Thesis Resume  

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       With the growing awareness of the threat of CO2 emissions, geopolymer-type materials are emerging as an obvious alternative in many areas. Geopolymers are assimilated to inorganic polymers which exhibit excellent fire resistance, low density, environmentally friendly nature, and excellent thermal properties. These materials have received special attention in recent years as a promising new type of material for a wide variety of applications.

In the industrial field, attempts mainly propose geopolymer materials as candidates in the replacement of Portland cement as a building material. This results from the fact that the chemistry of geopolymers is very different from that of cements. In fact, CO2 emissions are six times lower during the manufacture of geopolymers when compared to the preparation of traditional cement. From a chemical point of view, geopolymers are part of the aluminosilicate family. They are produced at room temperature and at atmospheric pressure. Whether used pure, filled, or reinforced, geopolymers are already finding applications where traditional materials (metals, ceramics, organic polymers) do not provide a satisfactory solution.

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         In the development of geopolymers, metakaolin, calcined kaolin, is generally used as a raw material due to its simple chemical composition. Metakaolin can be activated with a strong alkaline solution (NaOH, KOH, etc.) to obtain an alkaline-based geopolymer. These geopolymers are known for their high ionic conductivity which allows them to be used in batteries and as electrochemical sensors.

Metakaolin can also be activated with a phosphoric acid solution to prepare a new type of geopolymer. This new type presented mechanical, thermal, and dielectric properties different from those obtained for geopolymers produced through basic etching. These materials are used in electronic packaging and encapsulation materials. Little scientific research has been interested in this type of geopolymeric materials, especially with regard to dielectric characterization. Due to the growing interest in geopolymeric materials, we have chosen in this work to develop geopolymers from calcined kaolinitic clay (pharmaceutical kaolin) by activation with phosphoric acid. The main objective of the thesis is to obtain a geopolymer material with a non-conductive dielectric character compared to geopolymers obtained by basic etching. Three geopolymers with three different Si / P molar ratios equal to 1.25; 1.5 and 1.75 (with Si: number of moles of silicon existing in metakaolin and P: number of moles of phosphor existing in phosphoric acid) were developed.

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      The effect of varying the Si / P molar ratio on the different properties of geopolymer materials has been investigated. Scanning electron microscope observations showed that the decrease in the Si / P ratio noticeably favored the geopolymerization phenomenon and subsequently the structure becomes denser. X-ray diffraction spectra showed that the synthesized geopolymers are essentially amorphous regardless of the Si / P ratio. In addition, they underlined the presence of certain crystalline products resulting from the geopolymerization reaction.

Thermal studies carried out by differential scanning calorimetry (DSC) supplemented by differential thermal analysis (DTA) confirmed the thermal stability of geopolymers up to temperatures of 1000 ° C and indicated the existence of water under several forms in these materials (free water, pore water and bound water).

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       A dielectric study at room temperature in a frequency range [0.1-106] Hz showed that the geopolymer samples produced are characterized by low dielectric permittivity and electric conductivity compared with those of traditional geopolymers. This analysis made it possible to highlight, on the one hand, the different dielectric relaxations and the ionic conduction phenomena existing in these materials. On the other hand, it allowed us to optimize the Si / P molar ratio (1.75) to obtain a geopolymer with low dielectric permittivity, ionic conduction, and dielectric losses.

In order to better understand the attributed dielectric phenomena, a study of the molecular mobility of different samples in a temperature range from -50 ° C to 200 ° C and a frequency range from 0.1 Hz up to 1 MHz was realized. We detected various relaxation phenomena: A water dipolar polarization in the ice state towards low temperatures, a dielectric polarization observed around 120 ° C which was attributed to the movements of the reorientation of the free water molecules appearing in the geopolymer, a dielectric polarization observed above 150 ° C which was attributed to the movements of the reorientation of the interstitial water molecules. In addition, a percolation phenomenon originating from the migration of ions through the pores in geopolymeric materials has been observed in the temperature range from 60 ° C to 110 ° C. In order to better study dipole polarization in geopolymeric materials, we calculated the activation energies of each dielectric relaxation process. The model applied for the relaxation of ice water molecules towards low temperatures was the Arrhenius model, while the polarization of water molecules detected at high temperatures followed the Macedo and Litovitz model. In order to seek to reinforce the non-conductive character of the geopolymer whose molar ratio chosen is Si / P = 1.75, we varied some experimental parameters such as the temperature of the heat treatment, the age of the hardening, and the raw material.

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      In the first part, geopolymer sample was exposed to heat treatment for different temperatures (150 ° C, 450 ° C, 750 ° C, and 1000 ° C). For high temperatures, new crystalline phases appeared in the processed sample. In addition, dielectric parameters such as dielectric permittivity and electrical conductivity decreased with increasing heat treatment temperature. The main reason for this is the elimination of water in its various forms. Dielectric losses have also decreased especially at high frequencies allowing geopolymeric materials to be used in several dielectric and industrial applications such as packaging and insulation.

 

        In the second part, the curing age of the specimen was extended from 7 days to 28 days. This led to an evolution of the geopolymerization reaction resulting in more amorphous and more compact structures. In addition, the dielectric parameters have been minimized with the development of the geopolymerization reaction.

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      In the third part, we carried out a comparative dielectric study between two geopolymers synthesized from two different types of clay: metakaolin and a calcined natural Tunisian clay extracted from the Tabarka region. The results revealed a big similarity in the dielectric behavior of the two geopolymers. This then enabled us to develop natural resources.

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 Publications 

 

1) Douiri, H.; Ghorbel, N.; Raihane, M.; Arous, M.; Ilsouk, M.; Lahcini, M.; Yagci, Y.; Tasdelen, M. A.; Kallel, A. Effect of Clay on the Dielectric Properties of Novel Fluorinated Methacrylate Nanocomposites. 9.

DOI: 10.1002/pc.25192

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2) Douiri, H.; Louati, S.; Baklouti, S.; Arous, M.; Fakhfakh, Z.

Structural and Dielectric Comparative Studies of Geopolymers Prepared with Metakaolin and Tunisian Natural Clay.

Applied Clay Science 2017, 139, 40–44.

DOI: 10.1016/j.clay.2017.01.018.

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3) Douiri, H.; Kaddoussi, I.; Baklouti, S.; Arous, M.; Fakhfakh, Z.

Water Molecular Dynamics of Metakaolin and Phosphoric Acid-Based Geopolymers Investigated by Impedance Spectroscopy and DSC/TGA.

Journal of Non-Crystalline Solids 2016, 445–446, 95–101. 

DOI: 10.1016/j.jnoncrysol.2016.05.013.

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4) Douiri, H.; Louati, S.; Baklouti, S.; Arous, M.; Fakhfakh, Z.

Enhanced Dielectric Performance of Metakaolin–H 3 PO 4 Geopolymers.

Materials Letters 2016, 164, 299–302.

DOI: 10.1016/j.matlet.2015.10.172.

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5) Douiri, H.; Louati, S.; Baklouti, S.; Arous, M.; Fakhfakh, Z. Structural

Thermal and Dielectric Properties of Phosphoric Acid-Based Geopolymers with Different Amounts of H3PO4. Materials Letters 2014, 116, 9–12.

DOI: 10.1016/j.matlet.2013.10.075.

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