Atomistic simulation studies of the cement paste components

230 p.El cemento y sus derivados, como los morteros o el hormigón, son generalmente considerados materiales de bajo nivel tecnológico. A pesar de ser el material manufacturado más empleado en el mundo, otros como los plásticos, los metales, el algodón, la lana, la madera e incluso las piedras, se valoran más en el día a día. De hecho, el cemento es comúnmente considerado como una pasta gris, con la única característica de endurecerse cuando se seca, y que se empleada para construir edificios. Probablemente, el hecho de que sea barato, disponible, común y haya sido empleado satisfactoriamente durante siglos, contribuye a su percepción como material de bajo perfíl tecnológico. Sin embargo, esa visión se aleja de la realidad. La pasta de cemento es un compuesto complejo y heterogéneo, con diferentes características a diferentes escalas de tamaño. El mecanismo por el cual el clínker al entrar en contacto con el agua se convierte en una pasta endurecida incluye cientos de reacciones químicas y procesos físicos. El componente principal de la pasta de cemento, el gel C-S-H, es una fase amorfa con una determinada porosidad intrínseca, y su nanoestructura aún se desconoce. Curiosamente, el gel C-S-H presenta claras similitudes con otros sistemas de interés tecnológico. Por ejemplo, la estructura del gel es habitualmente descrita en términos de minerales naturales tobermorita y jennita. Estos minerales presentan una estructura laminar similar al de las arcillas montmorillonita-esmectita, que son utilizadas con objetivos catalíticos, como parte de los nano- y bio-composites, o como absorbentes de residuos contaminantes. La morfología del gel C-S-H en la microescala se parece también a la de la hidroxiapatita, que es el principal componente de los huesos. Tal semejanza proviene de su composición análoga: silicato-calcico-hidratado (C-S-H) en la matriz de cemento, y fosfato-calcico-hidratado (C-P-H) en hidroxiapatita. De hecho, tanto el gel C-S-H como la hidroxiapatita sufren un proceso de descalcificación, conocida como lixiviación de calcio en el cemento y osteoporosis en los huesos. Pero hay analogías adicionales con otros sistemas biológicos. La posición y el papel del agua en el gel C-S-H y en ciertas proteínas cristalinas son similares. Las moléculas de agua pueden estar en diferentes posiciones y asociadas con fuerzas diferentes, actuando como una parte estructural o como una solución en los poros. Estos ejemplos ilustran porque el interés de la estructura y las propiedades del gel C-S-H son comparables a los de otros materiales. La investigación en cemento incluye muchos aspectos diferentes, desde la reducción de los gases de efecto invernadero emitidos durante el proceso de fabricación, a la investigación de la nanoestructura del material, incluyendo el desarrollo de nuevos cementos que utilizan desechos como materias primas, o la modificación y mejora de las propiedades del cemento Portland ordinario. Debido a su naturaleza heterogénea, la pasta de cemento es un material multiescalar. El cemento presenta diferentes rasgos y características a escalas nano-, micro- y macro-, y su comportamiento en dichas escalas dista de ser el mismo, Además, la investigación del cemento es un campo multidisciplinar en el que están implicados ingenieros, químicos, físicos y geólogos. Ese ambiente cooperativo, así como la naturaleza de multiescalar de los problemas a estudiar, implican el uso de numerosas técnicas experimentales en la investigación del material. La evolución de las técnicas experimentales en los últimos años nos permite estudiar la pasta de cemento a escalas cada vez más pequeñas, con la apertura al cemento de un campo como la nanotecnología. En nanotecnología, los métodos de simulación atomística han demostrado ser un instrumento numérico indispensable. Estos métodos nos permiten estudiar la nanoescala de un material o molécula con gran detalle. Sin embargo, los métodos de simulación atomística apenas se han aplicado en la investigación de aspectos relacionados con el cemento. La misma complejidad que dificulta las investigaciones experimentales de los materiales en base cemento en la nanoescala, como su naturaleza amorfa y heterogénea, es un problema en la simulación atomística, ya que la posición exacta de los átomos es información necesaria para los cálculos. No obstante este problema ha sido parcialmente solucionado por el incremento de la capacidad computacional y el desarrollo de nuevas técnicas y métodos de cálculo. En esta Tesis, se han empleado métodos de simulación atomísticos para estudiar diversos aspectos de los componentes de pasta de cemento, como son sus propiedades elásticas, reactividad, estructura y formación, prestando una atención especial al gel C-S-H

Mechanisms and Dynamics of Mineral Dissolution: A New Kinetic Monte Carlo Model

Mineral dissolution is a fundamental process in geochemistry and materials science. It is controlled by the complex interplay of atomic level mechanisms like adatoms and terraces removal, pit opening, and spontaneous vacancy creation that can be gradually activated at different energies.This work was done under the umbrella of the Basque Country Initiative for Cement and Concrete Research (BASKRETE), and supported by the ELKARTEK and EMAITEK programs of the Basque Country Government. J.S.D. acknowledges the funding from the Spanish Ministry of Economy, Industry and Competitiveness (project Ref-201860I057) and the Spanish Ministry of Science, Innovation and Universities (project Ref RTI2018-098554- B-I00). P.M. acknowledges support from the Ph.D. scholarship Tecnalia Research & Innovation’s grant. All the simulations have been carried out at the high performance computing service of Basque Country i2basque

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

A comprehensive review of C-S-H empirical and computational models, their applications, and practical aspects

[EN] The C-S-H gel is an elusive material. Its variable composition and disordered nature complicate a complete characterization of its atomic structure, and the elaboration of models is key to understanding it. This work aims to review those proposed models, dividing them into empirical and computational models. After a brief description of related crystalline calcium silicate hydrates, empirical C-S-H models based on interpretation of experimental data are presented. Then, we focus on the historic development of atomistic models to study the C-S-H, until the current state of the art. We describe current computational C-S-H models built from the empirical models and computer simulations. We review common applications of these computational models: the aluminum incorporation, the elastic and mechanical properties, the diffusion of water and ions in nanopores, and C-S-H/organic composites. Finally, we discuss some practical aspects of the computational models and their interpretation, as well as possible future directions.The authors would like to acknowledge funding from “Departamento de Educación, Política Lingüística y Cultura del Gobierno Vasco” (Grant No. IT912-16 and IT1639-22) and the technical and human support provided by the Scientific Computing Service of SGIker (UPV/EHU/ ERDF, EU). E.D.-R. also acknowledges the postdoctoral fellowship from “Programa Posdoctoral de Perfeccionamiento de Personal Investigador Doctor” of the Basque Government

Water Adsorption on the β-Dicalcium Silicate Surface from DFT Simulations

beta-dicalcium silicate (beta-Ca2SiO4 or beta-C2S in cement chemistry notation) is one of the most important minerals in cement. An improvement of its hydration rate would be the key point for developing environmentally-friendly cements with lower energy consumption and CO2 emissions. However, there is a lack of fundamental understanding on the water/beta-C2S surface interactions. In this work, we aim to evaluate the water adsorption on three beta-C2S surfaces at the atomic scale using density functional theory (DFT) calculations. Our results indicate that thermodynamically favorable water adsorption takes place in several surface sites with a broad range of adsorption energies (-0.78 to -1.48 eV) depending on the particular mineral surface and adsorption site. To clarify the key factor governing the adsorption of the electronic properties of water at the surface were analyzed. The partial density of states (DOS), charge analysis, and electron density difference analyses suggest a dual interaction of water with a beta-C2S (100) surface including a nucleophilic interaction of the water oxygen lone pair with surface calcium atoms and an electrophilic interaction (hydrogen bond) of one water hydrogen with surface oxygen atoms. Despite the elucidation of the adsorption mechanism, no correlation was found between the electronic structure and the adsorption energies.National Natural Science Foundation of China (No. 51602148), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Program for Innovative Research Team in the University of Ministry of Education of China (No. IRT_15R35), the financial support from the Departamento de Educacion, Politica Linguistica y Cultura del Gobierno Vasco (IT912-16) and the ELKARTEK project

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

Relationship between Atomic Structure, Composition, and Dielectric Constant in Zr-SiO2 Glasses

[EN]Computational methods, or computer-aided material design (CAMD), used for the analysis and design of materials have a relatively long history. However, the applicability of CAMD has been limited by the scales of computational resources generally available in the past. The surge in computational power seen in recent years is enabling the applicability of CAMD to unprecedented levels. Here, we focus on the CAMD for materials critical for the continued advancement of the complementary metal oxide semiconductor (CMOS) semiconductor technology. In particular, we apply CAMD to the engineering of high-permittivity dielectric materials. We developed a Reax forcefield that includes Si, O, Zr, and H. We used this forcefield in a series of simulations to compute the static dielectric constant of silica glasses for low Zr concentration using a classical molecular dynamics approach. Our results are compared against experimental values. Not only does our work reveal numerical estimations on ZrO2-doped silica dielectrics, it also provides a foundation and demonstration of how CAMD can enable the engineering of materials of critical importance for advanced CMOS technology nodes.This research was enabled in part by support provided by Compute Canada (www.computecanada.ca). Computations were performed on the Niagara supercomputer at the SciNet HPC Consortium. SciNet is funded by the Canada Foundation for Innovation, the Government of Ontario, Ontario Research Fund.Research Excellence, and the University of Toronto

Exploring Mechanisms of Hydration and Carbonation of MgO and Mg(OH)2 in Reactive Magnesium Oxide-based Cements

Reactive magnesium oxide (MgO)-based cement (RMC) can play a key role in carbon capture processes. However, knowledge on the driving forces that control the degree of carbonation and hydration and rate of reactions in this system remains limited. In this work, density functional theory-based simulations are used to investigate the physical nature of the reactions taking place during the fabrication of RMCs under ambient conditions. Parametric indicators such as adsorption energies, charge transfer, electron localization function, adsorption/dissociation energy barriers and the mechanisms of interaction of H2O and CO2 molecules with MgO and brucite (Mg(OH)2) clusters are considered. The following hydration and carbonation interactions relevant to RMCs are evaluated i) carbonation of MgO, ii) hydration of MgO, carbonation of hydrated MgO, iii) carbonation of Mg(OH)2, iv) hydration of Mg(OH)2 and v) hydration of carbonated Mg(OH)2. A comparison of the energy barriers and reaction pathways of these mechanisms shows that the carbonation of MgO is hindered by presence of H2O molecules, while the carbonation of Mg(OH)2 is hindered by the formation of initial carbonate and hydrate layers as well as presence of excessed H2O molecules. To compare these finding to bulk mineral surfaces, the interactions of the CO2 and H2O molecules with the MgO(001) and Mg(OH)2 (001) surfaces are studied. Therefore, this work presents deep insights into the physical nature of the reactions and the mechanisms involved in hydrated magnesium carbonates production that can be beneficial for its development

ænet-PyTorch: A GPU-supported implementation for machine learning atomic potentials training

In this work, we present ænet-PyTorch, a PyTorch-based implementation for training artificial neural network-based machine learning interatomic potentials. Developed as an extension of the atomic energy network (ænet), ænet-PyTorch provides access to all the tools included in ænet for the application and usage of the potentials. The package has been designed as an alternative to the internal training capabilities of ænet, leveraging the power of graphic processing units to facilitate direct training on forces in addition to energies. This leads to a substantial reduction of the training time by one to two orders of magnitude compared to the central processing unit implementation, enabling direct training on forces for systems beyond small molecules. Here, we demonstrate the main features of ænet-PyTorch and show its performance on open databases. Our results show that training on all the force information within a dataset is not necessary, and including between 10% and 20% of the force information is sufficient to achieve optimally accurate interatomic potentials with the least computational resources.This work was supported by the “Departamento de Educación, Política Lingüística y Cultura del Gobierno Vasco” (IT1458-22), the “Ministerio de Ciencia e Innovación” (Grant No. PID2019-106644GB-I00), and the Project HPC-EUROPA3 (Grant No. INFRAIA-2016-1-730897), with the support of the EC Research Innovation Action under the H2020 Programme. The authors acknowledge technical and human support provided by SGIker (UPV/EHU/ERDF, EU) and the Duch National e-Infrastructure and the SURF Cooperative for computational resources (National Supercomputer Snellius). J.L.-Z. acknowledges financial support from the Basque Country Government (PRE_2019_1_0025). N.A. acknowledges funding from the Bayer AG Life Science Collaboration (“!AIQU”)