Efforts to tune the performance of organic/inorganic composites are hindered owing to a lack of knowledge related to the interfacial interaction mechanisms. Here we investigated the interfacial ...structure, dynamics, energetics and mechanical properties between calcium silicate hydrates (C-S-H) and polymers by molecular dynamics (MD) simulation. In this work, polyethylene glycol (PEG), polyvinyl alcohol (PVA) and polyacrylic acid (PAA) are intercalated into nanometer channel of C-S-H sheets to construct the model of polymer/C-S-H composite. In the interfacial region, the calcium ions near the surface of C-S-H play mediating role in bridging the functional groups in the polymers and oxygen in the silicate chains by forming Os-Ca-Op bond. In addition to ionic bonding, the bridging oxygen (C-O-C) in the PEG, hydroxyl (C-OH) in the PVA and carboxyl groups (-COOH) in the PAA provide plenty oxygen sites to form H-bonds with silicate hydroxyl, interlayer water and calcium hydroxyl in C-S-H substrate. The interfacial binding energy is dependent on polarity of functional groups in the polymers, the stability of the H-bond and Ca-O bond, ranking in the following order: E(PAA)> E(PVA) > E(PEG). The PVA with small number of H-bonds formed between oxygen in PVA and water molecules, resulting in increasing the mobility of confined water in the interlayer region. On the other hand, PAA and PVA, with strong polarity, can provide more number of non-bridging oxygen sites that widely distributed along the polymer chains to associate with more calcium ions and H-bonds. Furthermore, uniaxial tensile test is utilized to study the mechanical behavior of the composites. The incorporation of polymers, strengthening the H-bonds in the interfacial region and healing the defective silicate chains, can inhibit the crack growth during the loading process, which both enhance the cohesive strength and ductility of the C-S-H gel. In particular, the intercalated PAA increases the Young's modulus, tensile strength and fracture strain of C-S-H gel to 22.27%, 19.2% and 66.7%, respectively. The toughening mechanism in this organic/inorganic system can provide useful guidelines for polymer selection, design, and fabrication of C-S-H/polymer nanocomposites, and help eliminate the brittleness of cement-based materials from the genetic level.
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Calcium silicate hydrate (C-S-H) is the most important hydration product of cement-based materials. In the nanostructure of C-S-H, structural water molecules are distributed in the interlayer region ...and determine the mechanical performance of C-S-H gel. In this study, C-S-H gels with different water contents expressed as the water/Ca ratio are characterized in the light of molecular dynamics. In order to study the influence of the water molecules, the structures of 12 C-S-H gel samples with water/Ca ratios from 0 to 0.95 are investigated. It is found that the penetration of water molecules transforms the C-S-H gel from an amorphous to a layered structure by silicate depolymerization as the water content gradually increases. The structures are then tested for mechanical properties by simulated uniaxial tension and compression. The mechanical tests associated with structural analysis reveal that the structural water molecules can greatly weaken the stiffness and the cohesive force by replacing the ionic–covalent bond with unstable H-bond connections. By studying the tensile failure mechanism of C-S-H gels at different humidity levels, the disconnecting role of the structural water molecules is comprehensively interpreted. Because the interlayer water molecules prevent reconstruction of the bonds between the Caw and the silicate chains, the plasticity of the C-S-H gels is reduced significantly in the change from a dry state to a saturated state. In addition, the compressive strength of a C-S-H gel in the saturated state is much larger than the tensile strength. This provides molecular evidence for the tensile weakness of cement paste.
•Channel water shows good orientation, low diffusion rate and ordered organization.•H bonds donated by ONB atoms influence the structure and dynamics of channel water.•Tobermorite surface prefers ...adsorbing Ca ions and repulsing chloride ions.•Ca-Cl cluster formation accelerates the Ca ions dissociating from surfaces.
Water and ions transport in the cement hydrate determines the durability of cementitious material. Due to the structural similarity, tobermorite, an important mineral analogue of the main phase of cement hydrate, was used to investigate transport behaviour at the molecular level. In this study, the structural and dynamical properties of the water/ions and the tobermorite interface were studied by the molecular dynamics (MD) simulation method. On the (001) surface of tobermorite, water molecules diffusing in the channel between the silicate chains demonstrate a number of structural water features: large density, good orientation preference, ordered interfacial organization and low diffusion rate. The dipole vector of type 1 water molecules point upwards due to the attraction from the calcium sheet, while type 2 water molecules turn downwards due to the restrained H-bonds donated from ONB atoms in the silicate chains. The stable H-bonds connected with ONB in the silicate chains restrict the mobility of the channel water molecules. The significant reduction of the diffusion coefficient matches well with the experimental results obtained by NMR, QENS and PCFR techniques. With increasing distance from the channel, the structural and dynamical behavior of the water molecules vary and gradually translate into bulk water properties at distances of 10–15Å from the liquid–solid interface. In the respect of the ions and tobermorite interaction, there is Cl− repulsion and Ca2+ adsorption from the surface. In addition, in simulation time longer than 1ns, Cl− can diffuse to the surficial Ca2+ and forms the unstable Ca-Cl cluster, which accelerates the desorption of the surface Ca atoms.
Calcium silicate hydrate (C–S–H) is a mesoporous amorphous material with water confined in the gel pores, which provides the medium for investigating the structure, dynamics, and mechanical ...properties of the ultraconfined interlayer water molecules. In this study, C–S–H gels with different compositions expressed in terms of the Ca/Si ratio are characterized in the light of molecular dynamics. It is found that with increasing Ca/Si ratio, the molecular structure of the silicate skeleton progressively transforms from an ordered to an amorphous structure. The calcium silicate skeleton, representative of the substrate, significantly influences the adsorption capability, reactivity, H-bond network, and mobility of the interlayer water molecules. The structures were tested for mechanical properties by simulated uniaxial tension, and the mechanical tests associated with structural analysis reveal that the stiffness and cohesive force of C–S–H gel is weakened by both breakage of silicate chains and penetration of water molecules. In addition, the reactive force field is coupled with both the mechanical response and chemical response during the large tensile deformation process. On the one hand, the silicate chains, acting in a skeletal role in the layered structure, depolymerize to enhance the loading resistance. On the other hand, water molecules, attacking the Si–O and Ca–O bonds, dissociate into hydroxyls, which are detrimental to the cohesive force development.
The transport and adsorption behavior of ions and water in nanometer pores is influenced by the composition of the substrate. In this paper, to understand the effect of Al species on the properties ...of confined nanofluids, molecular dynamics is utilized to study the structure, dynamics and interfacial adsorption behavior of NaCl solution confined in the gel pores of C-S-H and C-A-S-H. The bridging silicate tetrahedron substituted by the aluminate species enhances the hydrophilic properties of C-S-H gel. As compared with water on the C-S-H surface, the water layered packing is densified and the magnitude of dipole moment is enlarged for water located in the vicinity of the C-A-S-H surface. This is mainly attributed to the increasing number of H bonds contributed by oxygen atoms in the aluminate silicate chains sharing more negativity. Furthermore, C-A-S-H gel immobilizes more sodium and chloride ions on the surface than C-S-H. Sodium ions can coordinate with about two to three oxygen atoms in the aluminate tetrahedron tessellated in the narrow vacancy of the silicate channel, forming inner adsorbed species. Weak interactions between Cl and the substrate are due to the few ionic pairs between Cl ions and surface-accumulated Na ions. Due to the strong Na-Os binding on the C-A-S-H surface, the diffusion coefficient of the Na ions confined in the nanometer pores is reduced by 50% and the hydration time for the Na ions associated with surrounding water is increased by 40% as compared with bulk solution.
Al species incorporated in silicate chains enhance hydrophilicity and cation immobilization ability of the C-S-H gel.
•Surface water shows higher dipole moment due to hydrophilic C-S-H layers.•The mobility of interlayer water, hydroxyl is reduced greatly due to H-bond restriction.•The oxygen in defective silicate ...chains influences water orientation and packing behavior.•Surface Ca ions and OH groups are dissociated from the C-S-H surface.
The movement of water molecules confined in the calcium silicate hydrate gel determines the chemical and physical properties of cement-based materials. In this paper, the molecular dynamics was utilized to investigate the structure and dynamic properties of interlayer water, surface adsorbed water and the capillary water molecules. The molecular structure of water molecule ultra-confined in the C-S-H gel or in the vicinity of calcium silicate surface is changed dramatically as compared with capillary water: enlargement of packing intensity, increasing magnitude of dipole moment, enhancement of the order interfacial organization, improvement of orientation preference and the extending of water-water spatial correlation. These structural features are attributed to the hydrophilic nature of the C-S-H surface that can provide non-bridging oxygen atoms in the silicate chains to accept H-bonds from the surface adsorbed water molecules and highly solvated calcium atoms in the interlayer region to associate confined water molecule to form hydration shell. Furthermore, three types of water molecules can be clearly distinguished by the diffusion coefficient derived from the mean square displacement of water molecules: Interlayer water (0.003×10−9m2s−1)≪surface water (1.2×10−9m2s−1)<capillary water (2×10−9m2s−1). The slow dynamic properties of interlayer water resemble that in the glassy state: water molecules are hard to escape from the “cage” constructed by the H-bonds and ionic bonds network.
Calcium silicate hydrate (C–S–H) is the most important binding phase of cement paste and can determine the mechanical properties of cementitious material. The multi-scale nature of C–S–H gel brings ...about challenges in interpreting the mechanical properties of C–S–H. C–S–H models at the nano scales are commonly formulated by the aggregation of colloids that consist of layered molecular structures. In this study, the mechanical properties of C–S–H were investigated at the molecular level (∼5 nm). Uniaxial tension testing in the x, y, z directions was applied to the layered structure of C–S–H. The structural difference in the layer leads to heterogeneous mechanical properties in three directions. Particularly in the z direction, the C–S–H layer connected by H-bonds and Ca2+, has the weakest tensile strength. Resulting from the investigation, it can be concluded that the simulation can provide molecular insights into the linkage between molecular and larger scale structures.
•C–S–H is the intrinsic component of cement-based materials and the cohesive force is investigated by molecular dynamics.•At nano-scale, the simulated model shows the heterogeneous mechanical properties at three directions.•Ca–O bond plays great role in loading resistance and damage results from the breakage of silicate chains and calcium ions.•Interlayer structure in z direction shows weakest loading resistance, due to unstable H-bond network and shielding Ca2+.•Displacement of water layers under tension loading results in shrinkage and creep of cement at macro-level.
The bonding performance between epoxy resin and cement-based materials affects the load transfer capacity between fiber-reinforced polymers (FRP) and concrete. FRP-reinforced concrete structures are ...inevitably exposed to aggressive environments during service, triggering interfacial deterioration and premature structural failure. To assess the stress state of FRP-reinforced concrete structures and to predict their long-term durability, a fundamental investigation of the bonding performance of the epoxy/concrete interface in aggressive environments is required. In this study, the bonding performance between epoxy resin and calcium silicate hydrate (CSH) in water and salt environments is investigated using molecular dynamics (MD) simulations. The adhesion energy of epoxy/CSH interface calculated via combine MD simulation and Bell’s model theory is dramatically reduced in water and salt environments. Analysis of the static structures and dynamic properties reveals that the adhesion between the epoxy resin and CSH matrix is mainly dependent on the O–Ca–O and H-bond connections, while the water and salt environments significantly weaken these connections and disrupt the integrity of the epoxy/CSH interface. This study provides a molecular-level understanding of the bonding degradation at the epoxy/CSH interface in aggressive environments.
Graphical abstract
This study provides a molecular level understanding of the bonding degradation between epoxy and CSH matrix in aggressive environments.
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•Two-scale representation of the capillary pore structure of hydrated cement paste.•Two-scale random walk simulation for the computation of paste transport properties.•Experimental ...tests-based validation of the proposed computation scheme.•Empirical model as a simple way to estimate transport properties of cement paste.•Transport properties can be derived from realistically represented microstructure.
Predicting transport properties of cement-based materials directly from the microstructure is very challenging, due to the problems of bridging length scales and the difficulties of realistically representing the microstructure. Based on a two-scale representation of the microstructure, a scheme is proposed in this paper to model the transport properties of cement paste through two-scale random walk simulation. A random walk algorithm is firstly applied at the sub-micro-scale to determine the diffusion tortuosity of the outer C-S-H layer. This is then up-scaled to the micro-scale to compute the diffusion tortuosity of cement paste. Based on physical laws, the diffusion tortuosity is transformed into the formation factor, and further into the electrical conductivity and the chloride diffusion coefficient of cement paste, and subsequently validated. It is proven that a more realistic representation of the microstructure makes it possible to derive transport properties of cement paste, directly and accurately, from the microstructure.
Reactive force field molecular dynamics was utilized to simulate the reactivity, structure and dynamics of water molecules confined in calcium-silicate-hydrate (C-S-H) nano-pores of 4.5 nm width. Due ...to the highly reactive C-S-H surface, hydrolytic reactions occur in the solid-liquid interfacial zone, and partially surface adsorbed water molecules transforming into the Si-OH and Ca-OH groups are strongly embedded in the C-S-H structure. Due to the electronic charge difference, the silicate and calcium hydroxyl groups have binomial distributions of the dipolar moment and water orientation. While Ca-OH contributes to the O
w
-downward orientation, the O
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atoms in the silicate chains prefer to accept H-bonds from the surface water molecules. Furthermore, the defective silicate chains and solvated Ca
w
atoms near the surface contribute to the glassy nature of the surface water molecules, with large packing density, pronounced orientation preference, and distorted organization. The stable H-bonds connected with the Ca-OH and Si-OH groups also restrict the mobility of the surface water molecules. The significant reduction of the diffusion coefficient matches well with the experimental results obtained by NMR, QENS and PCFR techniques. Upon increasing the distance from the channel, the structural and dynamic behavior of the water molecules varies and gradually translates into bulk water properties at distances of 10-15 Å from the liquid-solid interface.
Structural and dynamic properties of surface water molecules.
