Abstract
Geopolymers are alternative binders to cement and form a promising class of materials for civil and nuclear engineering. Based on aluminosilicate, they can develop strong mechanical properties that are of great interest for the storage of radioactive wastes or isolating foams. The geo-polymerization process starts with the dissolution of a solid aluminosilicate source with an alkaline solution of high pH. Oligomers of few nanometers in size form and aggregate in a 3D percolating porous network at the mesoscale (hundreds of nm). Mechanical setting is reached in about 5h, analogous of setting in neat cement pastes. Several questions remain open regarding the complex process of geopolymerisation such as : What originates cohesion ? What is the water behavior at the grain-grain interface within the gel ? How to model the complex gel phase at the atomistic level ? In this thesis, molecular simulations are used to investigate the formation mechanism of geopolymers at the atomistic (~1nm) and meso scale (~100nm), hardly reachable by experiment. The starting point of the atomistic scale simulations are aluminosilicate nano-grains generated with a reactive interaction potential. The Potential of Mean Force (PMF) quantifies the potential energy of interactions betweentwo geopolymer grains. It can be computed in various charge conditions to mimic the pH effects, using either aperturbative approach or well-tempered metadynamics. Both methods are compared and their limitations are discussed in the framework of amorphous aluminosilicate nano grains immersed in an electrolyte. Under such analysis, metadynamics is chosen for more thorough investigation of grains cohesion. At low deprotonation level/grain charge attraction is observed. These results shed light on the role of oligomers deprotonation that appears to be key point for the gel formation.A view of the system as “meta-grains” is proposed based on the evolution of grain geometry and ionic polarization. In this view, we decompose the global PMF into fundamental interaction terms at the grain scale, in terms of coulombic, dipole-dipole and dispersion/van der Walls interactions. Metadynamics results are recovered with water dielectric constant chosen based on the water content at the grain-grain interface. At large deprotonation level, the Poisson-Boltzmann repulsion is recovered, with a wet interface and a strong Debye screening. At very short range and low deprotonation level, we found that grain-grain attraction can be related to the formation of very local hydrogen bonds between the hydroxyls of the grains, in a dry interface. Grains then link through iono-covalent bonds with the release to the solution of water molecules and OH groups. This attracto/repulsive behavior is supported by experiments and discussed in regards to the cohesion of clays and cement hydrates.In the last part of the thesis, mesoscale models for the growth of geopolymers are proposed based on the obtained PMFs in attractive conditions. Structures resulting from Coarse-Grained Monte Carlo simulations are characterized interms of pore size distributions and small angle neutron scattering, discussed with experimental data.