Etude théorique et expérimentale de la synthèse et caractérisation des géopolymères à base d’aluminosilicates riches en fer dans des conditions drastiques et de la formation des pores.

Abstract

This study focuses on the use of iron-rich aluminosilicate:Laterites, abundant raw materials in tropical zones and particularly in Cameroon and a metakaolin-hematite model for the synthesis and characterization of new geopolymeric structural and functional functional geopolymeric matrices under a wide range of environmental conditions. Initially, a theoretical study was carried out to evaluate the effect of Al ( III) substitutions by Fe (III) and Fe (II) and the energy expenditures of three pore-forming agents in the geopolymeric foams synthesis. For this purpose, a graphical software (Gausview) and a calculation software (Gaussian09) were used, implementing the Density Functional Théory (DFT) method. The results showed that iron makes the molecule more reactive, as the reactivity indices calculated from the HOMO and LUMO values decrease considerably. Furthermore, of the three pore-forming agents used (hydrogen peroxide, aluminum powder and silica fume), aluminum powder cosumes less energy, as the entropy of the reaction system is lower. Then, four laterite samples (LC25, LN25, LS25 and LW25) from four regions of Cameroon (Centre, North, South and West) were used. For a proper study, these laterites (lateritic curasses) were crushed, ground and sieved to 200 µm before being calcined at 550, 575, 600 and 700°C. An alkaline solution (NaOH-8M) was used to test their reactivity. Several techniques were used to characterize the raw materials and obtained products: X-Ray Fluorescence (XRF), X-Ray Diffraction (XRD), Fourier Transform Infrared (FTIR), Thermogravimetric and Thermal Analysis (TGA/TTA), Particle Size Analysis (D10, D50 and D90), Specific Surface Area Measurements (SBET), Scanning Electron Miscroscopy (SEM). The results show that, despite the different climates in which the samples were collected, the reactivities remain similar and that all laterites can be used as solid precursors and, like metakaolin, can be used as raw materials for cement substitution. For further work, a single laterite was used (Latérite du center). Geopolymer composites were synthesized from uncalcined laterite combined with additives such as rice husk ash and quarry sand. The alkaline solution in this case consisted of the mixture of NaOH-8M and sodium silicate in the ratio ½. The samples obtained underwent three curing modes: room temperature, 80°C oven drying and humidity control (65%). Mechanical results and physical tests showed that the humidity-controlled curing mode was the most efficient, confirming that water plays an important role in the geopolymerization process. Indeed, the optimum results give flexural strength values equal to 16.4, 28.82 and 56.41 MPa for the three modes respectively. xxxvii Finally, new materials such as geopolymer foams were developed, with laterite calcined at 700°C and metakaolin-hematite mixtures as solid precursors. Two alkaline solutions were used: sodium hydroxide + sodium silicate and potatium hydroxide + potatium silicate. These matrices were obtained by introducing a surfactant (triton H66) and a pore-forming agent (aluminum powder) into the dense matrix, and placed in an oven at 70°C. The resulting compounds were then placed in drastic conditions (fire resistance at 1175° C and freeze-thaw cycle between -20 and 20°C). The results obtained show that foams based on potassium mixtures are lighter than those based on sodium mixtures (this is easily detected with the volume expansion and thermal conductivity results). The XRD results indicate new crystalline phases such as Nepheline in the sodic medium and Leucite in the potassium medium when the samples are placed at temperature; this leads to densification of the matrices as can be seen with the mechanical resists. However, the freeze thaw cycle does not affect the properties of the products obtained. The Ashby diagram plotted with thermal conductivity and compressive strength data shows that these matrices are beneficial compared with those made from polymers, as they emit fewer VOCs. Furthermore, the foams synthesized in this work are comparable to fine ceramics (referring to their position in the Ashby diagram).