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

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

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

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”)

Normal and Anomalous Self-Healing Mechanism of Crystalline Calcium Silicate Hydrates

The origin of different stability of crystalline calcium silicate hydrates was investigated. The tobermorite crystal has been used as an analog of cement hydrate that is being mostly manufactured material on earth. Normal tobermorite is thermally unstable and transforms to amorphous at low pressure. Meanwhile, anomalous tobermorite with high Al content does not significantly transform under high pressure or high temperature. Conducted X-ray absorption spectroscopy explains the weak stability of normal tobermorite which was originally hypothesized by the role of zeolitic Ca ions in the cavities of silicate chains. Atomic simulations reproduced the experimentally observed trend of pressure behavior once the ideal structures were modified to account for the Al content as well as the chain defects. The simulations also suggested that the stability of tobermorite under stress could be rationalized as a self-healing mechanism in which the structural instabilities were accommodated by a global sliding of the CaO layer.J.M. acknowledges support by a grant (20SCIP-C159063-01) from Construction Technology Research Program funded by Ministry of Land, Infrastructure and Transport of Korean government. H.M. acknowledges the financial support from the Gobierno Vasco (project IT912-16). The work in San Sebasti ' an (R.D., J.S.D, A.A) was supported by the Spanish Ministry of Science and Innovation with RTI2018-098554-B-I00, PID2019-105488GB-I00 and PCI2019-103657 research grants, the Gobierno Vasco UPV/EHU (Project No. IT-1246-19), and the European Commission from the NRG-STORAGE project (GA 870114). The Institute of Engineering Research in Seoul National University provided research facilities for this work. The Ca-XAS and HPXRD experiments were performed at XAFCA beamline in Singapore Synchrotron Light Source (SSLS) and 12.2.2 beamline in Advanced Light Source (ALS), respectively. The ALS supported by a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231 and the Consortium for Materials Properties Research in Earth Sciences under NSF Cooperative Agreement EAR 1606856. The authors thank Prof. Simon M. Clark, Dr. Yonghua Du, and Dr. Shibo Xi for helpful discussions and beamline experimental supports

Synergistic theoretical and experimental study on the ion dynamics of bis(trifluoromethanesulfonyl)imide-based alkali metal salts for solid polymer electrolytes

Model validation of a well-known class of solid polymer electrolyte (SPE) is utilized to predict the ionic structure and ion dynamics of alternative alkali metal ions, leading to advancements in Na-, K-, and Cs-based SPEs for solid-state alkali metal batteries. A comprehensive study based on molecular dynamics (MD) is conducted to simulate ion coordination and the ion transport properties of poly(ethylene oxide) (PEO) with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt across various LiTFSI concentrations. Through validation of the MD simulation results with experimental techniques, we gain a deeper understanding of the ionic structure and dynamics in the PEO/LiTFSI system. This computational approach is then extended to predict ion coordination and transport properties of alternative alkali metal ions. The ionic structure in PEO/LiTFSI is significantly influenced by the LiTFSI concentration, resulting in different lithium-ion transport mechanisms for highly concentrated or diluted systems. Substituting lithium with sodium, potassium, and cesium reveals a weaker cation-PEO coordination for the larger cesium-ion. However, sodium-ion based SPEs exhibit the highest cation transport number, indicating the crucial interplay between salt dissociation and cation-PEO coordination for achieving optimal performance in alkali metal SPEs.The research was supported by funding as a part of the DESTINY PhD program, funded by the European Union's Horizon2020 research and innovation program under the Marie Skłodowska-Curie Actions COFUND (Grant No. 945357), and funding through the Basque Government PhD Grant. The authors also acknowledge funding from ‘Departamento de Educación, Política Lingüística y Cultura del Gobierno Vasco’ (Grant No. IT1358-22), the Basque Government (PRE_2022_1_0034), and thank SGI/IZO-SGIker UPV/EHU for providing supercomputing resources

Molecular-Level Insight into Charge Carrier Transport and Speciation in Solid Polymer Electrolytes by Chemically Tuning Both Polymer and Lithium Salt

The advent of Li-metal batteries has seen progress toward studies focused on the chemical modification of solid polymer electrolytes, involving tuning either polymer or Li salt properties to enhance the overall cell performance. This study encompasses chemically modifying simultaneously both polymer matrix and lithium salt by assessing ion coordination environments, ion transport mechanisms, and molecular speciation. First, commercially used lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt is taken as a reference, where F atoms become partially substituted by one or two H atoms in the −CF3 moieties of LiTFSI. These substitutions lead to the formation of lithium(difluoromethanesulfonyl)(trifluoromethanesulfonyl)imide (LiDFTFSI) and lithium bis(difluoromethanesulfonyl)imide (LiDFSI) salts. Both lithium salts promote anion immobilization and increase the lithium transference number. Second, we show that exchanging archetypal poly(ethylene oxide) (PEO) with poly(ε-caprolactone) (PCL) significantly changes charge carrier speciation. Studying the ionic structures of these polymer/Li salt combinations (LiTFSI, LiDFTFSI or LiDFSI with PEO or PCL) by combining molecular dynamics simulations and a range of experimental techniques, we provide atomistic insights to understand the solvation structure and synergistic effects that impact macroscopic properties, such as Li+ conductivity and transference number.The authors acknowledge support from the European Commission grant for Erasmus Mundus Joint Master’s Degree MESC+ under Framework Agreement Number 2018-1424/001-001-EMJMD, the EU Marie Sklodowska-Curie COFUND DESTINY project under Grant Agreement No. 945357, and the Basque Government PhD Grant. H.M. acknowledges funding from the “Departamento de Educación, Política Lingüística y Cultura del Gobierno Vasco” (Grant IT1358-22). They also thank SGI/IZO-SGIker UPV/EHU for supercomputing resources

A potential C-S-H nucleation mechanism: atomistic simulations of the portlandite to C-S-H transformation

The nucleation of the C-S-H gel is a complex process, key to controlling the hydration kinetics and microstructure development of cement. In this paper, a mechanism for the crystallization step during the C-S-H gel non-classical nucleation is proposed and explored by atomistic simulation methods. In the proposed mechanism portlandite precursor monolayers undergo a chemically induced transformation by condensation of silicate dimmers, forming C-S-H monolayers. We studied by DFT and nudged elastic band the structural transformation from bulk portlandite to a tobermorite-like calcium hydroxide polymorph, and the silicate condensation reaction at portlandite surface. Then, both processes are studied together, investigating the topochemical transformation from a portlandite monolayer to a C-S-H monolayer at room conditions using targeted molecular dynamics and umbrella sampling methods. Comparing the free energy of the process with thermodynamic data we conclude that the proposed mechanism is a potential path for C-S-H formation.This work was supported by the "Departamento de Educacion, Politica Lingueistica y Cultura del Gobierno Vasco'' (IT1458-22) and the "Ministerio de Ciencia e Innovacion"(PID2019-106644GB-I00). The authors thank for technical and human support provided by SGIker (UPV/EHU/ERDF, EU). X.M.A. acknowledges the financial support from the University of the Basque Country, UPV/EHU (PIF17/118), and J.L.-Z. the financial support from the Basque Country Government (PRE_2019_1_0025)