Clay materials analysed (Figure 1) in the present work were collected from three distinct localities (Kaliwa (C1), Kabé (C2) and Malo (C3)) in the commune of N’Djamena in the Chad republic as displayed on the map of Figure 2.

Sample clay materials C1, C2 and C3 consist of 42.9% - 63.1% sand, 614.7% - 28.2% silt and 22.2% - 28.9% clay, while sample C1 and C1 exhibits a high percentage of sand than C3. The elevated content percentage of sand in these samples enable us the attest that, the studied materials is a clayey sand exhibiting homogeneous and dispersed particle size. The particle size distribution of individual studied CMs and the ATTERBERG limit test details are presented in Table 1. The CMs exhibits a plasticity index in between 27 and 31.0, that is: 31.0%,

28.0% and 27.2% respectively C1, C2 and C3 samples. The data from Table 1 were used to plot the Holtz and Kovacs diagrams presented in Figure 3, which indicate that, that sample CMs samples C1, C2 and C3 exhibits a kaolinitic character and are highly plastic.

In addition, CMs pottery and extrusion brick-making potential was studied. The position of the CMs in the diagram of Bain and Highley is presented in Figure 4. From Figure 4, it is obvious that, the studied CMs exhibits good properties for pottery and acceptable for extrusion brick-making. However, the chemical composition, the nature of the mineralogical phases, the size of the grains and the firing parameters are the main factors which influence the quality of pottery and extrusion brick-making. Furthermore, mixing and drying of the raw material is important in the process of producing pottery objects.

The geochemistry study was carried out on C1, C2 and C3. The chemical elements distribution and major mineral elements in the CMs are displayed in Table 2. From Table 2, it obvious that, the overall samples exhibit the same mineral composition but with variable proposition.

The geochemical analysis of Kaliwa site (C1) revealed that the major elements are silica (SiO₂: 57.28%), alumina (Al₂O₃: 14.36%) and iron oxides (Fe₂O₃: 9.23%). SiO₂ is the dominant mineral compared to the other minerals. The presence of alkaline and alkaline earths metals is remarkable: potassium (K₂O: 3.64%), titanium (TiO₂: 1.40%), sodium oxide (Na₂O: 1.08%), calcium oxide (CaO: 1.05%) and magnesium oxide (MgO: 0.73%). Other constituents such as manganese oxide (MnO: 0.17%) and Phosphorus oxide (P₂O₅: 0.14%), although present, are negligible. The loss on fire for C1 is relatively low (~10.92%).

As far as the sample (C2) from Kabé district site is concerned, the major minerals are composed of silica (SiO₂: 65.81%), alumina (Al₂O₃: 12.29%) and iron oxides (Fe₂O₃: 6.16%). The proportion of silica with 65.81% highest dominant mineral compared to other minerals in respective samples (Table 1), while Fe₂O₃ with only 6.16% remain the lowest. Alkaline and alkaline earth metals are essentially the same but with a slight difference in term of concentration. K₂O (4.10%), TiO₂ (1.09%), Na₂O (1.26%), CaO (1.31%) and MgO (0.75%) are present, while

Mn₂O₃ (0.11%) and P₂O₅ (0.11) are still negligible. The loss on fire is 0~7.03% and is lower for Kabé site. Analogously to the Karewa (C1) and Kabé (C2) sites, the Malo (C3) site results present the same constituents for which the comparative proportions are almost similar (see Table 2).

The geochemical analysis presents a chemical composition of the materials that constitute the CMs studied. Major elements and fire loss are presented in Table 2. It appears that Kabé site contains more silica than others. The soils of Malo and Karewa display almost identical alumina values. Iron is higher in Karewa soil. Alkalis and alkaline earths show mixed results. The fire loss is lower in Kaliwa and even lowest in Kabé.

Thegeochemistry analysis gives a precise idea of the geochemical composition of the clay formations on the Kaliwa, Kabé and Malo sites. Indeed, these samples present similar geochemical characteristics from one site to another. According to the results obtained, variations in the contents of major elements are relatively high in the overall samples inthepresent investigation. The main oxides listed in order of their content’s importance are SiO₂, Al₂O₃, Fe₂O₃, K₂O, CaO, MgO, TiO₂ and SiO₂ varying from 57.28 to 65.81% in the studied sites. The highest rate is from the sample collected from Malo district (C2), followed by that fromKabé district (C3) and finally sample from the Kaliwa district (C1).

The XRF results suggested in general that, the studied CMs are essentially composed of alumina silicate with an excess of quartz-forming silica. The obtained results are in accordance with those obtained by Mache et al., and Kagonbé et al. [4] [21] , who reported that, the CMs from the Sudano-Sahelian areas are mainly composed of silica. However, the occurrence of Al₂O₃ in the materials studied depend on the existence of CMs such as kaolinite, while the presence of kaolinite in these minerals highlights this geochemical process called mono-sialization followed by bis-sialization. The presence of Fe₂O₃ at a relatively high concentration suggests the presence of hematite phase in good agreement with the literature [4] . CaO and MgO are quantitatively low in the samples studied due the low content of carbonate minerals observed in most of the samples. The potassium and sodium oxide contents present in the clay raw material could probably be attributed to the presence of illite [22] .

The Loss on Ignition results is in the range of 7 to 11% in overall samples. The highest rate of loss on ignitionis obtained for sample from Kaliwa. These CMs would have undergone dehydroxylation reactions, combustion of organic matter and decomposition of CMs carbonates [23] . In addition, other melting minerals likeillite and smectite are suggested to be present in agreement with earlier results in the literature [24] .

The XRD patterns of the CMs samples, displaying variable peaks intensities are presented in Figure 5. Figure 5 exhibits a perfect similarity between the XRD patterns of the entire studied samples with a basal reflection at higher (~14.66 Å)

and lower (~1.49 Å) diffraction angle, attesting of the occurrence of smectite. The weak peaks recorded at ~10.22 Å confirm the presence illite and those at ~7.31, ~2.56 and at ~1.45 Å are characteristics of kaolinite phase. The peaks characteristic of quartz minerals appears at 3.34 Å and 4.24 Å, while the signal corresponding to the feldspars is detected consecutively in the diffraction angle located at 3.77 and at 3.46 Å respectively. The remaining shrill peaks are essentially linked to presence of quartz. The XRD analysis revealed that, major crystalline phases detected are mainly made up of smectite, Illite (K₂O∙2H₂O, 2Al₂(Si₃Al)O₁₀(OH)), kaolinite (Si₂O₅Al₂(OH)₄), quartz (SiO₂) and Felds paths (Ba, Ca, Na, K, NH₄)(Al, B, Si)₄O₈.

The XRD results for all samples indicate that smectite has been characterized as dioctahedral smectite, confirmed by the presence of kaolinite at ~7.31 Å mineral generally phase associated with smectite. In the overall samples, quartz (~3.34 Å) and K-feldspars (~2.90 - ~3.77 Å) coexist as accessory minerals. The mineralogical composition of the CMs in this study reveals that, the samples possess high content of Quartz (50%) followed by feldspar (20%), kaolinite (15%), smectite (10%) and illite (5%). The observed free silica in the CMs is due to the detected quartz as it is confirmed by the calculation of the SiO₂/Al₂O₃ ratio. This quartz content with the contribution of kaolinite are definitely an asset in the compressed CMs bricks formulation.

The peaks shape characteristic of the as-studied minerals account of their crystallized nature. The intensity of peaks characteristics to quartz attest on their preponderance throughout the mineralogical sphere. The variations observed in the mineralogy composition of the studied CMs account on the difference on their erodibility character. Indeed, soils whose mineralogy is impacted by the presence of 2:1 type clay suchas smectite and kaolinite are sensitive to runoff and erosion once in contact with water [25] [26] . This physical character will have an impact on the mechanical properties of the as-studied CMs materials.

The infrared spectra of the overall samples are almost similar and include the presence several absorption bands as depicted in Figure 6(a), Figure 6(b). The adsorption bands ranging from 3750 to 3500 cm⁻¹ are characteristic of the elongation of the hydroxyl groups; while those at ~3696, ~3642 and ~3620 cm⁻¹ are characteristic of kaolinite phase [25] [26] . The observed two significant vibration bands suggest that, the kaolinite phases present in the CMs exhibits a disordered structure [27] . An elongation vibration bands ranging from ~3642 to ~3696 cm⁻¹, are attributed to inter-layer hydroxyl groups. It is worth to mention that, the intensities and positions of those hydroxyl groups are usually sensitive to the intercalation of organic molecules. The Al-OH and Al-O-Si deformation vibration bands appear from ~905 cm⁻¹ in perfect agreement with the literature. [28] [29] while the shoulders at ~675 cm⁻¹ can beassigned to the presence of quartz [29] . The vibration bands around ~3420 and~3460 cm⁻¹ can be attributed to the Si-O vibrations, while that located at ~525 and ~765 cm⁻¹ are assigned to different vibration modes of Si-O-Al bonds (where Al is tetra coordinated) in kaolinite. The band at 905 cm⁻¹ characteristic of the δ Al-OH group deformation of [30] , will disappear to give way to a wide band up to 995 cm⁻¹, belonging to the Si-O-Si bonds of the kaolinite. This modification would be due to the formation of an amorphous structure as already noticed with the XRD analysis (Figure 3).

FTIR results are in good agreement with XRD data and therefore, confirm the presence of Quartz, Kaolinite and Illite in the CMs studied. The Si-O-Si bonds of kaolinite are also identified at 645 cm⁻¹ [31] [32] .The deformation vibrations of

the Al-O bonds appear at 690 cm⁻¹ [33] . Quartz and feldspars absorb very little radiation in the visible or near-IR ranges, hence their sober appearance [34] . The intensity of the peak broad band at 750.57 cm⁻¹ corroborates with the abundance of Quartz [35] in line with XRD results.

Thermogravimetric and differential analyzes (TGA/DSC) for all samples were carried out and the resulting curves are presented on Figure 7 and Figure 8. Figure 7 presents the heat flow variation with temperatures for the three samples. From the Figure 7 two prominent endothermic peaks located between the temperature of 0 to 260˚C and 400 to 600˚C as well as another barely perceptible exothermic peak in between 900 and 1000˚C were observed for the overall samples. The observed picks can be assigned to the water loss, the decomposition of hydroxides and organic matter and finally the quartz transformation and the loss into carbonates. The first peak at 87˚C corresponds to the water hygroscopic departure from the illite sheets [36] [37] [38] . The second endothermic picks at ~ 482, 488, 494 and 553˚C for sample C1, C2 and C3 respectively followed by mass losses are due to hydroxylation (OH) of kaolinite (transformation into metakaolinite and smectite.

The exothermic peaks of thermograms observed around 561˚C, 545˚C and 557˚C respectively for the three samples C1 C2 C3, illustrate the hydroxylation of illite and smectite (Figure 7). The quartz content of the three samples could not be compared due to the overlapping peaks similarity from different minerals. Quartz is identified here by its progressive disintegration in the temperature range of 927 to 948˚C, with a shift in its content around 1100˚C; although the

effective temperatures at which the carbonate decomposes can sometimes fluctuate [38] [39] . This exothermic phenomenon is related to the metakaolinite structural reorganization.

The three curves in Figure 8 highlight the mass losses as a function of the peaks observed on Figure 4. From the TGA curve on Figure 8, the mass loss of 1.33, 1.18 and 1.29% was observed for C1, C2 and C3 respectively. The second endothermicpeaks between 453 and 550˚C (C1), between 450 and 530˚C (C2) and between 486 and 546˚C (C3) following the mass losses 6.1, 4.22 and 5.86% respectively are due to hydroxylation kaolinite and their transformation into

metakaolinite. The overall CMs exhibits characteristic of water constituent elimination, resulting from the release of structure dihydroxylation [39] [40] [41] .

Scanning electron microscopy is a useful tool used to observe the texture and microstructure of the clay soil in the sample and characterize the mineralogical assemblages. The morphological configuration of soil samples C1 from Kaliwa C2 from Kabé and C3 from Malo was analyzed and present Figure 9. Their textures display clay particles with various heterogeneous shapes surrounded with porous and open cavities. Several stacked observed on top of each other in adjacent crystals to form clusters were detected. The irregularity of the particle sizes is characteristic of the illite. a great numberof clusters scattered on each samples micrograph, suggesting the presence of Al, Si, Mg, Fe, Na, Ca, K, Ti and O chemicals constituents present in the entire CMs samples in the form of oxides (Al₂O₃, SiO₂, Fe₂O₃, MgO, Na₂O, CaO, K₂O and TiO₂ already observed above by XRF and XRD analysis. The high content of SiO₂, Al₂O₃ and FeO in some grains confirms the presence of kaolinite. The microstructural analysis of the set of three samples with SEM revealed the presence of smectites in the form of clusters in superimposed sheets where undulations are perceptible [42] [43] .