhitskii, Reference Lazorenko and Kasprzhitskii2026).

In Cameroon, clay resources are mainly used in traditional pottery, the construction of artisanal kilns and the manufacture of cosmetic products. Despite their availability, the sector remains underdeveloped (Kagonbé et al., Reference Kagonbé, Tsozué, Nzeukou and Ngos2021; Ndjigui et al., Reference Ndjigui, Mbey, Fadil-Djenabou, Onana, Bayiga, Embom and Ekosse2021; Njindam et al., Reference Njindam, Njuhou, Pountouenchi, Mandou, Mfomboum, Mountapbeme and Njoy2024). The main limitations are related to inadequate research infrastructure, scarcity of reliable geological data, limited industrial implementation of scientific findings and the preference for imported products over locally manufactured alternatives. These constraints partly explain the country’s strong dependence on foreign ceramic products (Pountouenchi et al., Reference Pountouenchi, Njoya, Njoya, Rabibisao, Mache and Yongue2018; Kalguem et al., Reference Kalguem, Wouatong, Njopwouo, Kemteu and Ekosse2019; Ndjigui et al., Reference Ndjigui, Mbey, Fadil-Djenabou, Onana, Bayiga, Embom and Ekosse2021). Studies conducted in the Foumban region have identified clays dominated by kaolinite, with quartz and feldspars, although some deposits require compositional modification before being suitable for ceramic applications (Wouatong et al., Reference Wouatong, Tchounang, Hawa, Yerima, Melo and Njopwouo2015; Bomeni et al., Reference Bomeni, Njoya, Ngapgue, Wouatong, Yongue Fouateu, Kamgang Kabeyene and Fagel2018; Nkalih et al., Reference Nkalih, Pilate, Yongue, Njoya and Fagel2018; Njindam et al., Reference Njindam, Njuhou, Pountouenchi, Mandou, Mfomboum, Mountapbeme and Njoy2024; Pountouenchi et al., Reference Pountouenchi, Ndzana, Mountapbeme, Njuhou, Njoya and Fagel2024). Nevertheless, several neighbouring deposits, particularly those in the Kouoptamo area, remain poorly documented, and their mineralogical and physicochemical characteristics are not yet well-established. The lack of such information limits their rational exploitation and integration into local industrial sectors. Therefore, comprehensive characterization of local clay resources is essential for their effective utilization in ceramic applications (Ndjigui et al., Reference Ndjigui, Mbey, Fadil-Djenabou, Onana, Bayiga, Embom and Ekosse2021). In this study, clays collected from the Kouoptamo area (West Cameroon) were characterized to evaluate their suitability for ceramic applications.

Materials and methods

Raw materials

The clayey materials (denoted as ABK, AJK, ARK and AVK) examined in this study come from the locality of Kouoptamo, West Cameroon (Fig. 1), more precisely in the Noun division, located ∼40 km west of Foumban, along the D66 departmental road. The area is part of the Bamoun Plateau and is characterized by moderately dissected landforms as well as weathered volcanic and metamorphic formations. The materials resulted mainly from the gradual decomposition, under the combined action of water, heat and time, of the metamorphic and volcano-sedimentary rocks that predominate in this region (Bidias et al., Reference Bidias, Nguegni, Ilouga, Kenna, Moundi and Kamgang2023).

Figure 1.

Map of the Kouoptamo area, with the study area, according to Bidias et al. (Reference Bidias, Nguegni, Ilouga, Kenna, Moundi and Kamgang2023). The red arrow indicates the collection area of samples ABK, AJK, ARK and AVK.

Kouoptamo area in West Cameroon showing sample collection sites and volcanic features. See long description.
Figure 1 Long description

The Kouoptamo area in West Cameroon highlights the sample collection area with a red arrow and yellow circle. The map includes various volcanic features labeled numerically: 1) Santa-Ndomanik maar, 2) Njimouapeu volcano, 3) Petpenoun domain, 4) Petpenoun lake, 5) Tchoua maar, 6) Kechouamin volcano, 7) Nkoe-Ngoum volcano. The elevation is indicated with a gradient from low (1000 meters) to high (1625 meters). The left inset map shows the location within West Africa, with the sample collection area marked in red. Localities and roads are also depicted. Spatial distribution of volcanic features is concentrated around Kouoptamo, with notable features like Mount Kouoptamo and Nkouemboum nearby.

Methods

The selection of a suitable sampling area was based on the properties (texture, thickness) of the clayey materials that could be used in the manufacture of ceramic products, as well as on the site’s accessibility in relation to the main road and the presence of tracks allowing rapid removal of these clays. The samples were collected at depths ranging from 0.5 to 1.5 m to avoid the surface layer, which is affected by organic matter. Each sample was manually taken using an auger and subsequently stored in labelled plastic bags to minimize alteration or contamination. The raw materials were air dried at ambient temperature and then oven dried at 105°C. They were subsequently ground in a mill with a grinding charge within a cylinder rotating about its horizontal axis for 5 h. The various powders obtained after 5 h of grinding were sieved through a 100 µm sieve.

The particle-size distribution was determined with a laser diffraction particle size analyser (Mastersizer 2000 equipped with Hydro-G dispersion unit, Malvern Instruments, Ltd). Each sample was dispersed in water; 10% sodium metaphosphate was added to improve the dispersion of agglomerates, and the dispersions were subjected to ultrasonic treatment for 30 s before being analysed.

The chemical analysis of the major elements of the clays was carried out using X-ray fluorescence (XRF) spectrometry with an ARL PERFORM-X 4200 XRF spectrometer with a standard error of <1%. Fourier-transform infrared (FTIR) spectra were collected on a Bruker FTIR Spectrometer in the range 4000–400 cm–1 to assess the structure and crystal order of the clay minerals.

The analysis of the mineralogical phases was performed using X-ray diffraction (XRD; Bruker D8 Advance diffractometer) with monochromatic Cu-Kα radiation (λ = 1.5406 Å), a scanning step of 0.020°2θ in the range 2–70°2θ, operated at 40 kV and 25 mA. Quantitative mineralogical analysis was performed using the Rietveld method implemented in Maud software. Reference structural models were extracted from the Crystallography Open Database (COD; Gražulis et al., Reference Gražulis, Daškevič, Merkys, Chateigner, Lutterotti and Quirós2012). Least squares refinement was articulated in incremental steps. Initially, only background, intensity scale factor and lattice parameters were refined for all phases. Subsequently, size/strain parameters, in the isotropic approximation, were added to the analysis. For clay phases, the preferred orientation was modelled using a single fibrous component aligned along the (00l) axis; for kaolinite, turbostratic disorder (Ufer et al., Reference Ufer, Roth, Kleeberg, Stanjek, Dohrmann and Bergmann2004) was also taken into account. Finally, common displacement factors for the significant elements were refined.

Thermogravimetric analysis (TGA)/derivative thermogravimetry (DTG) was carried out with a SETARAM Cahn apparatus, operating under the following conditions: heating rate of 5°C min–1, sample weight of 100 mg, in an air atmosphere from ambient temperature to 1000°C.

Scanning electron microscopy (SEM) was used to observe mineral morphology and size; the observations were carried out using an ESEM Philips XL-30 device. The images were obtained by a secondary electron detector after coating with gold powder by plasma spraying for an accelerating voltage scan.

To investigate the thermal behaviour of the clays, 50 g of each clay powder was moistened with 5 mass% of water and then mixed manually. The resulting pastes were uniaxially pressed at 30 MPa in a cylindrical mould with a diameter of 3 cm using a hydraulic press. The resulting products were then sintered at 1000°C, 1100°C and 1200°C for 4 h in an electric furnace with a heating rate of 5°C min–1.

For physical and mechanical characterization, firing shrinkage (R c) was evaluated from the difference between the sample height before and after thermal treatment. This was assessed with a calliper of ±0.01 mm sensitivity. Bulk density (ρa) and open porosity (Π) were determined using the water displacement method according to EN 993-1 standard (Norme ENV 993-1, 1997) using an analytical balance (±0.0001 g). Compressive strength was measured using a Toniversal 645–656 brand hydraulic testing machine (crushing speed of 0.005 N/mm2.s), from the maximum load sustained by cylindrical samples according to the EN 993-5 standard (Norme NBN EN 993-5, 1999). Measurements of physical and mechanical properties were taken on five cylindrical test specimens, each 3 cm in diameter and 3 cm in height.

Results and discussion

Raw material characterization

The particle-size distribution and classification of the raw materials based on the diagram illustrating the ideal particle-size distribution for tiles and bricks are shown in Fig. 2. All raw materials are mainly composed of silt-sized particles, with a limited clay fraction and minor amounts of fine and coarse sand. Sample ABK had the finest particle-size distribution, containing 17% clay and 76% fine silt. Such a composition is expected to increase plasticity and cohesion, which are favourable characteristics for ceramics shaping processes (Ngun et al., Reference Ngun, Mohamad, Shamsul, Kiyoshi and Zainal2011; Martín et al., Reference Martín, Pachepsky, García-Gutiérrez and Reyes2018; Nkalih et al., Reference Nkalih, Pilate, Yongue, Njoya and Fagel2018). In contrast, sample AVK contains a lower clay fraction (9%) and a slightly higher sand content, suggesting lower plasticity but improved dimensional stability during drying.

Figure 2.

(a) Particle-size distributions of the investigated raw materials, showing the relative contents of clay, fine silt, coarse silt and sand fractions. (b) Evaluation of the suitability of the raw materials based on the diagram of Winkler (1954) (Ngun et al., Reference Ngun, Mohamad, Shamsul, Kiyoshi and Zainal2011; Nkalih et al., Reference Nkalih, Pilate, Yongue, Njoya and Fagel2018).

A bar graph and a triangular diagram showing particle size distribution and material suitability by raw material source. See long description.
Figure 2 Long description

A bar graph with the vertical axis labeled, Part icle size distribution, ranging from 0 to 100. The horizontal axis is labeled, Raw material source, with categories ABK, AJK, ARK and AVK. A legend lists, Clay 0 to 2 micro meter, Fine silts 2 micro meter to 20 micro meter, Coarse silts 20 micro meter to 50 micro meter and Fine sands 50 micro meter to 200 micro meter. For ABK, the bars are approximately 15, 75, 10 and 0. For AJK, the bars are approximately 15, 65, 18 and 0. For ARK, the bars are approximately 12, 78, 12 and 0. For AVK, the bars are approximately 10, 68, 22 and 0. The image B showing a triangular diagram. The left corner label reads, 100 percent Clays, greater than 2 micro meter. The right corner label reads, 100 percent Fine silt, less than 20 micro meter. The top corner label reads, Coarse silt and sand, greater than 20 micro meter, 100 percent. Inside the triangle, region labels read, Solid bricks, Perforated bricks, Roofing tile and Not suitable, all products liable to excessive shrinkage on firing. A legend lists ABK, AJK, ARK and AVK with distinct marker symbols. Four markers appear in the lower right area of the triangle, with one marker near the Roofing tile label and three markers below it.

Compared with clays reported from other regions of Cameroon, the Kouoptamo clays are characterized by a high proportion of fine silt and low sand content, indicating a relatively fine and homogeneous texture. All four materials are projected within the suitability range for roofing tile production (Fig. 2b). In comparison with the sand-rich clays from Foumban and Feutap (Nkalih et al., Reference Nkalih, Pilate, Yongue, Njoya and Fagel2018; Pountouenchi et al., Reference Pountouenchi, Njoya, Njoya, Rabibisao, Mache and Yongue2018), the Kouoptamo clays may offer improved workability and cohesion, which might be advantageous for the manufacture of fine ceramic products. The clays from Bamenda and Ndop (Yongue-Fouateu et al., Reference Yongue-Fouateu, Ndimukong, Njoya, Kunyukubundo and Kemeng2016) contain greater clay fractions, which generally promote plasticity but may also increase drying shrinkage and cracking susceptibility. These variations highlight the diversity of Cameroonian clay deposits and their influence on processing behaviour and potential industrial applications.

The chemical compositions of the Kouoptamo clays are summarized in Table 1. The investigated samples shows notable compositional differences, although all are dominated by SiO2 (46–52 wt.%) and Al2O3 (25–37  wt.%). Sample ABK has the highest alumina content (36.6  wt.%) and the lowest iron content (0.78 wt.%). The SiO2/Al2O3 ratio of 2.16 indicates the predominance of alumina-rich minerals, mainly kaolinite, which may be associated with increased refractoriness and suitability for white ceramic products such as faience, tiles and porcelain (Sawadogo et al., Reference Sawadogo, Zerbo, Sawadogo, Seynou, Gomina and Blanchart2020; Kankao et al., Reference Kankao, Ngon, Mbog, Tehna, Bayiga, Mbaï and Etame2022). In contrast, sample AJK is distinguished by its high Fe2O3 content (8.7 wt.%) and a SiO2/Al2O3 ratio of 3.21, reflecting a matrix enriched in SiO2 and Fe2O3, typical of illitic clays. Such a composition is favourable for red bricks and roof tiles, where the iron oxide acts as both a flux and a colouring agent during firing (Zanelli et al., Reference Zanelli, Raimondo, Guarini and Dondi2011; Tibang et al., Reference Tibang, Waji, Shemang and Suh2021). The ARK and AVK samples have intermediate compositions. ARK, with a relatively high Al2O3 content (29.6 wt.%) and moderate Fe2O3 content (2.7 wt.%), may be suitable for semi-refractory ceramic applications. AVK has the highest SiO2 content (52.0  wt.%) and K2O content (2.1  wt.%), promoting fluxing behaviour and reducing firing temperature. Such compositional features may favour its use in utilitarian pottery and porous ceramic products (Bomeni et al., Reference Bomeni, Fotsing, Nzeugang, Kieufack and Fagel2025).

Table 1.

Chemical compositions (%) of the samples obtained from the Kouoptamo area.

Across four samples, SiO2 ranges from 46.59 to 51.99 percent, while Al2O3 ranges from 25.13 to 36.59 percent. See long description.
Table 1 Long description

Chemical composition percentages are listed for samples ABK, AJK, ARK, and AVK, with columns for Al2O3, SiO2, Fe2O3, TiO2, MnO, MgO, CaO, K2O, P2O5, LOI, and Si2O/Al2O3. For ABK, Al2O3 is 36.59, SiO2 46.59, Fe2O3 0.78, TiO2 0.60, MnO 0.01, MgO 0.14, CaO 0.19, K2O 0.39, P2O5 0.08, LOI 14.22, and Si2O/Al2O3 2.16. For AJK, Al2O3 is 25.13, SiO2 47.41, Fe2O3 8.71, TiO2 1.47, MnO 0.01, MgO 0.16, CaO 0.05, K2O 0.38, P2O5 0.14, LOI 14.38, and Si2O/Al2O3 3.21. For ARK, Al2O3 is 29.64, SiO2 49.32, Fe2O3 2.70, TiO2 1.55, MnO 0.01, MgO 0.20, CaO 0.05, K2O 0.47, P2O5 0.17, LOI 15.42, and Si2O/Al2O3 2.83. For AVK, Al2O3 is 26.46, SiO2 51.99, Fe2O3 2.55, TiO2 1.38, MnO 0.02, MgO 0.68, CaO 0.12, K2O 2.08, P2O5 0.11, LOI 13.80, and Si2O/Al2O3 3.34. Fe2O3 is highest in AJK at 8.71 and lowest in ABK at 0.78, while K2O is highest in AVK at 2.08 and is under 0.50 in the other three samples. LOI ranges from 13.80 in AVK to 15.42 in ARK, and MnO is 0.01 in ABK, AJK, and ARK and 0.02 in AVK.

LOI = loss on ignition.

Figure 3 presents the classification of the raw clays according to the ternary diagram proposed by Fiori et al. (Reference Fiori, Fabbri, Donati and Venturi1989). ABK, ARK and AVK are suitable for white stoneware, whereas AJK is better suited for red stoneware. Consequently, Kouoptamo clays may possess broad potential for ceramic applications, ranging from whitewares to structural ceramics. Compared with neighbouring deposits from Foumban, Feutap and Ndop, the Kouoptamo clays are characterized by higher Al2O3 contents and more variable Fe2O3 levels (Table 2). Foumban clays are generally richer in iron oxides (5–10 % Fe2O3) K2O and CaO, promoting vitrification and dark red colouration during firing (Nkalih et al., Reference Nkalih, Pilate, Yongue, Njoya and Fagel2018; Pountouenchi et al., Reference Pountouenchi, Njoya, Njoya, Rabibisao, Mache and Yongue2018). Feutap clays, which are slightly richer in sand and silica, are reported to be more suitable for artisanal pottery and porous bricks (Bomeni et al., Reference Bomeni, Fotsing, Nzeugang, Kieufack and Fagel2025). By contrast, Ndop clays, with higher Al2O3 (28–36%) and lower SiO2 contents, exhibit a more refractory character and are therefore commonly used in technical ceramics applications or as kiln supports (Yongue-Fouateu et al., Reference Yongue-Fouateu, Ndimukong, Njoya, Kunyukubundo and Kemeng2016).

Figure 3.

Ternary diagram of Fiori et al. (Reference Fiori, Fabbri, Donati and Venturi1989). Chemical classification of clay raw materials in ceramics: (a) white stoneware, (b) red stoneware, (c) cottoforte tile and (d) majolica tile. ‘Other oxides’ = Fe2O3, TiO2, MnO, MgO, CaO, Na2O, K2O and P2O5.

A ternary scatter diagram of clay raw materials, with four labeled samples clustered near the lower right side of the triangle.
Table 2.

Quantitative mineralogical composition (wt.%) of the studied clays.

Kaolinite is highest in sample ABK at 88.27 wt percent, while other samples range from 61.20 to 70.02 wt percent. See long description.
Table 2 Long description

Mineral contents are reported as weight percent for four samples, ABK, AJK, ARK, and AVK, with values given as a mean plus or minus an uncertainty, and an Rwp value per sample. ABK lists kaolinite 88.27 plus or minus 0.88, illite 6.52 plus or minus 0.06, quartz 4.47 plus or minus 0.04, and Rwp 13.76, with montmorillonite, goethite, hematite, rutile, and anatase shown as absent. AJK lists kaolinite 61.20 plus or minus 0.61, illite 8.17 plus or minus 0.08, quartz 22.32 plus or minus 0.22, goethite 7.64 plus or minus 0.08, anatase 1.47 plus or minus 0.01, and Rwp 16.55, with montmorillonite, hematite, and rutile shown as absent. ARK lists kaolinite 70.02 plus or minus 0.71, illite 7.08 plus or minus 0.08, quartz 19.03 plus or minus 0.20, hematite 2.70 plus or minus 0.01, rutile 1.55 plus or minus 0.01, and Rwp 14.43, with montmorillonite, goethite, and anatase shown as absent. AVK lists kaolinite 62.54 plus or minus 0.62, illite 12.18 plus or minus 0.11, quartz 20.71 plus or minus 0.21, montmorillonite 3.18 plus or minus 0.06, rutile 1.38 plus or minus 0.01, and Rwp 18.25, with goethite, hematite, and anatase shown as absent.

Rwp = weighted profile R-factor.

Figure 4 shows the XRD traces of the studied clay samples, while Table 2 summarizes their mineralogical composition. The Kouoptamo clays are composed mainly of kaolinite (61–88 wt.%), quartz (4–22 wt.%) and illite (7–12 wt.%). Minor phases, including montmorillonite, goethite, hematite, rutile and anatase, were identified in some samples. The quantitative phase assemblages are consistent with the chemical compositions reported in Table 1. Sample ABK has the highest kaolinite content (88.3 wt.%) and the sharpest reflections at 12.35°2θ and 24.80°2θ, indicating a high degree of crystal order. Such characteristics are commonly associated with refractory and whiteware clay deposits (Sawadogo et al., Reference Sawadogo, Zerbo, Sawadogo, Seynou, Gomina and Blanchart2020; Claudel et al., Reference Claudel, Rolland, Marguerite, Theodore, Parfait and Salomon2022; Kankao et al., Reference Kankao, Ngon, Mbog, Tehna, Bayiga, Mbaï and Etame2022; Pountouenchi et al., Reference Pountouenchi, Ndzana, Mountapbeme, Njuhou, Njoya and Fagel2024). Sample ARK also remains kaolinite-rich (70.0 wt.%), although its reflections of kaolinite are slightly broader than those of ABK, suggesting a somewhat lower crystal order. In addition, ARK contains appreciable amounts of quartz (19.0 wt.%) together with minor hematite (2.7 wt.%) and rutile (1.6 wt.%). By contrast, AVK contains lower kaolinite (62.5 wt.%) and higher illite and quartz contents, reflecting a more siliceous composition (i.e. SiO2/Al2O3 = 3.34), in agreement with its chemical composition.

Figure 4.

XRD traces of the (a) ABK, (b) AJK, (c) ARK and (d) AVK samples. A = anatase (TiO2, PDF 21-1272); Goe = goethite (α-FeO(OH), PDF 29-713); H = hematite (α-Fe2O3, PDF 33-664); I = illite ((K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)], PDF 26-911); K = kaolinite (Si2O5Al2(OH)4, PDF 14-164); M = montmorillonite; ((Na,Ca)0.3(Al,Mg)2Si4O10(OH)2·n(H2O), PDF 13-259); Q = quartz (SiO2, PDF 5-490); R = rutile (TiO2, PDF 21-1276).

XRD plots for ABK, AJK, ARK and AVK samples showing phase identification with dominant quartz peaks near 27 degrees 2 theta. See long description.
Figure 4 Long description

X-ray diffraction plots for samples ABK, AJK, ARK and AVK, used for phase identification. The x-axis is labeled 2 theta in degrees, ranging from 0 to 70. The y-axis is labeled Intensity in arbitrary units. Each plot shows a dominant peak near 27 degrees 2 theta, indicating quartz presence. The letter markers represent phases: K for kaolinite, Q for quartz, I for illite, Goe for goethite, H for hematite, A for anatase, M for montmorillonite and R for rutile. ABK: Shows sharp kaolinite peaks at approximately 12 and 25 degrees, with additional quartz peaks. AJK: Displays a prominent quartz peak at 27 degrees and secondary peaks for goethite and anatase. ARK: Features quartz and kaolinite peaks, with hematite and illite markers. AVK: Contains quartz and rutile peaks, with montmorillonite and illite markers. Overall, ABK has the sharpest kaolinite peaks, while AJK shows notable goethite presence. ARK and AVK have more diverse phase markers, indicating varied mineral compositions.

Sample AJK has the lowest kaolinite content (61.2 wt.%) and the highest quartz, together with a high goethite content and minor anatase. The high quartz content and the absence of swelling clay minerals may contribute to its improved dimensional stability during firing. These characteristics may favour the use of AJK in brick and roofing tile production (Celik, Reference Celik2010; Zanelli et al., Reference Zanelli, Raimondo, Guarini and Dondi2011; Lamouri et al., Reference Lamouri, Labadi, Boukoffa, Chouaf, Djouder, Djaiz and Bouabsa2022; He et al., Reference He, Atheba, Allou, Drogui, El Khakani and Gbassi2023; Wembe et al., Reference Wembe, Ngueyep, Nkuissi, Langollo, Tiwa and Pliya2024). Anatase in kaolin deposits is commonly interpreted as an authigenic phase formed by alteration of Fe–Ti-bearing precursor minerals, whereas rutile generally occurs as a detrital accessory mineral derived from the source rocks (Sayin & Jackson, Reference Sayin and Jackson1975; Baioumy, Reference Baioumy2014). Anatase is a metastable TiO2 polymorph that may transform into the thermodynamically stable rutile phase upon heating (Hanaor & Sorrell, Reference Hanaor and Sorrell2011).

The progressive decrease in kaolinite content in the order ABK > ARK > AVK > AJK, accompanied by an increase in siliceous phases, is in accord with the chemical and granulometric results. Compared with the clays from Foumban and Feutap (Nkalih et al., Reference Nkalih, Pilate, Yongue, Njoya and Fagel2018; Pountouenchi et al., Reference Pountouenchi, Njoya, Njoya, Rabibisao, Mache and Yongue2018), the Kouoptamo clays usually have higher kaolinite and lower quartz contents, suggesting more extensive kaolinization of the parent materials. Although Bamenda clays are also rich in kaolinite, the greater kaolinite crystal order may be advantageous for refractory ceramic applications (Yongue-Fouateu et al., Reference Yongue-Fouateu, Ndimukong, Njoya, Kunyukubundo and Kemeng2016).

The FTIR spectra of the ABK, AJK, ARK and AVK samples are shown in Fig. 5. The spectra are broadly similar and can be divided into several frequency domains, corresponding to specific structural entities. In the 3600–3800 cm–1 region, absorption bands associated with O–H stretching vibrations are observed, which are characteristic of kaolinitic materials (Maier et al., Reference Maier, Beuntner and Thienel2021). The band at 3623 cm–1 observed in AJK, ARK and AVK is also consistent with the presence of illite (Bodian et al., Reference Bodian, Mactar, Ndeye, Sambou and Younouss2021; El Halim et al., Reference El Halim, Daoudi, El Ouahabi and Fagel2022). Broader features near 3400 cm–1 correspond to the hydroxyl groups involved in hydrogen bonding or to adsorbed water, whereas the sharper features at higher wavenumbers (≈ 3600–3700 cm–1) can be associated with isolated hydroxyl groups. The band at 1630–1650 cm–1 is assigned to the H–O–H bending vibration of adsorbed or non-structural water (Rahayu et al., Reference Rahayu, Sholeha and Rahmah2023).

Figure 5.

FTIR spectra of the samples (ABK, AJK, ARK, and AVK) acquired from Kouoptamo. Full spectra in the 4000–400 cm–1 region highlighting the characteristic bands of clay minerals, quartz, iron oxides and TiO2-related vibrations.

A multi line graph of transmittance spectra for ABK, AJK, ARK and AVK, with multiple troughs across the wavenumber range. See long description.
Figure 5 Long description

Transmittance (a.u.) versus Wavenumber (cm superscript minus 1) line graph with four stacked spectra labeled AVK, ARK, AJK and ABK. The x-axis is labeled, Wavenumber (cm superscript minus 1), ranging from 4000 at the left to 0 at the right. The y-axis is labeled, Transmittance (a.u.). AVK spectrum: A curve labeled AVK with numeric annotations near the left side at 3698, 3616 and 3431. A numeric annotation near the middle reads 1645. Near the right side, a text label reads Saturated signal. Additional numeric annotations near the right side read 792, 692, 541 and 468. ARK spectrum: A curve labeled ARK with a numeric annotation near the middle reading 1646. Near the right side, numeric annotations read 798 and 465. AJK spectrum: A curve labeled AJK with a numeric annotation near the middle reading 1645. ABK spectrum: A curve labeled ABK with a numeric annotation near the middle reading 1643. Near the right side, numeric annotations read 465 and 415.

The 1200–400 cm–1 region shows characteristic vibrations of clay minerals, quartz, iron oxides and TiO2 (Fig. 5b). Between 1200 and 800 cm–1, the spectra are dominated by Si–O stretching vibrations (He et al., Reference He, Atheba, Allou, Drogui, El Khakani and Gbassi2023), while Al–OH deformation bands appear at ∼953–943 cm–1. Goethite O–H deformation bands are expected between 900 and 700 cm–1; however, their identification in AJK is complicated due to overlap with Si–O and Al–OH vibrations (Salama et al., Reference Salama, El Aref and Gaupp2015). Weak bands in the 800–600 cm–1 region are attributed to quartz Si–O stretching and kaolinite O–H deformation (Rahayu et al., Reference Rahayu, Sholeha and Rahmah2023). The 600–400 cm–1 region is associated with Fe–O vibrations of iron oxides (i.e. goethite and hematite), Ti–O stretching vibrations of rutile/anatase and Si–O–Si deformation modes of kaolinite and quartz (Ihekweme et al., Reference Ihekweme, Shondo, Orisekeh, Kalu-Uka, Nwuzor and Onwualu2020).

Figure 6 shows the TGA/DTG curves of the clayey materials studied. The TGA/DTG curves of the four clay samples display similar profiles, characteristic of kaolinitic materials (Claudel et al., Reference Claudel, Rolland, Marguerite, Theodore, Parfait and Salomon2022; Mostafa et al., Reference Mostafa, Hayatulla, Biswas, Rahman, Rana and Alam2025). The first mass-loss event, occurring between 75°C and 150°C, corresponds to the release of adsorbed water and does not induce structural changes in the minerals (Hazarika et al., Reference Hazarika, Huang and Babaahmadi2024; Idoum et al., Reference Idoum, Tiskatine, Mounir, Bammou and Aharoune2024). The pronounced loss at ∼278°C in sample AJK reflects the decomposition of goethite into hematite (Bomeni et al., Reference Bomeni, Njoya, Ngapgue, Wouatong, Yongue Fouateu, Kamgang Kabeyene and Fagel2018). A major dehydroxylation step is observed between 400°C and 600°C in all samples and corresponds to the breakdown of kaolinite and illite. This reaction leads to the collapse of the clay layered structure and the formation of amorphous metakaolinite (El Kasmi et al., Reference El Kasmi, Zriouil, Ahmamou and Barka2016; Mesrar et al., Reference Mesrar, Laila and Raouf2023; Hazarika et al., Reference Hazarika, Huang and Babaahmadi2024; Idoum et al., Reference Idoum, Tiskatine, Mounir, Bammou and Aharoune2024; Gauly et al., Reference Gauly, Mariame, Ségnéninhinténin, Koffi, N’Dri, Sanou and Trokourey2025).

Figure 6.

TGA/DTG curves of the raw clayey materials: (a) ABK, (b) AJK, (c) ARK and (d) AVK.

A set of four line graphs showing TGA and DTG curves for ABK, AJK, ARK and AVK across temperature. See long description.
Figure 6 Long description

A line graph labeled ABK with two curves and a legend listing TGA and DTG. The TGA curve is drawn as a solid line and the DTG curve is drawn as a dashed line. The horizontal axis is labeled, Temperature with unit degree Celsius, ranging from 0 to 1000. The left vertical axis is labeled, Weight loss with unit percent, ranging from 80 to 105. The right vertical axis is labeled, DTG with unit percent per minute, ranging from positive 2 to minus 18. The TGA curve slopes downward with a small change near the text label 74 degree Celsius, then a larger downward step centered near the text label 533 degree Celsius and then becomes nearly flat at higher temperatures. The DTG curve shows a small dip near 74 degree Celsius and a deeper dip near 533 degree Celsius. The image B showing a line graph labeled AJK with two curves and a legend listing TGA and DTG. The TGA curve is drawn as a solid line and the DTG curve is drawn as a dashed line. The horizontal axis is labeled, Temperature with unit degree Celsius, ranging from 0 to 1000. The left vertical axis is labeled, Weight loss with unit percent, ranging from 80 to 105. The right vertical axis is labeled, DTG with unit percent per minute, ranging from positive 2 to minus 14. The TGA curve slopes downward with a small change near the text label 86 degree Celsius, another change near the text label 278 degree Celsius and a larger downward step near the text label 524 degree Celsius, followed by a flatter segment at higher temperatures. The DTG curve shows dips near 86 degree Celsius and 278 degree Celsius and a deeper dip near 524 degree Celsius. The image C showing a line graph labeled ARK with two curves and a legend listing TGA and DTG. The TGA curve is drawn as a solid line and the DTG curve is drawn as a dashed line. The horizontal axis is labeled, Temperature with unit degree Celsius, ranging from 0 to 1000. The left vertical axis is labeled, Weight loss with unit percent, ranging from 80 to 105. The right vertical axis is labeled, DTG with unit percent per minute, ranging from positive 2 to minus 16. The TGA curve slopes downward with a small change near the text label 95 degree Celsius, then a larger downward step near the text label 547 degree Celsius and then becomes nearly flat at higher temperatures. The DTG curve shows a small dip near 95 degree Celsius and a deeper dip near 547 degree Celsius. The image D showing a line graph labeled AVK with two curves and a legend listing TGA and DTG. The TGA curve is drawn as a solid line and the DTG curve is drawn as a dashed line. The horizontal axis is labeled, Temperature with unit degree Celsius, ranging from 0 to 1000. The left vertical axis is labeled, Weight loss with unit percent, ranging from 80 to 105. The right vertical axis is labeled, DTG with unit percent per minute, ranging from positive 2 to minus 12. The TGA curve slopes downward with a small change near the text label 92 degree Celsius, then a larger downward step near the text label 522 degree Celsius and then becomes nearly flat at higher temperatures. The DTG curve shows a small dip near 92 degree Celsius and a deeper dip near 522 degree Celsius. Across the four graphs, each sample shows a small event labeled near 74 to 95 degree Celsius and a larger event labeled near 522 to 547 degree Celsius. The AJK graph includes an additional labeled event at 278 degree Celsius that is not labeled in the other three graphs.

Figure 7 shows the morphology of the raw materials as observed by SEM. ABK consists of closely packed, flat, lamellar particles, typical of kaolinite-rich clays (Daou et al., Reference Daou, Lecomte-Nana, Tessier-Doyen, Peyratout, Gonon and Guinebretière2020; Zhang et al., Reference Zhang, Zheng, Wei, Diao and Yin2021; El-Rafei & Mansour, Reference El-Rafei and Mansour2023), suggesting strong cohesion, consistent with the high proportion of fine particles observed in the granulometric analysis (Fig. 2a). By contrast, AJK is composed of coarser, angular grains, reflecting a higher quartz content (Daou et al., Reference Daou, Lecomte-Nana, Tessier-Doyen, Peyratout, Gonon and Guinebretière2020; Tibang et al., Reference Tibang, Waji, Shemang and Suh2021; Marlan et al., Reference Marlan, Putri, Pratiwi and Jaya2023), in agreement with XRD and chemical analyses. ARK exhibits a granular and strongly aggregated texture, with particles forming rounded clusters, possibly indicating intermediate cohesion and plasticity between ABK and AJK.

Figure 7.

SEM images of the clayey raw materials: (a) ABK, (b) AJK, (c) ARK and (d) AVK. Red arrows indicate flat lamellae of kaolinite in the ABK and AVK clays, angular quartz grains in AJK and granular texture in ARK.

Content of image described in text/caption.

AVK has a highly lamellar but less compact microstructure, possibly due to its high kaolinite and illite contents (Table 2). It also has the highest SiO2/Al2O3 ratio (3.34), resulting in the coexistence of phyllosilicates and quartz. The presence of illite may improve workability and plasticity, but it may also increase susceptibility to drying-induced cracking (Kalguem et al., Reference Kalguem, Wouatong, Njopwouo, Kemteu and Ekosse2019; Tibang et al., Reference Tibang, Waji, Shemang and Suh2021; Yang et al., Reference Yang, Zheng, Ma, Cheng, Wang and Qin2024). The observed microstructural differences reflect the mineralogical and chemical compositions of the clays, the abundant lamellae particles in the kaolinite-rich samples (ABK and AVK), the coarser siliceous grains in the quartz-rich clays (AJK) and the aggregated textures in ARK.

Fired-product characterization of the Kouoptamo clays

Figure 8 shows the XRD traces of the fired bodies. The phase evolution observed during firing is influenced by the clays’ initial mineralogical compositions. Mullite formation is the dominant transformation process and results from the dehydroxylation of kaolinite followed by the crystallization of mullite from metakaolin above ∼950°C (Sánchez-Soto et al., Reference Sánchez-Soto, Martínez-Martínez, Pérez-Villarejo and Garzón2022). As expected, ABK demonstrates the most pronounced mullite formation owing to its higher kaolinite content (El-Rafei & Mansour, Reference El-Rafei and Mansour2023, Pountouenchi et al., Reference Pountouenchi, Ndzana, Mountapbeme, Njuhou, Njoya and Fagel2024). Cristobalite is detected in all samples, probably due to the partial transformation of free silica or crystallization of a glassy phase. Residual quartz remains, particularly in AJK, due to its stability over the investigated firing range (El Ouahabi et al., Reference El Ouahabi, Daoudi, Hatert and Fagel2015, Xue et al., Reference Xue, Dong, Fan, Ma and Yao2023; Yang et al., Reference Yang, Zheng, Ma, Cheng, Wang and Qin2024). Rutile is also observed in the AJK clay after firing. Although goethite is present in the raw clay of AJK, no iron-containing crystalline phase was identified after firing. This suggests that the iron was redistributed during the heat treatment, possibly via dissolving in the vitreous phase and/or, to a limited extent, via being incorporated into the mullite structure. These possibilities cannot be verified by XRD alone. In fact, a refinement allowing a partial substitution of iron (9.1% ± 0.25%) in the tetrahedral sites of the mullite gave a lower weighted profile R-factor (Rwp) value (from 17.9% to 16.1%), indicating that such substitution is consistent with the experimental data, although additional evidence is needed to confirm this interpretation.

Figure 8.

XRD traces of the fired bodies at different temperatures (1000°C, 1100°C and 1200°C): (a) ABK, (b) AJK, (c) ARK and (d) AVK. C = cristobalite (SiO2, PDF 39-1425); M = mullite (Al6Si2O13, PDF 15-778); Q = quartz (SiO2, PDF 5-490); R = rutile (TiO2, PDF 21-1276).

X-ray diffraction plots for ABK, AJK, ARK and AVK samples at 1000, 1100 and 1200 degrees Celsius, showing phase changes and peak intensities. See long description.
Figure 8 Long description

X-ray diffraction traces of fired bodies at different temperatures: ABK, AJK, ARK and AVK. The y-axis is labeled Intensity, arbitrary units. The x-axis is labeled 2 theta, degrees, ranging from 10 to 70. Each sample has traces for 1000, 1100 and 1200 degrees Celsius, stacked for clarity. For ABK, peaks are prominent at 1200 degrees Celsius, indicating increased crystallinity with markers for mullite, cristobalite and quartz. AJK shows rutile presence at 1200 degrees Celsius, with peaks for mullite and cristobalite. ARK displays consistent mullite and cristobalite peaks across temperatures, with sharper peaks at higher temperatures. AVK has clear mullite and cristobalite peaks, with increased intensity at 1200 degrees Celsius. The traces reveal phase evolution and increased crystallinity with temperature, highlighting differences in mineral composition and transformation processes among samples.

Figure 9 presents the physical and mechanical properties of the studied materials as a function of temperature. The firing behaviour of the Kouoptamo clays varies according to their chemical (Table 1), mineralogical (Table 2) and granulometric characteristics (Fig. 2a). For all samples, increasing the firing temperature promotes densification, resulting in higher bulk density and linear shrinkage, accompanied by lower porosity due to enhanced vitrification (Zanelli et al., Reference Zanelli, Raimondo, Guarini and Dondi2011; El-Rafei & Mansour, Reference El-Rafei and Mansour2023).

Figure 9.

Densification behaviour of the fired products versus temperature: (a) firing shrinkage, (b) bulk density, (c) open porosity and (d) compressive strength.

Different types of data visualizations such as four line graphs of firing shrinkage, bulk density, open porosity and compressive strength versus temperature. See long description.
Figure 9 Long description

A line graph with the legend, ABK, AJK, ARK and AVK. The horizontal axis label is, Temperature left parenthesis degree c right parenthesis, with values 1000, 1100, 1200. The vertical axis label is, Firing shrinkage left parenthesis percent right parenthesis, with values 0, 2, 4, 6, 8, 10, 12, 14. The plotted values are shown at 1000, 1100 and 1200 for each series. ABK: 1000 about 2, 1100 about 4, 1200 about 4. AJK: 1000 about 4, 1100 about 10, 1200 about 12. ARK: 1000 about 3, 1100 about 6, 1200 about 7. AVK: 1000 about 1, 1100 about 4, 1200 about 4. The image B showing a line graph with the legend, ABK, AJK, ARK and AVK. The horizontal axis label is, Temperature left parenthesis degree c right parenthesis, with values 1000, 1100, 1200. The vertical axis label is, Bulk density left parenthesis g slash cm superscript 3 right parenthesis, with values 1.4, 1.6, 1.8, 2.0, 2.2, 2.4. The plotted values are shown at 1000, 1100 and 1200 for each series. ABK: 1000 about 1.55, 1100 about 1.65, 1200 about 1.70. AJK: 1000 about 1.65, 1100 about 1.95, 1200 about 2.15. ARK: 1000 about 1.60, 1100 about 1.75, 1200 about 1.78. AVK: 1000 about 1.52, 1100 about 1.62, 1200 about 1.70. The image C showing a line graph with the legend, ABK, AJK, ARK and AVK. The horizontal axis label is, Temperature left parenthesis degree c right parenthesis, with values 1000, 1100, 1200. The vertical axis label is, Open porosity left parenthesis percent right parenthesis, with values 0, 10, 20, 30, 40, 50. The plotted values are shown at 1000, 1100 and 1200 for each series. ABK: 1000 about 43, 1100 about 42, 1200 about 39. AJK: 1000 about 35, 1100 about 27, 1200 about 22. ARK: 1000 about 36, 1100 about 35, 1200 about 33. AVK: 1000 about 35, 1100 about 30, 1200 about 27. The image D showing a line graph with the legend, ABK, AJK, ARK and AVK. The horizontal axis label is, Temperature left parenthesis degree c right parenthesis, with values 1000, 1100, 1200. The vertical axis label is, Compressive strength left parenthesis MPa right parenthesis, with values 0, 10, 20, 30, 40, 50, 60. The plotted values are shown at 1000, 1100 and 1200 for each series. ABK: 1000 about 15, 1100 about 18, 1200 about 22. AJK: 1000 about 32, 1100 about 45, 1200 about 50. ARK: 1000 about 20, 1100 about 28, 1200 about 32. AVK: 1000 about 25, 1100 about 35, 1200 about 38.

ABK had the lowest linear shrinkage with relatively high porosity. These characteristics indicate limited vitrification and good dimensional stability during firing. Although its composition suggests good potential for refractory applications, refractory behaviour cannot be confirmed solely from the present results and should be evaluated through pyrometric cone-equivalent (PCE) measurements. Further investigations are therefore required to assess its refractory performance. Such properties may also be advantageous for whiteware and thermally insulating ceramics (Muñoz et al., Reference Muñoz, Morales, Mendívil and Muñoz2014; El-Rafei & Mansour, Reference El-Rafei and Mansour2023; Pountouenchi et al., Reference Pountouenchi, Njoya, Mbey, Mache, Njoya and Yongue2023). By contrast, AJK shows the most pronounced densification behaviour, reaching a bulk density of 2.11 ± 0.02 g cm–3 with the highest compressive strength (49.3 ± 0.6 MPa) at 1200°C, possibly due to the presence of illite and iron-rich phases, which facilitate densification during firing (Pountouenchi et al., Reference Pountouenchi, Njoya, Njoya, Rabibisao, Mache and Yongue2018; Ndzana et al., Reference Ndzana, Njuhou, Pountouenchi, Lecomte-Nana and Njoya2024). Such characteristics are generally favourable for the production of red bricks and roofing tiles (Gu & Ling, Reference Gu and Ling2024). ARK demonstrates an intermediate firing behaviour, characterized by moderate shrinkage, porosity and mechanical strength, which may be advantageous for domestic ceramics (Nugroho et al., Reference Nugroho, Indra, Vidya, Pra, Milzam and Rizki2021). AVK exhibits moderate densification and satisfactory mechanical performance. These properties may support the production of utilitarian pottery, porous tiles and ceramics requiring controlled permeability (Zanelli et al., Reference Zanelli, Raimondo, Guarini and Dondi2011; Gu & Ling, Reference Gu and Ling2024; Wembe et al., Reference Wembe, Ngueyep, Nkuissi, Langollo, Tiwa and Pliya2024; Yusuf et al., Reference Yusuf, Abdulmumin, Adanlawo, Telejaiye, Ogbonnaya and Idris2026). Therefore, the observed differences in firing behaviour and properties can be related to variations in the chemical, mineralogical and granulometric characteristics of the investigated clays.

Conclusions

Clays from the Kouoptamo area, West Cameroon, were characterized to assess their potential for ceramic applications. The investigated clays exhibited distinct chemical, mineralogical and granulometric characteristics, resulting in different firing behaviours and physical and mechanical properties. Among the investigated materials, ABK (the most kaolinite-rich sample) possessed the lowest linear shrinkage (≈3%) and relatively high porosity (38%), reaching a compressive strength of 25 MPa after firing at 1200°C. On this basis, ABK might be considered a potential candidate for refractory applications. By contrast, AJK (the most goethite-rich sample) achieved the highest bulk density (2.11 g cm–3) and the greatest shrinkage (≈13%) at the same temperature, yielding a compressive strength of ∼50 MPa. AJK may be suitable for the production of roofing tiles and other structural ceramic products. ARK and AVK showed intermediate firing behaviours and mechanical properties. The potential applications of the Kouoptamo clays range from refractory and whiteware products to roofing tiles, depending on their composition and firing behaviour. This study provides the first systematic classification of these clays and establishes a basis for their future utilization in ceramic applications. Future work should focus on application studies to validate the performance of these clays in refractory, whiteware and structural ceramic products.

Acknowledgements

The authors thank the Argillaceous, Geochemistry Sedimentary Environments (AGEs) research unit at the University of Liège for the material characterization, as well as the Department of Industrial Engineering at the University of Trento for providing the facilities and support for the ceramic testing.

Financial support

The authors gratefully acknowledge the financial support provided by the Italian Ministry of Foreign Affairs and International Cooperation (MAECI).

Competing interests

The authors declare none.

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Figure 0

Figure 1. Map of the Kouoptamo area, with the study area, according to Bidias et al. (2023). The red arrow indicates the collection area of samples ABK, AJK, ARK and AVK.Figure 1 long description.

Figure 1

Figure 2. (a) Particle-size distributions of the investigated raw materials, showing the relative contents of clay, fine silt, coarse silt and sand fractions. (b) Evaluation of the suitability of the raw materials based on the diagram of Winkler (1954) (Ngun et al., 2011; Nkalih et al., 2018).Figure 2 long description.

Figure 2

Table 1. Chemical compositions (%) of the samples obtained from the Kouoptamo area.Table 1 long description.

Figure 3

Figure 3. Ternary diagram of Fiori et al. (1989). Chemical classification of clay raw materials in ceramics: (a) white stoneware, (b) red stoneware, (c) cottoforte tile and (d) majolica tile. ‘Other oxides’ = Fe2O3, TiO2, MnO, MgO, CaO, Na2O, K2O and P2O5.

Figure 4

Table 2. Quantitative mineralogical composition (wt.%) of the studied clays.Table 2 long description.

Figure 5

Figure 4. XRD traces of the (a) ABK, (b) AJK, (c) ARK and (d) AVK samples. A = anatase (TiO2, PDF 21-1272); Goe = goethite (α-FeO(OH), PDF 29-713); H = hematite (α-Fe2O3, PDF 33-664); I = illite ((K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)], PDF 26-911); K = kaolinite (Si2O5Al2(OH)4, PDF 14-164); M = montmorillonite; ((Na,Ca)0.3(Al,Mg)2Si4O10(OH)2·n(H2O), PDF 13-259); Q = quartz (SiO2, PDF 5-490); R = rutile (TiO2, PDF 21-1276).Figure 4 long description.

Figure 6

Figure 5. FTIR spectra of the samples (ABK, AJK, ARK, and AVK) acquired from Kouoptamo. Full spectra in the 4000–400 cm–1 region highlighting the characteristic bands of clay minerals, quartz, iron oxides and TiO2-related vibrations.Figure 5 long description.

Figure 7

Figure 6. TGA/DTG curves of the raw clayey materials: (a) ABK, (b) AJK, (c) ARK and (d) AVK.Figure 6 long description.

Figure 8

Figure 7. SEM images of the clayey raw materials: (a) ABK, (b) AJK, (c) ARK and (d) AVK. Red arrows indicate flat lamellae of kaolinite in the ABK and AVK clays, angular quartz grains in AJK and granular texture in ARK.

Figure 9

Figure 8. XRD traces of the fired bodies at different temperatures (1000°C, 1100°C and 1200°C): (a) ABK, (b) AJK, (c) ARK and (d) AVK. C = cristobalite (SiO2, PDF 39-1425); M = mullite (Al6Si2O13, PDF 15-778); Q = quartz (SiO2, PDF 5-490); R = rutile (TiO2, PDF 21-1276).Figure 8 long description.

Figure 10

Figure 9. Densification behaviour of the fired products versus temperature: (a) firing shrinkage, (b) bulk density, (c) open porosity and (d) compressive strength.Figure 9 long description.