Clay samples were collected from a well located at latitude 9.285126˚ North and longitude 13.455767˚ East. Sample from Lobo was denoted LO and sample from Ngoya was denoted NG. About 2.0 kg of each sample was collected and placed in small polyethylene bags. 1.0 kg of each sample was dried, pulverized and sieved before analysis. Documentary studies [9] suggested the clay nature of the grayish material.

Sieve series, a densimeter, a pycnometer, X-ray diffraction and infrared spectrometry were used to study the particle size as well as the physical and chemical properties of the material.

Table 1 summarizes the particle size distribution of different materials studied.

Sample clay material LO consists of 1.5% gravel, 55.92% sand, 6.08% silt and 36.49% clay while sample clay material NG consists of 0.6% gravel, 57.63% sand, 17.56% silt and 24.14% clay. The high percentage of sand in these samples allows us to deduce that it is clayey sand having homogeneous and spread particle size. [10] .

The results of the ATTERBERG limit test are shown in Table 2.

These clay materials have a plasticity index in the range of 18 to 47. The sample LO has a plasticity index of 26.22% while the NG sample has a plasticity index of 17.52%. These values from Table 2, make it possible to conclude that sample clay materials LO is plastic (Ip = 26.22%) while sample NG is not (Ip = 17.52%). The low plasticity of the NG sample is explained by the high proportion of sand (57.63%) [11] .

Figure 1 and Figure 2 show the diffractograms of the clay materials. LO clay

Gravel: (f > 2 mm), Sand: (0.02 mm < f > 2 mm), Silt: (0.002 mm < f > 0.02 mm) Clay: (f < 0.002 mm)

material consists of quartz (in the form of sand) associated with clay minerals (illite, kaolinite) while in NG clay material, in addition to quartz (in the form of sand) and kaolinite, other minerals such as k-feldspars and hematite are also present [12] .

Figure 3 and Figure 4 show the complete infrared spectra of the clay materials LO and NG respectively.

These spectra reveal two zones of absorbance; the first zone is between 4000 and 3500 cm⁻¹ and the second zone of absorbance is between 2000 and 500 cm⁻¹. These bands are characteristic of the elongation vibrations of the external hydroxyl (OH) bonds of kaolinite network (Al-OH-Al) [13] . The band at 3620 cm⁻¹ could also reflect the presence of illite [14] . The second absorbance zone is between 2000 and 500 cm⁻¹, with absorption bands at 1030, 1000, 912, 795, 748, 685 and 533 cm⁻¹ for LO clay sample and at 1630, 1020, 1000, 912, 791, 745, 675 and 535 cm⁻¹ for NG clay sample. The band at 1630 cm⁻¹ observed on NG spectrum represents the vibration of the hydroxyl (OH) groups of the water molecules

[13] and the bands at 1030 and 533 cm⁻¹ for LO are characteristic of the Si-O bond vibrations. The bands between 795 and 675 cm⁻¹ represent the bands relating to the quartz doublet. The 912 cm⁻¹ bands for each sample are characteristic of Al-O bond vibration.

The results of the thermogravimetric analysis are shown in Figure 5 and Figure 6.

These thermograms show five zones of anomalies: The first zone is between 100˚C and 150˚C and has endothermic peaks centered at 97˚C for LO and at 98˚C for NG. These peaks correspond to the mass loss of water contained in the studied materials; the second anomaly zone is between 150˚C and 450˚C with endothermic peaks centered at 199˚C and 290˚C for LO clay material, then at 220˚C and 360˚C for NG clay material; these peaks correspond to the dehydroxylation of phyllosilicates; the third anomaly zone is between 450˚C and 600˚C with endothermic peaks centered at 540˚C for LO clay and at 520˚C for NG clay material. These peaks are due to the dehydroxylation of kaolinite; the fourth anomaly zone is between 650˚C and 800˚C with endothermic peaks centered at 790˚C for LO clay and 760˚C for NG clay; these peaks come from the dehydroxylation of 2/1 clay minerals such as illite. Losses of mass below 180˚C are related

to the disappearance of adsorbed or capillary water. Between 150˚C and 450˚C, losses of mass are due to dehydroxylation of phyllosilicates. Mass losses observed between 450˚C and 650˚C are due to dehydroxylation of kaolinite to give metakaolinite which has a very poorly organized structure [14] . Mass losses recorded between 650˚C and 800˚C could result from the dehydroxylation of type 2/1 minerals such as illite.

Organic matter content of LO clay material is 6.03% with a pH of 7.73 while NG sample has an organic matter content of 2.40% with a pH of 5.56. The chemical composition of the clay materials studied is summarized in Table 3.

It can be seen from Table 3 that LO clay consists mainly of silica (64.09%), followed by aluminum oxide (20.24%) and titanium oxide (1.87%). iron oxide (1.82%), potassium oxide (1.19%). Other oxides are present in trace such as

magnesium oxide (0.28%), manganese oxide (0.02%), calcium oxide (0.06%), phosphorus oxide (0.09%). NG clay is also mainly composed of silica (59.12%), aluminum oxide (22.06%), iron oxide (2.34%), titanium oxide (1.81%), potassium oxide (1.19%). Other oxides are also present in trace such as magnesium oxide (0.19%), potassium oxide (0.17%), manganese oxide (0.03%), calcium oxide (0.11%). The SiO₂:Al₂O₃ ratio is greater than 2%; 3.16% for LO clay and 2.67% for NG clay, reflecting the abundance of sand in the studied clay materials [15] [16] .

The quality control tests carried out are designed to evaluate certain ceramic properties such as linear shrinkage during cooking, bulk density, compressive strength, and water absorption rate. Figure 7 shows the variation in linear shrinkage of clay materials as a function of temperature.

Linear shrinkage is a parameter that reflects the reactivity of material during firing [17] . From Figure 7, it follows that the linear shrinkage increases with temperature. The shrinkage values for our materials are relatively low, therefore favorable for the manufacture of refractory bricks of good quality.

The resulting curves of apparent density are shown in Figure 8.

Figure 8 shows that apparent density increases with temperature. For LO clay, density varies very little from 2.13 g/cm³ at 1000˚C to 2.23 g/cm³ at 1100˚C, then from 2.27 g/cm³ to 1200˚C. For NG clay, the bulk density varies from 1.97 g/cm³ at 1000˚C to 2.09 g/cm³ at 1100˚C and at 1200˚C the apparent density is 2.36 g/cm³. Sintering clay materials leads to the reorganization of the matrix of the material during the appearance of the glassy phases at increasing kinetics with temperature [18] . The clay materials studied can then be favorable for the manufacture of ceramic products.

From Figure 9, it is noted that the values of compressive strengths decrease as the temperature increases. This implies that these materials melt at low temperatures and recrystallization occurs at temperatures above 1000˚C [19] .

Water absorption rates are shown in Figure 10.

Generally, the water absorption of ceramic products should decrease with the increase in firing temperature caused by the gradual disappearance of porosity during sintering [20] . The water absorption is closely related to the densification of the clay matrix, and gives an idea of the sintering rate. It can be seen that the rate of water absorption decreases as a function of temperature. For LO clay, for example, the absorption rate range from 16.35% at 1000˚C to 14.26% at 1100˚C and drops to 12% at 1200˚C. On the other hand, for NG clay, the water absorption

rate is 12.5% at 1000˚C, 11.33% at 1100˚C and 11.17% at 1200˚C. The NG sample absorbs less water than the LO sample.