Three natural clay samples of different mineralogical procession are collected from the Oued Abiod section. These clays are suspended in water and sieved by a 40 µm sieve to remove impurities, and then dried in an oven at 60˚C. A mixture of 15 g of sieved clay and a quantity of Na₂CO₃ (5%) is soaked in 150 ml with distilled water then put under continuous agitation for an hour. The better results of activation for clays were obtained with 5% of added Na₂CO₃. The selection of 5% Na₂CO₃ at 75˚C as the optimal condition was determined based on experimental results from doctoral research of [17]. These parameters were optimized as part of this study and are also reported in a previous publication ([17]), where detailed experiments were conducted to validate this condition. For that reason, we present in this work only the results of the activation of clays realized with 5% of basic added in 75˚C during one hour ([17]; [17]). Raw and treated natural clays are characterized by various analysis techniques: X-ray Diffraction (XRD), specific surface area measurement by nitrogen adsorption at 77 K (BET), X-ray fluorescence (XRF), chemical analysis, pH measurement, and scanning electron microscopy (SEM).

3.2.1. Specific Surface Area and Porous Volume

The BET surface analysis and the porosity of the clays (before and after activation) were performed at the “Laboratory of Sorptiometry” using a Quantachrom-Autosorb Specific surface area analyzer. The nitrogen gas sorption method was used at 77 K for specific surface area determination. The pore space volumes were evaluated Barrett-Joyner-Halenda’s method, from the adsorption branch. The samples were outgassed at 393 K under a vacuum at 10^–3 mm Hg for 3.5 h.

3.2.2. Chemical Analysis: X-Ray Fluorescence

The chemical analysis of raw and activated clay of the Oued Abiod section were conducted by X-ray Fluorescence techniques on fused glass pearls using type Oasis 9900, Thermo-Fisher, in the Control and Quality Laboratory at the Gabès Cement Company. The XRF is a rudimentary analysis technique of the sample, representing an excitation of the sample with X-rays when the analyzed material emits secondary X-ray emission, called fluorescence X. From the characteristic X-ray spectra of the sample composition, the elemental composition can be deduced, i.e., the mass concentrations of the elements. The identification of the major elements requires sample preparation in the form of pellets in a transparent lithium tetra borate. The weight loss on ignition was measured by calcination at 1000˚C.

3.2.3. X-Ray Diffraction

The clay fraction was separated from larger particles by decantation and centrifugation ([3]). Such fine fraction was subjected to a series of analyses. In fact, the X-ray diffraction analyses were carried out on two types of preparations. With respect to the first one, it concerns the diffractograms of oriented preparations, in which the phase identification is based on the diagrams of fine fractions after physico-chemical treatments (normal, glycoled and heated to 550˚C) ([16]). As for the second type, it relates to the preparation of powder diffractograms of raw and treated clay. The latter presents the set of rays (hkl) of all minerals composing the sample. The estimate of the percentage of each mineral is affected by the integration of the area of the most intense peak ([18], 2017). All XRD data were collected under the same experimental conditions using an X’pert HighScore plus PANalytical diffractometer. Philips PW type diffractometer PANalytical X Pert Pro MPD (θ - 2θ) system, equipped with a copper anticathode, was used CuKα radiation source under 40 kV/40mA and an angular range of 3˚ - 70˚ 2θ for bulk rocks and 3˚ - 30˚ 2θ for fine fraction, a step size of 0.017˚2θ, and a counting time of 10 s/step to determine mineral composition.

3.2.4. SEM Analysis

The morphology of raw clays treated by Na₂CO₃ samples (HOA1, HOA2 and HOA3) of the Oued Abiod section was observed by an electronic microscope of the type HITACHI SC-2500’s sweeping. The principle of this technique is based on the strong interaction between the spread emitted and retro secondary electrons which allow to reconstitute the image of the object. The working tensions are generally between 10 and 30 kV, which allows to have a swelling that can go to 30.000 times. The set is silvered by a gold layer to make it conductive before the observation.

3.2.5. PH Meter

The pH of six samples of raw and activated clays is measured by a pH measure, Typifies NeoMet 200 L in the microbiological laboratory of the Faculty of Science of Sfax.

3.2.6. Micro-Grading Analysis by Diffraction Laser

The particle size distribution of the raw clays and those activated with Na₂CO₃ was analyzed using laser diffraction. This analysis was carried out using a wet method at the Tunisian Chemical Group Factory in Gabès. The particle size distribution was measured with a Mastersizer S laser particle size analyzer.

Table 1 shows that the specific surface by BET of raw clays varies from 4 to 30.41 m²/g (HOA1: SBET = 4 m²/g; HOA2: SBET = 11 m²/g; HOA3: SBET = 30.41 m²/g). After basic activation of these three clays, the specific surface increases in a significant way. Better results were obtained at the top of this section (HOA3: SBET = 150 m²/g).

Table 1. Specific Surface (BET) of clay samples before and after activation.

Table 2 shows that the chemical composition changes upon activation, with CaO decreasing from 6% to 3.8% and Na₂O increasing from 1.2% to 7.9%. This is due to the replacement of Ca²⁺ ions by Na⁺ ions. Furthermore, the increase in Na₂O is more pronounced than the decrease in CaO. Other divalent ions, such as Mg²⁺ and Fe²⁺ (with MgO and Fe₂O₃ decreasing from 5.22% to 1.86% and from 4.34% to 2.7%, respectively), as well as trivalent ions (Al³⁺ and Fe³⁺) present in the samples, are also partially replaced by Na⁺. These substitutions, promoted by Na₂CO₃, lead to the formation of sodic smectite and hydrated aluminosilicate microporous materials, such as zeolites. The synthesized products exhibit high porosity and lower permeability.

Table 2. Results of the chemical analyses of sample HOA3 from Oued Abiod: raw and activated.

4.3.1. DRX Analysis

The mineralogical analysis by XRD of the HOA1 sample before basic activation shows a clay procession prevailed by the smectite (80%) to which the kaolinite is joined (Figure 3 and Figure 4).

Figure 3. XRD patterns (CuKα radiation) of the raw clay of the HOA1, HOA2 and HOA3 samples (Oued Abiod). (Sm) smectite; (Ka) Kaolinite; (Acm) all clay minerals; (Ca) calcite; (Do) dolomite; (Qr) quartz; (Fr) francolite; (Gy) gypsum.

Figure 4. XRD patterns (CuKα radiation) of three oriented clay fractions of the HOA1, HOA2 and HOA3 samples (Oued Abiod) (Sm: smectite; Sep: sepiolite; Pa: palygorskite, Ka: kaolinite, Il: illite).

The non-clay procession is composed of dolomite, quartz and gypsum. After basic activation by Na₂CO₃ (5%) at 75˚C, the studied clay shows a decrease of the intensity of the peaks of the smectite and the kaolinite, the appearance of a low peak of feldspar characterized by its main ray in 3.19 Å, the persistence of secondary mineral peaks (quartz and dolomite) and the disappearance of the gypsum after this basic activation by Na₂CO₃.

The calcic or rather magnesian smectite before activation, characterized by its main ray in 15.71 Å, becomes a sodic smectite after activation, characterized by a main ray 12.37 Å (Figure 5(a)).

In the middle part of the summit of Oued Abiod section (Figure 3), the diffractogramme of the HOA2 sample presents intense peaks of smectite and kaolinite before treatment, while the illite appears track there. The quartz shows a slightly intense peak against a low calcite peak. All these minerals persist after basic activation at 75˚C (Figure 5(b)), but with lower proportions than at the beginning. The appearance of the intense peaks (12.47 - 3.19 - 3.06 Å) after this activation by Na₂CO₃ (Figure 5(b)) could be explained by the beginning of zeolitisation of the activated sample HOA2 confirmed by the observation by SEM.

Figure 5. XRD patterns (CuKα radiation) of the clay (<40 µm) (raw and activated with 5% Na₂CO₃ at 75˚C for one hour) of HOA samples: (a) HOA1; (b) HOA2; (c) HOA3. (Sm) smectite; (Po) porous materials; (Ze) zeolite; (Pa) palygorskite; (Sep) sepiolite; (Ka) kaolinite; (Acm) all clay minerals; (Ca) calcite; (Do) dolomite; (Qr) quartz.

At the upper part of this section, the HOA3 sample (Figure 4) shows a high percentage of smectite (80%), a low rate of kaolinite and tracks of illite, palygorskite and sepiolite. The non-clay procession of this sample presents a high percentage of dolomite, a small proportion in quartz and francolite (Figure 3). After basic attack by solid Na₂CO₃, all the peaks of clay and non-clay minerals change intensity (Figure 5). The confrontation of the obtained results by the X-rays and those obtained by SEM confirm that this activated clay in the best already mentioned conditions was transformed to mesoporous materials or zeolite.

4.3.2. SEM Observations

1)Beforetreatment

The HOA1 sample selected from Oued Abiod section, allows to underline an abundance of rhombohedral dolomite crystals. These dolomites are well crystallized and mixed in a clayey matrix (Figure 6(a) and Figure 6(b)). The photomicrographs (b), detail of the photomicrographs (a), expose a richness in foliated clay of smectite nature. These minerals, characterized by their main reflection 15.29 Å, are detected in the normal aggregate.

Figure 6. ((a), (b)) HOA1, (a) an abundance of rhombohedral dolomite in a clay matrix; (b) layered texture showing a richness in smectite. (a1) Their EDX analysis reveals Ca, Mg, Si, and Al. ((c), (d)) HOA2, (c) flaky and platelet-like clay; (d) abundance of kaolinite in the form of platelets. ((e), (f)) HOA3, (e) fibrous clays interspersed with a cotton-like texture; (f) fibrous clays (palygorskite and sepiolite) in the form of spines. (e1) EDX reveals Mg, Si and Al.

The sample HOA2, taken in the middle of the summit part (Figure 6(c) and Figure 6(d)), expresses a richness in smectite and kaolinite mixed with the pyrite.

A fragment of the sample HOA3 shows the abundance of rhombohedral crystals of dolomite scattered in a clayey matrix (Figure 6(e) and Figure 6(f)), and which are sometimes altered. The microanalysis X confirms the dolomitic nature with the presence of the peaks of the Ca and Mg. These crystals are surrounded by smectites (Figure 6(e)). Frequent fibrous clays show a thorny aspect and a lifeless appearance. These fibers are associated with a foliated matrix (Figure 6(f)). These results are confirmed by XRD (Figure 4).

2)Aftertreatment

The SEM observation of the morphology of clays from the HOA1 sample activated with 5% Na₂CO₃ at 75˚C reveals the significant dissolution of smectite layers, particularly rhombohedral dolomite (Figure 7(a) and Figure 7(b)). It is observed that the basic activation leads to the partial dissolution of phyllosilicates and attacks the dolomite crystals (Figure 7(a)and Figure 7(b)). Considering that dolomitization involves the transformation of calcite to dolomite through the circulation of fluids rich in magnesium salts, the dissolution of dolomite in clays activated under the described conditions does not result in the formation of calcite. Instead, it promotes zeolitization and the formation of smectite-Na (Figure 7(b)).

Figure 7. Scanning Electron Microscopy (SEM) observation of HOA samples activated by Na₂CO₃. ((a), (b)) HOA1, showing significant dissolution of smectite sheets (a); dolomite undergoing dissolution ((a), (b)); perforated and altered dolomite disseminated within the Na-smectite (b). The X-ray microanalysis of a dolomite rhombohedron in micrograph (b) shows very intense peaks of Mg and Ca. ((c), (d)) HOA2, reveals a porous texture of zeolites in the form of a sponge, with EDX analysis (a) revealing the presence of Si, Al, Na; (c) smectite sheets and kaolinite platelets in a smectitic matrix. ((e), (f)) HOA3 shows an amalgam of crystals in the form of aggregates and tubular structures; (e) zeolites in the form of tubules crossed by pseudohexagonal tunnels of varying lengths; (f) this micrograph displays several types of porosity (micropores, macropores, mesopores, and macropores) in a new homogeneous phase. X-ray microanalysis reveals two intense peaks of Si and Na, a small peak of Al, and traces of Mg and Ca.

The HOA2 sample after treatment exhibits a porous, sponge-like texture, with calcite crystals that are partially or fully altered, and sometimes dissolved. It also shows a clay with a foliated and porous appearance, as well as plates of kaolinite. The structures observed after this basic activation, as seen in micrograph (c) of Figure 7, resemble honeycomb patterns formed by pores, cages, and channels. These are zeolites, which were identified by XRD (Figure 7(c) and Figure 7(d)). The X-ray microanalysis of the micrographs (c) reveals the characteristic energy lines of Si, Al, and Na. This zeolitic structure imparts specific properties, including those of a molecular sieve and a selective catalyst.

The SEM observation of the activated HOA3 clay sample reveals the presence of an almost homogeneous crystalline phase, corresponding to a zeolite (Figure 7(e) and Figure 7(f)). This crystallization is associated with the simultaneous presence of clay minerals (smectite, palygorskite, and sepiolite) and non-clay minerals such as dolomite and quartz in an alkaline environment with a pH of 10. Elongated tubular crystals of varying size and shape indicate the presence of one or more varieties of zeolite (Figure 7(e) and Figure 7(f)).

The pH of the studied clays before activation varies from 7.32 to 8.31 (Table 3). Clays of samples HOA3 before treatment are slightly basic (pH = 7.31); after activation by the base Na₂CO₃ in the conditions of activation (5% Na₂CO₃, 75˚C, 1 hour), the pH of these clays mounts to 10 (Table 3). These clays with ascendancy of smectite and contain the palygorskite, the sepiolite and the dolomite, are transformed into zeolite ([17]). On the contrary, sample HOA1 and HOA2 is basic before activation, their pH is upper to 8 and their mineralogical procession of departure is formed by the smectite, the kaolinite and the calcite. After activation, these clays give sodic smectites rather than zeolites, though the pH reached 10.

Table 3. pH of raw and activated clay by 5% Na₂CO₃ (1 h at 75˚C).

The particle size distribution of the two samples, HOA1 and HOA2, from the Oued Abiod section is bimodal both before (Figure 8(a1) and Figure 8(a2)) and after activation (Figure 8(a2) and Figure 8(b2)). This can be explained by the mineralogical composition before treatment. The recrystallization of smectite and kaolinite in the presence of calcite is minimal after basic treatment for both HOA1 and HOA2 samples.

The change in particle size distribution of sample HOA3, shifting from a unimodal distribution before activation (Figure 8(c1)) to a bimodal distribution after activation with solid Na₂CO₃ (Figure 8(c2)), can be explained by the redistribution and recrystallization of the composite atoms within the crystal lattice of these smectitic and fibrous clays, in the presence of dolomite and quartz, which are sources of Mg and Si, respectively. This granulometric redistribution in this sample is attributed to the formation of Na-smectite and zeolites. This result is confirmed by mineralogical analyses (XRD and SEM).