3.1. Characterization of Raw Material
3.1.1. X-ray powder diffraction
The chemical composition of kaolin was determined with Thermo Nitron FXL (Table 1).
Table 1
Chemical composition of kaolin (% of mass).
SiO2
|
Al2O3
|
FeO3
|
MgO
|
Na2O
|
CaO
|
K2O
|
49.17
|
44.05
|
0.69
|
0.27
|
1.43
|
0.81
|
2.13
|
The XRD pattern of kaolin (Fig. 1) was analyzed by Le Bail method, without atoms, using the pattern matching routine of the FULPROF program [15]. The profile refinement results indicate that the diffraction pattern corresponds to a mixture of two phases: Kaolinite (C 1; a = 5.055(1) Å, b = 8.984(5) Å, c = 7.376(4) Å, α = 91.730(6)°, β = 104.92(3)°, γ = 89.67(4)°, and Muscovite (C 2/c; a = 5.180(2) Å, b = 8.932(3) Å, c = 20.034(3) Å ,α = 90.0°, β = 95.14(8)° and γ = 90.0°). Semi-quantitative analysis using the RIR (reference intensity ratio) method shows that the commercial kaolin is a mixture of 94% of kaolinite and 6% of Muscovite.
3.1.2. IR Spectroscopy
Kaolinite with general formula: Al2(OH)4Si2O5 is 1:1 type layer silicates and consists of a two-layer arrangement of tetrahedra and octahedra sheets. The tetrahedral layer is composed of SiO4 linked in a hexagonal array. The bases of the tetrahedra are approximately coplanar and the apical oxygen atoms are linked to a second layer containing aluminum ions and OH groups. The aluminium ions located in the center of octahedra are surrounded by 4(O) and 2(OH) [16].
A major component of the separation force is the hydrogen-bonding force between hydroxyl groups on octahedral sites and oxygen atoms of SiO4 tetrahedra, as van der Waals force among the atoms.
The identification of proton positions in the structure of layer silicate are important to interpret infrared spectroscopy data of kaolin.
The kaolinite contains four (OH) groups in the primitive unit cell. The observed band at 3620 cm−1 can be assigned to the (OH) stretching vibration of one hydroxyl group which lies within the layers and is nearly parallel to the layer surface, a (001) plane [17–18]. The other three (OH) groups, lie on the upper surface of the layers, and are oriented at angles of 60-73° to the (001) plane [17–18]. The three hydroxyl groups couple to give a strong in-phase symmetric stretch, observed, in our case, at 3691 cm−1 and two weak out-of-phase vibrations at 3669 cm−1 and 3652 cm−1 [19]
The IR spectrum of kaolin (Fig. 2) exhibits additional bands at 3677, 3727, 3730, 3740 cm−1 in the (OH) stretching region that are not reported at the previous research works. These extra bands were observed in the Raman spectrum of kaolinite at 3686 and 3710 cm-1 [18], and assigned to transverse optical modes involving in-phase coupled vibrations of the layer-surface hydroxyl groups. Farmer suggested that the corresponding IR bands have transition moments nearly perpendicular to the layer surface, and appear in the higher frequencies range of the longitudinal optical modes of macroscopic crystals.
The Infrared vibration bands of kaolin below 1200 cm−1 displayed in Fig. 3, represent cooperative motions of structural ionic groups. The IR vibration bands may divided into vibrational, rotational and translational motions of anion-cation associations. Although, the proportion of each factor in the production of vibrations is difficult to evaluate at the present time. That is why the detailed vibrational assignments of bands in this region are controversial. To reveal the general characteristics of these network vibrations we have adapted assignments that present a general agreement.
The moderately strong broad band at 1033 cm−1 and the strong sharp band at 1004 cm−1 correlated to the antisymmetric and symmetric stretching modes of Si-O-Si, respectively [20]. A medium band appeared at 1117 cm−1 correspond to the apical Si-O vibrations [20–22]. The weak bands at 940 and 912 cm−1 assigned to the deformation modes of inner and inner-surface of Al2O-H [21–22].
The domain observed from 800 to 400 cm−1 is the skeleton vibrational modes region. There is a general agreement that the medium band observed at 428 cm−1, the weak band at 650 cm−1 and the strong bands at 460 and 528 cm−1 belong to Si-O-Si deformation banding modes [21–22].
Three medium bands appeared at 688, 752 and 795 cm−1 were assigned to the deformation modes of AlVI-O-Si bands [21–22].
A weak band near of 413 cm−1 ascribed to out-of-plane OH liberations [23].
The XRD pattern of geopolymer showed the typical amorphous structure of metakaolin. In addition, the characteristic diffraction peaks of muscovite remained after geopolymerization, which suggests that muscovite did not participate in the geopolymerization reaction. While, the amorphous structure of metakaolin transformed from one structure to another structure.
A broad peak characteristic of amorphous structure with two peaks corresponding to muscovite and zeolite phases were observed at 300°C and persisted until 600°C.
After treatment at 900°C, muscovite crystalline phase remained stable, while the zeolite disappeared (Fig. 5).
After heating at 950°C, Na-geopolymer transformed to crystalline material consisting predominantly in nepheline (NaAlSiO4) with a few low intensities, minor peaks are ascribable to the muscovite crystalline phase. On heating at 1100°C, the amount of the amorphous phase increases with the decrease of crystalline peaks intensities of the nepheline phase. The relative decrease should be ascribed to the appearance of the liquid phase in the matrix. The liquid will affect crystalline phases by partial dissolution (Fig. 6).
3.2.2. IR Spectroscopy:
Figure 7 illustrates the infrared spectra of kaolin, metakaolin and geopolymer treated at different temperatures for 1 h.
The IR spectrum of kaolin showed a dihydroxylation phenomenon during its transformation to metakaolin correlated by the appearance of peaks near of 3650 cm-1.
On the IR spectrum of metakaolin, a peak appeared at 1037 cm-1 corresponds to a large concentration of Si-O bonds. Two infrared bands are located at 569 cm-1 and 648 cm−1 assigned to Si-O-Al stretching vibrations; a peak at 439 cm-1 arises from Si-O-Si bending vibration. Indeed, an IR band observed at 806 cm−1 is assigned to the Al-O bending mode of [AlO6] octahedra [20–21].
Due to the presence of water molecules in geopolymer, the strong characteristic peaks at approximately 3331 cm− 1 and 1631 cm− 1 were attributed to stretching and bending vibrations of hydroxyl groups, respectively [22, 23]. After geopolymerization, the chemical environment around regular arranged chain structures of the Si-O bond altered, along with the formation of Al-O-Si bonds. Subsequently, the strong asymmetrical stretching vibration peak of the Si-O bond on the IR spectrum of metakaolin (1037 cm−1) shifted to a lower wavenumber (960 cm−1). This indicated that the solidification process of the geopolymer is a chemical reaction, according to a generation of new substance.
Table 2
The wavenumber of kaolin, metakaolin and geopolymer treated at different temperature
Kaolin
|
Métakaolin
|
Geopolymer
|
100°C
|
300°C
|
900°C
|
1100°C
|
1200°C
|
3740
3730
3727
3677
-
-
1117
1033
1004
940
912
-
795
752
688
650
528
460
413
|
-
-
-
-
-
-
1037
-
-
806
-
-
648
569
-
439
|
3331
1631
-
-
-
-
-
960
843
-
-
697
545
470
421
|
3344
1647
1564
1379
-
-
-
962
853
-
716
-
-
558
462
-
|
3347
1654
1549
1371
1221
-
-
968
848
-
729
-
-
572
467
-
|
-
-
1367
-
-
975
-
-
708
-
-
555
456
-
|
-
-
-
-
970
-
-
-
682
654
556
465
-
|
-
-
-
-
-
991
846
-
-
699
636
574
442
-
|
The principal new band appeared at 960 cm−1 is assigned to the asymmetric stretching vibration of Si–O–T band links in the geopolymer frameworks (T:Si or Al in tetrahedral coordination). This band is known to be sensitive to connectivity and Si/Al ratio [24–25], and in this case, it is observed at a wavenumber consistent with the presence of predominantly Si–O–Al bonds, which agrees well with the stoichiometry showed in previously studied systems [24–25].
It was also found that the broad and strong peak observed at approximately 802 cm− 1; which belongs to the stretching vibration of hexa-coordinate Al(VI)-O in metakaolin; almost disappeared after geopolymerization.
A new peak located at 697 cm−1 corresponds to the bending vibration of tetra-coordinated Al(IV)-O-Si in a cyclic structure emerged on the FT-IR spectra of the geopolymers. This phenomenon signified the formation of aluminosilicate networks with the transition from hexa-coordinated Al(VI) to tetra-coordinated Al(IV) during the geopolymerization process, as observed by Sitarz et al. [26].
After heating at 100°C, the geopolymer produces a marked decrease in the water bands (Fig. 7).
Apart from this obvious change in the sample hydration on heating to 1100°C, small changes also occur in the Al-Si-O region, especially the development of the broad zeolites band at about 725 cm−1 (see Figure.7, T = 300 and 600°C, consistent with the XRD results).
In summary, all the results indicate that the removal of hydration water from a well-treated Na-PSS1 polymer brings about no significant change to the structure. Furthermore, the structure displays a high degree of thermal stability at higher temperatures, retaining its X-ray amorphous character and the atomic environment of its constituents, Al, Si and Na components.
Finally, on the spectrum of 1200°C a double band observed at 635 and 592 cm−1 (Fig. 7) relative to the presence of crystalline corundum (α-Al2O3) phase [27] (already detected through XRD). It supports a hypothesis of the conversion of the structure of the material from sialate [-Si-O-Al-O] to sialate-siloxo [-Si-O-Al-O-Si-O-] according to the appearance of α-Al2O3 crystalline phase. The principal band at 960 cm−1 characteristic of sialate shifted to a lower wavenumber (846 cm−1). The principal new band at 846 cm−1 is consistent with the presence of sialate-siloxo [24–25].
3.2.3. Thermal analysis by TG-DTA
The thermal analysis curves TG and DTA of Na-geopolymer are shown in Figure 8.
Below 250°C, the TG curve indicates that the sample present about 14% of mass loss related to the evaporation of free water (~70%). This phenomenon is well evidenced on the DTA thermogram by the presence endothermic peaks appearing from ambient temperature until approximately 250°C with a minimum values occurring at 50°C and 150°C.
The remainder is either more tightly bound or less able to diffuse to the surface, and continues to evolve gradually up to about 600°C.
Between 250 and 550°C, an endothermic reaction occurring on the material, caused by the dihydroxylation of the octahedral sheet (constitutive water) of muscovite. On the TG curve, the hydroxyl groups of the tetrahedral sheet of muscovite are gradually removed up to 850°C. An exothermic peak resulting by the crystal reformation (zeolite phase), then appears between 550 and 750°C. It can also be inferred that the exothermic peaks at 850-1150°C are due to the formation of nepheline (NaAlSiO4).
A large and intense exothermic peak started from 1150°C attributed to the structural reorganization.
3.2.4. Electrical conductivity of Na-Sialate geopolymer
As mentioned above, from 300 to 900°C, Na-sialate geopolymer presents the same disordered structure of the nepheline. The crystal structure of Nepheline is characterized by layers of six-membered tetrahedral rings of exclusively oval conformation. The rings are built up by regularly alternating AlO4 and SiO4 tetrahedral. The stacking of the layers parallel to the c axis results in a three-dimensional network containing channels that are occupied by the Na cations [28]. This topology is in favor of an easy movement of Na+ ions throughout the structure. For this raison ionic migration in nepheline is extensively reported [29–31].
The electrical measurements were performed on geopolymer pellets sintered at 300°C to remove free water.
Figure 9 shows the impedance plot at different temperatures of the Na-sialate geopolymer. The data was refined with an electrical equivalent model, the polarization part of the sample and the electrode polarization were modeled by a constant phase element (CPE), while the migration part was represented by a resistance R. The complex impedance spectra were characterized by the appearance of a semi-circle centered below the x-axis. At room temperature, the diagram shows two contributions, with two tangled semi-circles. These two relaxations, modeled by two parallel R//CPE circuits, correspond to ion migration in the grains and in the grain boundaries. The refinement of Na-Sialate geopolymer at room temperature gives bulk high ionic conductivity of about 5.10−5 S. cm−1. At higher temperatures, the characteristic frequencies become so close that it is impossible to distinguish the contributions. A total resistance comprising both grain and grain boundaries contribution is then determined.
According to these results, the temperature dependence of electrical conductivity can be analyzed following an Arrhenius plot (Fig. 10).
Showing Fig. 10, two different behaviors of the geopolymer can be evidenced. At low temperature, the total conductivity slowly decreases under 200°C. This phenomenon can be attributed to the loss of water which was absorbed on the surface of the pellet leading to protonic conduction. Then, from 200°C to 725°C, the conduction properties seem to approximately follow an Arrhenius law both on heating and cooling with deduced activation energies which are 0.26 eV and 0.28 eV respectively.
Thus, Na-Sialate geopolymer presents lower σ than the specific ionic conductivity of single crystals of Na-Nepheline along the crystallographic c-axis [28] but higher than the value measured in the polycrystalline sample [6, 30–31]