A new and economic approach to synthesize and fabricate anorthite based ceramics using kaolin and CaCO

A new and economic approach to synthesize and fabricate anorthite based ceramics using kaolin and

CaCO 3

Soumia Zaiou^a, Ismahan. Serradj^a, Abdelhamid. Harabi^a

aCeramics Lab, Faculty of Exact Science, Physics department, Mentouri University of Constantine, Constantine 25000, Algeria. E-Mail :zaiou_21 @yahoo.fr

Abstract— This new and economic approach to fabricate resistant anorthite based ceramics consists of Algerian kaolin and calcite (CaCO3). The anorthite (CaO·Al2O3·2SiO2) based ceramics were obtained by solid state reaction. The starting powders were sintered at different temperatures (800-1100

°C) for 1 h. The optimum sintering conditions gave a relatively higher density (2.64 g/cm³). Different techniques were used to investigate the physical proprieties of the prepared anorthite such as: scanning electron microscopy, X- ray diffraction, Raman spectroscopy and tensile strength. The best obtained 3 points flexural strength value was about 202 MPa for the samples sintered at 1000 °C for 1 h. Furthermore, the best value of Vickers micro-hardness of the samples sintered at 1000 °C was 7.1 GPa. Finally, a correlation between microstructure and mechanical properties of elaborated supports has been discussed.

Keywords— anorthite; kaolin; calcite; sintering; Vickers micro-hardness; tensile strength.

I. INTRODUCTION

Algeria is one of the countries in the world that have abundantly available raw materials, such as calcium carbonates (CaCO3), bones (natural derived hydroxyapatite (HA) : (Ca10(PO4)6(OH)2), kaolin, feldspar and quartz. Many works have already been published for valorizing these native raw materials, for the production of advanced ceramics [1-3], ceramic membranes [4-5] and bioceramics [6-7]. Anorthite (2SiO2.Al2O3.CaO) is a mineral of the feldspar group consisting of aluminum and calcium silicate (Ca Al2Si2O8). In feldspars, anorthite is part of the group called "plagioclase", which means that it contains sodium or calcium, with

reference to the tripartite system inclination crystalline (Triclinic). Usually, anorthite is prepared by several methods such as sintering of mixtures , mechano-chemical, treatments and sol-gel, controlled devitrification of glass and sometimes with addition of a nucleation agent . All these methods carry their own advantages and disadvantages .

Consequently, this study is mainly focused on the preparation of anorthite based ceramics using a modified milling system by using Algerian natural materials (kaolin (DD2) and calcium carbonate (CaCO3)) mixtures less expensive and which are abundantly available in our country (Algeria).

II. EXPERIMENTAL PROCEDURES

the anorthite were prepared from domestic kaolin (DD2)(Al2O3.2SiO2.4H2O) and calcium oxide extracted from calcium carbonates (CaCO3) obtained from the Algerian sites of .Guelma and Constantine, respectively.

All anorthite samples were elaborated using 80 wt% kaolin (DD2) and mixed with CaO (20 wt%) extracted from local CaCO3. After its calcination at 900 °C for 12 h, using an original homemade vibratory milling system for 17 h, dried and calcined at 800°C, for 1 h.

After drying, the test specimens for firing tests, shaped as discs of about 13 mm diameter and 2 mm thickness, were obtained after uniaxial pressing of powders at 75 MPa. The dried samples were sintered at 800, 850, 900, 1000 and 1100°C for 1 h. The device used in this milling system was manufactured at the laboratory using simple parts, as well detailed elsewhere [7]. It was mainly composed of 4 parts: a metallic cylinder, a Teflon bottle, a motor and a rotational

system. The latter makes the cylinder to rotate with a constant speed. In order to avoid the adhesion of the powder on its sides, the bottle has been placed obliquely in the middle of the cylinder within a certain angle. This device has also four springs fixed at 4 corners of its support in order to allow the bottle to vibrate gently or strongly. This system works smoothly and silently, contrarily to the usual noisy vibratory milling system due to its motor. These last two properties are at the origin of the obtained multidirectional rotation. As a result, powerful and continuous collisions may occur between the hard milling balls and the powder particles which substantially reduce the powder particles size.

Different characterization techniques were used to investigate the properties of raw powders and anorthite samples.

A. X-ray diffraction

The phase compositions of the powders and prepared samples were identified by using X-ray diffraction (XRD) (BRUKER, D8 ADVANCE) (Karlsruhe, Germany) with a CuKα radiation (λ = 0.154 nm) and a Ni filter, working voltage 40 kV and working current 30 mA.

B. SEM analyses

The morphology and the microstructure of kaolin, calcite powders and sample surfaces were observed by an SEM (Hitachi, JSM-6301 F) (Tokyo, Japan) working at 7 kV as an accelerating voltage. Before SEM observation, all samples were gold coated.

C. Raman spectroscopy

A Raman micro-spectrometer (BRUKER Raman SENTERRA R200L) (Germany) was optimized for maximum throughout, detection sensitivity and fluorescence suppression.

The argon ion laser provided a 25 mW incident light at 785 nm.

D. Sintering technique

The bulk density was determined using the Archimedes method (water displacement method).

The relative density was calculated by the following relation:

(ρexp/ρtheo).100 = (%)... (1)

Where ρexp(g/cm^-3) is the bulk density and ρtheo(g/cm^-3) is the theoretical density of the sample.

E. Evaluation of properties

The tensile strength testing of specimens was obtained using a diametral compression test (FORM TEST SEIDNER D 79- 40) (Germany). One of the fundamental aspects of this test is the relatively small proportion of the specimen volume which reaches the peak stress at fracture.

In its simplest form, a right circular cylindrical specimen compressed diametrally between two flat platens. A biaxial stress state is produced within the test specimen and on the assumption of ideal line loading the vertical plane is subjected to a uniform horizontal tensile strength of magnitude

σt= 2P/πdt... (2)

where σt (MPa) is the maximum tensile stress, P (N) is the applied load at fracture, d (mm) is the specimen diameter and t (mm) is the specimen thickness.

The correspondence between measured tensile strength (σt) value and its equivalent 3 point flexural (bending) strength (σf) is given by the following equation:

σf(MPa) =2.7σt(MPa) ... (3) This equation was also confirmed by Harabi [8].

III. RESULTS AND DISCUSSION

The sintering process is also an important step in the preparation of the ceramic samples, where the sintering temperature has a significant effect. Fig. 1 illustrates that the bulk density increases with the increase in the sintering temperature (800-1100 °C). In fact, the bulk density increases sharply between 800 and 1000 °C, where the bulk density varies from 1.68 to 2.64 g/cm-³. Afterwards, the relative density decreases significantly at 1000 °C. Additionally, a relative density of about 96% of the theoretical one was reached for anorthite samples using the proposed activating process. This result highlights the importance of using the modified milling system and native natural materials (kaolin DD2 type and CaCO3). When this process was applied, the powder particles were drastically smaller. Moreover, the morphology of particles had elongated and spherical shapes.

800 850 900 950 100010501100 60

70 80 90 100

Bulk density(gcm-3)

Relative density (%)

Sintering temperature (°C) 1.6 1.8 2.0 2.2 2.4 2.6

20 30 40 50 60

Intensity (a.u.)

2(°)

400 500 600 700 800 900 1000 1100 1200

Wavenumber(cm-1) Fig. 1. Bulk density of anorthite samples sintered at different temperatures

A typical SEM micrograph of the fracture surface of samples sintered at 1000 °C for 1 h is given in Fig. 2. This figure confirms that the more densified samples may correspond to the more resistant ones.

Fig. 2. SEM micrograph illustrating a submicron anorthite grains for powders sintered at 1000 °C

The XRD spectrum for samples sintered at 1000 °C is illustrated in Fig.3. The main phase detected in samples fired at1000 °C was only anorthite (CaO·Al2O3·2SiO2). Moreover, after a careful examination of these XRD spectrum , one can remark that fortunately there are no any free CaO traces.

Fig. 3. XRD spectrum of anorthite samples sintered at 1000 °C

Additionally, Fig. 4 confirms again a typical anorthite structure sintered at 1000°C, using Raman spectroscopy.

Indeed, all the vibration regions, shown in this figure, entirely coincide with those reported in the literature [9], in the low wave number region of 400–1200 cm⁻¹. The prominent peaks in this region represent vibrations within the anorthite [9].

Fig. 4. Raman spectrum of anorthite samples sintered at 1000 °C

Fig.5 shows the tensile strength of anorthite samples, prepared according to the proposed process, as a function of

800 850 900 950 1000 1050 1100 30

40 50 60 70 80

Sintering temperature (°C) ⁰

50 100 150 200 250

Flexural strength (MPa)

0 2 4 6 8 10

80 100 120 140 160 180 200 220

Tensile strength (MPa)

sintering temperature. This figure shows that the 3PFS increases gradually from 81 to about 202 MPa for samples sintered at temperatures ranging from 800 to 1000 °C. It was observed that a flexural strength about 202 was also obtained for samples sintered at 1000 °C. Afterwards, the flexural strength decreases significantly at 1000 °C. Additionally, typical SEM micrograph of samples sintered at 1000 °C is given in Fig. 2. This figure confirms that the more densified samples correspond to the more resistant ones.

Fig. 5. Flexural strength of samples sintered at different temperatures

IV. CONCLUSIONS

The attractiveness in the present work is the development of anorthite (CaO·Al2O3·2SiO2) based ceramics using economical natural local raw materials. The anorthite was formed by extrusion of a ceramic paste from kaolin and calcium carbonates mixtures, the samples were sintered at different temperatures ranging between 800 and 1100 °C. The

importance of using both the modified milling system and native natural materials (kaolin DD2 type and CaCO3) were put into evidence. Finally, the relatively promising characteristics mentioned above may extend further the application domains of these elaborated anorthite based ceramics.

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