MASKE: A kinetic simulator of coupled chemo-mechanical processes driving microstructural evolution

The microstructure of materials evolves through chemical reactions and mechanical stress, often strongly coupled in phenomena such as pressure solution or crystallization pressure. This article presents MASKE: a simulator to address the challenge of modelling coupled chemo-mechanical processes in microstructures. MASKE represents solid phases as agglomerations of particles whose off-lattice displacements generate mechanical stress through interaction potentials. Particle precipitation and dissolution are sampled using Kinetic Monte Carlo, with original reaction rate equations derived from Transition State Theory and featuring contributions from mechanical interactions. Molecules in solution around the solid are modelled implicitly, through concentrations that change during microstructural evolution and define the saturation indexes for user-defined chemical reactions. The structure and implementation of the software are explained first. Then, two examples on a nanocrystal of calcium hydroxide address its chemical equilibrium and its mechanical response under a range of imposed strain rates, involving stress-driven dissolution and recrystallization. These examples highlight MASKE's distinctive ability to simulate strongly coupled chemo-mechanical processes. MASKE is available, open-source, on GitHub

Creep of Bulk C--S--H: Insights from Molecular Dynamics Simulations

Understanding the physical origin of creep in calcium--silicate--hydrate (C--S--H) is of primary importance, both for fundamental and practical interest. Here, we present a new method, based on molecular dynamics simulation, allowing us to simulate the long-term visco-elastic deformations of C--S--H. Under a given shear stress, C--S--H features a gradually increasing shear strain, which follows a logarithmic law. The computed creep modulus is found to be independent of the shear stress applied and is in excellent agreement with nanoindentation measurements, as extrapolated to zero porosity

Functional Renormalisation Group for Brownian Motion I: The Effective Equations of Motion

We use the functional Renormalisation Group (fRG) to describe the in and out of equilibrium dynamics of stochastic processes, governed by an overdamped Langevin equation. Exploiting the connection between Langevin dynamics and supersymmetric quantum mechanics in imaginary time, we write down renormalisation flow equations for the effective action, approximated in terms of the Local Potential Approximation and Wavefunction Renormalisation. We derive \textit{effective equations of motion} (EEOM) from the effective action (EA) $\Gamma$ for the average position $\left\langle x\right\rangle$, variance $\langle \left(x- \langle x \rangle\right)^2\rangle$ and covariance. The fRG flow equations outlined here provide a concrete way to compute the EA and thus solve the derived EEOM. The obtained effective potential should determine directly the exact equilibrium statistics, name the position, the variance, as well as all higher order cumulants of the equilibrium Boltzmann distribution. This first paper of a two part series is mostly concerned with setting up the necessary formalism while in part two we will numerically solve the equations derived her and assess their validity both in and out of equilibrium.Comment: 11 + 4 pages, 2 Appendices. Part one of two part series. Significant changes, added discussion on non-equilibrium correlation functions, moved numerical results on equilibrium to part tw

Nanoscale shear cohesion between cement hydrates: The role of water diffusivity under structural and electrostatic confinement

[EN] The calcium silicate hydrate (C-S-H) controls most of the final properties of the cement paste, including its mechanical performance. It is agreed that the nanometer-sized building blocks that compose the C-S-H are the origin of the mechanical properties. In this work, we employ atomistic simulations to investigate the relaxation process of C-S-H nanoparticles subjected to shear stress. In particular, we study the stress relaxation by rearrangement of these nanoparticles via sliding adjacent C-S-H layers separated by a variable interfacial distance. The simulations show that the shear strength has its maximum at the bulk interlayer space, called perfect contact interface, and decreases sharply to low values for very short interfacial distances, coinciding with the transition from 2 to 3 water layers and beginning of the water flow. The evolution of the shear strength as a function of the temperature and ionic confinement confirms that the water diffusion controls the shear strength.We gratefully acknowledge the financial support by "Departamento de Educacion, Politica Linguistica y Cultura del Gobierno Vasco" (IT912-16, IT1639-22). E.D.-R. acknowledges the postdoctoral fellowship from "Programa Posdoctoral de Perfeccionamiento de Personal Investigador Doctor" of the Basque Government. The authors thank for technical and human support provided by i2basque and SGIker (UPV/EHU/ERDF, EU), for the allocation of computational resources provided by the Scientific Computing Service

Autogenous healing in cement: A kinetic Monte Carlo simulation of CaCO3 precipitation

Autogenous healing induced by the dissolution of C-S-H and CH in a cracked cement paste was modelled in this study, at the mesoscale of tens of nanometres. The pore solution contains carbon dioxide (CO2) resulting in the precipitation of calcium carbonate (CaCO3) into the crack. The simulations were performed using MASKE, a recently developed coarse-grained Kinetic Monte Carlo framework where the molecules of the solid phases are modelled as mechanically interacting particles that can also precipitate and dissolve. The precipitation of CaCO3 molecules was initially observed in tiny gel pores within the C-S-H, but eventually extends completely filling the crack. The mechanical properties of the healed system were also investigated by straining the simulation box, computing the corresponding virial stress, and plotting the resulting stress-strain relationship

Molecular model of geopolymers with increasing level of disorder in the atomic structure

Concrete is the most used building material on Earth, but the production of its main binding component, cement, is responsible for 8% of worldwide CO2 emissions. A greener alternative cementitious material is provided by geopolymers, which can be synthetized from calcined clays and industrial by-products. A key issue, that limits the applicability of geopolymers in the construction sector, is an insufficient understanding of the relationship between their chemistry and development of long-term properties. Reducing these uncertainties requires an integrated approach combining modelling and experimentation. The binding phase of geopolymers often consists of sodium-alumino-silicate-hydrates (N-A-S-H), obtained through the reaction of a sodium silicate solution with an alumino-silicate source. Theoretical models describe this structure at the molecular scale as “pseudo-crystalline” [1] but, the existing models, based on solely amorphous or crystalline structures, are not always in agreement with this definition and with experimental results. For this reason, a defective crystalline structure is proposed here as a baseline geopolymer cell, featuring both amorphous and crystalline attributes (Figure 1). This new structure is created by creating vacancies in a sodalite crystalline cage, which is then stress-relaxed and reorganised to achieve full polymerisation of Al and Si tetrahedra while respecting the Loewenstein\u27s principle. Results are compared with experimental data and with other simulation results for amorphous and crystalline molecular models, showing that the newly proposed structures better capture important structural features with impact on mechanical properties, reconciling experiments with the “pseudo-crystalline” model. Specifically, the comparison with the experiments addresses the effect of Si:Al molar ratio and water content on a range of structural and mechanical properties such as skeletal density, ring structure, bong-angle distribution, X-ray diffraction (Figure 1) and X-ray pair distribution function. The simulation results confirm the necessity of a defective structure able to detect both order and disorder in geopolymers experiments. The proposed defective molecular model provides a starting point for the multiscale understanding of geopolymer cements, as well as for investigating the molecular interactions between geopolymer cements and various adsorbates, e.g. for applications in environmental engineering and nuclear engineering. Please click Additional Files below to see the full abstract