4.1- Mineralogical composition
The X-ray patterns of bulk samples (Fig.3), showed the predominance of kaolinite, illite-muscovite and quartz in all samples of the three deposits clays. In Tamazert deposit, clay minerals (21-87%), quartz (7-25%), and muscovite-illite (8-60%). In Hadj Ali deposit, quartz (9-14%), clay minerals (30-44%), muscovite-illite (28-50%). In Chekfa deposit, quartz (9-13%), clay minerals (14-20%), muscovite-illite (34-47%), K-feldspar and plagioclase are more abundant than in the two previous deposits (25-43%). Kaolinite and muscovite-illite are identified by the (001) basal reflection and other secondary reflections respectively (7.12Å and 3.58Å) (9.98Å, 4.99Å and 3.32Å), quartz with their reflection (101) occur at (3.34Å) and the second reflection at (4.25Å). However K-feldspars at (3.18Å) and plagioclase at (3.22Å) are absent in Tamazert clays (TM), or rarely present together in the Hadj Ali (HA) and Chekfa (CH) clay samples. The X-ray patterns of the 3 clay deposits show a similar mineralogical composition of the raw samples with kaolinite, quartz, muscovite-illite. K-feldspars and plagioclase are only present in Hadj Ali and Chekfa deposits.This gives a first indication of the degree of alteration that affects each kaolinized deposit.
X-ray patterns of the clay fraction (˂2µm) show dominance of kaolinite and illite, accessory minerals as chlorite and smectite are also present at small quantities (≤ 0.5%) in some samples of (CH and HA). Chlorite is identified by the persistence of the second reflection at (7Å) during heating. Smectite is evidenced by its characteristic peak at 14Å under natural condition that migrate to16Å after glycolation and collapses to 10Å after heating. The semi-quantitative abundance of the minerals in all samples was estimated from the height of a diagnostic peak multiplied by a corrective factor as reported (Table 3) by (Cook et al.,1975; (Boski et al,.1998 and Fagel et al., 2003). The semi-quantitative data of the clay minerals of the TM, CH and HA deposits are shown in (Fig.4).These data show that, TM samples are enriched in clay minerals (57% on average) in regard with the samples of CH and HA (14% and 35% on average).
4.2 Plasticity test
Plasticity of clays is one of the most important parameters affecting the determination and process of clay production (Murray, 2007; Barış et al., 2020). The Evaluation of the plasticity index (PI) is very important in determining the suitability of clays for the ceramic industry. The plasticity index of Tamazert and Hadj Ali clays was calculated from the arithmetic difference of LL and PL. these Clay materials are characterized by moderate plasticity index 9% and 8% respectively. we could not perform the plasticity test on the Chekfa samples because of the high quartz content. In the diagram of Holtz and Kovacs (1981) (Fig. 5), the clays of Tamazert and Hadj Ali are ploted in the zone of midium plasticity,the samples plot as kaolintic clay. The PI of all samples clay is < 10%, samples were found unsuitable for bulding-related ceramics production due to the risk craks during extrusion (Nyakairu et al., 2002)
4.3 Infrared spectroscopy
Infrared spectra (Fig.6), show marked differences between the three studied samples. The first difference is marked in region of high absorption. Four bands at 3697, 3668, 3653 and 3621cm-1 are observed in the spectrum of the Tamazert sample whereas only three bands are present in the spectra of Chekfa and Hadj Ali samples 3698, 3652 and 3621cm-1. These four bands are assigned to OH-stretching of kaolinite (Farmer 1974; Van Der Marel & Beutelspacher 1976). The second difference marked in the region of low absorption bands 1115-913 cm-1 corresponding to the Si-O-Si and Si-O-Al stretching.
4.4 Thermal analysis
TGA curves (Fig.7) showing the samples mass loss (TM, CH, HA). A significant mass loss (4%) is observed above 400°C for the sample (TM) in comparison with loss of 1.5% for samples (CH and HA). This mass loss is attributed to the dehydroxylation structure (Nahdi et al., 2002b) and dehydroxylation of kaolinite due to its transformation to metakaolinite (Wang et al,.2011). A slightly loss is observed around 180°C in all samples due to dehydroxylation of micaceous phase (Caillère et al,.1976), the mass loss below 180°C is due to the removal moisture water. In all t samples, values of loss mass is lower than the theoretical values of pure kaolin 1.5-4% (13.96 wt,%, Ptáček et al,.2010) reflecting the presence of impurities, especially free quartz.
4.5 Scanning electron micrscopy
The micrographs (Fig. 8) obtained by the different samples showeds the dominance of kaolinite pseudo-hexagonal particles organized as platelets or tight packages. EDS microanalyses reveal some small relics of feldspars and quartz in TM kaolinite whereas fresh or slightly altered K-feldspars minerals are evidenced in both CH and HA kaolinites. The better crystallization of TM and HA samples evidenced by ESEM is consistent with X-ray diffraction and IR spectroscopy data.
4.6 Physico-chemicals properties (SSA and CEC)
The CEC of Tamazert clay (11.58 meq/100g) is higher than for Chekfa and Hadj Ali (5.42 meq/100g and 6.46 meq/100g (Table.2). Such CEC are in the range of the values reported in literature for kaolin deposits (3 to 15 meq/100g, in Grim 1968). Specific surface areas of CH and HA kaolinites (~45.5 m2/g) are higher than that of TM kaolinite (28.7 m2/g). This reflects the small grain size of Chekfa and Hadj Ali kaolinites compared to Tamazert kaolinite.
4.7 Particles-size distribution
The suitability of clays for different industrial applications is based on their particle size distribution.
For ceramic products, the finer fraction (<2 μm) is of particular attention (Mahmoudi et al., 2008). The particle size distribution of the different samples shows that these clays are poor in fine fraction (2–7%), while the silty sands and sandy loam fractions are abundant (28 to 63% and 34 to 64%, respectively). The samples from Chekfa and Hadj Ali show low fine fraction contents (<2 μm) compared to those from Tamazert. In the ternary diagram widely used in the ceramic industry (Dondi et al., 1992), the clay samples studied were classified as silty sand (samples H1, H3 and C1) and sandy silt clay (sample T2, T3, T6 , C1, C2 and H5, Fig. 9). On Winkler's (1954) ternary diagram (Fig.10), almost all of the samples show an aptitude for making construction products such as common bricks.
4.8 Chemical analysis
The chemical composition of the clay samples are showen (Table 4). The clay show most abundant of SiO2 and Al2O3. SiO2 ranged from 49 to 64% for TM, 61 to 68% for HA and 64 to 69% for CH. Al2O3 varies between 34 to 21% for Tamazert, 24 to 20% for Hadj Ali and 19 to 22% for Chekfa. The content in Al2O3 within clays depends on the intensity of hydrolysis. More hydrolysis gives more kaolin minerals and, therefore higher Al2O3 content (Yanik, 2011). Fe2O3 is low for TM (<1%) and ranges between 0.85 to 1.63% for HA and CH. CaO elements, MgO, TiO2 and P2O5 are present in small quantities often lower than the detection limit, in all sites. Na2O presents small variations from one site to another (< 1% for TM and CH, > 2 % for most HA samples). In contrast, K2O presents higher fluctuations, ranging between 1.8 and 5.1% for the 3 sites. LOI values from kaolin samples ranged from 5.7 to 7.0% for Chekfa, 4.2 to 5.0% for Hadj Ali and 6.6 to 13.8% for Tamazert. The low CaO, MgO, Na2O values reflect an important leaching of calco-alcaline elements. It is probably due to their high mobility during kaolinization process and it is compatible with an advanced argillic alteration system close to hydrothermal kaolin deposits (Meyer and Hemley, 1967; Meunier et al., 1983. Inoue, 1995; Dill et al., 1997, 2000). However, the enrichment in K2O observed in all samples is probably due to the release of the potassium content by feldspar dissolution.