Molecular dynamics investigation of the mechanical, thermal and surface properties of tricalcium silicate and its early hydration

• Autor(a) principal: Claverie, Jérôme
• Data de Publicação: 2019
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Repositório Institucional da UNESP
• Texto Completo: http://hdl.handle.net/11449/191301
• topic: Tricalcium silicate; Hydration; Molecular dynamics; Cleavage energy; Silicato tricálcico; Hidratação; Dinâmica molecular; Energia de clivagem; Interface

The energetical and environmental problematic related to the cement production is a very sensitive issue. As every other field, the construction industry must go through drastic change in the design of concrete and cement-based materials. The understanding of the physical and chemical properties of the Portland cement (PC) clinker is important to improve its design. Tricalcium silicate C3S, or alite, is the main phase of PC clinker and has been largely studied since it is the first responsible for the strength development of the cement paste. On the other hand, the development of computational methods at the molecular scale has made possible the modelling of structural, dynamical and energetic properties, sometimes hardly measurable by experimental means. Such methods are relatively new in the field of cement chemistry, but have been increasingly employed over the last 15 years. In this project, density functional theory (DFT), classical molecular dynamics (MD), and ab initio molecular dynamics (AIMD) are employed towards a better understanding of mechanical, thermal, and superficial properties of monoclinic C3S, as well as C3S/water interface features. The present thesis consists of five chapters. The first chapter presents a review of the literature on the chemistry of cement, and more particularly on the hydration process modeling. The various phases which compose the Portland cement clinker are introduced, then the the different polymorphs of C3S are described, in particular the M1 and M3 forms, which are studied in this thesis. Then, several models of hydration are discussed, from the first single particle models, to the last attempts of investigations at the atomic scale. The second chapter provides an overview of the fundamental principles related to atomistic simulations. It describes the fundamentals of calculations based on the electronic density in many particle systems. The Density Functional Theory (DFT) revolutionized the study of these systems since it considers the electronic density as only calculation variable, greatly reducing the calculation time. The classical molecular mechanics is then introduced, with the notion of fields of empirical forces to describe interatomic forces. Then, the calculation methods based on the minimization of energy are explained, as well as the molecular dynamics (MD), where the integration of Newton's equation of motion allows to simulate a system at finite temperature. The principles of \emph{ab initio} molecular dynamics (AIMD), based on calculations of interatomic forces by DFT, are explained. Then some concepts of statistical mechanics, important in MD, are introduced. Finally, different force fields, already used to describe the C3S/water interface and the initial hydration of \ce {C3S} are reviewed. The third chapter presents the results of calculations of cleavage energy, and of mechanical and thermal properties of C3S M1 and M3. The elastic constants are firstly calculated from the stiffness matrices, using the Voigt-Reuss-Hill homogenization method. These calculations are made by a static method, minimizing the energy of the unit cell at each deformation step. In addition, the calculation of the bulk modulus is performed by equilibrium molecular dynamics (EMD) under different hydrostatic pressure. The stress-strain curves are also obtained by non-equilibrium MD (EMD), applying a continuous deformation during the dynamics of the system. The elastic constants are also deduced from these calculations. The specific heat is determined by a direct method, from the rate of change of enthalpy as a function of the temperature of the system, constant pressure. The expansion coefficients are also calculated according to the variation of the volume with respect to the temperature under the same conditions. A more accurate method, based on the density of phonon states (DOS) is employed. DOS are calculated from the Fourier transform of the autocorrelation function of atomic velocities. In general, the calculated mechanical and thermal properties were in good agreement with the experimental measurements. Finally, the cleavage energies of C3S M1 and M3 were calculated from the difference in energy of unified and cleaved systems. The superficial ions are relaxed to their minimum energy configuration applying steep temperature gradients. The equilibrium shapes of the M1 and M3 crystals are constructed from the energies calculated, employing the Wulff construction method, which is based on the minimization of free energy during crystal growth. The fourth chapter presents a study by MD of the behavior of interfacial water as a function of the degree of hydration of the (040) surface of the \ce{M3 C3S}, with a hydration model including the typical range of pH values of cement solutions. The interface energy is calculated according to the hydration degree of the surface. The structure and dynamics of the water molecules at the interface are evaluated by analysis of the obtained trajectories. Atomic densities at the interface, as well as the different binding modes of water molecules on the surface are highlighted. The orientation of water molecules and coordination spheres of the different chemical species present at the interface are analyzed. The dynamics of water molecules is quantified from the diffusion coefficient as a function of their distance from the C3S surface. Analyses also make possible the quantification of the number and the type of hydrogen bond at the interface, as well as their life time. It is observed that, during hydration, the behavior of the water at the interface changes radically. The hydrogen bond network existing between the water molecules in contact with the anhydrous C3S decomposes upon protonation of oxide ions and silicates. The fifth chapter presents an AIMD study of proton transfers at the C3S/water interface, on the same surface and the same polymorph as previously. Three types of hydroxyl groups are analyzed: hydroxides formed on oxide ions, hydroxyls in silanol groups and hydroxides resulting from the dissociation of water molecules. Hydroxides formed on oxide ions are very stable. Conversely, the number of two other types fluctuate according to proton transfers. These transfers have been quantified in terms of frequency and energy barrier. Furthermore, the importance of the environment of the superficial oxide ions on their protonation, this parameter was not considered in the previously used model, which only account for the pKa of hydroxide and silicic acid in solution. In addition, analyses show that the orientation of water molecules on the surface greatly influenced by its topology. Electron density analysis allows to highlight regions of abundance depletion of electrons due to the adsorption of water molecules, and occurring during proton exchanges. The size of these regions around hydroxyl groups is a function of the stability of the group. Generally speaking, the results obtained in this thesis allow for a better understanding of the behavior of C3S at the atomic scale and its early hydration, occurring systematically even before mixing with water.