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

Physical Origins of Thermal Properties of Cement Paste

Despite the ever-increasing interest in multiscale porous materials, the chemophysical origin of their thermal properties at the nanoscale and its connection to the macroscale properties still remain rather obscure. In this paper, we link the atomic- and macroscopic-level thermal properties by combining tools of statistical physics and mean-field homogenization theory. We begin with analyzing the vibrational density of states of several calcium-silicate materials in the cement paste. Unlike crystalline phases, we indicate that calcium silicate hydrates (CSH) exhibit extra vibrational states at low frequencies (<2  THz) compared to the vibrational states predicted by the Debye model. This anomaly is commonly referred to as the boson peak in glass physics. In addition, the specific-heat capacity of CSH in both dry and saturated states scales linearly with the calcium-to-silicon ratio. We show that the nanoscale-confining environment of CSH decreases the apparent heat capacity of water by a factor of 4. Furthermore, full thermal conductivity tensors for all phases are calculated via the Green-Kubo formalism. We estimate the mean free path of phonons in calcium silicates to be on the order of interatomic bonds. This satisfies the scale separability condition and justifies the use of mean-field homogenization theories for upscaling purposes. Upscaling schemes yield a good estimate of the macroscopic specific-heat capacity and thermal conductivity of cement paste during the hydration process, independent of fitting parameters.Portland Cement AssociationNational Ready Mixed Concrete Association (Research and Education Foundation

Rigidity Transition in Materials: Hardness is Driven by Weak Atomic Constraints

Understanding the composition dependence of the hardness in materials is of primary importance for infrastructures and handled devices. Stimulated by the need for stronger protective screens, topological constraint theory has recently been used to predict the hardness in glasses. Herein, we report that the concept of rigidity transition can be extended to a broader range of materials than just glass. We show that hardness depends linearly on the number of angular constraints, which, compared to radial interactions, constitute the weaker ones acting between the atoms. This leads to a predictive model for hardness, generally applicable to any crystalline or glassy material

Combinatorial molecular optimization of cement hydrates

Despite its ubiquitous presence in the built environment, concrete’s molecular-level properties are only recently being explored using experimental and simulation studies. Increasing societal concerns about concrete’s environmental footprint have provided strong motivation to develop new concrete with greater specific stiffness or strength (for structures with less material). Herein, a combinatorial approach is described to optimize properties of cement hydrates. The method entails screening a computationally generated database of atomic structures of calcium-silicate-hydrate, the binding phase of concrete, against a set of three defect attributes: calcium-to-silicon ratio as compositional index and two correlation distances describing medium-range silicon-oxygen and calcium-oxygen environments. Although structural and mechanical properties correlate well with calcium-to-silicon ratio, the cross-correlation between all three defect attributes reveals an indentation modulus-to-hardness ratio extremum, analogous to identifying optimum network connectivity in glass rheology. We also comment on implications of the present findings for a novel route to optimize the nanoscale mechanical properties of cement hydrate.National Ready Mixed Concrete Association (Research sponsorship)Education Foundation (N.J.) (Research sponsorship)Portland Cement Association (Research sponsorship

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