Ameni Gharzouni

The effect of an activation solution with siliceous species on the chemical reactivity and mechanical properties of geopolymers

Precursors are critical parameters in geopolymerization mechanisms because they govern the reaction kinetics as well as the working properties of the final materials. This study focuses on the effect of alkaline solutions on geopolymer formation. Toward this end, several geopolymer samples were synthesized from the same metakaolin and various alkaline solutions. First, the solutions were characterized by thermogravimetric analysis as well as DTA–TGA, infrared spectroscopy, and MAS-NMR spectrometry. The structural evolution of the formed geopolymers was investigated using infrared spectroscopy. The measurement of mechanical strength was tested by compression. The results provide evidence of relationships between the chemical composition, the extent of depolymerization of the alkaline solutions, the kinetics of Si–O–Si bond substitution by Si–O–Al and the compressive strength. For a given aluminosilicate source, the nature and the quantity of siliceous species in the activation solution appear to lead to variation in the reactivity and, consequently, to the formation of various networks that control the kinetics of formation of geopolymers and their mechanical properties.

Determination of the polymerization degree of various alkaline solutions: Raman investigation

Alkaline silicate solutions are used for different industrial applications. Structural studies of these solutions performed by fourier transform infrared and nuclear magnetic resonance spectroscopies have shown the presence of cyclic species. However, these techniques do not allow determining the variations of chain and ring size depending on the content of alkali cations and water. The objective of this study was thus to use Raman spectroscopy to obtain additive information, such as variations of ring and chain size to evaluate the reactivity of the silicate solutions. For this, several solutions were prepared according to different preparation methods (commercial or laboratory) with different cations and Si/M ratios (Mu2009=u2009Na and/or K). The structural results obtained by Raman spectroscopy showed that the preparation method has an effect on the variation of the amount of rings with 3 and 4 tetrahedra. It has also been demonstrated that the cation had a stronger influence on the variations of these rings for commercially available solutions compared to their laboratory-made counterparts. Moreover, in the case of mixed solutions (Mu2009=u2009Nau2009+u2009K), the alkali cation influenced the formation of this type of rings. Then, the effect of the Si/M ratio on the different species was highlighted and, in particular, the formation of rings at the expense of chains. Finally, this study has demonstrated that Raman spectroscopy is a crucial technique for understanding the structure of silicate solutions and their reactivity.Graphical Abstract

Effect of the Reactivity of the Alkaline Solution and the Metakaolin on the Geopolymer Formation

The choice of precursors is a key parameter in the geopolymerization mechanism since it governs the kinetic of the reaction as well as the working properties of the final materials. This study focuses on the effect of the alkaline solution reactivity and metakaolin properties on the geopolymer formation. For this purpose, several geopolymer samples were synthesized from two alkaline solutions and three metakaolins. The structural evolution of formed geopolymers was investigated using FTIR spectroscopy and the following of the pH value during the formation. The measurement of mechanical strength was tested by compression. The results allow to evidence that for a less reactive alkaline solution, the reactivity of metakaolin governs the geopolymerization reaction. However, the alkaline solution is the steering reaction when it is highly reactive. Therefore, the extent of depolymerization of the alkaline solution and the reactivity of the metakaolin seem to control the rate of polycondensation and the mechanical properties of the geopolymer materials.