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Etude des mécanismes de dissolution/polycondensation lors de la géopolymérisation : réactivité du métakaolin et influence de la solution d'activation

Abstract : Geopolymers are alternative aluminosilicates binders prepared from metakaolin and alkaline silicate solutions. However, the origin of their setting through a massive polycondensation of aluminosilicates remains unclear. The influence of hydroxide ions and of the stoichiometry Si-Al-alkali in solution has to be clarified. But these species are also implicated in the dissolution of metakaolin. First, interactions between hydroxides and silicates in the activating solution were expressed by means of the Hammett function H- to quantify free hydroxides, and of the buffering capacity β to quantify hydroxide bonded to silicates. Then the dissolution was dissociated from the polycondensation by progressively diluting the metakaolin mixed with the solution. The dissolution was then monitored by microcalorimetry and static 27Al NMR. As a complement, the geopolymerization was described as occurring in several steps using rheometry, conductimetry, SAXS and QENS. Third, a microcalorimetry-NMR correlation exhibited that the dissolution pathway remained unchanged whatever the initial silicate and sodium concentrations and the nature of the alkali used, thus whatever the initial H- and β. On the contrary, the higher those parameters, the higher the dissolution rate. The rheological setting time of geopolymers is also directly dependent on the dissolution rate. The suggested mechanisms involves that the initially bonded hydroxides in the activating solution, expressed by β, are progressively released in solution according to a dissolution-condensation loop process.
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Julien Aupoil. Etude des mécanismes de dissolution/polycondensation lors de la géopolymérisation : réactivité du métakaolin et influence de la solution d'activation. Matériaux. Université Paris sciences et lettres, 2019. Français. ⟨NNT : 2019PSLET014⟩. ⟨tel-02863870⟩

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