Unravelling CSH atomic structure via computational and experimental physical chemistry

Calcium Silicate Hydrate (CSH) is the main binding phase for the cement paste, which is responsible for its strength and creep behavior. This is a nonstoichiometric hydration phase with calcium to silicon ratio (C/S) ranging from 1 to 2.2. At low C/S ratios, the molecular structure of CSH resembles to that of Tobermorite minerals, whereas in high C/S ratios it mostly looks like disordered glasses. By taking advantage of tools of statistical physics, it is shown that CSH at a given C/S can be associated with degenerate molecular structures called CSH polymorphs. Polymorphs are energetically competitive, i.e., they have the same free energy content, which means they can coexist under equilibrium conditions. To start, SiO2 groups are randomly removed from the layered atomic structure Tobermorite 11A. One hundred and fifty structures are created. Grand Canonical Monte Carlo simulation of water adsorption is performed to adsorb water in the interlayer spacing and nanoscale porosities in defected CSH structures. The amount of adsorbed water scales linearly with the number of defects in the calcium–silicate layer. Samples are relaxed using a reactive potential in canonical and isothermal–isobaric ensembles. We observe that the confined water reacts with the free interlayer calcium atoms and nonbridging oxygen to form hydroxyl groups. The number of hydroxyl groups scales linearly with the amount of defects. The amount of water in CSH and Ca‑OH content match well with drying and Neutron Scattering experiment. Although the reactive modeling of CSH impacts the water molecules in CSH’s nanoconfinement environment, it does not significantly affect the silica chain length. This means that the reactive atomistic modeling does not affect the calico-silicate backbone of CSH structures. The silica mean chain length from atomistic simulation aligns perfectly with experimental NMR data. The elastic properties and hardness of all CSH polymorphs are measured at a given C/S and are directly compared with nano-chemo-mechanical testing via coupled nanoindentation and X-ray WDS. Atomistic simulation matches with the experimental data in both elastic and plastic regimes. The correlation of mechanical properties to structural observables of the molecular structures such as dimer content, mean silicate chain length, density, basal distance, water content, number of hydroxyl groups, and topological constraints parameter are calculated. No direct correlations were found at short ranges. The search was extended to the medium range order analysis and it is found that the polymorphism is closely related to the medium range order of Si‑O bonds

Bottom-up analysis of infrastructure materials and systems

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2015.Cataloged from PDF version of thesis.Includes bibliographical references (pages 261-303).Civil infrastructure is and continues to be the backbone of our society to meet our needs in housing, transportation, water and electricity supply, and so on. However, its functions are recently revisited in response to rising concerns about its certain sustainability aspects. These aspects include and are not limited to excessive greenhouse gas emissions, unreasonably high energy footprint, relatively short service life, low durability and poor resilience. This presents us with an exclusive opportunity to take these detrimental aspects seriously and turn them into exciting venues for research in the realm of civil and environmental engineering. These opportunities are disseminated across the entire infrastructure landscape, spanning several length scales starting from the molecular structure of construction materials to the entire global transportation network. From Atoms to Cities is intended to provide a multiscale bottom-up framework to seamlessly connect the heat transport through the molecular structure of construction materials to thermal energy losses at the city level. Separated by twelve orders of magnitude in length scales, from nanometers to kilometers, this provides a chance to link ideas in mechanics and physics of materials to analysis of complex systems. Two major impediments hinder any progress in pursuit of such a hierarchical multiscale model: the absence of a realistic molecular structure of construction materials such as Calcium-Silicate-Hydrates (C-S-H), the glue of concrete, and the multiplicity of factors affecting heat losses at large scales. The first is an indispensable requirement in statistical mechanics as it shapes the energy landscape. The second makes it rather impossible to quantitatively assess the impact of sustainability initiatives at the city scale. By combining the tool of statistical physics with combinatorial screening technique, we first construct a database of realistic molecular structures of C-S-H with varying calcium-to-silicon ratios and compare them against an extensive array of nanotextural and nano-mechanical experiments. A comprehensive analysis of this database reveals a deeper level of connection between cement science and glass physics. This includes the existence of anomalies in mechanical properties similar to that observed in rigidity transition windows in binary glasses and the presence of extra atomic vibrational modes at low THz regime known as Boson peak. These models are further utilized to calculate the heat capacity and transport properties using Green-Kubo formalism in equilibrium molecular dynamics. While considering other phases in cement paste, the use of mean-field homogenization technique enables us to upscale thermal properties from the nanoscale to the engineering macroscale. The macro-level thermal properties are subsequently compared with those measured experimentally throughout the cement's hydration process. Afterwards, we show that the building envelope's heat transport property is among the set a few influential parameters that affect heat losses at the city scale. This subset of key parameters makes it feasible to construct a high fidelity mechanistic-based reduced order model of heat losses at the building level. Together with energy consumption data of more than 6,200 buildings in Cambridge, MA, this model paves the way to find the shortest path to reduce heat losses in city's building block through retrofit.by Mohammad Javad Abdolhosseini Qomi.Ph. D

Research Brief: A Resilience Assessment of Structures Using Molecular Dynamics

Between 1993-ˇ2012, more than 75% of catastrophic losses in the United States were caused by windstorms (1). The Congressional Budget Office estimated an average annual damage amounting to $28 billion (0.16 percent of GDP), with a potential rise to$38 billion by 2075 – 55% of which is attributed to coastal development (2). This economic impact of wind related events calls for reevaluation of engineering approaches. Traditional structural mechanics approaches evaluate wind damage of structural elements (i.e. beams, plates, walls) in relation to a design code, while not accounting for the contribution of non-ˇstructural elements (i.e. sheathings, windows) which clearly reflect on building integrity (3). While more detailed frameworks accounting for all elements do exist, such as the Federal Emergency Management Agency’s HAZUS-ˇMH (FEMA 2016), they are only limited to specific building types and qualitative damage description (i.e. slight, moderate, extensive damage, and so on) (4). This motivates the development of an approach that can quantitatively address the complexity of buildings in element scale (i.e. structural/non-ˇstructural elements), and system scale (accounting for any building use and geometry)

Reactive force fields for modeling oxidative degradation of organic matter in geological formations.

In an attempt to better explore organic matter reaction and properties, at depth, to oxidative fluid additives, we have developed a new ReaxFF potential to model and describe the oxidative decompositions of aliphatic and aromatic hydrocarbons in the presence of the oxychlorine ClO n - oxidizers. By carefully adjusting the new H/C/O/Cl parameters, we show that the potential energies in both training and validation sets correlate well with calculated density functional theory (DFT) energies. Our parametrization yields a reliable empirical reactive force field with an RMS error of ∼1.57 eV, corresponding to a 1.70% average error. At this accuracy level, the reactive force field provides a reliable atomic-level picture of thermodynamically favorable reaction pathways governing oxidative degradation of H/C/O/Cl compounds. We demonstrate this capability by studying the structural degradation of small aromatic and aliphatic hydrocarbons in the presence of oxychlorine oxidizers in aqueous environments. We envision that such reactive force fields will be critical in understanding the oxidation processes of organic matter in geological reservoirs and the design of the next generation of reactive fluids for enhanced shale gas recovery and improved carbon dioxide adsorption and sequestration

Advances in atomistic modeling and understanding of drying shrinkage in cementitious materials

International audienc

Intermolecular Forces between Nanolayers of Crystalline Calcium-Silicate-Hydrates in Aqueous Medium

Calcium-silicate-hydrate (C–S–H) is the major binding phase responsible for strength and durability of cementitious materials. The cohesive properties of C–S–H are directly related to the intermolecular forces between its layers at the nanoscale. Here, we employ free energy perturbation theory (FEP) to calculate intermolecular forces between crystalline C–S–H layers solvated in aqueous medium along face-to-face (FTF) and sliding reaction coordinates. Contrary to mean-field theories, we find that our counterion-only system exhibits an oscillatory behavior in FTF interaction. We correlate these oscillations with the characteristic length scale comparable to the distance between interfacial water layers at the hydrophilic surface of crystalline C–S–H. We attribute the sliding intermolecular forces to the atomic level roughness of crystalline C–S–H layers stemming from the local arrangement of nanoscale structural motifs. These intermolecular forces provide a direct access to the key mechanical properties, such as surface energy, cohesive pressure and elastic properties. The simulation results are in close agreement with the available experimental measurements. Furthermore, we present these intermolecular forces in a mathematical framework to facilitate coarse-grain modeling of crystalline C–S–H layers. These results provide a novel route that paves the way for developing realistic mesoscale models to explore the origins of chemophysical properties of crystalline C–S–H