Mineralogical analysis was carried out with a Philips Xpert Pro MD standard beam with copper anti-cathode tube. The mineralogical characterization identifies the percentages of clay minerals (<2 µm) and associated minerals [6] [7] .

The chemical analysis of major elements was carried out by atomic absorption Perkin Elmer apparatus with acetylene peroxide flame. This analysis allows us to determine the percentages of CaO, Fe₂O₃, Al₂O₃, SiO₂, K₂O, MgO, Na₂O, SO₄, and loss on ignition.

This analysis was carried out under water and the refusal of each sieve was dried and weighed [8] . The micro-size analysis was performed with the Mikromeritics SediGraph 5120 particle size analyzer, size range 0.1 - 300 μm [9] . The particle diameter was determined by measuring the speed of sedimentation of suspended particles according to Stokes law [10] .

Plasticity was determined by the Atterberg limits. Those physical constants define the threshold for passing from a liquid state to a plastic state expressed in water content. Limit liquidity and plastic limit were measured according to the [11] . The plasticity index represents the broad field of plasticity clays [12] . The evaluation of plasticity was performed by Atterberg limits method. The plasticity index was calculated as the arithmetic difference of liquid limit and plastic limit. This analysis was carried out with a Casagrande apparatus.

Specific surface area by the methylene blue method was determined according to the standard EN ISO 10545-4 [13] . Specific surface area is directly related to the rate of phyllosilicates [14] . The adsorption test is used to classify clays [15] [16] .

Drying curve (Bigot) was determined using a D124 barellatographe apparatus while dilatometer curve is carried out using a dilatometer Adamel Lhomargy type BI. The registration was completed when the drying recording of clay is not accompanied by shrinkage. Drying and dilatometric curves allow highlighting the variations in weight and volume as a function of temperature [17] .

For the manufacture of bricks, different bricks underwent a drying operation performed in the open air and then in an oven at 100˚C and firing was done by means of an electric furnace at various levels of temperature 800˚C, 850˚C, 900˚C and 950˚C. The rise of temperature increased from 30˚C/h. For the manufacture of tiles, different tiles underwent a heating operation in an electric furnace at different temperatures 850˚C, 900˚C, 950˚C and 1000˚C with a gradient of 300˚C/h. The determination of water absorption and flexural strength were carried out following the Standards [18] [19] .

The results of mineralogical analysis (Figure 2) showed a clear change in the bottom to the top of the outcrops which distinguishes three mineralogical units (Table 1).

The unit A Mkt consisted mainly by illite and low smectite and Kaolonite. The H Mkt was marked by the

increase in the content of illite and the percentage of smectite. The unit S Mkt was marked by the presence of illite and the kaolinite. Associated minerals were present at levels nearly homogeneous throughout the outcrops

and were mainly represented by calcite and a percentage of quartz. Mineralogical results showed the dominance of illite presenting favorable proprieties for ceramic use. The content of quartz was very tolerable since it can be easily digested by vitreous flow during firing operation [20] .

Chemical analysis of the major elements was performed on a mixture of clays (Table 2). The results showed high content of CaO and Fe₂O₃. The percentage of SiO₂ indicated the importance of detrital contribution. Al₂O₃ had a relatively low percentage. Its origin cannot be the tetrahedral layer of clay minerals. The K₂O content was very high; it’s probably related to the presence of micas. Levels of alkaline fluxes (Na₂O and K₂O) present high level for all clay mixture due to the relatively larger amount of illite and orthoclase [21] . By comparing this clay with a fireclay, whose alumina rate was higher than 45%, the alkaline fluxes lower than 4%, it can be remarked that the clay of Makthar area was very plastic and was cooked red since the percentage of Fe₂O₃ is higher than 1.5%. The problem of sulfur will be corrected by the addition of barium carbonate to the paste, which will form a more stable barium sulfate. The high loss on ignition (17%) associated with low SiO₂ and high Al₂O₃ contents were due to the significant content of clay minerals. Besides its fluxing role, Fe₂O₃ also provides the fired products the characteristic reddish colour. However, Fe₂O₃ is not the only factor responsible for the coloration of ceramic wares, as also other constituents such as CaO, MgO, MnO and TiO₂ can appreciably modify the colour of fired clays [22] . The temperature of firing, the amount of Al₂O₃ relative to a range of other constituents, and the furnace atmosphere all play an important role in the development of colour in the fired clay products [23] .

The particle size distribution of clay is a factor in determining its suitability for various applications, and particular attention should be given to the finer fraction (<2 μm) for ceramic products [9] . Granulometric analysis of clay indicated a homogeneous distribution. These results showed that the clay samples had sand fraction about 15% for unit A Mkt, 9% for the H Mkt and 5% for unit S Mkt. The percentage of elements whose diameter higher than 50 µm was negligible and approximately reached 5% for all units (Figure 3).

The microgranulometric analysis of clay mixtures showed that the elements whose diameter was lower than 2 µm was very high and reached 30%, for the unit H Mkt, 37% for the unit A Mkt and 35% for the unit S Mkt. The granulometry of all studied clays was excellent for use in ceramics. The problem arising by the presence of particles in the coarse sand fraction (200 - 2000 μm) can be solved simply by grinding.

The micro-size curves (Figure 4) for clay units H Mkt and A Mkt showed a parabolic facies indicating a

transport operation either by turbidity currents at medium speed or by suspension graduated suspension [24] .

The values of the limits of plasticity and plasticity index values indicated that all mixture clay of Makthar area plastic-type (Table 3). These clay mixtures were located in the area of illitic minerals as shown on Holtz and Kovacs diagram (Figure 5). This is of importance for applications since it indicated the minimum moisture content necessary to reach a plastic condition. At a high plastic limit, the samples were more difficult to dry. On

the other hand, the use of high plastic clays reduced the wearing down of the equipment for grinding and conformation (extruder). High plasticity was associated with bodies with greater mechanical strength [21] . Figure 5 shows the position of these clays on the Holtz and Kovacs diagram [25] . The almost same PI value for all clays may be related to higher plastic limit of illite (35% - 60%) and montmorillonit (50% - 100%) than kaolinite (25% - 40%) [26] .

The mixture of clays of all clay units had specific surface which reflect illitic character (Table 4). We noted that the value of the specific surface was strongly influenced by the nature of the clay minerals and associated minerals. However, the presence of high levels of calcium carbonate can significantly diminish the value of the specific surface.