Dynamics of Alumino-Silicates Fluids – DYNAMISTE

This project relies on a synergic approach coupling both experiments and modelling. In order to access all the spatio-temporal phenomena of such systems, the experimental part consists in studies based on (i) rheology techniques coupled with scattering techniques: Dynamic light scattering (DLS), small and wide angle x-ray and (or) neutron scattering and diffraction (SWAXS, SANS and XRD), and on (ii) multiscale NMR approaches (from Å to few tens µm). At the same time, the theoretical part is based on multiscale methods coupling molecular dynamics and coarse-grained simulations, allowing for accessing the structural and dynamical properties of these fluids at the microscopic and mesoscopic scales.

The characterization of the solutions (content of alkali, alkali silicates, and sodium aluminates) has been carried out by x-ray scattering techniques (WAXS) and RAMAN spectroscopy. A UV-Visible spectrocolorimetry study has been set up to determine the free hydroxide content as a function of the formulation parameters, and in particular the alkali content. The results have shown that the more the alkali content increases the more the silicates depolymerized which implies that the silicates become more reactive with respect to the aluminate solution. At the same time, the gelation kinetics processes have been studied by rheology and SAXS on Synchrotron. The results pointed out that the reaction kinetics is very rapid. In addition, rheology studies on the influence of the aluminium content and the temperature enlightened that the kinetics processes were fast and the strength of the network increased as the aluminium content increased. On the other hand, the temperature only changes the setting time of the gel without affecting the structure and the strength of the network formed.
At the same time, classical molecular dynamics (MD) simulations have been performed for aqueous solutions of alkali hydroxide (Li+, Na+, K+, and Cs+) at different concentrations. We calculated the theoretical SAXS spectra and compared them to the experimental ones for NaOH. This study allows for validating the polarizable force field used for describing OH- ions by classical molecular dynamics. Furthermore, MD simulations of solutions containing different silicate oligomers (SiOH4, SiO(OH)3 and Si2O(OH)5-) at different concentrations are currently in progress. The first results pointed out that the oligomers present in solution tend to form aggregates having different sizes similar to those observed experimentally, and this only by ionic bonding and van der Waals interactions.

Following the experimental studies performed on the gelling kinetics processes by rheology and SAXS (Synchrotron), we will focus on the aluminium content effect and the nature of the alkali ion on the kinetics and the structure of the gels formed at the mesoscopic scale by SAXS experiments. Therefore, a SAXS campaign at the Synchrotron Soleil is under progress. At the same time, the first multi-scale NMR studies that are currently under progress, will allow for obtaining information about the nature of the interactions involved between the species present in solution and the effect of temperature on the stability of the solutions.
From a theoretical point of view, in order to better understand the influence of the nature of the alkali cation on the structure of the solutions, we will focus on the calculation and the comparison of the theoretical SAXS spectra obtained from molecular dynamics simulations for KOH and CsOH aqueous solutions. Moreover, in order to quantify the ratio of the different aggregates of silicates observed in solution by molecular dynamics, we plan to predict the NMR spectra from our simulations and compare them to the experimental NMR data obtained in this project.
The experimental and theoretical developments in this project and applied in the context of the soil stabilisation will be transferable and adaptable to further important industrial application systems, such as dispersion of clays, geopolymer binders, ecological mineral paints and concrete acceleration.

The first experimental results obtained from rheology have been the subject of an oral communication at the US SOR Congress in Tampa (FL, USA) in February 2017.

The project DYNAMISTE aims at developing experimental and theoretical tools in order to optimise the industrial processes in which alkali activation solutions of aluminosilicates are involved such as liquefaction of ceramics, binders for mineral paints, refractory mortars or soil stabilization among others and in an attempt of developing sustainable and clean industry. This project gathers three academic laboratories recognized for their expertise in the physical chemistry of condensed matter, the Institut de Chimie Séparative de Marcoule with competency in “green chemistry” science (ICSM – UMR 5257), a CEA department for the waste retreatment and conditioning (CEA/DEN/DTCD Marcoule) and specialist for the characterization and formulation of cement-based materials, and a CNRS team at Ecole Polytechnique expert in the physics of irregularities (CNRS-PMC – UMR 7643), in collaboration with the German industrial partner Wöllner GmbH & Co.KG, who is one of the leaders in production of alkali silicate solutions.
Thanks to their environmental acceptability and their adaptability over a wide range of applications, alkali solutions of aluminosilicates are increasingly used. During the last 10 years, they have indeed increased their important role as inorganic and water based binders, notably for the production of mineral based, ecological materials for the building and construction industry. Especially during the last two years alkali-silicate solutions became more and more important for the alkali activation in geopolymer application, which is considered as green chemistry. Another important application is the use of sodium aluminosilicate gel for the so-called “ground stabilisation” and as “sealing layers” in order to avoid the inflow of groundwater in construction pits or the reinforcement of sandy ground. Although such solutions are increasingly used in the industry, there remain outstanding questions regarding their stabilities, and more precisely concerning the gelation process that is driven by the composition of the solution. It is therefore crucial to provide a clear and realistic description of such fluids during the gelation process, which remains quite not well known, and has to be confirmed experimentally and theoretically.
So, this project relies on a synergic approach coupling both experiments and modelling. In order to access all the spatio-temporal phenomena of such systems, the experimental part will consist in studies based on (i) rheology techniques coupled with scattering techniques: Dynamic light scattering (DLS), small and wide angle x-ray and (or) neutron scattering and diffraction (SWAXS, SANS and XRD), and on (ii) multiscale NMR approaches (from Å to few tens µm). At the same time, the theoretical part will be based on multi-scale methods coupling molecular dynamics and coarse-grained simulations, allowing for accessing the structural and dynamical properties of these fluids at the microscopic and mesoscopic scales.
The experimental and theoretical developments in this project and applied in the context of the soil stabilisation will be transferable and adaptable to further important industrial application systems, such as dispersion of clays, geopolymer binders, ecological mineral paints and concrete acceleration.