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Vol. 8. Issue 5.
Pages 4010-4018 (September - October 2019)
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Vol. 8. Issue 5.
Pages 4010-4018 (September - October 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.07.009
Open Access
Effect of chamotte on the structural and microstructural characteristics of mullite elaborated via reaction sintering of Algerian kaolin
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Alima Mebreka,
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a.mebrek@crti.dz

Corresponding author.
, Hadda Rezzaga, Sihem Benayachea, Afef Azzia, Yasmina Taïbib, Sabrina Ladjamaa, Naima Touatia, Azzedine Grida, Sabri Bouchouchaa
a Research Center in Industrial Technologies CRTI, P.O. Box 64, Cheraga 16014, Algiers, Algeria
b Ecole Nationale Supérieure des Mines et de la Métallurgie, Ex CEFOS Chaiba, BP 233 RP Annaba, W129, Sidi Amar, Algeria
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Tables (5)
Table 1. Phases, lattice parameters (a, c), relative deviation (Δa, Δc), average crystallite size <l> , microstrains <σ </l>2>1/2, relative proportion of the chamotte.
Table 2. Chemical composition of the DD3 kaolin (wt.%).
Table 3. Phases, lattice parameters (a, c), relative deviation (Δa, Δc), average crystallite size <l> , microstrains <σ </l>2>1/2, relative proportion of the DD3 kaolin.
Table 4. Phases, lattice parameters (a, c), relative deviation (Δa, Δc), average crystallite size <l> , microstrains <σ </l>2>1/2, relative proportion of the sintered samples C1 and C2.
Table 5. Elemental analysis (EDX) of the DD3 kaolin.
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Abstract

Mullite [3Al2O3 2SiO2] was synthesized by reaction sintering of Algerian kaolin of Djebbel Debbagh-Guelma located in the North-East of Algeria and chamotte. The mixture powders of kaolin and chamotte were milled, dried and cold compacted using a uniaxial press. The green samples were sintered for 2 h at 1350 °C. Powders’ morphology, structure and microstructure of the sintered samples were studied by means of X-ray fluorescence spectrometry (XRF), X-ray diffraction (XRD), scanning electronic microscopy (SEM), thermogravimetric and thermodifferential analysis (DTA/TGA) and the helium pycnometer (AccuPyc II 1340).

The results showed that DD3 kaolin has a silica content of 41% and alumina of 35%. X-ray analysis revealed that the major phases of the DD3 kaolin was kaolinite along with a muscovite and todorokite. After sintering, the ceramics composite composed mainly of mullite and cristobalite. The added chamotte increases the mullite formation, the apparent density from 2.7 to 2.75 g/cm3, decreases the porosity from 4.70 to 4.43% and a densification higher than 95%.

Keywords:
DD3 Kaolin
Ceramic
Mullite
XDR
Full Text
1Introduction

The Djebbel Debbagh kaolin deposit located in North East of Algeria (Guelma) is natural clay which consists of a filling of karst cavities with predominantly kaolin clays containing halloysite. Its quality varies with the extraction methods. We can find a pure kaolin with concentration of metal oxide impurities less than 5% and sometimes concentrations less than 1%. The DD kaolin of Djebbel Debbagh are rich in silica and alumina and they are classified in three categories: DD1, DD2 and DD3 according to the concentration of the metal oxide impurities and they are differentiated by their colors, white and gray because they contain manganese oxide. Due to the specific properties, DD Kaolin can be used also in several fields such as: building materials, paper, medical and cosmetic applications, thermal insulating, chemical producers, pulp and paper, food production-related industries and anything involving heat and/or hot products …. etc. The kaolin from Djebbel Debbagh (DD3) is the main refractory clays deposits mined in Algeria for manufacturing of silica–alumina refractories. These products are commonly used in the ceramics industry to manufacture kilns, and as kiln furniture, coating of laboratory furnace and refractory supports.

The main minerals constituents in kaolin are kaolinite or halloysite. The pure kaolinite theoretical formula Si2Al2O5(OH)4 is often presented in the form: Al2O32SiO22H2O [1]. This clay mineral is of great interest to the production of different types of ceramic materials for having properties interesting. Most important of these properties one is the phase mullite formation during sintering of ceramics which makes the material hard with good mechanical strength [2]. Kaolinite is one of the most used raw materials for alumina-silicate based ceramics, due to its abundance and good availability. Natural kaolinite coexists with minor constituents such as mica and quartz [3]. Halloysite have a similar composition (Al2O32SiO2nH2O; n ≥4) with some excess water molecules present between the layers and often exhibits a tubular morphology [4]. At the temperature of 40 °C, halloysite loses its water to form the meta-halloysite, a structure similar to kaolinite which is very difficult to differentiate. This halloysite morphology distinguishes it from the hexagonal grains of kaolinite.

The aim of the present work is to study the chamotte effect on the mullite formation and the structural and microstructural properties mullite elaborated via reaction sintering of Algerian kaolin of Djebbel Debbagh-Guelma located in the North-East of Algeria. Hence, a mixture of kaolin and chamotte powders with two compositions was mechanically milled in a high energy planetary bell mill, at room temperature for 1 h. The obtained powders were compacted into pellets and sintered at 1350 °C for 2 h.

2Materials and experimental procedure2.1Raw materials

The studied material was kaolin grey clay designated by DD3 extracted from Djebbel Debbagh (North-east of Algeria) (Fig. 1). The elaborated samples were produced following the steps indicated in the flow chat in Fig. 2. The chamotte was obtained by calcination of powder DD3 kaolin at 1350 °C for 1 h with a heating rate of 5 °C/min to yield mullite and cristobalite (Fig. 3). The chamotte composed in mullite (74.62%) and (25.38%) of cristobalite (Table 1), leads to a better volume stability during sintering treatment. The chamotte was added to the DD3 kaolin according to the following compositions:

  • Composition "C1" = 75% DD3 kaolin + 25% chamotte.

  • Composition "C2" = 50% DD3 kaolin + 50% chamotte.

Fig. 1.

DD3 kaolin.

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Fig. 2.

Schematic representation of samples elaboration.

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Fig. 3.

Rietveld refinement of XRD of the chamotte.

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Table 1.

Phases, lattice parameters (a, c), relative deviation (Δa, Δc), average crystallite size <l> , microstrains <σ </l>2>1/2, relative proportion of the chamotte.

Phases  a (nm) ± 10−4  b (nm) ± 10−4  c (nm) ± 10−4  c/a  Δa (%)  Δc (%)  <l> (nm) ± 3 </l>  2>1/2 × 10−3  Volume fraction (%) 
Mullite  0.75257  0.76536  0.28700  0.38  −0.82  −0.67  121.84  0.59  74.62 
Low cristobalite  0.50324  –  0.69588  1.38  1.19  0.47  260.162  4.39  25.38 

The mixture of two compositions C1 and C2 was dried at a temperature 120 °C for 8 h, and then milled for 1 h in a planetary ball mill Fritsch pulverisette 7 at a rotation speed of 400 rpm. The milled powder was compacted into pellets of 13 mm in diameter and 2 mm thickness under a pressure of 1 t/m² using a uniaxial hydraulic press. After drying, the green samples were sintered in an electrical muffle furnace at 1350 °C for 2 h and a temperature rise rate of 10 °C/min.

2.2Methods

The chemical composition was carried out by X-ray fluorescence spectrometry XRF (Bruker S8 Tiger). Differential thermal (DTA) and thermogravimetric (TGA) analysis were carried out by means of a TA Universal Analysis 2000 apparatus from 27 to 1200 °C at a heating rate of 10 °C/min. The microstructural characterizations of the powder DD3 kaolin and sintered samples were made by X-ray diffraction on a Rigaku diffractometer using Cu radiation (λCu = 0.15406 nm) in a (θ–2θ) Bragg Brentano geometry with a step size of 0.01°. The lattice parameters, volume fraction, crystallite size, <l> and microstrains, <σ </l>2>1/2 were obtained from the Rietveld refinement of the XRD patterns by using the MAUD program [5] which is based on the Rietveld method [6]. Morphological changes of the powder DD3 kaolin were characterized by scanning electron microscopy (SEM,EVO/MA25-ZEISS) coupled with dispersive X-ray spectrometry (EDX).The apparent density and open porosity of sintered samples were measured by the Archimedes method, and the absolute density was determined by using an automatic helium pycnometer (AccuPycII 1340).

3Results and discussions3.1Chemical composition

The chemical composition of the powder DD3 kaolin given in weight percentages (wt.%) is indicated in Table 2. One notes that the DD3 kaolin is a hydrothermal rock [7], constituted mainly of two major chemical elements which are: 42% silica and 36% alumina. The great value of fire loss (17%) is due to the presence of water in the various minerals existing in kaolin such as: kaolinite and todorokite [8]. This fire loss is greater than that associated with the dehydroxylation of an ideal kaolinite (13.76%). In fact, it icloser to the ideal value of the fire loss recorded during the dehydroxylation of halloysite (16.85%) [9]. The high percentage of MnO (2.25%) is explained to the blackish coloration of DD3 kaolin and also supposes that the content of the associated ore (todorokite) exceeds 5%. Also, a significant amount of Cao (1.16%) is detected and a oxide impurity concentration of less than 1%. The chemical composition of a pure kaolinite expressed as a weight percentage of oxide corresponding to: SiO2 = 46.38%; Al2O3 = 39.8%; H2O = 13.9% and SiO2/Al2O3 ratio = 1.16% [2]. In our case, the SiO2/Al2O3 ratio of DD3 kaolin is close to the theoretical value of a pure kaolinite = 1.16%.The particles size distribution of powder DD3 kaolin measured by the method air-jet sieve (Fig. 4) is predominantly composed of fine particles (˜32 µm), which gives it a high plasticity.

Table 2.

Chemical composition of the DD3 kaolin (wt.%).

Compounds  Mass% 
SiO2  41.70 
Al2O3  35.80 
MnO  2.25 
CaO  1.16 
Fe2O3  0.801 
K20.301 
SO3  0.230 
MgO  0.183 
P2O3  0.141 
TiO2  0.070 
Cr2O3  – 
Calc. loss  17.364 
Fig. 4.

Particles size distribution of the DD3 kaolin.

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3.2Analysis XRD3.2.1Phase formation of the milled powder

The Rietveld refinement of the XRD patterns is presented in Fig. 5. The powder DD3 kaolin show broad diffraction peaks with an reduction of their intensity because of the crystallite size reduction down to the nanometer scale, crystal distortion and accumulation of the structural defects such, grain boundary, dislocations, interstitials, vacancies, etc. The obtained parameters from the Rietveld refinement of the powder DD3 kaolin are summarized in Table 3.

Fig. 5.

Rietveld refinement of XRD of the DD3 kaolin.

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Table 3.

Phases, lattice parameters (a, c), relative deviation (Δa, Δc), average crystallite size <l> , microstrains <σ </l>2>1/2, relative proportion of the DD3 kaolin.

Phases  a (nm) ± 10-4  b (nm) ± 10-4  c (nm) ± 10-4  c/a  Δa (%)  Δc (%)  <l> (nm) ± 3 </l>  2>1/2 × 10−3  Volume fraction (%) 
Kaolinite  0.51136  0.89465  0.72533  1.42  12.21  0.45  12.21  0.45  85.1 
Muscovite  0.51594  0.89454  1.99972  3.88  14.48  0.26  14.48  0.26  9.3 
Todorokite  0.97654  0.28423  0.95395  0.98  14.98  2.34  14.98  2.34  5.6 

The XRD result show that the powder DD3 kaolin consists essentially of kaolinite (˜85%) of chemical formula [Al2Si2O5 (OH4)] or associated with halloysite [Al2Si2O5 (OH) nH2O n ≥ 4] [10,11].The kaolinite phase has triclinic structure, space group P1, lattice parameter a0 = 0.513 nm, b0 = 0.889 nm, c0 = 0.725 nm, α = 91.66°, β = 104.66° and γ = 90°.

The broad band observed for 11.5 <2θ> 12.5 is characteristic peak of kaolinite and present a strong disorder in (001) plans. The kaolinite is presented in the form of hexagonal lamellar particles formed by the stack of sheets. The elemental layer is formed by a silica tetrahedral layer (Si2O5)−2 resting on anoctahedral hydroxide layer (Al(OH)4)+2 connected by common edges. These polyhedral consist by the superposition of three layers of oxygen atoms and hydroxides.

(Si2O5)−2 + [Al(OH)4]+2 → [Al2Si2O5 (OH4)] (1)

The crystallite size the kaolinite is (˜12 nm) and the relative deviation of the lattice parameter reaches as much as Δa = −0.31% and Δc = 0.61%. In addition to these major phase (kaolinite), the DD3 kaolin contains two phases: (i) muscovite (9.29%) of chemical formula 2[Si3AlO10Al2(OH)2 K],with monoclinic structure, space group C2/c:b1, lattice parameter a0 = 0.185 nm, b0 = 0.896 nm, c0 = 2.01 nm, β = 95.66°, (ii) todorokite (5.55%) with monoclinic structure, space group P2/m, lattice parameters a0 = 0.97570 nm, b0 = 0.28419 nm, c0 = 0.95684 nm, β = 94.074°. The todorokite phase is black mineral contains two types of manganese oxides of the chemical structural formula described as follows: 2 (RO·MnO2·2H2O)3 (Mn2O3·3MnO·2H2O) with R = Ca, Mg, Ba, Mn) [8,12,13].

3.2.2Phase formation of the sintered samples

The XRD patterns of the sintered samples C1 and C2 (Fig. 6) exhibit a well crystallized samples having sharp diffraction peaks due to the recovery, strain relaxation, decrease of the crystalline defects and the increase of the crystallite size. Furthermore, the intensity of diffraction peaks of sintered sample "C1" are relatively reduced compared to those of sintered sample "C2".

Fig. 6.

XRD patterns of the sintered samples C1 and C2.

(0.16MB).

The Rietveld refinement of sintered samples "C1" and "C2" (Fig. 7) is composed of mullite (3Al2O3 2SiO2) as major phase and α-low cristobalite. The reaction of silica and alumina at high temperature provides to form the mullite (3Al2O3 2SiO2) and α-low cristobalite. The nucleation and crystals growth of mullite is done by the diffusion of silicon and aluminum at the grain interfaces. So, kaolinite undergoes a series of reactions to form the mullite which the phases transformations begins by the dissociate kaolinite from the temperature of 700 °C according to the following reaction:

Fig. 7.

Rietveld refinement of the sintered samples C1 and C2.

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When the kaolinite is heated, the adsorbed water is liberated at above 100 °C and the weakest part of the chemical bond is perturbed or broke. The dehydroxylation to metakaolinite (Al2O3 2SiO2) occurs between 400 and 550 °C. For kaolinite, dehydroxylation might result in the disturbance of the Al(O,OH)6 octahedral sheet by the extern hydroxyls, but does not have much effect on the SiO4 tetrahedral sheet due to the more stable inner hydroxyls groups. The extern hydroxyls of octahedral sheets may be more easily eliminated by heating than inner ones that will maintain a more ordered SiO4 tetrahedral group in structure during dehydroxylation. It might result in the disturbance of the Al(O,OH)6 octahedral sheet by the external hydroxyls, but does not have much effect on the SiO4 tetrahedral sheet due to the more stable inner hydroxyls groups [14].

The metakaolinite (Al2O3SiO2) has a monoclinic cell with parameters lattice a = 0.1514 nm, b = 0.890 nm, c = 0.68 nm and β = 100.2 [15-17]. It begins to dissociate from the temperature of 700 °C to form unstable amorphous silica, which will react at high temperatures according to three reactions:

  • 1

    A part with γ alumina to form the mullite around 980 °C, but inhomogeneities at the nanoscale may delay its formation up to 1300 °C [18]. The mullite growth is accelerated by instantaneous nucleation process and the short distance diffusion. It exhibits a orthorhombic structure with lattice parameters a0  = 0.7588 nm, b0 = 0. 7688 and c0  = 0.28895 nm and space group Pbam. The mullite is the main crystalline and only stable intermediate phase in the alumina silica system, and the Al2O3-SiO2 at atmospheric pressure. Mullite-based ceramics is characterized by excellent chemical and physical properties at high temperatures. It presents a good corrosion resistance, a low-dilatation coefficient and thermal conductivity, a good creep and thermal shock resistances [19]. These characteristics confer to these refractories a potential use in several industries and the ability to bear service conditions encountered in different applications. The studies showed that mullite formation process is a solution precipitation process which multiple-step process in various forms of silica–alumina mixtures [20]. They found that initial form of the alumina and silica has a large influence on mullite development because the interdiffusion of aluminum and silicon atoms between grains develops liquid phase [21].

  • 2

    The other part is crystallizing in cristobalite form, it is the β-high cristobalite with triclinic structure and represents the most stable form. It has a space group P1 and lattice parameters a0 = 0.7156 nm, b0 = 0.7156 nm, c0 = 0.7156 nm, α = 90°, β = 90° and γ = 90°. The β-high cristobalite appears at about 1075 °C and goes on increasing in between 1200 °C and 1300 °C after that it decreases.

  • 3

    This β-high cristobalite is subsequently transformed into α-low cristobalite between 170 °C and 240 °C. It has a tetragonal structure, space group P41212 and lattice parameters a0  = 0.49733 nm and c0  = 0.69262 nm.

The most intense peak of cristobalite (21.6°) corresponding to the direction (101) represents the preferred direction of development of cristobalite crystals. Between 1100 °C and 1400 °C, the cristobalite is in the form of a glass phase and as an amorphous phase above 1400 °C. An excess of the amorphous phase reduces the creep resistance and lowers the thermo-mechanical properties of the refractory at high temperatures. For this reason, it is necessary to reduce the rate of this phase.

Also, it is noted that the presence of manganese in the chemical composition of kaolin DD3 probably favors the formation and the development of cristobalite, so, the manganese plays a precursor role for the formation of the crystalline phase of cristobalite [12,22]. However, the CaO element insures the cristobalite vitrification at low temperatures and favors the mullite formation [23,24].

The Rietveld refinement of the sintered samples is summarized in Table 4. The mullite volume fraction increase with increase of chamotte content, thus, the C2 pellet (92.68%) is higher than that of the C1 pellet (91.08%). Indeed, the volume fraction of α-low cristobalite is about 8.92% and 7.32% for C1 and C2 pellets, respectively. The sintering treatment favors the crystallite size growth up to ˜150 and 200 nm for mullite and α-low cristobalite respectively. In addition to the relaxation of microstrains that are induced into the crystal lattice by the annihilation of various structural defects.

Table 4.

Phases, lattice parameters (a, c), relative deviation (Δa, Δc), average crystallite size <l> , microstrains <σ </l>2>1/2, relative proportion of the sintered samples C1 and C2.

  Phases  a (nm) ± 10−4  b (nm)±  ± 10−4  c (nm)  ± 10−4  c/a  Δa (%)  Δc (%)  <l> (nm)  ± 3 </l>  2>1/2 × 10−3  Volume fraction (%) 
C1Mullite  0.75525  0.77041  0.28866  0.38  −0.47  −0.10  147.9  0.69  91.1 
Low cristobalite  0.50909  –  0.69268  1.36  2.37  0.01  229.1  3.55  8.9 
C2Mullite  0.75394  0.76790  0.28770  0.38  −0.64  −0.43  145.1  0.39  92.7 
Low cristobalite  0.50204  –  0.70005  1.39  0.95  1.07  212.7  2.95  7.3 

The relative deviation of the lattice parameter of mullite reaches as much as Δa = −0.46% and −0.63% for C1 and C2 pellets respectively. The expansion/contraction of the crystal lattice of the sintered samples, which can be evaluated by the relative deviation (Δa = (a − ao)/ao) of the lattice parameters (a, c) from those of the perfect crystal (ao, co), can be linked to the preparation method. The variation of the lattice parameter value might be related to the heavy plastic deformation, crystal defects and/or the excess/deficiency of Al and Si atoms. The X-ray results samples sintered are in good agreement with DTA-TGA analysis.

One observes that the apparent density values of sintered pellets are lower that the absolute density (Fig. 8). The apparent density obtained by Archimedes method of sintered pellets C1 and C2 are 2.70 g/cm3 and 2.75 g/cm3, respectively. The absolute density obtained by using helium pycnometer show that the sintered samples C1 and C2 presented the densities equal to 2.76 g/cm3 and 2.83 g/cm3 respectively. Absolute density of mullite obtained in this work is less than that of theoretical density of pure mullite (˜3.16 g/cm3) [1]. Usually, the increase in density is attributed to the elimination of pores during sintering. This can be explained, when the sintering temperature increases, there are transformation of cristobalite into a crystalline phase (d = 2.5 g/cm3) and an increase of mullite content that takes place simultaneously. The samples porosity C1 and C2 is about 4.70% and 4.43% respectively.

Fig. 8.

Absolute and apparent density of the sintered samples C1 and C2.

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Sahnoune et al. [25] reported that a complete transformation of kaolinite to mullite require a sintering treatment at a temperature of 1600 °C for 4 h to obtain a high densification (94%). This may be due to the fact that the initial powder was milled for 5 h which led to the homogeneous powder formation with fine particles that reduced diffusion distances and increased the driving force for sintering. But Gonon et al. [26] showed that the absolute density of kaolin DD3 decreases as the sintering temperature rises beyond 1200 °C. On the other hand, Imai et al. [27] found that during the halloysite sintering at a temperature of 1400 °C for 50 h, two phases are obtained which are mullite and cristobalite. Another study revealed that when the increasing pressure, the density increase, and this increasing of density is proportional inversely to sample thickness [8].

3.3Differential thermal and thermogravimetric analysis

DTA is based on energy changes, it can detect changes in heat occurring during decomposition, recrystallization and polymorphic transformation phenomena during heating of a kaolinite. Differential thermal analysis scan of powder DD3 kaolin (Fig. 9) reveals the existence of several endothermic and exothermic peaks in the temperature range (50–1000) °C. The first endothermic peak at about 80 °C can be attributed to the release of interstitial water in the kaolinite (2SiO2Al2O32H2O). The dehydroxylation of kaolinite to metakaolinite (Al2O32SiO2) occurs between 400 and 550 °C. The kinetics of the dehydroxylation process depends on the degree of kaolinite structural organization. A disordered kaolinite dehydroxylates rapidly and a few residual OH remain in the metakaolinite. However, a well ordered kaolinite dehydroxylates slowly because the structure tends to retain the last hydroxyl groups [13].

Fig. 9.

DTA–TGA curves of the DD3 kaolin.

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The exothermic peak at 995 °C can be ascribed to the metakaolinite restructuring. The phase transformation of DD3 kaolin can be described as follows [28]:

Thermogravimetric analysis is based on weight change, it can detect a weight loss during decomposition of kaolinite when heated at a specific rate.

The TGA analysis show a first weight loss at about 0.8% due to the evaporation of adsorbed water, and corresponding to the first endothermic peak in DTA. The second weight loss is around 13.4% in the heating temperature 350–700 °C, and correlating with the second endothermic peak at 500 °C. This weight loss is lower than that associated with the dehydroxylation of an ideal halloysite (16.85%). In fact, it is closer to the ideal value of the weight loss recorded during the dehydroxylation of kaolinite (13.76%) [9]. This reaction associated with an important weight loss observed by the thermogravimetric analysis (˜14.2%) according to the following reaction [4]:

kaolinite [Si2Al2O5(OH)] → (Metakaolinite, amophe) [Si2Al2O5] + H2O (4)

3.4SEM observations

The SEM observations of the powder DD3 kaolin (Fig. 10) are mainly composed of large and small grain formed by irregular shapes with various different sizes. This imperfection structure is related to certain disturbances of the octahedral layer in which iron can replace aluminum [4]. The morphology of the DD3 kaolin is simultaneously composed of kaolinite and having a needle-like form microstructure with an entanglement in all directions of halloysite fibers [11].

Fig. 10.

SEM micrographs of the DD3 kaolin.

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Chemical composition was studied by EDX analysis (Table 5) in order to investigate the qualitative analysis in the sintered samples. The EDX analysis confirm the predominance of three principal elements which are: oxygen ((O=53 at.%)), aluminum (Al=22.at%) and silicon (Si=19 at.%). These elements composed of principal substances of alumina Al2O3 and silica SiO2, and that determine the main heat characteristics of refractories. One notes also the presence of other elements in very small proportion such as: Ca, Mn and Fe. The SEM observations of sintered pellet (Fig. 11) reveal a heterogeneous structure with spherical pores ranging in size from 2 to 3 μm [29].

Table 5.

Elemental analysis (EDX) of the DD3 kaolin.

Chemical elements  Wt.% 
53.07 
Al  21.94 
Si  18.93 
Ca  0.50 
Mn  4.09 
Fe  1.37 
Mg  0.10 
Fig. 11.

SEM micrographs of the sintered pellet C2.

(0.34MB).
4Conclusion

In this paper, mullite was prepared by solid-state reaction from Algerian kaolin of Djebbel Debbagh-Guelma (North-East of Algeria) and chamotte. The sintered samples composed mainly of mullite and cristobalite. The results of this present study indicate that the chamotte activates the sintering by the mullite formation, and increases the density to maximum value. The sintered samples yielded a dense mullite ceramics with a relative densification higher than 95%, an apparent density in the range 2.7–2.75 g/cm3 and a porosity from 4.70 to 4.43%.

Conflicts of interest

The authors declare no conflicts of interest.

.

[16].

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