Molecular Modeling of Capillary Transport in the Nanometer Pore of

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Molecular Modeling of Capillary Transport in the Nanometer Pore of Nanocomposite of Cement Hydrate and Graphene/GO Dongshuai Hou, Qingen Zhang, Jianhua Zhang, and Pan Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02539 • Publication Date (Web): 06 Jun 2019 Downloaded from http://pubs.acs.org on June 7, 2019

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The Journal of Physical Chemistry

Molecular Modeling of Capillary Transport in the Nanometer Pore of Nanocomposite of Cement Hydrate and Graphene/GO

Dong shuai Houa,Qing en Zhangb, Jian hua Zhangc,Pan Wangd* a. Professor, email address: [email protected], affiliation:

(1) Department of

Civil Engineering, Qingdao Technological University ， No. 11, Fushun Road, Shibei District, Qingdao 266033, China;(2) Collaborative Innovation Center of Engineering Construction and Safety in Shandong Blue Economic Zone, Qingdao266033, China b. Master of Engineering, email address: [email protected],

affiliation:

Department of Civil Engineering, Qingdao Technological University ， No. 11, Fushun Road, Shibei District, Qingdao 266033, China; c. Associate Professor, email address: [email protected]; affiliation: Harbin Engineering University, Harbin, China; d. Corresponding author, Associated professor, email address: [email protected], affiliation: (1) Department of Civil Engineering, Qingdao Technological University ， No. 11, Fushun Road, Shibei District, Qingdao 266033, China;(2) Collaborative Innovation Center of Engineering Construction and Safety in Shandong Blue Economic Zone, Qingdao266033, China

Abstract

The excellent impermeable nature of graphene-based nanomaterial makes it potential membrane to repel detrimental ions invasion. To evaluate the ions resisting properties by different graphene materials, molecular dynamics (MD) is utilized to investigate the water and ions transport in nanometer channel of the calcium silicate hydrate (C-S-H) substrate impregnated with single-layer graphene sheet, graphene oxide sheet functionalized by hydroxyl and carboxyl (GO-OH, GO-COOH). The transport rate and diffusivity of fluid is greatly dependent on the types of functional groups in coating sheet. The van der Waal interaction between graphene sheet and C-S-H gel is

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weakened dramatically by the invading of ions and water molecules, resulting in the dissociation of graphene sheet from the C-S-H surface. The detached graphene sheet contributes little to repelling water and ions. On the other hand, the hydroxyl and carboxyl groups in the GO sheets provide plenty of oxygen sites to accept the H-bonds and to associate with the neighboring sodium ions, which immobilizes the water molecules and ions on the GO surface. The GO-COOH sheets, deeply rooted on the C-S-H, further block the transport channel connectivity and “cage” the water and ions in the entrance region of gel pore. Hopefully, this study can provide valuable insights on the design of graphene oxide membrane to enhance water resistance for sustainable cementitious composite.

Keyword: Graphene oxide membrane; calcium silicate hydrate; capillary transport; H-bond; ionic pair; Time correlated function.

Introduction

Cement-based materials have been ubiquitously utilized in construction industry and infrastructure all over the world. The manufacture of the cement-based material requires burning the limestone and clay at high temperature (1450o C) [1]. Around 1.5 tons of raw materials are needed in producing every ton of Portland Cement (PC), while about 1 ton of carbon dioxide (CO2) is released in to the environment during the production[2]. Currently, the manufacture of the cement-based material results in about 6-8% of the yearly man-made global CO2 emissions[3]. In the respect of sustainability, the production of PC is extremely resource- and energy-intensive, and environmental unfriendly. Despite of great efforts paid on the development of the supplementary cementitious materials[3] in concrete and recycling techniques of concrete[4], there are no materials to replace the PC as the main construction material and the consumption of PC grows steadily every year. Therefore, in order to effectively lower the carbon foot print during cement production, it is necessary to enhance the durability of cement-based material. The durability of cementitious materials is weakened by


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various types of damage, and most of these deterioration processes are mainly attributed to migration of water and detrimental ions in the pores of cement hydrate[5] [6].

Recent study showed that incorporation of the hydrophobic graphene-based

material into cement matrix can enhance the water resistance and inhibit the ingress of corrosive ions in the cement-based materials[7]. The defect-free graphene sheet, impermeable to all gasses and liquid, makes it promising candidate as a barrier material [8] [9]. However, it is difficult to produce large area of defective free graphene sheet, which restricts the application of graphene as the protective membrane[10] [11]. The effective solution is to substitute the graphene sheets with laminates of its chemical derivative called graphene oxide(GO) that are easily produced and cheaply coated by spraying GO solution on the surface of cement-based material layer by layer[12]. The oxygen functional groups, attached on the basal planes and edges of GO sheets, significantly alter the van der Waals interactions between the GO sheets and therefore improve its dispersion in water. Good dispersion guarantees that the GO sheet remains the nanoscale nature, such as high surface area, good water resistance, and mechanical strength. In this way, the GO sheets can fill the nanometer pores of cement hydrate and the functional groups in the GO sheets can interact with the atoms in the surface of cement hydrate[13]. In particular, the functionalized GO sheets have unique transport properties and act as the molecular sieves that allow small solution species transport and reject large ones [14].

Many experimental efforts have been devoted to investigate on incorporation of GO material into the cement hydrate. Mohammed et al.[15] have dispersed graphene oxide into cement mortar to produce graphene oxide cement composite using incorporation of 0.01%, 0.03% and 0.06% by weight of cement. The transport related properties, such as water sorptivity, chloride penetration have been systematically studied for GO cement composite. Experimental results indicated that low fraction of GO (0.01%) incorporation can effectively hinder chloride ingress. Similar chloride penetration resistance and the ions transport resistance of GO reinforced cement composite have been reported by He and Han[16]. Furthermore, Tong et al.[17] have conducted



durability-related tests including the deterioration and freeze-thaw tests on the GO and cement composites. They found that GO sheets have the potentials to decelerate the chemical attack induced by an acidic solution and can improve the freeze and thaw performance of cementitious materials. Based on the experimental findings, the good ions transport resistant ability of GO and cement composite can be attributed to four primary reasons. Firstly, the graphene oxide sheets with high specific surface area, playing nuclei role, contribute to the cement hydration and reshape the morphology of the cement hydrate[18]. This can further influence on the tortuosity of the water migration channel. Secondly, well-dispersed GO solution can heal the defective region in the cement matrix by forming the interlocked layered structure. This can improve the pore size distribution, reduce the total porosity and block the channel connectivity[19]. The third reason is the strong chemical bonding between the GO sheet and the main hydration product of cement material. To better illustrate the intrinsic interfacial interaction, computational chemistry methods such as molecular dynamics (MD) have been conducted to investigate the interfacial atomistic structures, reactivity and dynamic properties between cement hydrate and graphene oxide[20-22]. The simulation work sheds valuable nanoscale lights on the critical role that the chemical bonds play in bridging cement hydrate and functionalized GO sheets.

Despite many studies on the reinforced cement material by GO sheets, the research is limited on the water and ions transport in the nanometer channel of GO and cement composite, and the fundamental transport mechanism has not been completely understood. In the marine environment, the cement-based concrete material often suffers the steel corrosion induced by chloride penetration[23] [24] and degradation of hydration products caused by sulfate attacking[25]. The sulfate and chloride ions transport in pores of cement hydrate dramatically influences the durability of cement-based material. Calcium silicate hydrate (C-S-H) gel, accounting for about 60% of hydration products, is the most important binding phase in cement-based material. It is a porous gel with gel pores characterized in sizes ranging from 0.5 nm


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to 10 nm, where the water and ions can transport through[26]. The purpose of the paper is first to reveal mechanism of water and ions migration in gel pore of C-S-H and to effectively prevent the water and ions penetration by incorporating graphene-based material. In this study, molecular dynamics is utilized to investigate the NaCl and Na2SO4 solution transport in nanometer channel of the C-S-H, and C-S-H channel with interior surface grafted by graphene sheet and graphene oxide sheet. To consider the effect of functional groups on the transport properties of ions and water, the incorporated GO sheets are functionalized by hydroxyl (C-OH) and carboxyl (COOH).

Simulation method

Model construction. The water and ion transport model consists of three parts: C-S-H gel pores, NaCl solution, GO/G sheets. As shown in Figure 1, the C-S-H gel pores are constructed by two C-S-H layers separated by 3.5, 4.5 and 5.5 nm, respectively. The selected pore width is in the range of gel pore size that distributes from 0.5 to 10 nm, as proposed in previous research[26]. The double layers are parallel to the y-axis. The thickness of one side of the C-S-H substrate is about 17.5 Å. The C-S-H layers are perpendicular to the NaCl solution. Subsequently, two GO/G sheets are placed at the interior C-S-H surface at the entrance of the C-S-H pore. The distance from the graphene substrate to the adjacent C-S-H layer is 10 Å. Different from directly putting the graphene sheet close to C-S-H layer, the setting of the distance of 10 Å allows graphene substrate to freely associate with the adjacent C-S-H layer. Graphene and GO sheets can be immobilized by C-S-H layer at the energy preferable positions rather than arbitrary arrangement. The size of single layer GO/G plate was 19.68 Å × 21.3 Å. The volume of the solution is 22.6 Å × 100 Å × 70 Å, which contains 5196 water molecules. The number of water molecules in the solution region is assumed to satisfy the density of bulk aqueous solution under ambient conditions (1 g/cm3). 90 Cl- ions, and 90 Na+ ions are added to simulate 1 mol/L of



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NaCl solution, and 90 Na+ and 45 SO42- ions are added to model 0.5 mol/L of Na2SO4 solution. The usage of lower concentration of Na2SO4 solution is attributed to that the sulfate ions at high concentration are easily to associate with Na ions and grow to large ionic cluster. The C-S-H substrates are obtained from the cleaved surface structure of the tobermorite 11 Å crystal along [0 0 1] direction as used in previous study15. Earlier many researchers have used similar slit pore models to investigate C-S-H cohesion and water dynamics, and it is widely accepted that the interlayer region of tobermorite is exposed to the solution

[21, 27, 28].

Based on the intrinsic

laminar structure of tobermorite, it is common to expand the system preserving the intralaminar configuration and increasing the basal distance. Due to the structural similarity found from X-ray data[29] [30], the layered tobermorite crystal has been taken as the mineral analogue of C-S-H and long served as a model for C-S-H to study the interfacial behaviour of cement hydrate and solutions. It should be noted that there also exist some differences between C-S-H and tobermorite in the respect of the chemical composition, the silicate chain morphology and the like. This study is a preliminary attempt to investigate the transport of water and ions and the influence of the substrate will be further studied in the short future.

Additionally, part of the

non-bridging oxygen atoms in the silicate tetrahedron are hydroxylated in the C-S-H surface to maintain the charge balance. The graphene model was based on a single sheet of the graphite crystal[31, 32] with cell parameters of a=2.46 Å, b=4.26 Å, c=3.4 Å and α = 90°, β = 90° and γ =90°. The unit cell was then replicated by 8 ×5×1 times along x, y and z directions to obtain the final graphitic structure. Experimental studies reported that a typical fraction of oxygen-rich functional species relative to the amount of carbon atoms in GO is about 20%[33]. Thus, the coverage of 20% (sp3 C/total C) for hydroxyl and carboxyl groups on GO surface is constructed. The GO construction procedure follows the methodology mentioned in the reference[21], and the GO sheet is constructed according to the following requirement: 1) most of carbon atoms located at the edge of GO sheet are functionalized by the carboxyl group or hydroxyl group; 2) the carbon atoms in the surface region are randomly connected with functional groups; 3) the number of functional groups pointing upward is quite


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close to that of groups downward. The periodic boundary condition (PBC) is set in x and z direction for the simulation box. In non-periodic y direction, two invisible walls are fixed and placed more than 50 Å away from the C-S-H gel pore and the solution, respectively. This setting can avoid the atoms escaping to the boundary of the simulation box. Force field and molecular dynamic procedure.In this simulation, the ClayFF force field[34] is employed to describe the interaction between atoms in the NaCl solution and the solid cementitious substrate. Due to its good transferability and reliability, ClayFF has been widely applied to model the interfacial structure, dynamics and energetic properties between minerals and solutions

[35] [36].

The parameters for the

ClayFF model, including the inter-molecular van der Waal interaction and Coulombic interaction, and the intramolecular bond interaction can be obtained from the reference[34] [37]. The inter-atomic potential in graphene phase is described by using the CVFF force field. The CVFF force field includes diagonal terms to describe the energy of deformation of bond lengths, bond angles, torsion angles, and inversions, and non-bonded terms (van der Waals and electrostatic interactions). This force field has been found to be applicable to graphene oxide structures[38]. The potential interaction energy of the Lennard-Jones (12–6) term, modeling the short-range van der Waals dispersive interactions, follows the geometric rule for combination between ClayFF and CVFF force field. The combination of CVFF force field and ClayFF force field has been found to be successfully applied to the interaction between grapheme/graphene oxide and C-S-H/ions solution[38-43]. All parameters in this study were provided in Table S1 in supporting information. The LAMMPS (Large-Scale Atomic/Molecular Massively Parallel Simulator) molecular dynamics code/MD simulator is used to perform the MD modeling on the liquid and solid system. First, an invisible wall is placed at the entrance of the gel pore to block the water molecules and ions in the bulk solution. Afterwards, system is equilibrated at a temperature of 300K for 2 ns using canonical ensemble, so that the atoms in NaCl solution reach an equilibrium state, and the C-S-H/GO and C-S-H/G



interfaces interact to obtain the optimized structure. Finally, the interface wall blocking the gel pore was removed, allowing water and ions to transport freely through the nanometer channel for 3000 ps. During the capillary adsorption stage, the entire simulation system was set up with NVT ensemble, and the trajectories of atoms were recorded every 100 fs for structural and kinetic analysis. The time step is set to 1 fs. The cutoff distance of 10 Å was adopted for the evaluation of the van der Waals interactions. To check the statistical stability for the transport system, the large transport model is simulated for 20 ns and the related analysis is in the supporting information file.

Results and discussion Capillary adsorption of NaCl solution. Figure 2a shows the water molecules progressively migrate from the bulk solution into the 3.5 nm gel pore of C-S-H substrates during 2000 ps. The forward meniscus of water in the nanopore is clearly observed, which is a typical feature of capillary adsorption, and the contact angle of the forward boundary less than 90° means that the C-S-H surface is hydrophilic. Figure 2b shows the water invasion into the nanometer pore of C-S-H grafted with single-layer graphene sheet. As compared with the water and ions penetration process in C-S-H gel pore without graphene treatment, the NaCl solution migration in the nanometer pore is slightly retarded, and it takes more than 2000 ps for the solution to fill the gel pore. It should be noted that the graphene sheet, originally located at the entrance of the C-S-H gel pore, dissociates from the calcium silicate surface and diffuses into the gel pore solution, with the penetration of the NaCl solution. After 2000 ps, the graphene sheets are completely detached from each side of C-S-H substrate. It implies that the invading water and ions push the graphene sheet forward, weaken the van der Waal connection between C-S-H and graphene sheet, and further results in the graphene dissociation from the substrate. Even though graphene sheet has strong hydrophobic behavior, the detached graphene sheet has little contribution to repelling the water and ions.


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On the other hand, as shown in Figure 2c and Figure 2d, the water and ions transport rate is significantly slowed down, as they penetrate into the C-S-H gel pore with the entrance arranged by the graphene oxide sheets. As shown in Figure 2c, the ions and water molecules have just climbed around 5 nm after 2000 ps in the nanometer pore of the graphene oxide sheet modified by the hydroxyl functional groups. In particular, as shown in Figure 2d, water and ions remain in the entrance region of the nanometer pore of GO sheet with COOH groups from 1000 ps to 2000 ps. After 3000 ps, NaCl solution only transports forward less than 3 nm due to the inhibition role from the GO-COOH sheets. It should be noted that different from the graphene sheet dissociation from C-S-H surface, both GO-OH and GO-COOH sheets are stably bonded with the atoms in the calcium silicate substrate, when they are subject to the disjoining pressure from the NaCl solution. The intrinsic binding structure between silicate chains, calcium ions and functional groups in GO sheets will be discussed in the following sections. The dramatically slow water and ions migration can be attributed to narrowing of the ink-bottle shape gel pore with inner surface rooted with the graphene oxide sheets. Even though the functional groups such as –OH and – COOH enhance the hydrophilicity of the graphene sheet [44], water molecules and ions are hard to penetrate into the C-S-H gel pore. It is unexpected that the C-S-H impregnated with hydrophilic GO sheets shows better water resistance than that with hydrophobic graphene sheets. It is valuable to note that the water repelling effectiveness of the coating material on cement hydrate is not only dependent on the water impervious ability of the coating material, but also relies on the life time that the coating remains on the C-S-H surface.

The time dependent invading depth of solution is detected by the trajectory of water and ion boundaries. As shown in Fig. 2a, the lowest point in the center of the capillary menisci is defined as the solution advancing front. Fig. 3a shows that the penetration depth of water and ions in the gel pore increases with time. It can be found that the water and ion transport curves are almost overlapped with each other, indicating the simultaneous transport of solution species. The capillary transport process of water



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and ions in the C-S-H gel pores can be divided into two stages. In the first 250 ps, water penetration depth increases linearly with time. This constant velocity of invading water is consistent with the theory proposed by Bosanquet

[45]

that the

inertial drag and capillary force accelerate the fluid at the very beginning. The negative charged non-bridging oxygen atoms in silicate chains and the surface cation form the electronic field. It can produce strong coulombic dragging force on the water molecules. Subsequently, the menisci advancing front invades slowly, gradually departs from the constant velocity regime and follows a parabolic relation with time that is consistent with the Lucas–Washburn equation (LW)[46] in classic capillary adsorption theory. The reduction of the capillary adsorption rate is mainly attributed to the viscosity resistance as the solution penetrates in ultra-confined nanometer pore.

Figure 3b, 3c and 3d exhibits the imbibition of water, Cl and Na, respectively, in C-S-H gel pore with different graphene and graphene oxide treatment. As shown in Figure 3b, in the constant velocity regime, the water penetration depth and rate in coated C-S-H gel are same with those in C-S-H gel without coating. During the first 250 ps, water molecules rapidly advance around 3 nm in the C-S-H gel pore grafted by the graphene and graphene oxide sheets. After the short constant velocity stage, it can be observed an obvious plateau where the water penetration depth remains almost unchanged. This plateau in the temporal penetration curve is mainly attributed to the pinned effects of graphene or graphene oxide from the graphene and graphene oxide edge that strongly restricts the movement of the contact line (the three-phase boundary) for NaCl solution[47]. This resembles the “cage” stage for the mobility of water molecules ultra-confined in the nanometer channel

[48].

The cage stage length

depends on the confinement from the graphene and graphene oxide sheet. The cage stage takes around 200 ps, 500 ps and more than 2000 ps for the NaCl solution transport in the graphene, GO-OH and GO-COOH interior surface, respectively. It should be noted that the graphene sheet dissociates from the interior surface of C-S-H and water molecules can transport through the gel pore entrance rapidly, as shown in Figure 2b. After the cage stage, the penetration process steps into the visco-inertia


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stage. As compared with water transport in C-S-H gel without coating, the transport rate is reduced in the visco-inertia stage for capillary adsorption of water in the graphene/ GO treated C-S-H gel. Similarly, as shown in Figure 3c and 3d, the Na and Cl ions transport in different nanometer pores resembles the capillary adsorption process of water molecules, and their transport rate ranks in the following order: Graphene > GO-OH > GO-COOH. In addition to penetration depth of the water and ions in advancing front, the change of the penetrated water density and ionic concentration are characterized by the evolution of y-direction intensity profile of water molecules, Na and Cl ions in the supporting information file (Fig.S1). In addition, similar dynamic transport process for Na2SO4 solution was also studied as presented in supporting information. The inhibition of ingress for sulfate by the GO sheets provides another evidence that the GO nanosheets can help improve the durability of cement-based materials.

Considering that C-S-H gel has complicated

nanometer channels, the pore size effect on transport properties of NaCl solution is studied by modeling 3.5nm, 4.5 nm, 5.5 nm gel pores and these gel pores impregnated with GO sheets, as presented in supporting information. For solution transport in gel pore with larger size, the length of plateau in penetration curves turns shorter. In the 3.5nm gel pore, both the narrowing effect and the immobilizing effect resist the ingress of solution species. In the gel pore with size larger than 4 nm, the narrowing effect turns quite weaker and the attraction from the GO-COOH nanosheets play predominated role in repelling solution.

Local structure of water and ions on the surface of C-S-H and GO sheets. The density profile normal to the water transport direction can provide insights on the molecular structures between solution species and the C-S-H substrate. Figure 4 shows the density distribution of solution species confined in the C-S-H gel pore along z direction. Correspondingly, Figure 5 exhibits the molecular structure of NaCl solution confined in the C-S-H gel and graphene/GO sheets. As shown in Figure 4a, there are pronounced peaks of the water density profile on each side of the interior C-S-H surface. It reflects the layered structure of water packing in the vicinity of



hydrophilic C-S-H surface. The sharp intensity peak of sodium ions is observed at the interfacial region, implying that the sodium ions prefer to adsorbing on the negative charged C-S-H substrate. As compared with sodium ions, there are less chloride ions distributed within 5 Å from the C-S-H substrate, which reflects repulsing effect on the anions. As shown in Figure 4b, the graphene sheet disturbs the water distribution in great extent. Since the graphene sheet is not fixed on the interior surface, water intensity peak is not clearly distinguished at the interface between graphene and solution. On the other hand, as shown in Figure 4c and 4d, the sharp peak with high intensity for carbon atoms indicates that the graphene oxide sheets are associated with C-S-H surface and fixed in the channel entrance, which reduces the gel pore size from 3.5 nm to around 2 nm. Multi-peaks of the water and ions can be observed in their intensity profile. Because the pore size of C-S-H gel is quite narrow, there are only a small number of ions penetrated into the gel pore. The penetrated ions are distributed randomly on each side of the layers instead of symmetry distribution. This asymmetry distribution turns more pronounced in gel pores coated with GO sheets. Because the atoms in the solid substrate substitute part of the neighbors of water molecule and ions, the H-bond structure of water and hydration structure are dramatically changed for ions in the interfacial region of the C-S-H surface. This will be further discussed in the following section. Additionally, as shown in Figure 5b, 5c and 5d, there are water molecules between graphene/ GO sheets and the C-S-H gel. Water invading in the interfacial region can weaken the bond strength between functional groups and the calcium silicate structure. In previous study, the interfacial binding energy is dramatically reduced by the percentage of water content, as epoxy-silicate composite is immersed into water solution

[49].

In the respect of chemical bonds, the water

molecules, with high mobility, can frequently attack the interfacial connection, and weaken the stability of silicate or graphene oxide skeleton. The hydrolytic weakening effect in calcium silicate hydrate and graphene oxide sheets has been widely found[50],[51].

The radial distribution function (RDF) between solution species and atoms in the solid


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substrate can further help understand the local structure of water and ions in the vicinity of C-S-H and graphene oxide substrate. As shown in Figure 6a, the RDF of Os-Ow (Os: oxygen atom in silicate chains; Ow: oxygen atom in water molecule) and Os-Na have the sharp peak locate at 2.70Å, and 2.35Å, respectively. The former one represents the H-bond formed between surface water molecules and oxygen atoms in silicate tetrahedron. The later one reflects the strong spatial correlation between sodium atoms and silicate oxygen. In Figure 7a, the silicate chain can provide the non-bridging oxygen atoms to accept the H-bond from surface water, and the hydroxyl can donate H-bond to neighboring water. Meanwhile, the C-S-H surface can associate with the sodium ions by forming Na-Os connection. It can be observed in Figure 6b that the RDF of Caw-Cl and Caw-Ow(Caw: Ca atoms in the interface) has the pronounced peak with the position of 2.45Å, and 2.90Å, respectively. This represents the hydration structure of the surface calcium ions, including the neighboring water molecules and chloride ions in the ionic pairs, as shown in Figure 7a. As shown in Figure 6c, no obvious peaks can be observed in spatial correlation of C-Ow, C-Cl, C-Na and C-Os. Water molecules and ions, distributed more than 3Å away from the graphene sheet, reflects the hydrophobic characteristic of the graphene surface. This explains that the dissociated graphene sheet, blocking the nanometer channel, prevents the water and ions from invading into the gel pore.

Figure 6d and Figure 6e show the RDF between oxygen atoms in functional group in GO sheets and solution species. The RDF of Oc-Ow (Oc: oxygen atom in the functional groups of GO sheet) in GO-OH and GO-COOH has a sharp peak located at distance of 2.70 Å and 2.75 Å, respectively. This is due to that Oc atoms in the functionalized group accept H-bond from surface water or Hc atoms (Hc : hydrogen atom in the functional group of GO sheet) donate H-bond to water. It implies the hydrophilic nature of the GO-OH sheet and GO-COOH sheet. On the other hand, as shown in Figure 6d, only a small shoulder exists at the distance of 2.5Å for the Oc-Na RDF, implying that the sodium ions are not affinity to adsorbing on GO-OH sheet. In particular, the RDF of Oc-Cl indicates that the chloride ion is distributed 3 Å away



from the GO-OH sheets.

As compared with GO-OH, the GO-COOH sheets show different interactions with neighboring ions by the RDF analysis. The RDF of Oc-Na has three pronounced peaks located at the distance of 2.40 Å, 3.45 Å, and 5.0Å. The carboxyl group has two oxygen sites that are more probable to capture the sodium ions. While the first two peaks in the RDF represent the direct connection between Na and Oc atoms, the third one is from the indirect connection of Oc-water-Na. It should be noted that RDF of Oc-Cl has a sharp peak with high intensity located at 3.15 Å. This is partly due to the H-bond formation between Cl and hydrogen atom in COOH, partly due to the fact that the multi-layer adsorption of sodium ions in the GO-COOH surface also contributes to binding the chloride ions by forming ionic pairs of Na-Cl. The discrepancy between GO-OH and GO-COOH indicates that the ions immobilizing ability of the graphene oxide sheets is greatly dependent on the polarity of the functional groups. The ions are probable to adsorb and accumulate on the GO-COOH sheets with more oxygen sites.

Furthermore, the RDF is also utilized to investigate interfacial connection between C-S-H and GO sheets. As shown in Figure 8a, the RDF of Oc-Caw and Oc-Os has the first peak at the distance of 2.54 Åand 2.58 Å, respectively. It means that the Caw in the silicate sheets can form bond with oxygen atoms in the functional group of GO-OH. As shown in Figure 9a, the Caw atoms play intermedia role in bridging the non-bridging oxygen atoms in the silicate chains and the oxygen atoms in the GO sheets. The Oc and Os atoms form coordinate atoms of central Caw, and the Oc-Ca-Os connection strengthens the binding between C-S-H and GO sheet. In addition to the Ca-Oc bond, the peak of Oc-Os also indicates that Si-OH and C-OH can form H-bond, contributing to the interfacial binding.

It can be observed in Figure 8b that two high intensity peaks of Ca-Oc RDF are located at 2.47 Å and 2.68 Å, respectively. The double peaks are attributed to the fact


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that the interlayer calcium ions have more coordinate oxygen atoms from the carboxyl groups. As compared with Ca-Oc in GO-OH, more coordinate number of Oc atoms can greatly improve the interfacial cohesive strength. In the RDF of Ca-Oc in both GO-OH and GO-COOH, the sharp peaks can extend as long as 8 Å, implying the strong spatial correlation between Ca atoms and atoms in the GO sheet in the long range.

Dynamic properties of the water and ions confined in the nanometer channel.The mean square displacement (MSD)

[52],

characterizing the translational motion

behavior for the atoms in the solution species, is calculated according to the following equation.

MSD(t ) 

ri (t )  ri (0) 2

(1)

Where ri(t) represents the position of atom I at time t，ri(0) is the original position for atom i. MSD (t) describes that atoms deviate from their initial position as the function of time, which is commonly used to characterize the diffusion behavior for liquid. As shown in Figure 10a, the MSD of water molecules transport in nanometer pore ranks in the following order: C-S-H > Graphene > GO-OH > GO-COOH. It is consistent with the transport rate obtained from the temporal penetration curve in Figure 3. It means that the graphene-based material, coated on the gel pore, can significantly reduce the diffusivity of water and ions. It reflects the excellent water repelling ability of the graphene-based material.

Furthermore, as shown in Figure 10b, the MSD of carbon atoms is calculated as the function of time in graphene, GO-OH and GO-COOH sheets. After 3000 ps, while the MSD of graphene sheets rapidly increases to nearly 3000 Å2, the MSD of GO-OH and GO-COOH remains at around 5 Å2. The graphene sheets, escaping from the C-S-H surface, transport freely with water molecules. On the other hand, the atoms in GO



Page 16 of 38

sheets, restricted on the C-S-H gel surface by the Oc-Ca-Os and H-bond, can only vibrate and rotate on the fixed positions. The small MSD value of GO sheet also confirms the stable interfacial structure between C-S-H and GO sheets.

Furthermore, time correlated function (TCF)

[53],

describing pair dynamics of

ion-water, ion-solid and ion-ion, is calculated by the following equation. C (t ) 

b(t )b(0) b(0)b(0)

(2)

Where δb(t) has binary value, if bond is connected, the value is equal to one, otherwise equal to zero. TCF can also evaluate the average lifetime for different chemical bonds and ionic pairs. Correspondingly, the resident time for ionic pair can be obtained by integrating the TCF. As shown in Figure 11a, the TCF of Oc-Hw (Hw : hydrogen atom in water molecule) in the GO-OH and GO-COOH degrades quite faster than that of Os-Hw. The resident time for water remaining on the functional groups of C-OH, -COOH and silicate chains is 14.66 ,17.7and 39.67 ps, respectively. The strength of H-bond formed between water and functional groups on the graphene oxide sheet is quite weaker than that formed by oxygen atoms in silicate chains and surface water. It also reflects that the calcium silicate surface is more hydrophilic than that of graphene oxide sheet. Furthermore, it can be observed in Figure 11b that the TCF of Oc-Na and Os-Na decreases more slowly, as compared with that of Oc-Hw and Os-Hw. It implies that the sodium ions remain at the C-S-H and GO-COOH sheet for longer time due to the stable Na-O bonding. In particular, the TCF of Oc-Na remains constant during 100 ps. The sodium ions are firmly immobilized on the carboxyl groups that provide plenty of non-bridging oxygen sites to associate with Na ions.

It means that the functional groups immobilize the ions on the interior surface and reject the ingress of ions. In previous study, Graphene nanopores can be designed to select cations and anions by functionalizing the nanopore with either nitrogen and fluorine, or hydrogen, respectively

[54].

Recent simulation studies have also shown

altering the size of graphene nanopores and the functional groups can be used to


Page 17 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


generate selective pores

[55].

The nanometer channel could be coupled with the

rejection of salts and other water contaminants, which can be applied to water purification.

Additionally, the strength of interfacial bond between C-S-H and GO sheet is evaluated by TCF of Os-Oc and Ca-Oc. It can be observed in Figure 11c that TCF of Os-Oc and Ca-Oc slightly reduces to 0.8 and 0.9, respectively after 100 ps. It means that the H-bond and Ca-O ionic bond are hard to break in the water environment. As compared with H-bond connectivity, more stable Ca-O bond plays essential role in bridging the C-S-H gel and GO sheet. In previous study[56][57][58], the reinforcement mechanism of GO sheets on the cementitious is mainly attributed to chemical bonds between C-S-H and GO sheets. In the GO-COOH sheet, most of COOH groups with high polarity and hydrophilicity of the GO sheets form stable COOCa bond with neighboring Ca ions. The COOH can also accept H-bonds from the Si-OH in the C-S-H gel, strengthening the interfacial connection. Furthermore, uniaxial tensioned test on different C-S-H/GO models reveals that C-S-H reinforced with GO-COOH and GO-OH have better interfacial cohesive strength and ductility as subject to tensile loading. On the other hand, the weakest mechanical behavior of the Graphene and C-S-H composite is attributed to poor bonding, dissociation of the functional groups and the instability of atoms in the interface region.

Conclusion In this study, molecular dynamics is utilized to investigate the capillary transport in nanometer channel of the C-S-H substrate impregnated with graphene-based material. To elucidate the transport mechanism of water and ions in nanometer pores, the interfacial molecular structure and dynamics of water and ions in the vicinity of C-S-H surface and graphene oxide substrate have been systematically investigate. Also, three factors influencing capillary transport have been studied: the solution types (NaCl and Na2SO4), the gel pore width (3.5, 4.5 and 5.5 nm), and the types of graphene-based material (graphene, GO-OH and GO-COOH). Following conclusions



can be made:

1. The early stage of water and ions imbibition depth as the function time follows constant-velocity and visco-inertia regimes for water transport in the nanometer channel of C-S-H. The C-S-H surface provides plenty of oxygen sites to accept the H-bond from surface water molecules and associate with the sodium ions. The C-S-H immobilization on the sodium ions results in accumulation of the ions, increasing the ionic concentration of local region of gel pore. 2. The incorporation of graphene and GO sheets in the C-S-H interior surface can resist both chloride and sulfate ions transport in the gel pore. The transport rate of fluid in the nanometer channel ranks in the following order: C-S-H gel > graphene sheet > GO-OH > GO-COOH. The mobility of fluid is greatly dependent on the types of functional groups in coating sheet impregnated in the inner surface of C-S-H gel. 3. The van der Waal interaction between graphene sheet and C-S-H gel is weakened dramatically by the invading of ions and water molecules, resulting in the dissociation of graphene sheet from the C-S-H surface. The detached graphene sheet contributes little to repelling water and ions. 4. The hydroxyl and carboxyl groups in the GO sheets provide plenty of oxygen sites to accept the H-bonds and to associate with the neighboring sodium ions, which resists the ingress of the water molecules and ions. In particular, the surface calcium atoms in the C-S-H gel play medium role in connecting the non-bridging oxygen site in silicate chains and oxygen functional groups in GO sheet, which strengthens interfacial chemical bond. The GO-COOH sheets, deeply rooted on the C-S-H, further block the transport channel connectivity and “cage” the water and ions in the entrance region of gel pore. Hopefully, the molecular dynamics study can help evaluate the water repellent ability of graphene-based material, and guide the selection of the appropriate GO sheets in durability design of nanocomposite of cement-graphene material.


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Acknowledgement Financial support from National Natural science foundation of China under Grant 51678317, 51420105015, the China Ministry of Science and Technology under Grant 2015CB655100, Natural science foundation of Shandong Province under Grant ZR2017JL024, The Fok Ying-Tong Education Foundation for Young Teachers in the Higher Education Institutions of China (Grant No. 161069).

Supporting Information Force field parameters; Evolution of intensity for NaCl solution; Pore size influence on capillary adsorption; Capillary transport of Na2SO4 solution; Simulation time effects. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figures

(a)



Graphene

GO-OH

Page 24 of 38

GO-COOH

(b) Figure 1 (a)Capillary adsorption model includes aqueous NaCl solution, calcium silicate sheets and the graphene or graphene oxide sheets.(b) Molecular structure diagram of graphene or graphene oxide sheets. (The gray stick represents the carbon-carbon bond, the white-red stick represents the hydroxyl bond, the yellow-red stick represents the silicate bond, the green ball represents the calcium atom, and the pale green and purple ball in the solution represent the chlorine and sodium atoms, respectively.)

(a)


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(b)

(c)



(d) Figure 2 molecular structure of water and ions transport in the 3.5nm channel of C-S-H gel at 0 ns, 1ns, 2 ns, 3 ns (a) without water repellent treatment (b) with graphene sheet grafted on the inner surface of C-S-H pore (c) with GO-OH grafted on the inner surface (d) with GO-COOH grafted on the inner surface during 3000 ps.

100

Mean penetration depth

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 38

Ow Cl Na 0.5 y=a1x

80

y=a2x

60

40

20

0 0

400

800

1200

Time (Ps) (a)


1600

H2O G OH COOH

100 80 60 40 20 0 0

500

1000

1500

2000

2500

3000

2000

2500

3000

Time(Ps) (b)

Mean penetration depth of Cl(Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


water penetration depth(Å)

Page 27 of 38

100

H2O G OH COOH

80

60

40

20

0 0

500

1000

1500

Time(Ps) (c)


100

Page 28 of 38

H2O G OH COOH

80

60

40

20

0 0

500

1000

1500

2000

2500

3000

Time(Ps)

(d) Figure.3 (a) penetration depth of water, Na and Cl ions with the evolution of simulation time in the C-S-H gel pore; time dependent penetration depth of (b) water (c) Cl ions (d) Na ions in the C-S-H gel pore (H2O) , C-S-H gel pore impregnated with graphene sheet (G) , C-S-H gel pore with GO-OH (OH) , C-S-H gel pore with GO-COOH (COOH)

Intensity(ab.units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Mean penetration depth of Na(Å)


Os Ow Cl Na

8

4

0 0

15

30

45

Distance along z direction(Å) (a)


60

Page 29 of 38

Intensity(ab.units)

15

Os Ow cl Na C

10

5

0 0

10

20

30

40

50

60

Distance along z direction(Å) (b) 15

Intensity(ab.units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Os Ow cl Na C

10

5

0 0

10

20

30

40

50

60

Distance along z direction(Å) (c)



15

Intensity(ab.units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 38

Os Ow cl Na C

10

5

0 0

10

20

30

40

50

60

Distance along z direction(Å) （d） Figure 4 intensity profile for water and ions confined in (a) the C-S-H gel pore; (b) C-S-H gel pore with graphene sheet; (c) C-S-H gel pore with GO-OH; (d) C-S-H gel pore with GO-COOH. (Ow - the oxygen atom in water, Os - the oxygen atom in the silicate chain, Caw - the calcium ion at the surface of tobermorite, C - Carbon atoms on the G/GO substrate.)

(a)

(b)


Page 31 of 38

(c)

(d) Figure 5 Molecular structure of Na, Cl and water molecules confined in the nanometer pore of (a) C-S-H (b) graphene sheets (c) GO-OH sheet (d) GO-COOH sheet

Os-Ow Os-Na

0.6

RDF(a.b.units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4

0.2

0.0 0

2

4

6

Distance(Å) (a)


8

10


Caw-Ow Caw-Cl

RDF(a.b.units)

8

6

4

2

0 0

2

4

6

8

10

8

10

Distance(Å) (b)

C-Cl C-Na C-Os C-Ow

1.6

1.2

RDF

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.8

0.4

0.0 0

2

4

6

Distance(Å) (c)


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2.5

Oc-cl Oc-Na Oc-Ow

RDF(a.b.units)

2.0

1.5

1.0

0.5

0.0 0

2

4

6

8

10

Distance(Å) (d)

Oc-Cl Oc-Na Oc-Ow

2.5 2.0

RDF(a.b.units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5 1.0 0.5 0.0 0

2

4

6

8

10

Distance(Å) (e) Figure 6. (a) RDF between Os atoms in C-S-H and atoms in solution (b) RDF between Caw atoms in C-S-H and atoms in solution (c) RDF between C atoms in graphene and atoms in solution (d) RDF between Oc atoms in GO-OH and atoms in solution (e) RDF between Oc atoms in GO-COOH and atoms in solution. (Ow - the oxygen atom in water, Os - the oxygen atom in the silicate chain, Caw - the calcium ion at the surface of tobermorite, C - Carbon atoms on the G/GO substrate,Oc - the oxygen atom on the graphene oxide functional group.)



(a)

(b) Figure 7. (a) local structure of water, Na and Cl ions adsorption on the C-S-H surface (b) on the GO-OH and GO-COOH surface 12

RDF(a.b.units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Oc-Caw Oc-Os

9

6

3

0 0

2

4

6

Distance(Å) (a)


8

10

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Oc-Caw Oc-Os 30

RDF(a.b.units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

10

0 0

2

4

6

8

10

Distance(Å) (b) Figure 8. (a) RDF between Oc atoms in GO-OH and Os (Caw) atoms in the C-S-H (b) RDF between Oc atoms in GO-COOH and Os (Caw) atoms in the C-S-H

(a)

(b) Figure 9 molecular structures of H-bond connection and Os-Ca-Oc connection between C-S-H and (a) GO-OH (b) GO-COOH



10000

H2O G OH COOH

2

MSD of water(Å )

8000

6000

4000

2000

0 0

500

1000

1500

2000

Time (Ps) (a)

1000

G OH COOH

100

2

MSD of C(Å )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10

1

0.1 500

1000

1500

2000

2500

Time(Ps) (b) Figure 10 Mean square displacement of (a)water (b)C in the 3.5nm channel of C-S-H gel during 2 ns


Os-Hw Ooh-Hw Ocooh-Hw

1.0 0.8 0.6 0.4 0.2 0.0 0

20

40

60

80

100

Time(Ps) （a）

1.0 0.8

Os-Na Ocooh-Na

0.6

TCF

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TCF of surface H-Bonds

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0.4 0.2 0.0

0

20

40

60

Time(Ps) (b)


80

100


1.0

0.8

TCF

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Os-Ocooh Ca-Ocooh

0.6

0.4

0.2

0.0 0

20

40

60

80

100

Time(Ps) (c) Figure 11 Time correlation function of (a) interfacial H-Bonds for C-S-H、GO-OH and GO-COOH (b)Na-O bond on C-S-H and GO-COOH Surfaces (c) H-bond and Ca-O bond in the interfacial region between GO and C-S-H (Os: oxygen atoms in silicate chain; Ocooh: oxygen atoms in the GO-COOH sheet; Ooh: oxygen atoms in the GO-OH sheet; Hw: hydrogen atoms in water molecule)

Table of Contents

The GO-COOH sheet, immobilizing the ions by functional groups and blocking the gel pore, inhibits the invasion of detrimental ions.


Molecular Modeling of Capillary Transport in the Nanometer Pore of

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