Confined Water Dissociation in Disordered Silicate Nanometer

Article pubs.acs.org/Langmuir

Confined Water Dissociation in Disordered Silicate NanometerChannels at Elevated Temperatures: Mechanism, Dynamics and Impact on Substrates Dongshuai Hou,*,† Dengke Li,† Tiejun Zhao,† and Zongjin Li‡ †

Department of Civil Engineering, Qingdao Technological University, Qingdao 266033, China The Hong Kong University of Science and Technology, Hong Kong, China

‡

ABSTRACT: The effects of elevated temperature on the physical and chemical properties of water molecules filled in the nanometer-channels of calcium silicate hydrate have been investigated by performing reactive molecular dynamics simulation on C−S−H gel subjected to high temperature from 500 to 1500 K. The mobility of interlayer water molecules is temperature-dependent: with the elevation of temperature, the self-diffusivity of water molecules increases, and the glassy dynamic nature of interlayer water at low temperature transforms to bulk water characteristic at high temperature. In addition, the high temperature contributes to the water dissociation and hydroxyl group formation, and proton exchange between neighboring water molecules and calcium silicate substrate frequently happens. The hydrolytic reaction of water molecules results in breakage of the silicate chains and weakens the connectivity of the ionic−covalent bonds in the C−S−H skeleton. However, the broken silicate chains can repolymerize together to form branch structures to resist thermal attacking.

1. INTRODUCTION Water molecules ultraconfined in nanometer-channels have been intensively studied in many natural and industrial processes. Ultraconfined water molecules play an essential role in a wide range of applications that involve hydrogen storage molecular-sieving catalysts,1 biological membranes for ultrafiltration or energy conversion,2 and detrimental ion stabilization in radioactive waste repositories.3 As compared with that in the bulk state, the ultraconfinement provides extremely different local physical and chemical environments of water molecules and water transport in nanometer-channel and demonstrates unique properties such as strong hydrogen connectivity,4 glassy-like dynamic nature,5 anomalous lubricity, and drying phase transition behavior.6 One important example is dissolution of water molecules ultraconfined in calcium silicate hydrate, the major binding phase in the cement paste that is ubiquitously utilized as the construction materials. Considering that 5% of the emission of global green house gases is due to the manufacture of cement, investigation of the water in C−S−H gels, involving the hydration mechanism and process improvement, is of crucial importance to environmental sustainability. The cohesive behavior7 and durability of concrete8 are greatly influenced by the movement of water molecules within the C−S−H gels that are closely related to the interior temperature, humidity, and surrounding environment of the materials. The service life of cement-based materials exposed to high temperature can be reduced to a great extent with degrading of the mechanical performance and physic/chemical © 2016 American Chemical Society

structures. The main chemical process responsible for the internal damage of concrete is the alteration of hydrates and the states of the interlayer water molecules confined in the C−S−H gel. By efforts from various experimental techniques, such as Xray diffraction (XRD), NMR and thermogravimetric analysis (TGA)9,10 the microstructure evolution for the C−S−H gel and the water state alteration have been investigated at elevated temperature. The dehydration of cementitious materials can be analyzed by using thermal analysis techniques such as TGA and differential scanning calorimetry (DSC). The microstructure of C−S−H gel changes can be quantified in terms of mass loss during the progressively elevated temperature. The evaporation of water and C−S−H dehydration lead to the weight loss of cement paste in the temperature range from 378 to 1273 K (105 to 1000 °C)1112.13 The wide range for the C−S−H gel dehydration is attributed to the various compositions of the water molecules including the free water in the capillary pore, the physical bond water near the surface, and the chemical bond water rooted in the calcium silicate layers14.15 This means that the dehydration process is a multistage reaction. The dehydration of the interlayer water molecules can further result in the variation of the crystal structure and the silicate morphologies. Using combined XRD and NMR characterization of heat-treated C−S−H samples, Alizadeh16 proposed that the high temperature results in [001] space shrinkage for Received: February 4, 2016 Revised: April 14, 2016 Published: April 14, 2016 4153

DOI: 10.1021/acs.langmuir.6b00444 Langmuir 2016, 32, 4153−4168

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Langmuir the C−S−H gel and further contributes to the repolymerization of the silicate chains, due to the absence of interlayer water molecules. The mean silicate chain length increase and basal spacing reduction have also been observed by Cong and Kirkpatrick.17 This analysis was contrary with Alonso’s research based on NMR study. Alonso proposed that the maximum transformation of C−S−H gel is at 733 K (450 °C), where Q1 and Q0 species, representing dimmer and monomer structures, are produced due to depolymerization.18 Thus, the experimental research alone cannot give comprehensive understanding of the behavior of water molecules in the C−S−H gel at different temperatures due to the complexity of the cementitious materials at nanoscale. In addition to the experimental investigation, computational methods can help unravel the molecular properties of materials. Molecular dynamics (MD) is able to give a more quantitative interpretation of the thermodynamic behaviors of ultraconfined water molecules in C−S−H gels. By using molecular dynamics and the Monte Carlo method, Bonnaud et al.19 investigated the water content in the C−S−H model and cohesive force evolution with elevated temperature. They proposed that the interlayer water molecules in the C−S−H gel evaporate at 746 K at normal pressure. Based on the idea of water content decreasing, Hou et al.8 studied the mechanical properties of C− S−H gel at the molecular level by molecular dynamics, finding that the intragranular cohesive force is enhanced as water loss from the interlayer region. However, these molecular dynamics studies are based on a force field that cannot allow bond breakage and formation in the simulation. In this respect, the chemical reactions of the water molecules suffering elevated temperature cannot be reflected in a realistic way. In this work, molecular structure, dynamics, and reactivity of water molecules confined in the C−S−H gel at elevated temperatures is studied by using reactive force field molecular dynamics. The aim of this paper is to capture the evolution of properties for ultraconfined water molecules in different thermal conditions. The impact of water dissociation on the substrate can be characterized by analyzing silicate chain morphology and chemical bonds at different temperatures. Furthermore, mechanical properties including the stiffness and cohesive force of the C−S−H gels can be achieved directly from the uniaxial tension simulation. Reactive force field molecular dynamics, coupling the chemical, mechanical, and thermal response, can unravel the behavior of interlayer water molecules in complicated local environments.

updated continuously to simulate the chemical reactions. The ReaxFF potential is composed of covalent or bonded interaction, and nonbonded van der Waal, Coulombic interactions. The covalent interactions including the bond energy, the bond angle energy, and the torsion angle energy, are determined solely from the bond orders so that all partial energy contributions related to valence interactions reduce to zero smoothly as the bond is broken. ReaxFF has been successfully used in several systems including the silica−water interfaces,21 C−S−H gels,22 and some crystals.23 Manzano et al. extended the ReaxFF force field to the simulation system of calcium silicate hydrate by integrating Ca−O−H potential with a Si−O−H set that was developed by van Duin et al.24 The parametrization and performance of the force field for Ca, Si, O, and H can be achieved from the data in previously published papers.25,26 2.2. C−S−H model. In the current study, the C−S−H model construction followed the procedures proposed by Pellenq27 and Manzano.24 First, due to the structural similarity, tobermorite 11 Å was considered as the beginning structure of C−S−H gels.28,29 The interlayer water molecules were removed. SiO2 atoms in the silicate chains were removed to obtain the Q species distribution with Q1 = 64.2%, Q2 = 35.8% and Q0 = 0%. In this respect, the mean silicate chain length (MCL = 2(Q2/Q1 + 1) = 3.12) matches well with experimental results from NMR testing30 and previous molecular dynamics simulation.31 The initial configuration of the C−S−H model is shown in Figure 1.

2. SIMULATION METHODOLOGY 2.1. Force Field. CSHFF Force Field. The CSHFF force field,20 developed for the calcium-silicate-hydrate family, is utilized to simulate the GCMC water adsorption in model construction. In this study, CSHFF employed a nonpolarizable but flexible version of the simple point charge (SPC) model that reproduced the thermal dynamic and structural properties of water molecules7.8 The core-only force field parameters can be obtained from the above literature. ReaxFF Force Field. ReaxFF, a reactive force field method, is used to model atomic structure construction and uniaxial tensile testing. It uses a bond order scheme to simulate chemical reactions. Different from other force fields, the connectivity is not assigned to fix for the covalent bonds in ReaxFF. In ReaxFF, the bond order is proposed to simulate the breakage and formation of chemical bonds. The parameter is derived from interatomic distances instantaneously, which are

Figure 1. Supercell 2 × 3 × 1 of initial tobermorite 11 Å with broken silicate chains. a = 22.32, b = 22.17, c = 22.77; α = 90°, β = 90°, γ = 90°. Yellow and red bonds represent the silicate chain (Si−O); the green and gray balls correspond to the calcium atoms in the sheet and interlayer region, respectively. The white red sticks are water molecules. Figure 2 and Figure 5 have same meanings for the balls and sticks; the red balls are dangling oxygen atoms due to dimmer omission.

Subsequently, water adsorption in the dry C−S−H gel is simulated by the Grand Canonical Monte Carlo (GCMC) method.7 GCMC simulations describe potential energy, molecule structure, and number of interlayer water molecules at constant volume that are equilibrated with a fictitious infinite region of liquid water. The GCMC method was conducted with the General Utility Lattice Program (GULP) code32 on the dry C−S−H sample. The water chemical potential (μ) was set as 0 4154


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Figure 2. (a) The interlayer distance increase with temperature; (b) fluid pressure evolution with elevated temperature; (c) atomic profile along the interlayer direction; (d) molecular configuration for the C−S−H gel at different temperatures.

The CSH-FF potential, including the flexible SPC model, was utilized to simulate the water adsorption process in the

eV, which corresponds to the bulk liquid phase with a density of 1 g/cm3 at 300 K by using the flexible SPC water potential.27 4155


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substrate results in the progressive separation of the interlayer structures. Additionally, the density profiles and corresponding molecular configurations, shown in Figure 2c,d, give further information about the molecular structure transformation. As shown in Figure 2c, when the temperature is lower than 1300 K, the alternative peaks of Ca, Si, Ow, and Hw in the density profile suggest the sandwich-like structure of the C−S−H gel. This means that the C−S−H gel can maintain layered orders even at a temperature of 800 K and show good fire-resistant capability. Previous experimental findings also proposed that the intrinsic properties for the C−S−H grains have not been changed even when the temperature rises to 573 K.34 However, at 1500 K, the distributions of Ca and Si overlap to a great extent along the z direction, and no obvious boundaries exist between the interlayer region and the calcium sheet, indicating a loss of the layered feature. As demonstrated in Figure 2d, the silicate chains are distributed across the z direction in a disorderly manner, which shows the amorphous nature of the structure in the form of silicate glass. It should be noted that interlayer water molecules also influence the order of layered structure. In previous research, Hou et al.8 simulated the C−S−H gel at different saturated states. At low humidity, the water molecules are gradually missing from the interlayer region, which leads to the repolymerization of silicate chains in the interlayer region. This contributes to the disorder of the C− S−H gel. The combined information from Figure 2a,b,c,d gives a preliminary understanding of structural evolution with rising temperature. At temperature 1300 K, the density profile demonstrates weak distribution of Hw in the intralayer of the C−S−H gel. The change of the density profile can be explained by the increase of the translational mobility of water molecules, which prevents interfacial anchoring. The mobility of the water molecules will be further discussed in section 3.2. The wider distribution of water indicates that the high temperature can give sufficient kinetic energy to overcome the local energetic barrier in the defect region of the calcium silicate sheet. When the temperature reaches as high as 1500 K, the water molecules can penetrate into calcium silicate sheets and form a water channel along the interlayer direction. Silicate Morphology Evolution. The local structure of silicon atoms is first analyzed by the radial distribution function (RDF) of Si−O. The differences in peak heights of radial distribution functions for Si−O at elevated temperature calculated from simulations are influenced by the inaccuracy of the volume used in the normalization for silicate chains. The increasing temperature can result in the expanding of the simulation cell volume. Additionally, while the silicate chains are restricted in the calcium sheet at low temperature and only occupy the layered region (small fraction of cell volume), the silicate chains turn amorphous at high temperature and grow across the interlayer region (large fraction of cell volume). On the other hand, the position and width of the peaks are greatly influenced by the chemical environment due to the thermal effect, and are important to be noted. The ReaxFF potential yields a bond distance of 1.635 Å, which is similar to previous experimental and simulation findings.35 The intense and sharp peak at low temperature gradually transforms to the small and broad one at high temperature. In particular, the RDF curves show wider shoulders at distance from 1.7 to 2.0 Å, implying that a few Si−O bonds are extremely stretched due to the elevated temperature. As shown in Figure 3a, with increasing

following procedure. First, at least 300 million GCMC steps were performed to reach system equilibrium, where both total energy and water number reach a stable state. Finally, the production run was followed by 100 million GCMC steps to obtain 10 000 equilibrium configurations for data analysis. For each GCMC run, the water molecules in the simulated volume can be inserted, removed, displaced, and rotated. The final chemical formula of the simulated model is (CaO)1.32(SiO2)·(H2O)1.3. The REAX package in LAMMPS33 was employed to perform the reactive force field MD run. Reactive potential, developed by Manzano,22 was used to describe the interaction between Ca, Si, O, and H atoms. The trajectories of atoms were calculated by the Verlet algorithm. The time step was set as 0.25 fs. At the very beginning, the Nosé−Hoover thermostat algorithm was used to simulate C− S−H gel in the canonical ensemble for 250 ps. When the system reached equilibrium, it turned to the NPT ensemble at 300 K and 1 atm for 100 ps. There were eight simulation cases, including T= 300, 500, 600, 700, 800, 1000, 1300 and 1500 K. Finally, a further 1000 ps NPT production run was performed to obtain the atomistic trajectories for analysis. 2.3. Uniaxial Tension Test. The uniaxial tensioned simulation was also performed by the REAX implement in the LAMMPS software. The C−S−H supercells, achieved by extending the model in section 2.2 by a factor 2, were subjected to uniaxial tensile strain through progressive elongation at strain rates of 0.08/ps. The tensile deformation of samples was obtained by a stepwise displacement of atoms. During the entire simulation period, an isobaric−isothermal ensemble was implemented using a Nose-Hoover thermostat and a Verlet integration scheme. As the C−S−H tensioned along the y direction, the pressure in the x and z directions was coupled to zero. This is considered the Poisson effect. The stress tensor component was calculated in eq 1: N

N

PIJ =

∑k mk vkI vkJ V

+

∑k rkIfkJ V

(1)

where V is the simulation box volume, I and J are equal to x, y and z; mkvkI, rkI, and f kI are the I components of the momentum, position, and force acting on the kth atom with mass mk.

3. RESULTS AND DISCUSSION 3.1. Molecular Structures. Layered Structure Transformation. The layered morphology variation during the temperature elevation is first characterized by the interlayer distance. As shown in Figure 2a, as the temperature rises from 300 to 1000 K, the cell size along the z direction increases slightly from 22.9 to 23.2 Å. The C−S−H gel swells significantly, and the interlayer distance reaches 24 Å while the temperature exceeds 1300 K. The interlayer distance is closely related with the fluid pressure in the interlayer region. The fluid pressure along the interlayer direction is calculated based on a similar stress tensor scheme on the interlayer water molecules as eq 1 described. The fluid pressure evolution with elevated temperature is demonstrated in Figure 2b. The positive pressure of all the cases from 300 to 1500 K indicates that the interlayer water molecules play a disjoining role in the C−S−H gel. With increasing temperature from 300 to 1500 K, the pressure gradually increases from 3.3 to 4.7 GPa. The strong disjoining pressure between water and calcium silicate 4156


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Figure 3. (a) RDF for the C−S−H gel at temperatures of 500, 600, 700, 800, 1000, 1300, and 1500 K; (b) the Q species percentage changes from 500 to 1500 K; (c) the reaction pathway for the Si−O bond breakage due to chemical adsorption at 1000 K; (d) the reaction pathway for bond breakage at 1300 K; (e) Q0 and Q3 species in the C−S−H gel heated at 1500 K. (The reacted water molecules and hydroxyl groups are emphasized by the white-red stick and balls. Other water molecules are described by the white-red lines.) 4157


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Figure 4. (a) Radial distribution function for intermolecule Ow−Hw atoms; (b) average HB number evolution with temperature; (c) local configuration of HB at 500 and 1000 K; (d) the hydroxyl number of Si−OH, Ca−OH, and water changes with temperature; (e) water molecule decomposes to Si−OH and Ca−OH; (f) water decomposes to two Si−OH groups; (g) Si−OH bond position in the silicate morphology.

silicate skeleton. Due to the skeleton role of the silicate structures, the layered structure, shown in Figure 2, can resist thermal influence by high temperature and remain undamaged. On the other hand, the ratio of Q2 reduces and that of Q1 increases, as the temperature rises higher than 800 K, implying a silicate morphology transformation from a long silicate chain to a short one. By employing the NMR technique, Alonso and Fernandez found that the silicate morphology in the C−S−H

temperature, the average Si−O bond distance increases from 1.64 to 1.66 Å. Silicate chain morphologies, characterized by the connectivity factor Qn, change due to elevated temperature. In the C−S−H gel, Q1 and Q2 occupy a predominated percentage. Figure 3b describes the Q species evolution with elevation of temperature. When the temperature is lower than 800 K, the Qn percentage remains unchanged, indicating the chemical stability of the 4158


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Langmuir gel at 723 K gradually transforms to dimmer structures.36 Additionally, experimental glass transition temperatures Tg,exp for the sodium silicate glass NS2 and NS3 are at 740 and 760 K, respectively.37 At around 800 K, the Q3 species in the sodium glasses begin to transform to Q2 species due to the Si−O bond breakage. At 1300 K, the silicate chains further break, and the Q2 ratio further degrades. It is observed that all the damages of the silicate chains occur in the bridging sites. This confirms previous energetic investigation on the C−S−H analogues that the bridging tetrahedron in silicate chains occupies lower binding energy than that of pair tetrahedrons. 38 The depolymerization mechanism caused by the temperature elevation can be categorized into two types: the chemical adsorption of water molecules and the thermal elongation of the silicate bonds. The first reaction occurs at relative low temperature. Figure 3c illustrates the reaction pathway: the hydroxyl group or water molecule is attracted by the neighboring silicon atoms, the Si− Ow connection is formed, water dissociates, and the Si−BO bond breaks. The water molecules play a role in attacking the stretched Si−O−Si bonds and result in the separation between the neighboring silicate tetrahedrons. In this process, the thermal energy is not high enough to stretch the broken Si−O bonds, but the stretching for the Si−O bonds results in the distorted tetrahedron structure (Si−O bond length increase and O−Si−O angle varies). Hence, the water molecules can diffuse near the loose silicate structure and become the coordinated atoms. Interestingly, the chemical reaction is reversible as the following sequence: the Si−O bond linking the pair and bridge site is reconnected, so the silicon tetrahedron can reverse to a 5-fold silicon structure, and finally the hydroxyl group decomposes. The second reaction occurs at relative higher temperature compared with the first reaction. The second reaction mechanism of silicate chain breakage is shown in Figure 3d: the Si−O−Si bond is tension broken, water molecules associate with the broken silicate chains, and adsorbed water molecules dissociate to form Si−OH groups. The newly produced Si−OH structures results in more irreversible Q1 species. This is one important difference from the first reaction mechanism. Furthermore, Figure 3a demonstrates that there are new species Q0 and Q3 in the C−S−H gel heated to 1500 K, which is also clearly shown in the molecular configuration in Figure 3e. The Q3 species plays a role in bridging the neighboring broken short chains and helps form the local branch skeleton. Different from the previous bond breakage mechanism for the silicate chain, the polymerization degree for the silicate chains increases at higher temperature. The anomalous chemical reaction mechanism of silicate chains has been found by experimental investigation on C−S−H gel.16 Combined XRD and NMR characterization of heat-treated C−S−H samples demonstrates better polymerization degree, compared with the untreated one. Based on the experimental results, Alizadeh proposed that the high temperature accelerates the absence of the interlayer water, the layered structure collapses, and finally the neighboring silicate tetrahedrons come in close proximity of each other, forming the cross-link structure. Despite no water escaping in the current simulation, the high temperature results in the collapse of the layered structure and contributes to the silicate tetrahedron rearrangement. Local Structures for Water Molecules. In addition to the silicate skeleton, the molecular structure of interlayer water is

influenced by rising temperature. As shown in Figure 4a, the first peak in the oxygen−hydrogen correlation, representing the H-bonds connection, gradually disappear as the temperature increase to 600 K. The double peaks in normal temperature, corresponding to the nearest two shells for individual water, merge to one broad peak. On average, the H-bond number for each water molecule has been calculated according to the following requirement: the H-bond length is smaller than 2.45 Å and the donator−hydrogen−acceptor angle is larger than 150°. Figure 4b demonstrates average H-bond number per interlayer water molecule, and its donator and acceptor components at different temperatures. The average H-bond number gradually reduces from 1.44 to 0.42, as the temperature increases from 300 to 1500 K. It means that the dense H-bond network in interlayer region is completely destroyed due to the thermal effect. At normal temperature, one water molecule donates 1.05 HB to neighboring oxygen atoms and accepts 0.39 HB from neighboring hydrogen atoms. As shown in Figure 4b, the HB breakage process can be divided into two stages: the HB from the water donator is first damaged as the temperature increasing to 800 K, while the HB contributed by the acceptor maintains integrity; subsequently, HB from the acceptor is progressively destroyed as the temperature exceeds 800 K. Figure 4c demonstrates the local configurations of the H-bond connectivity at 500 and 1000 K. At low temperature, the water molecules can form the H-bonds with neighboring nonbridging oxygen atoms in the silicate chains, which further contributes to the H-bond network formation in the interlayer region. At high temperature, the H-bond cannot grow through the interlayer region, weakening the connectivity. Furthermore, the reactivity of water molecules is also temperature-dependent. Figure 4d demonstrates oscillation of the number of Ca−OH, Si−OH and water molecules with elevated temperature. It can be observed from Figure 4d that more than half of the water molecules react and form Si−OH and Ca−OH groups. When the temperature is lower than 1000 K, the number of hydroxyl groups is not changed significantly with increasing temperature. It implies that the relative stable connection between calcium silicate sheet and hydroxyl groups. Si−OH percentage is equal to that of Ca−OH group. As the temperature further increase, the percentage of water molecules decreases and the Si−OH ratio becomes larger than that of Ca−OH. The temperature-dependent hydrolytic reactions of water molecules are based on two different mechanisms. In the C− S−H gel at relatively low temperature (∼1000 K), as shown in Figure 4e, H+ and OH− dissociated from water molecule are respectively bonded with the ONB atoms in the silicate chains and the neighboring interlayer calcium atoms, which contributes to the same number of Si−OH and Ca−OH bonds. On the other hand, at relatively high temperature (>1000 K), more silicate chains are broken as describe in the previous section, resulting in more Si−ONB sites. It can be observed in Figure 4f that the water molecules are dissociated near the ONB atoms and form two Si−OH bonds. That explains a larger percentage of Si−OH bonds than Ca−OH bonds at high temperature, as shown in Figure 4d. Different dissociation process at high temperature reflects transformation of chemical bond water including Si−OH and Ca−OH bonds. At temperature higher than 1000 K, the Si−OH and Ca−OH bonds turn unstable and are frequently broken and rebonded. The dynamic properties of the chemical bonds water will be discussed in section 3.2. 4159


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Figure 5. (a) Mean square displacement for Si at 500, 800, and 1500 K; (b) MSD for Ca at 500, 800, and 1500 K; (c) the trajectories in the xy plane for the interlayer Ca atoms and calcium sheet. (d) van Hove correlated function for the calcium atoms at 800 K from 0.25 to 25 ps. (e) van Hove function for the interlayer calcium atoms and atoms in the sheet at 800 K.

More importantly, the positions of the Si−OH groups have been marked in the C−S−H gel, which help monitor the hydrolytic reaction domain. As shown in Figure 4g, water molecules only dissociate and form Si−OH bonds near the ONB atoms penetrated in the interlayer region at 500 K. When the temperature is elevated to 1000 K, the water molecules can

react close to bottom of silicate channel so than the Si−OH bonds are deeply rooted in the calcium sheet. It should be noted that at normal temperature, the calcium sheet, constructed by the dense calcium octahedron, is organized in order state and has good chemical stability. Implanted hydroxyl in the calcium sheet indicates that the ordered calcium sheet 4160


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Figure 6. (a) The MSD of H atoms bonded with Si−O (Hb) and the nonbonded H atoms (Hnb); (b) MSD for hydrogen atoms at T from 500 to 1500 K; (c) self-diffusion coefficient evolution with temperature; (d) interlayer water molecule in cage stage at T = 500, 800, 1000, and 1300 K; (e) MSDx and MSDz for H atoms at T = 1000 K.

stability of the chemical bonds is further analyzed by investigating the dynamic properties of different atoms. Mean square displacement MSD(t),39 the function to estimate the mobility of different atoms, can be defined by eq 2:

has been distorted due to thermal effect. At 1500 K, because the calcium silicate sheet is partially destroyed, water molecules are decomposed near more ONB sites through the C−S−H gel. In particular, the silicate monomer, with four ONB sites, is more likely react with surrounding molecules. 3.2. Dynamical Properties. Mean Square Displacement for Ca and Si Atoms. In the previous three sections, it has been discussed that the layered molecular structures of the C−S−H gel are constructed by the Ca−O bond, Si−O bond, hydroxyl bond, and H-bond connections. In the following section, the

MSD(t ) = ⟨|ri(t ) − ri(0)|2 ⟩

(2)

Where ri(t) is the position of atom i at time t. As shown in Figure 5a, MSD evolution with time for the Si atoms at different temperatures shows three-stage glassy dynamics. The ballistic motion ⟨r2⟩ ∼ t2 can be observed as time shorter than 0.2 ps. It is explained by the inertia performance for atoms at 4161


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confirms that the movement of the interlayer atoms contributes to the jump behavior from MSD evolution. Diffusive Motions of Interlayer Water Molecules. The motion of the interlayer water molecules at 300 K is characterized by the MSD of hydrogen atoms, which is illustrated in Figure 6a. MSD is decomposed into the component of hydrogen atoms bonded (Hb) with silicate chains and the component of unbonded ones (Hnb). The MSD for both hydrogen atoms, having long cage stage, demonstrates the glassy nature, which was also proposed by Youssef by using the SPC water model.5 In the current simulation, the mobility for H atoms yielded by the reactive potential is better than that from the empirical SPC model due to free association with and dissociation from neighboring atoms for the hydrogen atoms. In addition, reactive force field also allows categorizing two types of hydrogen atoms by dynamic nature: the Hb atoms, restricted by the Si−O bonding, can only rotate and vibrate at the fixed positions, which show slower mobility than the hydrogen atoms diffusing in the interlayer region. As shown in Figure 6b, for the H atoms with elevated temperatures, the parabolic jump is directly followed by the linear behavior, represent the diffusive stage. This means that the high temperature can enhance the diffusivity of H atoms. When the temperature is lower than 800 K, the diffusivity for the hydrogen atoms does not change significantly. MSD values increase pronouncedly as temperature exceeds 1000 K. From the structural analysis in previous sections, the high temperature results in the breakage of silicate chains and expansion of the interlayer region, which reduces the chemical and geometric restricted influence on the water molecules. Furthermore, the self-diffusion coefficient (D) is derived from the diffusive regime from the MSD curves by following eq 4:

short time. After 0.2 ps, the MSD step to the cage stage and ⟨r2⟩ nearly plateaus, implying the slow down for the atomic motions. This is because the silicon tetrahedron collides with its neighboring atoms and is restricted in the cage that is formed by the surrounding silicate tetrahedron and calcium octahedron in the calcium silicate layers. The length of cage regime is dependent on the temperature: more than 100 ps for Si atoms heated at 500 K, while less than 1 ps for atoms heated at 1500 K. Finally, as Si atoms escape from the neighboring constraints, they undergo diffusive motion, where ⟨r2⟩ ∼ tβ takes place. As shown in Figure 5a, at temperature 1500 K, the MSD in diffusion stage includes the anomalous diffusion with β < 1 (t = 0.6 ps to 5 ps) and normal diffusion with β = 1 (t > 5 ps). The anomalous diffusion is the transition stage from the cage regime to the diffusion regime. The transition regime has been explained as the result of hopping behavior of particles in the supercooled liquid.40 It should be noted that the multistage dynamics of Si atoms is the characteristic of melting silicate glasses phase at high temperature. However, the motions of Si atoms in the silicate glass show long cage stage (∼10 ns) due to the firm restriction from neighboring Q4 species.41 Figure 5b records the evolution for the MSD with elapse of time for the Ca atoms at different temperatures. Generally, the displacements for Ca atoms are larger than those of Si atoms in the diffusive stage, implying worse thermal stability for Ca atoms. Different from MSD for the Si atoms, there is a pronounced jump from 0.4 to 1 ps directly after the ballistic regime. This dynamic feature at the initial diffusive stage for the Ca atoms resembles that of liquid. The heterogeneous dynamics for the Ca atoms in the layered C−S−H gel contributes to the jump in the MSD evolution. As shown in Figure 5c, the trajectories of Calcium atoms in the sheet and interlayer region are recorded. The interlayer calcium atoms, influenced by the unstable water molecules at high temperature, are more likely to escape the surrounding cage and exchange with neighboring atoms. On the other hand, the calcium atoms, rooted in the silicate skeleton, are coordinated with 6 to 8 ONB atoms and have slow mobility. When the Caw atoms step to the diffusive stage, most of the Cas atoms are still constrained in the cage. The jump in the MSD evolution reflects the inhomogeneous dynamic behavior for the Ca atoms. To further confirm the Jump mechanism of calcium atoms in MSD, the self-part of the van Hove function was calculated according to eq 3: Gs(r , t ) =

1 N

2D·t =

i=1

(4)

It can be observed in Figure 6c that the diffusion coefficient of interlayer water molecules increases from 0.0014 to 0.59 × 10−4 cm2/s, as temperature is raised to 1500 K. At normal temperature, the D value is around 100 times slower than that for the bulk water (0.2−0.4 × 10−4 cm2/s). This highlights the slow dynamics of the Si−OH and Ca−OH groups that are strongly implanted in the calcium silicate sheets. On the other hand, the elevated temperature can improve the diffusivity for interlayer water molecules and transform the glassy dynamic nature of water molecules to liquid-like properties. Figure 6d marks the water molecules and hydroxyl groups with individual square displacement less than 4 Å2 at 100 ps, which is assumed to be atoms restricted in the cage. As expected, the number of molecules confined in the interlayer region progressively decreases with elevated temperature. In particular, at 1000 K, most of the hydrogen atoms in the Si−OH group remain in the original positions, implying the ultrastrong O−H bond strength for the chemical adsorbed water. The only motions available to these hydroxyls are the rotations and vibrations at fixed positions, as the water molecules restricted in the crystalline. In Figure 6e, MSD curves are further decomposed into MSDx and MSDz, corresponding to the motion parallel and perpendicular to the nanopore. While MSDx and MSDz are consistent with each other at initial ballistic regime, the MSDx values become larger at diffusive regime. The discrepancy is attributed to the connectivity of C−S−H gels in the xy plane caused by the defective silicate structures. Therefore, the high

N

∑ ⟨δ(r − |ri(t ) − ri(0)|)⟩

1 ⟨|ri(t) − ri(0)|2 ⟩ 3

(3)

The physical meaning of 2πrGs(r,t) is the probability that the atom i has moved a distance of r with escaping time t.42 Figure 5d describes the van Hove functions of total calcium atoms in C−S−H gel at 800 K during 25 ps. At short time scale (∼0.25 ps), the calcium atoms are constrained in the cage. In the range of 0.25 to 1 ps, calcium atoms accumulate at the hopping site characterized by small shoulder of Gs(r,t) at 1.5 Å, which is half the length of Ca−O bonds. With increasing time, the shoulder shifts toward farther distance. When the time reaches 25 ps, some of the calcium atoms can move farther than 3 Å. Moreover, the Gs(r,t) at 25 ps is decomposed into Cas and Caw components. All displacement of Cas atoms is smaller than 1 Å, implying that the atoms remain in the cage during the simulation time. On the other hand, the distribution for Caw atoms shows a wider shoulder ranging from 1 to3.5 Å. It 4162


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Figure 7. (a,b,c) Time-correlated function for the Si−O bond, Ca−O bond, and O−H bond at different temperatures; (d) the reaction pathway for the proton exchange between silicate chains and interlayer water molecules.

sequence: Si−O > Ca−O > O−H bonds, according to the reduction extent of the C(t). In Figure 7a, the C(t) values remain constant at 500 K. There are few deviation for the C(t) values, as temperature is raised to 1000 K. C(t) can rapidly return to 1, implying quick healing for the silicate bonds. This process can be explained by the reversible Si−O breakage process due to chemical adsorption of water as discussed in section 3.2. As temperature exceeds 1300 K, C(t) fluctuates to some extent and decreases to about 0.97. It means that some Si−O bonds are completely stretched broken due to the thermal effect. The small reduction for the time correlated function indicates that silicate connectivity maintains integrity. That is why only a small percentage of Q species depolymerizes and polymerizes in the previous section. On the other hand, the C(t) for the Ca−O reduces to a greater extent, as compared with that of Si−O connections. The thermal instability for the Ca−O bonds is mainly attributed to the interlayer calcium atoms, which are restricted by the weak H-bonds. Nevertheless, in the presence of stable calcium sheets, the TCF can persist at around 0.9 even at 1300 K. Unexpectedly, the TCF for the O−H bonds degrades significantly. At normal temperature, in the equilibrium state,

temperature contributes to the diffusivity for the water molecules and the connectivity for the nanopores, and structural water molecules, ultraconfined by disordered calcium silicate substrates, can gradually escape from the interlayer region. The flow of water molecules in the C−S−H gel driven by the temperature can lead to the loss of the interlayer atoms, finally causing the collapse of the layered structures.8 This process of interlayer water missing will be further investigated in future work. Time-Correlated Function for Chemical Bonds. The time correlated function (TCF), as described in eq 5, is utilized to describe the dynamical properties of various chemical bonds. C(t ) =

δb(t )δb(0) δb(0)δb(0)

(5)

where δb(t) = b(t) − ⟨b⟩, b(t) is a binary operator that is equal to 1 if a chemical bond (e.g., Si−O, Ca−O, or H−O) is formed and otherwise zero, and ⟨b⟩ is the average value of b over the entire simulation time and all chemical bonds. The bond strength can be estimated by calculating the deviations from the value of 1 in the TCF. In general, as shown in Figure 7a,b,c, the thermal resistance for different bonds ranks in the following 4163


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Figure 8. Stress−strain relation for the C−S−H gel tensioned along the y direction at 500, 600, 700, 800, 1000, and 1300 K.

are completely dehydrated and Si−OH and Ca−OH groups are missing at 1200 K. 3.3. Mechanical Properties. Stress−Strain Relation. Stress−strain curves are used to assess the mechanical performance of C−S−H gel in the whole tensile loading process. Figure 8 lists the stress−strain curves of the C−S−H samples at different temperatures. At relative low temperature (500 K), stress increases to the failure strength 9 GPa at the strain around 0.15 Å/Å, directly followed by a quick drop to 6 GPa at strain 0.2 Å/Å. Subsequently, the stress steps into a plateau region and persists unchanged until the strain reaches 0.4 Å/Å. In the end, the stress reduces slowly. When the temperature is lower than 1000 K, the turning point at 0.2 Å/Å can be clearly observed in the stress−strain relations, implying the structural rearrangement for the C−S−H gel. With elevating temperature, the tensile strain at failure state gradually decrease to around 0.1 Å/Å and the fluctuations of stress become more pronounced. At 1300 K, no plateau can be observed at postfailure stage. It means that the rising temperature weakens the toughness for the C−S−H gel and result in different failure mode.

few hydrolytic reactions occur between the water molecules ultraconfined in C−S−H gel. The pronounced decrease for the TCF values is partly due to the water dissociation near the bond breakage and partly caused by the proton exchange between different hydroxyl groups in the C−S−H gel. At relatively low temperature, the proton in water molecules transfers to a neighboring one. Further elevated temperature allows proton transfer between silicate substrate and interlayer water molecules. As shown in Figure 7d, when the temperature is higher than 1000 K, the proton exchange pathway between Si−O chains and water molecule is in the following sequence: Si−OH bonds are stretched and free hydroxyl move close to the dissociated H atom; H+ and OH− associate to a water molecule; the water molecule diffuses Away. Meanwhile, the Si−O bond breakage can also accelerate the hydrolytic reactions, and proton exchange occurs frequently at high temperature. It should be noted that the proton escaping from the chemically bonded Si−OH and Ca−OH groups is the critical stage of dehydration of C−S−H gel. The breakage of OH bonds in silicate hydroxyl groups at high temperature is consistent with the experimental finding that the C−S−H gels 4164


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Figure 9. (a) Young’s modulus and (b) failure strength reduction with temperature.

Figure 10. Molecular structural evolution of C−S−H gel tensioned along the y direction at (a) 500 K and (b) 1300 K at strain 0.2, 0.4, 0.6, and 0.8 Å/Å.

Tensile Strength and Young’s Modulus. As shown in Figure 9a, with progressively increasing temperature from 500 to 1300 K, the Young’s modulus reduces from 80 GPa to less than 56 GPa. The mechanical properties calculated by the ReaxFF match well with experimental results from nanoindentation test (60 GPa)43 and other computational calculations (55−68 GPa).27 It should be noted that the moduli of samples at temperature less than 800 K remain at around 80 GPa, which

are much larger than other samples. No bond breakage and silicate connectivity degrade happens in these three samples, in which the silicate skeleton is relatively stable. When temperature is higher, water molecules themselves can also significantly weaken the stiffness of the C−S−H gel. Additionally, the macro-level mechanical experiments on heated and sealed concrete (do not allow water loss, similar to the current simulation case) show that the elevated temperature weakens 4165


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Langmuir the elastic moduli up to 1000 K.44 In previous research, Hou et al.8 investigated the mechanical properties of the C−S−H gel with different water amounts. It was proposed that losing interlayer water molecules can improve the chemical bond connections and further enhance the mechanical performance of C−S−H gel in the interlayer direction. The dehydration of C−S−H gel results in the absence of water molecules in the interlayer region at temperatures higher than 1000 K, so the mechanical properties of the C−S−H gel might be enhanced to some extent. Current research, not simulating the water loss at high temperature, underestimates the strength and modulus of C−S−H gel at 1300 K. A similar evolution trend of tensile strength can be observed in Figure 9b. Tensile strength decreases from 9 GPa in dry state to 5 GPa in saturated state. The difference of C−S−H gel tensile strength at low and high temperature states indicates different failure mechanisms. The stability of chemical bonds in the C−S−H gel determines the mechanical performance in different temperature states to a large extent. As discussed about TCF in the previous section, the bond strength for Si−O, Ca−O and OH is significantly weakened due to the elevated temperature. This means that the lifetime for the chemical bonds is shortened to a great extent. In particular, the H-bonds and interlayer Ca−O bonds are frequently broken and formed, losing the tensile resistance. With increasing temperature, the dynamic nature of interlayer water molecules transforms from a glassy feature to the liquid-like characteristic. The fluidity at high temperature further increases the disjoining pressure and results in the swelling of C−S−H gel. Molecular Structural Evolution. The configurations from strain = 0.2 Å/Å to strain = 0.8 Å/Å are listed in Figure 10a,b to qualitatively illustrate the fracture process of the C−S−H gel at 500 and 1300 K, respectively. As shown in Figure 10a, in the elastic regime, Si−O bonds are elongated, and the Si−O−Si angles are stretched open to carry the tensile loading in the C− S−H gel. In the yield region, the siloxane bonds are gradually stretched broken, resulting in transformation of the silicate skeleton. In the postfailure regime, the cracks in the C−S−H gel grow and coalesce at a very slow rate. Even though the strain reaches 0.8 Å/Å, the layered structure maintains a relative ordered state and is not a tensioned fracture. However, at high temperature, the cracks in the C−S−H gel develop rapidly. This is attributed to the fact that the chemical bonds become unstable and the bond strength is weakened significantly due to thermal effect. In particular, the silicate skeleton is depolymerized into separated short chains at high temperature, which results in the quick failure style of the C−S−H structure. Chemical Reactions during the Tensioned Process. The connectivity evolution of the silicate chains can be characterized by the Qn species. At 500 K, as the strain is less than 0.2 Å/Å, the Qn species persist unchanged, implying that the silicate chains are not stretched broken during the elastic and yield region. This corresponds to the Si−O−Si stretching observed in the structural evolution. As strain varies from 0.2 to 0.4 Å, the Q2 species suddenly decreases with increasing of Q1, indicating the long silicate chains breakage to short ones. The bond breakage is consistent with the stress-drop process in the stress−strain relation from 0.2 to 0.4 Å/Å. This indicates that at relatively low temperature, the silicate chains play an important role in resisting tensile loading. On the contrary, the Q species continuously changes during the tensile process, yet the variation ratio is very low. When the C−S−H gel suffers both tensile loading and thermal attacking, the skeleton role of Si−O

bonds is weakened to a great extent. As discussed in the Silicate Morphology section, the silicate chains have depolymerized at the bridging silicate tetrahedron. Especially, the 5-fold silicon structure in the chains is likely to be stretched broken with small tensile strain. As shown in Figure 11b, despite at very small strain less than 0.1 Å/Å, the Q2 species is still breaking. In this respect, the mechanical loading and thermal effect accelerate the failure of the structures. As shown in Figure 12a, at 500 K, the number of water molecules changes slightly as the strain is smaller than 0.2 Å/Å. As strain increases to 0.2, the number of water molecules

Figure 11. Q species evolution at (a) 500 K and (b) 1300 K; (c) mean silicate chain length reduced with tensile loading. 4166


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(2) Dynamically, the Si atoms play a skeleton role in bridging the structure and have strong thermally resistant ability even at 1500 K. The calcium atoms, categorized into interlayer calcium atoms and those rooted in the sheet, at thermal conditions demonstrate inhomogeneous dynamic behaviors. Additionally, at high temperature, the proton exchange between neighboring water molecules and calcium silicate substrate frequently happens. The hydrogen atoms ultraconfined in the nanopores, except those associated with ONB in the silicate chain, gradually lose their glassy dynamic nature and freely diffuse among the defective silicate channel. (3) Mechanically, both stiffness and cohesive strength are significantly weakened due to the breakage of silicate chains and reduced stability for the chemical bonds. Coupled thermal action from elevated temperature, tensile loading, and water attacking deteriorate the mechanical performance of chemical bonds such as Si− O, Ca−O, and H-bonds.

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research is financially supported from National Natural science foundation of China (51508292), Shandong Provincial Natural Science Foundation (2014ZRB01AE4), and China Ministry of Science and Technology (2015CB655100). All the organizations mentioned above are greatly appreciated.

Figure 12. Hydroxyl number changes during the tensile process at (a) 500 K and (b) 1300 K.

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REFERENCES

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reduced is accompanied by the increasing number of Si−OH and Ca−OH groups, implying widely hydrolytic reactions. Interestingly, the number of Si−OH and Ca−OH is consistent with each other at the initial stage, yet the number of Si−OH deviates to higher values after 0.2 Å/Å. This can be explained by the fact that the Si−O bond breakage (from 0.2 to 0.4 Å/Å) results in water dissociation near the nonbridging oxygen site and only Si−OH groups are produced. On the other hand, as shown in Figure 12b, the number of Si−OH and Ca−OH both increase simultaneously and the increased amount is the same at 1300 K. This means that the water dissociation widely occurs near the broken Ca−O bonds.4

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CONCLUSIONS The molecular structures, dynamics, and reactivity of water molecules ultraconfined in the C−S−H gels at temperatures from 500 to 1500 K were studied by using the reactive MD method. Many conclusions can be made in this study. (1) Structurally, the elevated temperature results in the swelling of the C−S−H gel along the interlayer direction due to the progressively increasing disjoining pressure. The silicate chain breakage occurs at the bridging silicate tetrahedron due to thermal extension of Si−O bonds. Water molecules diffuse from the interlayer region and penetrate to the distorted calcium silicate sheet, which further deteriorates the thermal damage. 4167


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