Nanoscale origins of creep in calcium silicate hydrates

Nanoscale Prediction of Physical Water Reducer: Lubricating the Cement System by the Electric Field

Fluidity is a critical property of cement that significantly impacts the performance of cement paste in construction engineering. Fluidity is typically enhanced through the application of chemical additives (e.g., water-reducing agents). While chemical additives can enhance the fluidity and workability of cement, their drawbacks, such as cost and environmental impact, must be carefully considered. Most of the current research focuses on the use of chemical admixtures, while studies on physical alternatives remain limited. This study employs molecular dynamics (MD) simulation to propose an innovative strategy for improving the fluidity of cement slurry by applying an electric field, which acts as a physical water reducer. This research investigates the lubricating effect and underlying mechanism of the electric field on cement hydration product C-S-H particles at the nanoscale. This work demonstrates that increasing the electric field strength significantly reduces friction between cement particles, thereby improving fluidity when ions are present at the particle interface. Atomic-level structural analyses reveal that the electric field promotes a denser C-S-H structure and facilitates ion desorption from the C-S-H surface, which acts as a lubricant between particles. This study provides new insights into how an electric field can serve as a lubricant in cement systems, offering a promising approach to enhancing concrete fluidity without relying on chemical admixtures.

The Effect of Relative Humidity on Creep Behavior of Cement Paste Microprism

Despite decades of extensive studies, the mechanism of concrete creep remains a subject of debate, mainly due to the complex nature of cement microstructure. This complexity is further amplified by the interplay between water and the cement microstructure. The present study aimed to better understand the creep mechanism through creep tests on microprisms of cement paste at hygral equilibrium. First, microprisms with dimensions of 150 mm × 150 mm × 300 mm were prepared by precision cutting from a cement paste specimen with a water-to-cement ratio of 0.4. Subsequently, uniaxial compression and creep tests were carried out on these microprisms in a chamber with controlled relative humidity (RH). To mitigate the impact of plasticity and damage, the applied peak load was set to generate a stress level that was approximately 40% of the compressive strength. Moreover, an analytical coefficient φ was formulated to account for the foundation effect on microprism creep, agreeing with the numerical analysis employing the finite element method. Our findings showed that the microscale creep compliance varied when the RH level was changed from 90% to 11%. Furthermore, logarithmic and power-law models were both applied to simulate creep curves. Lastly, the modeled creep behaviors were compared with those obtained by microindentation experiments in previous studies.

Molecular simulation of the confined crystallization of ice in cement nanopore

Freezing of water under nanoconfinement exhibits physical peculiarities with respect to the bulk water. However, experimental observations are extremely challenging at this scale, which limits our understanding of the effect of confinement on water properties upon freezing. In this study, we use molecular dynamic simulations to investigate how confinement affects the kinetics of growth of ice and the thermodynamic equilibrium of ice-liquid coexistence. TIP4P/Ice water model and CSH-FF model were applied to simulate ice crystallization in a confined cement system at temperatures down to 220 K. We adapted an interface detection algorithm and reparameterized the CHILL/CHILL+ algorithm to capture ice growth. The confinement leads to a shift of the maximum growth rate of ice to a higher temperature than for bulk water. Both the confinement and surface impurities contribute to slowing down the ice growth. For the ice-liquid coexistence at equilibrium, we derive a formulation of Thomson's equation adapted to statistical physics quantities accessible by molecular simulation, and we show that this adapted equation predicts accurately the melting line of bulk and confined ice Ih as a function of pressure. The confinement decreases systematically the melting temperature of ice of about 5 K compared with bulk ice Ih. A premelted water film about 1 nm thick is observed between the solid wall and ice, and its thickness is found to decrease continuously as temperature is lowered. We note that the surface impurities are key to the formation of the premelted water nanofilm when the temperature is lower than 250 K.

Nanoindentation Study of Calcium-Silicate-Hydrate Gel via Molecular Dynamics Simulations

The mechanical properties of calcium-silicate-hydrate (C-S-H) gels in cementitious materials are mainly realized by nanoindentation experiments. There is limited research on the dynamic response of the molecular structure of C-S-H under nanoindentation conditions. This study simulated the nanoindentation on the C-S-H gel samples by the molecular dynamics method considering the essential factors of modeling and loading process. The results demonstrate that the averaged elastic moduli we obtained had slight differences from those by experiments. In contrast to the experimental results, the gels showed bi-modulus and transverse isotropic with the material principal direction perpendicular to the C-S-H layers. The modulus in a direction increased with the loading speed, which indicates that C-S-H behaves viscous due to the water motion in the sample and the propagation of stress wave. The saturation of water influenced the moduli differently because more water in C-S-H will reduce the polymerization of silicon chains and then weaken the local stiffness. The conclusions provide a deeper understanding of the mechanism on the unique mechanical response of C-S-H gels.

Multi-layered cement-hydrogel composite with high toughness, low thermal conductivity, and self-healing capability

The inherent quasi-brittleness of cement-based materials, due to the disorder of their hydration products and pore structures, present significant challenges for directional matrix toughening. In this work, a rigid layered skeleton of cement slurry was prepared using a simplified ice-template method, and subsequently flexible polyvinyl alcohol hydrogel was introduced into the unidirectional pores between neighboring cement platelets, resulting in the formation of a multi-layered cement-based composite. A toughness improvement of over 175 times is achieved by the implantation of such hard-soft alternatively layered microstructure. The toughening mechanism is the stretching of hydrogels at the nano-scale and deflections of micro-cracks at the interfaces, which avoid stress concentration and dissipate huge energy. Furthermore, this cement-hydrogel composite also exhibits a low thermal conductivity (around 1/10 of normal cement) and density, high specific strength and self-healing properties, which can be used in thermal insulation, seismic high-rise buildings and long-span bridges.

The Analysis of WJ Distribution as an Extended Gaussian Function: Case Study

The double exponential WJ distribution has been shown to competently describe extreme events and critical phenomena, while the Gaussian function has celebrated rich applications in many other fields. Here we present the analysis that the WJ distribution may be properly treated as an extended Gaussian function. Based on the Taylor expansion, we propose three methods to formulate the WJ distribution in the form of Gaussian functions, with Method I and Method III being accurate and self-consistent, and elaborate the relationship among the parameters of the functions. Moreover, we derive the parameter scaling formula of the WJ distribution to express a general Gaussian function, with the work illustrated by a classical case of extreme events and critical phenomena and application to topical medical image processing to prove the effectiveness of the WJ distribution rather than the Gaussian function. Our results sturdily advocate that the WJ distribution can elegantly represent a Gaussian function of arbitrary parameters, whereas the latter usually is not able to satisfactorily describe the former except for specific parameter sets. Thus, it is conclusive that the WJ distribution offers applicability in extreme events and critical phenomena as well as processes describable by the Gaussian function, namely, implying plausibly a unifying approach to the pertinent data processing of those quite distinct areas and establishing a link between relevant extreme value theories and Gaussian processes.

Interactions between Reduced Graphene Oxide with Monomers of (Calcium) Silicate Hydrates: A First-Principles Study

Graphene is a two-dimensional material, with exceptional mechanical, electrical, and thermal properties. Graphene-based materials are, therefore, excellent candidates for use in nanocomposites. We investigated reduced graphene oxide (rGO), which is produced easily by oxidizing and exfoliating graphite in calcium silicate hydrate (CSHs) composites, for use in cementitious materials. The density functional theory was used to study the binding of moieties, on the rGO surface (e.g., hydroxyl-OH/rGO and epoxide/rGO groups), to CSH units, such as silicate tetrahedra, calcium ions, and OH groups. The simulations indicate complex interactions between OH/rGO and silicate tetrahedra, involving condensation reactions and selective repairing of the rGO lattice to reform pristine graphene. The condensation reactions even occurred in the presence of calcium ions and hydroxyl groups. In contrast, rGO/CSH interactions remained close to the initial structural models of the epoxy rGO surface. The simulations indicate that specific CSHs, containing rGO with different interfacial topologies, can be manufactured using coatings of either epoxide or hydroxyl groups. The results fill a knowledge gap, by establishing a connection between the chemical compositions of CSH units and rGO, and confirm that a wet chemical method can be used to produce pristine graphene by removing hydroxyl defects from rGO.

Atomistic insights into structure evolution and mechanical property of calcium silicate hydrates influenced by nuclear waste caesium

The fundamental mechanisms underlying the influence of nuclear wastes on concrete properties remain poorly understood, especially at the molecular level. Herein, caesium ions (Cs⁺) are introduced into calcium silicate hydrates (CSH) to investigate its effect using molecular dynamics simulation. Structurally, a swelling phenomenon is observed, attributed to the CSH interlayer expansion as Cs⁺ occupies larger space than Ca²⁺. The diffusion of interlayer water, Ca²⁺ and Cs⁺, following an order of water > Cs⁺ > Ca²⁺, is accelerated with increasing Cs⁺ content, owing to three mechanisms: expanded interlayer space, weakened interfacial interaction, and loss of chemical bond stability. Mechanically, the Young's modulus and strength of CSH are degraded by Cs⁺ due to two mechanisms^: (1) the load transfer ability of interlayer water and Ca²⁺ is weakened; (2) the load transfer provided by Cs⁺ is very weak. Additionally, a "hydrolytic weakening" mechanism is proposed to explain the mechanical degradation with increasing water content. This study also provides guidance for studying the influence of other wastes (like heavy metal ions) in concrete.

Mechanical behavior and phase change of alkali-silica reaction products under hydrostatic compression

Alkali-silica reaction (ASR) causes severe degradation of concrete. The mechanical property of the ASR product is fundamental to the multiscale modeling of concrete behavior over the long term. Despite years of study, there is a lack of consensus regarding the structure and elastic modulus of the ASR product. Here, ASR products from both degraded field infrastructures and laboratory synthesis were investigated using high-pressure X-ray diffraction. The results unveiled the multiphase and metastable nature of ASR products from the field. The dominant phase undergoes permanent phase change via collapsing of the interlayer region and in-planar glide of the main layer, under pressure >2 GPa. The bulk moduli of the low- and high-pressure polymorphs are 27±3 and 46±3 GPa, respectively. The laboratory-synthesized sample and the minor phase in the field samples undergo no changes of phase during compression. Their bulk moduli are 35±2 and 76±4 GPa, respectively. The results provide the first atomistic-scale measurement of the mechanical property of crystalline ASR products.

Spectral attributes of sub-amorphous thermal conductivity in cross-linked organic-inorganic hybrids

Organic-inorganic hybrids have found increasing applications for thermal management across various disciplines. Such materials can achieve thermal conductivities below the so-called "amorphous limit" of their constituents' thermal conductivity. Despite their technological significance, a complete understanding of the origins of this thermal conductivity reduction remains elusive in these materials. In this paper, we develop a prototypical cross-linked organic-inorganic layered system, to investigate the spectral origins of its sub-amorphous thermal conductivity. Initially, we study the atomic structure of the model and find that besides polymer chain length, the relative drift of the layers governs the reduction in computed basal spacing, in agreement with experimental measurements. We, subsequently, find that organic cross-linking results in up to 40% reduction in thermal conductivity compared to inorganic samples. An in-depth investigation of vibrational modes reveals that this reduction is the result of reduced mode diffusivities, which in turn is a consequence of a vibrational mismatch between the organic and inorganic constituents. We also show that the contribution of propagating modes to the total thermal conductivity is not affected by organic cross-linking. Our approach paves the path toward a physics-informed analysis and design of a wide range of multifunctional hybrid nanomaterials for thermal management applications among others.

Long-term creep deformations in colloidal calcium-silicate-hydrate gels by accelerated aging simulations

When subjected to a sustained load, jammed colloidal gels can feature some delayed viscoplastic creep deformations. However, due to the long timescale of creep (up to several years), its modeling and, thereby, prediction has remained challenging. Here, based on mesoscale simulations of calcium-silicate-hydrate gels (CSH, the binding phase of concrete), we present an accelerated simulation method-based on stress perturbations and overaging-to model creep deformations in CSH. Our simulations yield a very good agreement with nanoindentation creep tests, which suggests that concrete creep occurs through the reorganization of CSH grains at the mesoscale. We show that the creep of CSH exhibits a logarithmic dependence on time-in agreement with the free-volume theory of granular physics. Further, we demonstrate the existence of a linear regime, i.e., wherein creep linearly depends on the applied load-which establishes the creep modulus as a material constant. These results could offer a new physics-based basis for nanoengineering colloidal gels featuring minimal creep.

Monte Carlo Molecular Modeling of Temperature and Pressure Effects on the Interactions between Crystalline Calcium Silicate Hydrate Layers

The interactions of calcium silicate hydrates with water are at the heart of critical features of cement-based material behavior such as drying and autogenous shrinkage, hysteresis, creep, and thermal expansion. In this article, the interactions between nanocrystalline layers of calcium silicate hydrates are computed from grand canonical Monte Carlo molecular simulations. The effects of temperature, chemical potential, and pressure on these interactions are studied. The results are compared with simulation and experimental data found in the literature concerning surface energy, cohesive pressure, and out-of-plane elastic properties. The disjoining pressure isotherms of calcium silicate hydrates are negligibly affected by changes in water pressure under saturated conditions. The surface energy decreases with the temperature, the chemical potential of water, and the water pressure. Coarse-grained simulations are performed using the potential of mean force obtained at the molecular level. The mesostructure presents hysteresis with respect to mechanical and thermal loads. The anharmonicity of the interactions identified at the molecular scale translates to an asymmetry tension/compression and thermal expansion that are also observed at the mesoscale. These results leave room for a better understanding of the multiscale origin of physical properties of calcium silicate hydrates.

Two models based on local microscopic relaxations to explain long-term basic creep of concrete

In this study, we propose an exhaustion model and an adapted work-hardening model to explain the long-term basic creep of concrete. In both models, the macroscopic creep strain originates from local microscopic relaxations. The two models differ in how the activation energies of those relaxations are distributed and evolve during the creep process. With those models, at least up to a few dozen MPa, the applied stress must not modify the rate at which those relaxations occur, but only enables the manifestation of each local microscopic relaxation into an infinitesimal increment of basic creep strain. The two models capture equally well several phenomenological features of the basic creep of concrete. They also make it possible to explain why the indentation technique enables the quantitative characterization of the long-term kinetics of logarithmic creep of cement-based materials orders of magnitude faster than by macroscopic testing. The models hint at a physical origin for the relaxations that is related to disjoining pressures.