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Inhomogeneity in Cement Hydrates: Linking Local Packing to Local Pressure
Abstract
Nanoscale structural heterogeneities were recently revealed in computational and experimental studies of calcium silicate hydrates in hardened cement pastes. In this work their consequences for the mechanics are analyzed by computing local pressures in model samples at different overall densities, corresponding to different initial water-to-cement ratios. The correlations between pore size distributions, local density, local cohesive energy, and local pressure clearly show how in these materials structural heterogeneities may be the origin of significant mechanical heterogeneities. The results indicate that even at high density pressure, heterogeneities develop during the densification of cement hydrates and result in the coexistence of regions of high positive pressure with regions of negative pressure, in spite of the overall mechanical stability of the samples. Furthermore, the regions of negative pressure, prone to mechanical instabilities and local plastic processes, tend to be localized close to the surface of large mesopores and hence to be more significant at higher initial water content.
... In some cases the potentials and parametrizations have been used directly and individually, e.g. in Refs. [90][91][92]. In other cases they were combined and slightly modified, such as in Refs. ...
... The distribution of local volume fractions is also useful in homogeneous precipitation models as it allows to explore heterogeneities within the configuration. Such distributions have been computed in different models [56,89,90,92,123]. The definition of the local volume for the computation of the lvf differs between models. ...
... The definition of the local volume for the computation of the lvf differs between models. It is either defined in a spherical volume of few particles diameter around each particle [90,92] or based on a 3D grid [56]. Distributions of local volume fractions for spherical and ellipsoidal particle simulations are shown in Fig. 19. a and b respectively [56,92]. ...
The nano-to-micro mesoscale is crucial for cementitious materials; here reactions and interactions between molecules produce complex mechanisms that determine the behavior of cement minerals, especially C-S-H. This manuscript reviews the current state of the art in coarse-grained and mesoscale simulations of C-S-H. These simulations leverage a rigorous statistical mechanical framework, linking atomistic description with coarse-grained modelling through several pivotal concepts: potential of mean force, ion-ion correlations between charged surfaces, and grand canonical reactive ensemble. The second part of the manuscript discusses the effective interaction potentials between C-S-H particles that are currently used, followed by methods to simulate C-S-H formation. Structural, physical and mechanical properties predicted by the existing simulations are then presented. Finally the manuscript highlights opportunities for future research, which are driving the multi-scale modelling of C-S-H but also of other mesostructured materials.
... Indeed, although polydispersity has been shown to play a critical role in controlling the packing density (and, thereby, the mechanical properties) ( Masoero et al., 2014( Masoero et al., , 2012, the role of polydispersity at constant packing density remains unclear. Similarly, although structural and mechanical heterogeneity has been pointed out to impact the nanomechanics of C-S-H gels ( Ioannidou et al., 2014;Ioannidou et al., 2017;Masoero et al., 2014 ), the role of the extent of order and disorder on macroscopic properties remains poorly understood. Meanwhile, little is known about the role of the level of order in the mesostructure of C-S-H-which has been suggested to be higher in high-density C-S-H phases ( Constantinides and Ulm, 2007;Ioannidou et al., 2016aIoannidou et al., , 2014Jennings, 20 0 0 ). ...
... It should be noted that, in the thermodynamic sense, stress is only properly defined for a large ensemble of atoms, so the physical meaning of the "stress per grain" is unclear. Nevertheless, this quantity can conveniently capture the existence of local instabilities within the gel due to competitive inter-grain forces ( Ioannidou et al., 2017 ). ...
... Note that this local stress does not result in any macroscopic stress, namely, the grains experiencing some positive or negative shear stress mutually compensate each other so that the overall structure remains at zero stress. The presence of such eigenstress is a manifestation of the out-of-equilibrium nature of disordered C-S-H and arises from the fact that the grain precipitate in a random fashion, so that the addition of new grains in a preexisting rigid structure necessarily involves some non-optimal contact among grains and, hence, the formation of some local stress ( Ioannidou et al., 2017 ). A visual inspection of the local stress map (see Fig. 9 a) reveals that the stress distribution is highly heterogeneous, that is, most of the stress is concentrated in some local clusters of interconnected grains. ...
The colloidal calcium–silicate–hydrate (C–S–H) gel largely controls the strength of concrete. However, little remains known about how the structural features of the C–S–H gel control its mechanical properties. Here, based on coarse-grained mesoscale simulations, we investigate the effect of grain polydispersity and structural disorder on the nanomechanics of C–S–H. Our simulations offer a good agreement with nanoindentation data over a large range of packing density values. We show that, at constant packing density, stiffness and hardness are not affected by the polydispersity in grain sizes. In contrast, the level of disorder is found to play a critical role. We demonstrate that, in contrast to the case of ordered C–S–H models, the elastic response of disordered C–S–H gels is governed by the existence of stress heterogeneity and nanoyielding within its structure. These results highlight the intrinsically disordered nature of C–S–H and the crucial role of order and disorder in controlling gels’ mechanical properties.
... A. Mesoscale model of hardened cement paste. Ioannidou's et al. mesoscale C-S-H model (18,23) was used to calculate the water adsorption/desorption by DFT simulations. The precipitation of C-S-H nano-grains and settings was simulated using the approach recently proposed in Ref. (15). ...
... The effective interparticle forces between cement hydrates depends on the concentration of calcium ions in the solution and changes during the hydration (51,52). In precious works, the microstructure of C-S-H gels at early hydration stages was investigated using attracto-repulsive potential arising from ion-ion correlation forces (15,23). ...
Capillary effects such as imbibition-drying cycles impact the mechanics of granular systems over time. A multiscale poromechanics framework was applied to cement paste, that is the most common building material, experiencing broad humidity variations over the lifetime of infrastructure. First, the liquid density distribution at intermediate to high relative humidities is obtained using a lattice gas density functional method together with a realistic nano-granular model of cement hydrates. The calculated adsorption/desorption isotherms and pore size distributions are discussed and compare well to nitrogen and water experiments. The standard method for pore size distribution determination from desorption data is evaluated. Then, the integration of the Korteweg liquid stress field around each cement hydrate particle provided the capillary forces at the nanoscale. The cement mesoscale structure was relaxed under the action of the capillary forces. Local irreversible deformations of the cement nano-grains assembly were identified due to liquid-solid interactions. The spatial correlations of the nonaffine displacements extend to a few tens of nm. Finally, the Love-Weber method provided the homogenized liquid stress at the micronscale. The homogenization length coincided with the spatial correlation length nonaffine displacements. Our results on the solid response to capillary stress field suggest that the micronscale texture is not affected by mild drying, while local irreversible deformations still occur. These results pave the way towards understanding capillary phenomena induced stresses in heterogeneous porous media ranging from construction materials, hydrogels to living systems.
... Figure 3 shows such structural heterogeneities developed during the out-ofequilibrium process of particle precipitation interacting with LJ potential. Hardened cement paste exhibits a broad heterogeneity of local volume fractions ηlocal 15,53 . After thresholding the configuration for increasing local volume fractions, an underlying percolating network of highly packed (>64% RCP) particles is revealed. ...
... Here, we move further to understand also the role of particles with local volume fraction below 60%. Figure 4 shows the correlations between local volume fractions and local pressure Plocal in a configuration of hardened cement paste where the total eigenstress (at the level of the simulation box) has been relaxed (~10kPa) 53 . In the population of particles with local volume fractions larger than 60% a tail towards positive local pressure is observed, hence overall particles are under compression. ...
Cement hydrates named C-S-H are the main products of the reaction of cement with water. The C-S-H phase is the most important phase of cement paste as it glues all other phases together in a solid rock-like material. C-S-H gels form and densify via out-of-equilibrium precipitation and aggregation of nano-grains during cement hydration. In this chapter, the link between the making and densification of C-S-H gels and amorphous solids is discussed by coarse-grained models based on the evolution of interaction potentials and an out-of-equilibrium simulation approach for particle precipitation. In particular, we characterize and correlate mesoscale structural and mechanical heterogeneities resulting into residual local eigenstresses. This underlying microscopic picture explains recent macroscopic measurements of the volume change of hydrating cement in fully saturated conditions.
... and ff and sf are the interaction parameters of fluid-fluid and fluid-solid, respectively, which are imported from previous atomistic simulation data (Bonnaud et al., 2012). Model configurations made of packed spherical particles of different sizes targeted two fractions of solid: 0.52 to represent for low-density C-S-H, dominates at water/cement ratio around 0.45; and 0.63 to represent high-density C-S-H, which is representative of a paste with water/cement ratios around 0.4 (Jennings, 2004;Ioannidou et al., 2017). The fluid-fluid interaction parameter, ff , is related to the bulk critical temperature B c = ff /2, where ( = 6) is the value of the nearest neighbours of the cubic lattice. ...
Concrete is the most widely used construction material across the world. It is a heterogeneous material consisting of cement paste, fine and coarse aggregates, and it is a multi-phase porous media with dry air, liquid water, vapour and other kinds of fluids filling the pores. The hygro-thermo-chemo-mechanical behaviours of concrete under elevated temperature have been investigated for decades at the macroscale level using experimental methods and numerical simulations, including material strength, material properties, variables (e.g. pore pressure) and even microstructural properties. However, the findings are generally empirically in nature, and the mechanisms underneath all these macroscopic behaviours are still not clear. The objective of this thesis is to contribute to the understanding of the influences of microstructural processes on the macro-level behaviour of concrete at elevated temperatures through a combined numerical and experimental study. The work uses finite element analysis with a fully coupled hygro-thermo-chemo-mechanical model in combination with water vapour sorption isotherms measurements using the ‘Dynamic Vapour Sorption’ method. Numerical parametric studies have been conducted for nine properties rooted in the micro-scale. These have confirmed that: permeability is key to the development of gas pressures; the description of the heat and mass boundary conditions can have a considerable effect on the predicted results; the amount of water introduced into the system as a result of dehydration of the cement paste, the influence of micro-scale gas flow behaviour and the evolution of capillary pressures are all found to have a considerable effect on the development of macro-scale behaviours. Furthermore, the transient behaviour of moisture under elevated temperatures are found to be significantly affected by the formulation of the sorption isotherm, especially where that relates to microstructural behaviour. To further investigate this and potential contradictions between theoretical or observed microstructural behaviour and macro-scale model formulations, a series of experiments were conducted to measure water vapour sorption isotherms using the ‘Dynamic Vapour Sorption (DVS)’ method. Investigations were carried out to determine the potential microstructural changes of hardened cement pastes (CEMI with water to cement ratio of 0.4) subjected to different relative humidity ranges. The results indicate that the microstructure of cement paste is not affected by elevated temperatures until 80°C, after removing confounding effects from irreversible changes upon first drying and harsh drying. The only microstructural changes consistent with the presented results, during desorption and adsorption, appear to be reversible. The temperature dependency of sorption isotherms was investigated as well. The results confirm and extend the interpretation that the adsorption isotherm is near-equilibrium, and the desorption isotherm is not. These results are qualitatively confirmed by classical Density Functional Theory (DFT) theory. It is also confirmed that the adsorption isotherm is weakly temperature-dependent and desorption is much less temperature-dependent than from desiccator tests, where a marked increase of cavitation pressure is observed in the desorption branch with increased temperature. Additionally, the mechanism of interlayer water was investigated in this project by drying the samples to nominal RH=0%, instead of drying at 5% RH. The results suggested that the interlayer water play a significant role in the desorption range below 5% RH and the hysteresis in isotherms suggested that until the interlayer water was evaporated sufficiently, the interlayer spaces were never filled even when re-wetting to full saturation. All these results and their implications indicate the need for a revision of the models linking water content with humidity at high temperature, with possibly important implications for the understanding and prediction of temperature-induced damage in concrete. All these results indicated that microstructural processes have significant influences on the behaviours of concrete at the macroscale level when exposed to high temperature. However, these microstructural processes are still vague, which are strongly affected by the pore size distribution that needs to be explored and clarified further.
... Energies 2022, 15, 6045 ...
Neat well cement experience significant strength retrogression at high temperatures above 110 °C, especially at approximately 150 °C. To reveal the mechanism of performance degradation and guide the preparation of high-performance cement, we investigate the hydration process, mechanical behavior, and fracture process for well cement at the temperature of 150 °C based on molecular dynamics simulations and experiments. From triaxial pressure tests and Brazilian splitting tests, the strength, elastic modulus, and Poisson’s ratio of well cement decrease drastically with temperature increases from 80 °C to 150 °C. According to XRD, TG/DTG/DSC, and SEM, the hydration degree is insufficient, and larger pores exist in the microstructures. As the main binding phase of well cement, the mechanism of calcium silicate hydrates (C-S-H) influenced by curing temperatures is investigated through molecular dynamics simulations. C-S-H with calcium/silicon ratios (C/S) of 1.1 and 1.8 are simulated in the aqueous and solid states to investigate precipitation and mechanical behaviors. By reducing the C/S ratio to 1.1, the strength rebounds to a certain extent, and the adequacy of the hydration degree improved. It is found from the polymerization process that the increasing temperature promotes the polymerization rate, which is higher with C/S = 1.8 than that of 1.1. However, an increase in the C/S ratio will lead to a decrease in bridging oxygen content, thus a lower polymerization degree. The fracture simulations of C-S-H gels at different temperatures indicate that the failure of the C-S-H structure is mainly attributed to the disassembling of the calcium oxygen layers. With a higher temperature, there are fewer Ca-O bonds breaking, thus less strain energy consumed, resulting in worse performance. The elasticity of C-S-H, including Young’s and shear moduli, also exhibits certain degradations at a higher temperature. The elastic behavior of C-S-H with a low C/S ratio is generally higher than the high C/S.
... By the end of hydration, C-S-H becomes denser and denser, and its nanoscale features (including the interlayer distances and chemical compositions usually described in terms of Ca/Si ratio) play a predominant role in most observations and studies [16,21,29]. Nevertheless, the earlier stage mesoscale morphology controls the development of larger pores and contributes to local stresses in the initial gel network, which have consequences for the long term evolution of the material and its interactions with the environment [5,43,44,[50][51][52][53]. The change in shape of the nanoscale interactions, with competing attraction and repulsion and a striking increase of the attraction strength with surface charge density during hydration, largely controls the morphology of the mesoscale structures that build the gel network and can dramatically steer compressive or tensile stresses as the material progressively densifies and solidifies. ...
Cement is one of the most produced materials in the world. A major player in greenhouse gas emissions, it is the main binding agent in concrete, to which it provides a cohesive strength that rapidly increases during setting. Understanding how such cohesion emerges has been a major obstacle to advances in cement science and technology. Here, we combine computational statistical mechanics and theory to demonstrate how cement cohesion results from the organization of interlocked ions and water, progressively confined in nano-slits between charged surfaces of Calcium-Silicate-Hydrates. Due to the water/ions interlocking, dielectric screening is drastically reduced and ionic correlations are proven significantly stronger than previously thought, dictating the evolution of the nano-scale interactions during cement hydration. By developing a quantitative analytical prediction of cement cohesion based on Coulombic forces, we reconcile a novel fundamental understanding of cement hydration with the fully atomistic description of the solid cement paste and open new paths for science and technologies of construction materials.
... By the end of hydration, C-S-H becomes denser and denser, and its nanoscale features (including the interlayer distances and chemical compositions usually described in terms of Ca/Si ratio) play a predominant role in most observations and studies (16,21,29). Nevertheless, the earlier-stage mesoscale morphology controls the development of larger pores and contributes to local stresses in the initial gel network, which have consequences for the long-term evolution of the material and its interactions with the environment (5,43,44,(50)(51)(52)(53). The change in shape of the nanoscale interactions, with competing attraction and repulsion and a notable increase of the attraction strength with surface charge density during hydration, largely controls the morphology of the mesoscale structures that build the gel network and can markedly steer compressive or tensile stresses as the material progressively densifies and solidifies. ...
Cement is the most produced material in the world. A major player in greenhouse gas emissions, it is the main binding agent in concrete, providing a cohesive strength that rapidly increases during setting. Understanding how such cohesion emerges is a major obstacle to advances in cement science and technology. Here, we combine computational statistical mechanics and theory to demonstrate how cement cohesion arises from the organization of interlocked ions and water, progressively confined in nanoslits between charged surfaces of calcium-silicate-hydrates. Because of the water/ions interlocking, dielectric screening is drastically reduced and ionic correlations are proven notably stronger than previously thought, dictating the evolution of nanoscale interactions during cement hydration. By developing a quantitative analytical prediction of cement cohesion based on Coulombic forces, we reconcile a fundamental understanding of cement hydration with the fully atomistic description of the solid cement paste and open new paths for scientific design of construction materials.
... It should be noted that, in the thermodynamic sense, stress is only properly defined for a large ensemble of atoms, so that the physical meaning of the "stress per atom" is unclear. Nevertheless, this quantity can conveniently capture the existence of local instabilities within the gel due to competitive interatomic forces (Ioannidou et al., 2017). Based on this framework, we compute the local stress experienced by each Si atom, since the silicate chains constitute the rigidity backbone of C-S-H. ...
Topological constraint theory (TCT) classifies disordered networks as flexible, stressed-rigid, or isostatic based on the balance between the number of topological constraints and degrees of freedom. In contrast with the predictions from a mean-field enumeration of the constraints, the isostatic state—wherein the network is rigid but free of stress—has been suggested to be achieved within a range of compositions, the intermediate phase, rather than at a fixed threshold. However, our understanding of the nature and potential structural signatures of the intermediate phase remains elusive. Here, based on molecular dynamics simulations of calcium–silicate–hydrate systems with varying compositions, we seek for some mechanical and structural signatures of the intermediate phase. We show that this system exhibits a composition-driven rigidity transition. We find that the fracture toughness, fracture energy, mechanical reversibility, and creep compliance exhibit an anomalous behavior within a compositional window at the vicinity of the isostatic threshold. These features are argued to constitute a mechanical signature of an intermediate phase. Notably, we identify a clear structural signature of the intermediate phase in the medium-range order of this system, which is indicative of an optimal space-filling tendency. Based on these simulations, we demonstrate that the intermediate phase observed in this system arises from a bifurcation between the rigidity and stress transitions. These features might be revealed to be generic to isostatic disordered networks.
... The C-S-H data were isolated from the cement samples by spatially correlating nanoindentation data and energy dispersive X-ray spectrometry and elemental mapping data. The comparison of these data highlights the fact the densest part (η>0.6) of the material spans the whole structure in a complex, loadbearing network, which dominates the nano-indentation experiments 33,35 . ...
Cement is the most used building material on earth, yet its properties are inexactly understood and not fully controlled. Cement hydrates named C-S-H (Calcium-Silicate-Hydrate) are the most abundant phase in hydrated cement paste and are responsible for gluing all other hydration products and unreacted cement together. A source of complexity in modelling C-S-H is that the structure is amorphous, multiscale with fully interconnected porosity spanning from a few nm up to mm. The focus of this chapter is modeling approaches that allow connecting structure and mechanics of C-S-H at the mesoscale (the scale that spans from few nm up to the micron) from the early stages of hydration, the setting and up to the hardened cement paste. The modelling approach reviewed here is of reduced complexity based on coarse-graining with emphasis on the effective interactions between C-S-H particles. It addresses the mesoscale of C-S-H and has provided a unified framework for understanding the microstructure of C-S-H and reconciling data from many different experimental techniques. A consistent picture is presented covering (1) the reactive solidification of cement, (2) the origin of the observed microstructure of C-S-H, and (3) its link to mechanics. This provides a powerful predictive tool for nanoscale design of cement.
... Reactive heterogeneous materials such as cement have residual stresses due to the out-of-equilibrium solidification process [51][52][53]. The cement paste structure analyzed here has accumulated tensile eigen-stress of -47 MPa through the precipitation of nano-grains [54,55]. Figure 4 shows how capillary forces affect the relaxation of two different samples, young cement paste sample with initial tensile eigen-stresses (-47MPa) and hardened sample (the relaxed version of the young sample). ...
We propose a theoretical framework to calculate capillary stresses in complex mesoporous materials, such as moist sand, nanoporous hydrates, and drying colloidal films. Molecular simulations are mapped onto a phase-field model of the liquid-vapor mixture, whose inhomogeneous stress tensor is integrated over Voronoi polyhedra in order to calculate equal and opposite forces between each pair of neighboring grains. The method is illustrated by simulations of moisture-induced forces in small clusters and random packings of spherical grains using lattice-gas Density Functional Theory. For a nano-granular model of cement hydrates, this approach reproduces the hysteretic water sorption/desorption isotherms and predicts drying shrinkage strain isotherm in good agreement with experiments. We show that capillary stress is an effective mechanism for internal stress relaxation in colloidal random packings, which contributes to the extraordinary durability of cement paste.
... In return, the energy barrier is associated with the maximum magnitude of the eigenstress max | σ * |, with a temperature dependence according to Eq. (7) consistent with its role as an energy barrier. According to this conjecture, the eigenstress represents but a mean force field which -if relaxed-entails the measured incremental swelling or shrinkage, much akin to mesoscale colloidal simulations carried out in an NpT-ensemble ( Ioannidou et al., 2017a ). More specifically, in terms of Eq. (9) , repulsion is associated with the η 2 -term, and the attraction with the η 4 -term. ...
Volume changes in chemically reactive materials, such as hydrating cement, play a critical role in many engineering applications that require precise estimates of stress and pressure developments. But a means to determine bulk volume changes in the absence of other deformation mechanisms related to thermal, pressure and load variations, is still missing. Herein, we present such a measuring devise, and a hybrid experimental–theoretical technique that permits the determination of colloidal eigenstresses. Applied to cementitious materials, it is found that bulk volume changes in saturated cement pastes at constant pressure and temperature conditions result from a competition of repulsive and attractive phenomena that originate from the relative distance of the solid particles – much as Henry Louis Le Châtelier, the father of modern cement science, had conjectured in the late 19th century. Precipitation of hydration products in confined spaces entails a repulsion, whereas the concurrent reduction in interparticle distance entails activation of attractive forces in charged colloidal particles. This cross-over from repulsion to attraction can be viewed as a phase transition between a liquid state (below the solid percolation) and the limit packing of hard spheres, separated by an energy barrier that defines the temperature-dependent eigenstress magnitude.
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.
The colloidal calcium-silicate-hydrate (C-S-H) gel largely controls the mechanical properties of concrete. However, little remains known about how the structural features of the C-S-H gel control its mechanical properties. The aim of the current study is therefore to investigate the structure, mechanical properties and the crack propagation of the colloidal C-S-H gel at the mesoscale. To achieve this, Grand Canonical Monte Carlo methods were utilized to construct the colloidal C-S-H gel. The structure features were demonstrated by the pore size distribution. Subsequently, computational uniaxial tension tests were performed on the C-S-H gel specimens using a non-local peridynamics (PD) model. On the basis of the simulated stress–strain response, the Young’s modulus and tensile strength can be obtained. The modelling results show that both the strength and Young’s modulus grow exponentially with the packing fraction increasing which shows reasonable agreement with the literature, revealing the feasibility of the PD method for the investigation of mechanical performance of C-S-H gel at mesoscale.
Although calcium carbonate (CaCO3) precipitation plays an important role in nature, its mechanism remains only partially understood. Further understanding the atomic driving force behind the CaCO3 precipitation could be key to facilitate the capture, immobilization, and utilization of CO2 by mineralization. Here, based on molecular dynamics simulations, we investigate the mechanism of the early-stage nucleation of an amorphous calcium carbonate gel. We show that the gelation reaction manifests itself by the formation of some calcium carbonate clusters that grow over time. Interestingly, we demonstrate that the gelation reaction is driven by the existence of some competing local molecular stresses within the Ca and C precursors, which progressively get released upon gelation. This internal molecular stress is found to originate from the significantly different local coordination environments exhibited by Ca and C atoms. These results highlight the key role played by the local stress acting within the atomic network in governing gelation reactions.
In previous sections, the atomistic simulation methods have been introduced to the materials science for cement-based materials to decode the intrinsic building block of the cement hydrate at nanoscale. The molecular dynamics method exhibits the significant advantage in investigating the properties of cement-based concrete material at nanoscale and opens a novel pathway for design of construction and building materials.
Significance
Capillary effects in cement paste are associated with multiple degradation mechanisms. Using a framework, we investigated the role of capillary forces in cement paste under partial saturation from the nanograins level to the mesoscale. We show that the largest capillary forces concentrate at the boundary between gel pores and larger capillary pores, inducing nonaffine deformations with correlations up to a few tens of nanometers. Our results suggest a homogenization length scale common to liquid and solid stresses correlated with the scale of inherent structural heterogeneities. This opens the route to studying the poromechanics of complex multiscale media with explicit fluid–solid coupling.
An atomistic and mesoscopic assessment of the effect of alkali uptake in cement paste is performed. Semi-grand canonical Monte Carlo simulations indicate that Na and K not only adsorb at the pore surface of calcium silicate hydrates (C-S-H) but also adsorb in the C-S-H hydrated interlayer up to concentrations of the order of 0.05 and 0.1 mol/kg, respectively. Sorption of alkali is favored as the Ca/Si ratio of C-S-H is reduced. Long timescale simulations using the Activation Relaxation Technique indicate that characteristic diffusion times of Na and K in the C-S-H interlayer are of the order of a few hours. At the level of individual grains, Na and K adsorption leads to a reduction of roughly 5% of the elastic moduli and to volume expansion of about 0.25%. Simulations using the so-called primitive model indicate that adsorption of alkali ions at the pore surface can reduce the binding between C-S-H grains by up to 6%. Using a mesoscopic model of cement paste, the combination of individual grain swelling and changes in inter-granular cohesion was estimated to lead to overall expansive pressures of up to 4 MPa-and typically of less than 1 MPa-for typical alkali concentrations observed at the proximity of gel veins caused by the alkali-silica reaction.
Cesium-137 is a common radioactive byproduct found in nuclear spent fuel. Given its 30 year half life, its interactions with potential storage materials—such as cement paste—is of crucial importance. In this paper, simulations are used to establish the interaction of calcium silicate hydrates (C-S-H)—the main binding phase of cement paste—with Cs at the nano- and mesoscale. Different C-S-H compositions are explored, including a range of Ca/Si ratios from 1.0 to 2.0. These calculations are based on a set of 150 atomistic models, which qualitatively and quantitatively reproduce a number of experimentally measured features of C-S-H—within limits intrinsic to the approximations imposed by classical molecular dynamics and the steps followed when building the models. A procedure where hydrated Ca2+ ions are swapped for Cs1+ ions shows that Cs adsorption in the C-S-H interlayer is preferred to Cs adsorption at the nanopore surface when Cs concentrations are lower than 0.19 Mol/kg. Interlayer sorption decreases as the Ca/Si ratio increases. The activation relaxation technique nouveau is used to access timescales out of the reach of traditional molecular dynamics (MD). It indicates that characteristic diffusion time for Cs1+ in the C-S-H interlayer is on the order of a few hours. Cs uptake in the interlayer has little impact on the elastic response of C-S-H. It leads to swelling of the C-S-H grains, but mesoscale calculations that access length scales out of the range of MD indicate that this leads to practically negligible expansive pressures for Cs concentrations relevant to nuclear waste repositories.
Significance
Calcium–silicate–hydrate (C–S–H) nanoscale gels are the main binding agent in cement and concrete, crucial for the strength and the long-term evolution of the material. Even more than the molecular structure, the C–S–H mesoscale amorphous texture over hundreds of nanometers plays a crucial role for material properties. We use a statistical physics framework for aggregating nanoparticles and numerical simulations to obtain a first, to our knowledge, quantitative model for such a complex material. The extensive comparison with experiments ranging from small-angle neutron scattering, SEM, adsorption/desorption of N 2 , and water to nanoindentation provides new fundamental insights into the microscopic origin of the properties measured.
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
We investigate the development of gels under out-of-equilibrium conditions, such as calcium-silicate-hydrate (C-S-H) gels that form during cement hydration and are the major factor responsible for cement mechanical strength. We propose a new model and numerical approach to follow the gel formation upon precipitation and aggregation of nano-scale colloidal hydrates, whose effective interactions are consistent with forces measured in experiments at fixed lime concentrations. We use Grand Canonical Monte Carlo to mimic precipitation events during Molecular Dynamics simulations, with their rate corresponding to the hydrate production rate set by the chemical environment. Our results display hydrate precipitation curves that indeed reproduce the acceleration and deceleration regime typically observed in experiments and we are able to correctly capture the effect of lime concentration on the hydration kinetics and the gel morphology. Our analysis of the evolution of the gel morphology indicates that the acceleration is related to the formation of an optimal local crystalline packing that allows for large, elongated aggregates to grow and that is controlled by the underlying thermodynamics. The defects produced during precipitation favor branching and gelation that end up controlling the deceleration. The effects on the mechanical properties of C-S-H gels are also discussed.
Efforts to model and simulate the highly complex cementhydration process over the past 40 years are reviewed, covering different modeling approaches such as single particle models, mathematical nucleation and growth models, and vector and lattice-based approaches to simulating microstructuredevelopment. Particular attention is given to promising developments that have taken place in the past few years. Recent applications of molecular-scale simulation methods to understanding the structure and formation of calcium–silicate–hydrate phases, and to understanding the process of dissolution of cement minerals in water are also discussed, as these topics are highly relevant to the future development of more complete and fundamental hydration models.
The influence of water-to-cement mass ratio (w/c) on early-age properties of cement-based materials is investigated using a variety of experimental techniques. Properties that are critical to the early-age performance of these materials are tested, including heat release, semi-adiabatic temperature, setting time, autogenous deformation, and strength development. Measurements of these properties using a single cement are presented for four different w/c, ranging from 0.325 to 0.425. Some of the measured properties are observed to vary widely within this range of w/c ratios. The heat release and setting time behaviors of cement pastes are contrasted. While early-age heat release is relatively independent of w/c, the measured setting times vary by several hours between the four w/c investigated in this study, indicating the fundamental differences between a physical process such as setting and heat release which is purely a quantification of chemical reaction. While decreasing w/c certainly increases compressive strength at equivalent ages, it also significantly increases autogenous shrinkage and may increase semi-adiabatic temperature rise, both of which can increase the propensity for early-age cracking in cement-based materials.
Despite decades of studies of calcium-silicate-hydrate (C-S-H), the structurally complex binder phase of concrete, the interplay between chemical composition and density remains essentially unexplored. Together these characteristics of C-S-H define and modulate the physical and mechanical properties of this "liquid stone" gel phase. With the recent determination of the calcium/silicon (C/S = 1.7) ratio and the density of the C-S-H particle (2.6 g/cm(3)) by neutron scattering measurements, there is new urgency to the challenge of explaining these essential properties. Here we propose a molecular model of C-S-H based on a bottom-up atomistic simulation approach that considers only the chemical specificity of the system as the overriding constraint. By allowing for short silica chains distributed as monomers, dimers, and pentamers, this C-S-H archetype of a molecular description of interacting CaO, SiO2, and H2O units provides not only realistic values of the C/S ratio and the density computed by grand canonical Monte Carlo simulation of water adsorption at 300 K. The model, with a chemical composition of (CaO)(1.65)(SiO2)(H2O)(1.75), also predicts other essential structural features and fundamental physical properties amenable to experimental validation, which suggest that the C-S-H gel structure includes both glass-like short-range order and crystalline features of the mineral tobermorite. Additionally, we probe the mechanical stiffness, strength, and hydrolytic shear response of our molecular model, as compared to experimentally measured properties of C-S-H. The latter results illustrate the prospect of treating cement on equal footing with metals and ceramics in the current application of mechanism-based models and multiscale simulations to study inelastic deformation and cracking.
Although Portland cement concrete is the world's most widely used manufactured material, basic questions persist regarding its internal structure and water content, and their effect on concrete behaviour. Here, for the first time without recourse to drying methods, we measure the composition and solid density of the principal binding reaction product of cement hydration, calcium-silicate-hydrate (C-S-H) gel, one of the most complex of all gels. We also quantify a nanoscale calcium hydroxide phase that coexists with C-S-H gel. By combining small-angle neutron and X-ray scattering data, and by exploiting the hydrogen/deuterium neutron isotope effect both in water and methanol, we determine the mean formula and mass density of the nanoscale C-S-H gel particles in hydrating cement. We show that the formula, (CaO)1.7(SiO2)(H2O)1.80, and density, 2.604 Mg m(-3), differ from previous values for C-S-H gel, associated with specific drying conditions. Whereas previous studies have classified water within C-S-H gel by how tightly it is bound, in this study we classify water by its location-with implications for defining the chemically active (C-S-H) surface area within cement, and for predicting concrete properties.
Calcium-Silicate-Hydrate (or C-S-H), a phyllosilicate, is the major binding phase in cement pastes and concretes and a porous hydrated material made up of a percolated and dense network of cristalline nanoparticles of mean apparent spherical diameter ~5 nm that are each stacks of multiple C-S-H layers. Interaction forces between these nanoparticles are at the origin of C-S-H chemical, physical, and mechanical properties at the meso- and macroscales. These particle interactions and resulting properties may be affected significantly by nanoparticle density and environmental conditions such as temperature, relative humidity, or concentration of chemical species in the bulk solution. In this study, we combined grand canonical Monte Carlo simulations and an extension of the mean force integration method to derive the pair potentials. This approach enables realistic simulation of the physical environment surrounding the C-S-H particles. We thus constructed the pair potentials for C-S-H nanoparticles of defined chemical stoichiometry at 10% relative humidity (RH), varying the relative crystallographic orientations at a constant particle density of ρpart ~ 2.21 mmol/L. We found that cohesion between nanoparticles is affected strongly by both the aspect ratio and crystallographic misorientation of interacting particles. This method and findings underscore the importance of accounting for relative dimensions and orientation among C-S-H nanoparticles in descriptions of physical and simulated multiparticle aggregates or mesoscale systems.
Porous media offer many interesting problems in physics and engineering due to the interaction of phase transitions, surface effects and transport. In this thesis I examine two such problems: the degradation of lithium-ion batteries, and sorption and transport of fluids in porous materials. The dominant capacity fade mechanism in many lithium-ion batteries is the loss of cyclable lithium to a solid-electrolyte interphase layer on the surface of the negative electrode. I develop a single-particle model of this fade mechanism, based on diffusion of the reacting species through the growing layer and reaction at the surface of the active material. This analytical model is justified by comparison with a computational porous electrode model. Temperature is identified as the most important variable influencing the capacity fade rate, and the model is able to make predictions for accelerated aging tests as well as the effect of mismatched internal resistances in battery packs. The quantity of a fluid taken up by a porous material as a function of the partial pressure of the fluid relative to saturation can be used to measure the pore size distribution of the material. However, hysteresis between the wetting and drying paths complicates the interpretation of experimental results. I present a unified model of hysteresis that accounts for both single-pore and network effects, enabling the calculation of not only the pore size distribution but also a parameter measuring the connectivity between large and small pores. I then use the ideas of the model to examine drying shrinkage in hardened cement paste, demonstrating that the hysteresis in this shrinkage is primarily due to water inserted between molecular layers in calcium-silicate-hydrate. Finally, I outline a model of transport of a sorbing fluid with hysteresis, and suggest possible extensions to allow quantitative comparison with experimental results.
The risk of early-age fracture of cementitious materials in ever more challenging environments provides a unique opportunity to employ an experimental chemo-mechanical platform to develop functional relations between hydration degree, fracture and strength properties, assessed by isothermal calorimetry, micro-scratching, splitting and microindentation on white cement paste at various curing ages from 7 h to 28 days. We show that the modulus, tensile strength, fracture toughness and energy all evolve with a natural logarithmic dependence on the hydration degree. These trends are linked to the densification of the material during the hydration process, explained by compaction mechanics and free volume theory. We show that while the fracture process zone size is essentially constant during the hydration process, the ductility of the material, quantified by M/H, decreases, and is consistent with the evolution of Kc/H. Both quantities provide a convenient way to experimentally assess the fracture sensitivity of early-age cement-based materials.
Calcium silicates hydrates (C-S-H) are the most abundant hydration products in ordinary Portland cement paste. Yet, despite the critical role they play determining mechanical and transport properties, there is still a debate on their density and exact composition. Here, the site-specific mass density and composition of C-S-H in hydrated cement paste are determined with nanoscale resolution in a non-destructive approach. We used ptychographic X-ray computed tomography in order to determine spatially resolved mass density and water content of the C-S-H within the microstructure of the cement paste. Our findings indicate that the C-S-H at the border of hydrated alite particles have a higher density than the apparent inner-product C-S-H, which is contrary to the common expectations from previous works on hydrated cement paste.
Unlike other porous materials such as sandstone, brick, or porous glass, the interatomic bonding continuity of cement-based materials like concrete is far from obvious. When scrutinized at the micro- or nanoscopic level, the continuity of the ionic–covalent bonding in the solid phase is interrupted almost everywhere by water molecules or liquid water films. The same situation is found in set plaster.Yet, plaster and cementitious materials are able to withstand stresses of the same order of magnitude as rocks. Molecular simulation studies and direct-force measurements by atomic force microscopy provide strong arguments for predicting that short- and medium-range surface forces mediated by partially or totally hydrated calcium ions are the essential components of cement strength, with additional contributions from van der Waals and capillary forces. This provides a clue for understanding the nano- and mesostructure of cement-based materials and new levers for improving their properties.
Strengths of cement pastes with different mixture properties and maturities depend in a very similar overlinear fashion on the gel–space ratio, which is the ratio of the volume of hydration products over the volume of both hydration products and capillary pores. We here investigate the underlying microstructural effects by the experimentally validated micromechanics model of Pichler and Hellmich [CemConRes 41(5), 2011]. This model shows that the macrostrength of cement pastes are not only triggered by the capillary porosity, but also by a strengthening effect of unhydrated clinker “reinforcements” which are embedded as inclusions in the hydrate foam. The analysis is continued with quantifying the strength of the hydrates, in terms of an extended model validation activity. Satisfactory model performance is the motivation to present model predictions for the biaxial compressive failure envelopes of cement pastes, again as a function of gel–space ratio.
The investigations were performed on five hardened pastes of a type I (normal) portland cement, having initial weight ratios of water to cement of 0.35, 0.40, 0.50, 0.57, and 0.70, and hydrated for 12, 12, 12, , and years, respectively. The percentage hydration ranged from 90 to 98%.Nitrogen adsorption and desorption isotherms were determined for all pastes at the boiling point of nitrogen, from low pressures to almost the saturation pressure. The BET surface areas were calculated, and the pore space accessible to nitrogen was obtained by a slight extrapolation of the isotherms. In addition, pore structure analysis was made for three of the five pastes by the method of Cranston and Inkley.The specific surface areas of the five pastes were also determined by water vapor adsorption, and the total porosity to water was measured. For every paste, the water areas and porosities were greater than the nitrogen areas and porosities.The main conclusions from the results were as follows.(1) There is a wide range of pore sizes in the pastes.(2) Nitrogen is excluded from a part of the pore system by two mechanisms; some of the pores are too narrow and others have too narrow entrances to admit nitrogen.(3) The hydraulic radius (the volume of the pore system divided by the surface area of the pore walls) for the system of small pores increases with increasing water to cement ratio.(4) The hydraulic radius of the system of large pores also increases with increasing water to cement ratio, but the increase is smaller than for the small pores.(5) The paste with water to cement ratio of 0.35 has enough pore space for complete hydration.Some practical implications of the above conclusions are discussed.
The equations of hydrodynamics—continuity equation, equation of motion, and equation of energy transport—are derived by means of the classical statistical mechanics. Thereby, expressions are obtained for the stress tensor and heat current density in terms of molecular variables. In addition to the familiar terms occurring in the kinetic theory of gases, there are terms depending upon intermolecular force. The contributions of intermolecular force to the stress tensor and heat current density are expressed, respectively, as quadratures of the density and current density in the configuration space of a pair of molecules.
Both cryoporometry and relaxometry probe the surface-to-volume ratio of a porous material. Nuclear magnetic resonance (NMR) relaxometry uses the random motion of molecules, whereas cryoporometry uses the melting-point depression of a confined liquid. An NMR setup has been built to simultaneously perform cryoporometry and relaxometry measurements. Using materials with a well-defined pore size, i.e. silica gels, both methods are compared with the standard N2-adsorption technique, and a good correlation is found. The methods can be used in the pore size range between 1 and 100 nm. By performing NMR relaxometry during cryoporometry, more information about the pore-size distribution can be obtained. Besides for silica gels, this is demonstrated for mortar, which has a complicated pore structure.
It is well known from experiments that the uniaxial compressive strength of cementitious materials depends linearly on the degree of hydration, once a critical hydration degree has been surpassed. It is less known about the microstructural material characteristics which drive this dependence, nor about the nature of the hydration degree–strength relationship before the aforementioned critical hydration degree is reached. In order to elucidate the latter issues, we here present a micromechanical explanation for the hydration degree–strength relationships of cement pastes and mortars covering a large range of compositions: Therefore, we envision, at a scale of fifteen to twenty microns, a hydrate foam (comprising spherical water and air phases, as well as needle-shaped hydrate phases oriented isotropically in all space directions), which, at a higher scale of several hundred microns, acts as a contiguous matrix in which cement grains are embedded as spherical clinker inclusions. Mortar is represented as a contiguous cement paste matrix with spherical sand grain inclusions. Failure of the most unfavorably stressed hydrate phase is associated with overall (quasi-brittle) failure of cement paste or mortar. After careful experimental validation, our modeling approach strongly suggests that it is the mixture- and hydration degree-dependent load transfer of overall, material sample-related, uniaxial compressive stress states down to deviatoric stress peaks within the hydrate phases triggering local failure, which determines the first nonlinear, and then linear dependence of quasi-brittle strength of cementitious materials on the degree of hydration.
This article is concernedwith the calciumsilicate hydrates, including crystallineminerals and the extremely variable and poorly ordered phase (C-S-H)
that is the main binding phase in most concrete. Up-to-date composition and crystal-structure information is tabulated for the most important crystalline
calcium(alumino) silicate hydrates and related phases.Anumber ofmodels for the nanostructure of C-S-H are summarized and compared and it is shown
that there is much more of a consensus than might seem apparent at first sight. The value of the recently solved structures of 1.4 nm tobermorite and
jennite, together with those of jaffeite and metajennite, for visualizing the nanostructural elements present in themodels is demonstrated. The importance
of Hal Taylor's contribution to the solution of the structure of jennite is highlighted. The applicability of Richardson and Groves' model is demonstrated
using experimental composition-structure observations on the nature of C-S-H in a Portland cement-fly ash blend.
Herein, we present a comprehensive nanoindentation investigation of cement pastes prepared at substoichiometric water-to-cement (w/c) mass ratios between 0.15 and 0.4 with and without heat treatment. Based on a statistical indentation technique, we provide strong evidence of the existence of a statistically significant third hydrated mechanical phase in addition to the already known Low-Density (LD) and High-Density (HD) C–S–H phases. The nanomechanical properties of this third phase are found to follow similar packing density scaling relations as LD C–S–H and HD C–S–H, while being significantly greater. This third phase, whose nano-packing density is measured at 0.83 ± 0.01, is therefore termed Ultra-High-Density (UHD) phase. All three phases are present in concrete materials in different volume proportions: LD dominates cement-based materials prepared at high w/c mass ratios; HD and UHD control the microstructure of low w/c ratio materials. In addition, heat treatment favors the formation of HD and UHD. The insight thus gained into the link between composition, processing and microstructure makes it possible to monitor packing density distributions of the hydration products at the nanoscale.
Interest in microporous materials has risen in recent years, as they offer a confined environment that is optimal to enhance chemical reactions. Calcium silicate hydrate (C-S-H) gel, the main component of cement, presents a layered structure with sub-nanometer-size disordered pores filled with water and cations. The size of the pores and the hydrophilicity of the environment make C-S-H gel an excellent system to study the possibility of confined water reactions. To investigate it, we have performed molecular dynamics simulations using the ReaxFF force field. The results show that water does dissociate to form hydroxyl groups. We have analyzed the water dissociation mechanism, as well as the changes in the structure and water affinity of the C-S-H matrix and water polarization, comparing the results with the behavior of water in a defective zeolite. Finally, we establish a relationship between water dissociation in C-S-H gel and the increase of hardness due to a transformation from a two- to a three-dimensional structure.
Recently developed atomistic models of highly disordered nanoporous materials offer hope for a much more realistic description of the pore morphology and topology in such materials; however, a factor limiting their application has been the computationally intensive characterization of the models, particularly determination of the pore size distribution. We report a new technique for fast computation of pore size distributions of model materials from knowledge of the molecular coordinates. The pore size distribution (PSD) is defined as the statistical distribution of the radius of the largest sphere that can be fitted inside a pore at a given point. Using constrained nonlinear optimization, we calculate the maximum radii of test particles at random points inside the pore cavity. The final pore size distribution is then obtained by sampling the test particle radii using Monte Carlo integration. The computation time depends on factors such as the number of atoms, the sampling resolution, and the desired accuracy. However, even for large systems, PSDs with very high accuracy (>99.9%) are obtained in less than 24 h on a 3 GHz Pentium IV processor. The technique is validated by applying it to model structures, whose pore size distributions are already known. We then apply this method to investigate the pore structures of several mesoporous silica models such as SBA-15 and mesostructured cellular foams.
Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter--atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently --- those with short--range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed--memory parallel machine which allows for message--passing of data between independently executing processors. The algorithms are tested on a standard Lennard--Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers --- the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y--MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventi...
Thermodynamique et dynamique de l'eau d'un électrolyte confinés dans des nanopores: Application à l'hydrate cimentaire
- P Bonnaud