Reactive Molecular Simulation on Water Confined in the Nanopores of

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Reactive Molecular Simulation on Water Confined in the Nano-pores of the C-S-H Gel: Structure, Reactivity and Mechanical Property Dongshuai Hou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp509292q • Publication Date (Web): 23 Dec 2014 Downloaded from http://pubs.acs.org on December 31, 2014

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Reactive Molecular Simulation on Water Confined in the Nano-pores of the C-S-H Gel: Structure, Reactivity and Mechanical Property

Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:

The Journal of Physical Chemistry jp-2014-09292q.R2 Article 20-Dec-2014 Hou, Dongshuai; Qingdao Technological University (Cooperative Innovation Center of Engineering Construction and Safety in Shandong Blue Economic Zone), Zhao, Tiejun; Qingdao Technological University (Cooperative Innovation Center of Engineering Construction and Safety in Shandong Blue Economic Zone), Ma, Hongyan; Hong Kong University of Science and Technology, Civil and Environmental Engineering Li, Zongjin; Hong Kong University of Science and Technology,

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Reactive Molecular Simulation on Water Confined in the Nano-pores of the C-S-H Gel: Structure, Reactivity and Mechanical Property

Dongshuai Hou*a, Tiejun Zhaob, Hongyan Mac, Zongjin Lid a.

Dongshuai Hou, Corresponding author.

Qingdao Technological University (Cooperative Innovation Center of Engineering Construction and Safety in Shandong Blue Economic Zone), Qingdao, China; email address: [email protected]; telephone number: +8615012540759; fax number: +86 532 85071509.

b. Tiejun Zhao Qingdao Technological University (Cooperative Innovation Center of Engineering Construction and Safety in Shandong Blue Economic Zone), Qingdao, China;

c. Hongyan Ma Department of Civil and Environmental Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong.

d. Zongjin Li

Qingdao Technological University (Cooperative Innovation Center of Engineering Construction and Safety in Shandong Blue Economic Zone), Qingdao, China;



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Abstract

Calcium silicate hydrate (C-S-H) is a meso-porous amorphous material with water confined in the gel pores, which provides the medium for investigating the structure, dynamics and mechanical properties of the ultra-confined interlayer water molecules. In this study, C-S-H gels with different compositions expressed in terms of the Ca/Si ratio are characterized in the light of molecular dynamics. It is found that with increasing Ca/Si ratio, the molecular structure of silicate skeleton progressively transforms from an ordered to an amorphous structure. The calcium silicate skeleton, representative of the substrate, significantly influences the adsorption capability, reactivity, H-bonds network and mobile ability of the interlayer water molecules. The structures were tested for mechanical properties by simulated uni-axial tension, and the mechanical tests associated with structural analysis reveal that the stiffness and cohesive force of C-S-H gel is weakened by both breakage of silicate chains and penetration of water molecules. In addition, the reactive force field is coupled with both the mechanical response and chemical response during the large tensile deformation process. On the one hand, the silicate chains, acting in a skeletal role in the layered structure, de-polymerize to enhance the loading resistance. On the other hand, water molecules, attacking the Si-O and Ca-O bonds, dissociate into hydroxyls, which are detrimental to the cohesive force development.

Key words: H-bond, Ca/Si ratio, Uniaxial tension testing, De-polymerization, Hydrolytic reaction.


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1. Introduction

In biological, industrial and geological systems, water molecules confined in the nano-pores have been extensively investigated1-3. From water molecules in biological cells to molecules diffusing between the gel pores in cement hydrate, water molecules, influenced by the geometrical and electronic confinement, dramatically demonstrate different properties from the bulk counterpart. Calcium silicate hydrate (C-S-H), one of the main hydration products of cement-based material, is a porous gel with the gel pores characterized in size from 0.5 nm to 10 nm, in which the water and ions can diffuse. The motion of water in the pores relates directly to the cohesion of the C-S-H gel, determining the strength, creep, shrinkage, chemical and physical properties of cementitous materials. The properties of the water confined in the gel pores or in the vicinity of the C-S-H surfaces have been studied by various experimental techniques4-7. By using 1H nuclear magnetic resonance (NMR)4,8,9, the water in C-S-H gels was distinguished into three types: chemically bound water that is incorporated into the structure and forms a strong chemical bond with the calcium silicate structure, physical bound water that is deeply adsorbed near the surface and capillary water that is not bound and diffuses freely in the capillary pores. In addition, the quasi-elastic neutron scattering (QENS) technique5 characterizes the different water types by quantitatively using the diffusion coefficient. Furthermore, the behavior of water dynamics confined in C-S-H gel has been investigated by using broadband dielectric spectroscopy (BDS)7. The dielectric spectra revealed three different relaxation processes related to water molecules in the gel. Each process has its own dynamical characteristics and originated in different populations of water molecules. Especially, the process 2 is mainly attributed to the confined effects of the 1nm pore.

However, investigating the water structure and dynamics by experiment alone is challenging due to certain limitations, such as the material purity and instrument accuracy at the relevant sizes and timescales. Computational methods can help to



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interpret the experimental results and play a complementary role in understanding the structural and dynamic properties of water confined in the nanopore at the molecular level. With increasing knowledge of the molecular structure of C-S-H, water confined in the interlayer region has been extensively investigated by molecular dynamics simulation10. The simulation results showed that ultra-confined water molecules have glassy nature. The motion of the confined water molecules, influenced significantly by the calcium silicate substrate, shows a multi-stage dynamics, characteristic of super-cooled liquid11. Recently, Bonnaud et. al12 interpreted the cohesive force of C-S-H gel by analyzing the fluid pressure of the water molecules and the counter-ions in the interlayer region. Under varying humidity conditions, they found that the cohesive force mainly results from the negative pressure caused by the interaction between the interlayer calcium atoms and the calcium silicate sheets.

Previous research on water confined in C-S-H gels mainly focused on the structural and dynamic properties. Few attempts were made to investigate the mechanical properties, which are sensitively dependent on the water content in the C-S-H phase. In this work, the mechanical and chemical responses of the interlayer water molecules are investigated when the C-S-H gel is subject to large tensile deformation. The aim of the current research is to discover the molecular mechanism of the “hydrolytic effect” that influences the mechanical performance of porous materials saturated with water. To account for the structural complexity, ten calcium silicate skeletons, with different Ca/Si ratios, were constructed to represent the substrates for the ultra-confined water molecules. Uniaxial tension testing was performed on the C-S-H gels with different Ca/Si ratios to obtain the mechanical properties, such as Young’s modulus and tensile strength. It is worth noting that the reactive force field, coupling the mechanical and chemical responses for water molecules, can unravel the failure mechanism of the C-S-H gels at the molecular level.

2. Simulation method


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2.1 Reactive force field The Reactive force field (ReaxFF)13 was utilized to simulate the chemical reaction for both atomic structure construction and uniaxial tensile testing. The ReaxFF was originally developed by van Duin to make practical the molecular dynamics simulation of large scale reactive chemical systems for the hydrocarbons13. The ReaxFF has been applied to the inorganic system with the development of force field involving Si and O atoms14. Currently, it has been widely utilized in silica-water interfaces15, calcium silicate hydrate gel16 and nano-crystals17. The Reactive force field provides an advanced description of the interaction between Ca, Si, O and H atoms in the C-S-H gel. The short-range interactions for the ReaxFF force field are determined by a bond length-bond order scheme so that the bonds can be broken and formed, with the potential energy transforming into a smooth state18. On the other hand, the long-range coulombic interactions are determined by a 7th order taper function, with an outer cut off radius of 10 Å. The parameters for the force field have been obtained from fitting to the results of quantum chemical calculations on the structures and energy barriers for the atomic clusters and the condensed phases such as Si, SiO2 and CaO. It has been proven that the ReaxFF provides good description of the structure, reactivity and mechanical properties of the chemical systems13,14,16,19. The parameters of the force field for Ca, Si, O and H can be directly obtained from previous published reference data14,19.

2.2 C-S-H models with different Ca/Si ratios

The model construction in the present study is based on the procedures that combine the methods proposed by Pellenq20 and Manzano 21. Ten C-S-H models with Ca/Si ratios from 1.1 to 2.0 were constructed in three steps. Firstly, the layered analogue mineral of C-S-H, tobermorite 11 Å without water, was taken as the initial configuration for the C-S-H model22,23. Silicate chains were then broken to match the



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Q species distribution that is consistent with the results obtained both from NMR testing24 and molecular dynamics simulation25. It is worth noting that the Q0 percentage is controlled to less than 5%, also matching well with experimental results 26

. In order to construct the models with Ca/Si ratios from 1.1 to 2.0, some of the

bridging SiO2 units were first removed as proposed by Pellenq et al20. More importantly, some dimmer structures (Si2O4) were also removed to satisfy the Q species distribution, especially for the low Q0 percentage. After removal of the dimmer structure, to maintain charge neutrality, some oxygen atoms in the silicate chains have to remain in the form of dangling atoms. The dry calcium silicate skeletons with Ca/Si ratios of 1.1, 1.4, 1.7 and 2.0 are shown in Fig.1.

Subsequently, the Grand Canonical Monte Carlo (GCMC) method was utilized to investigate the structure of a dry calcium silicate skeleton immersed in water solution 12

. The dry sample obtained by the first step was utilized for simulation. GCMC

simulations determine the properties of the water molecules confined in the calcium silicate system at constant volume V, in equilibrium with a fictitious infinite reservoir of liquid bulk water solution, at chemical potential µ=0 eV and its temperature T=300 K20. The simulation process is analoguous to water adsorption in the micro-porous phases, such as calcium silicate hydrate and zeolite27.The simulation included 300,000 circles for the system to reach equilibrium followed by 100,000 circles for the production run. For each circle, it was attempted to insert, delete, displace and rotate the water molecules 1000 times in the constant volume calcium silica hydrate system.

(a)


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(b)

(c)

(d) Fig.1. Dry silicate skeletons with Ca/Si ratio 1.1, 1.4, 1.7 and 2.0. Yellow and red bond represents the silicate chain (Si-O) and the green ball is calcium atom. The silicate skeleton for every Ca/Si ratio is presented in the supplementary material.

After the GCMC simulation, the C-S-H model, saturated with water molecules, is obtained. F reactive force field molecular dynamic simulations under constant pressure and temperature (NPT) for 300 ps gave the structures of C-S-H gel at



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equilibrium states. A further 1000 ps NPT run was employed to achieve the equilibrium configuration for structural and dynamic analysis.

2.3 Uniaxial tension testing

Uniaxial tension testing was employed to investigate the mechanical behavior of the C-S-H gels. Super-cells, obtained by periodically extending the simulation model mentioned in section 2.2 by a factor two, underwent uniaxial tension in the x, y and z directions. The super-cells include around 6000 to 9000 atoms, of size around 40 Å × 40 Å × 40 Å. It should be noted that using a large number of atoms in this study can give stable statistical simulation results, especially in regard to reliable failure modes. In order to explore the failure mechanism of the layered structure, the stress-strain relation and the change of molecular structure were investigated in the loading process.

To obtain the stress-strain relation, the structure was subjected to uniaxial tensile loading through gradual elongation at two different strain rates of 0.08/ps and 0.008/ps. In the whole simulation process, NPT ensembles are defined for the system. Taking the tension along x direction for example, the super-cells were firstly relaxed at 300K and coupled to zero external pressure in the x, y, z dimensions for 500 ps. Then, after the pressures in the three directions reached equilibrium, the C-S-H structure was elongated in the x direction. Meanwhile, the pressure in y and z directions was kept at zero. Pressure evolution in the x direction was taken as the internal stress σxx. Setting the pressure perpendicular to the tension direction to zero can allow the normal direction to relax anisotropically without any restriction. The setting, considering Poisson’s ratio, can eliminate the artificial constraint for the deformation.


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3. Results and Discussion 3.1 Water dissociation in nano-pores

(a)

(b)



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(c)

(d) Fig.2 molecular structure of the C-S-H gel with Ca/Si ratio (a) 1.1 (b) 1.4 (c) 1.7 (d) 2.0. Yellow and red bond represents the silicate chain (Si-O), the green ball is calcium atom and red-white ball and stick is the water molecules and hydroxyl groups. The molecular structure for every Ca/Si ratio is presented in the supplementary material.


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(a)

Intensity (ab.units)

2000

1500

Ca CaSi1.4 Si Ow Hw

1000

500

0 12

15

18

21

Distance in z (Å)

(b)

2000

Intensity (ab. units)

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1500

Ca CaSi1.7 Si Ow Hw

1000

500

0 12

15

18

21

Distance in z (Å)

(c)

2000

Intensity (ab. units)

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1500

Ca casi2.0 Si Ow Hw

1000

500

0 12

15

18

21

Distance in z (Å)

(d)



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Fig.3 atomic profiles of calcium silicate hydrate with Ca/Si ratios (a) 1.1 (b) 1.4 (c) 1.7 (d) 2.0. The atomic profile for every Ca/Si ratio is presented in the supplementary material.

Four simulated C-S-H gel samples at different Ca/Si ratios are shown in Fig.2. Correspondingly, the intensity profiles of different atoms are plotted in Fig.3 versus the distance in the z direction. It can be clearly observed in Fig.2 that the calcium atoms (Cas) and the surrounding oxygen atoms (Os) form a Cas-Os octahedral, in constructing the Cas sheet. Defective silicate chains are grafted on both sides of the Cas sheet. Between the neighboring calcium silicate sheets, the interlayer calcium atoms (Caw) and water molecules (Hw, Ow) are distributed. The alternative maxima of Ca, Si, Ow and Hw in the density profiles (Fig. 3) indicate that C-S-H gel has a sandwich-like structure. In Fig. 3a, Ow and Hw atoms distributed from 12.5 to 19.5 Å, imply that the interlayer water molecules are ultra-confined in the calcium silicate nano-pores, with sizes less than 7 Å. The hydrogen atomic profiles show that the OH bonds of the water molecules point towards the calcium-silicate layers to form hydrogen bonds, with the oxygen atoms in the layer. In addition, some water molecules, distributed near the ONB atoms in the silicate chains, dissociate and form silicate hydroxyls. Both these two features indicate the hydrophilic nature of the C-S-H layers10,21. As shown in Fig.3, with increasing Ca/Si ratio, the intensity peak value of Si in the interlayer region gradually disappears (Ca/Si=1.7 and Ca/Si=2.0), due to the missing bridging silicate tetrahedron. The defective silicate chains are more likely to adsorb water molecules: while at the low Ca/Si ratio, water molecules only exist in the interlayer region. At high Ca/Si ratios, the water molecules penetrate into the cavities in the calcium silicate layer, due to the removal of the silicate chains. It can be observed in Fig. 2d that the water molecules in the neighboring nano-pores are connected in a direction perpendicular to the calcium silicate sheet. Furthermore, eliminating the silicate chains disturbs the calcium sheet arrangement and changes the layering of water ultra-confined in the nano-pores. As the Ca/Si ratio increases from 1.1 to 2.0, the Ow profile transforms from a binomial to a trinomial distribution. The


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double peaks of Ow in Fig. 3a correspond to the water molecules being strongly adsorbed by the top and bottom C-S-H layers. However, as shown in Fig.3d (Ow distribution), a third water layer, not directly forming H-bonds with the calcium silicate sheet, is located in the central position of the nano-pores. In this layer, the interaction between neighboring water molecules plays import role in determining the structure of the water molecules.

3.0

reaction degree water/Si OH/Si

2.5

40

2.0 1.5

20

1.0 0.5

0 1.0

1.2

1.4

1.6

1.8

2.0

1.8

2.0

Ca/Si ratio

(a) 1.0 Si-OH/Si Ca-OH/Ca Thomas et al.

0.8

0.6

0.4

0.2 1.0

1.2

1.4

1.6

Ca/Si ratio

(b)


Hydroxyl group(water)/Si ratio

Hydration degree (%)

60

Si-OH/Si (Ca-OH/Ca)

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(c) Fig.4 (a) Number of water and hydroxyl group, and hydrolytic reaction degree vary with Ca/Si ratio; (b) Number of Si-OH and Ca-OH change with Ca/Si ratio; (c) Snapshots of water dissociation in the C-S-H gel at Ca/Si 2.0 (top) and Ca/Si=1.1 (bottom).

In order to quantitatively analyze the water content in the C-S-H gel, the ratios of H2O/Si are calculated and plotted versus the Ca/Si ratios, as shown in Fig. 4a. Consistent with the result in the intensity profiles, while the Ca/Si ratio changes from 1.1 to 2.0, the H2O/Si ratio increases from 0.67 to 2.36, implying the better water adsorption ability of the C-S-H gel at high Ca/Si ratios. In particular, the chemical formula (CaO)1.7(SiO2)(H2O)1.82 matches well with the results obtained from SANS testing 6. Meanwhile, water molecules, confined in the nano-pores, dissociate into hydroxyl groups. The dissociation degree, defined as the dissociation ratio of the water molecules, can be used to estimate the reactivity of water in the C-S-H gel confinement. As shown in Fig.4a, the OH/Si ratio increases from 0.2 to 1.2 and the dissociation degree also increases from 30.7% to 52%. It implies that the water


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molecules have higher chemical reactivity in the C-S-H gel at high Ca/Si ratio. Both the adsorption ability and reactivity of water molecules are closely related with the geometric and chemical environment. In particular, the ONB atoms in the silicate chains are responsible for the water adsorption and dissociation. With increasing Ca/Si ratio, the silicate chain length gradually decreases and the more defective silicate chains have a larger number of ONB atoms. The hydrophilic nature of the ONB atoms leads to water adsorption and dissociation. On the other hand, the defective silicate chains at high Ca/Si ratio have a smaller number of OB atoms. In fact, during the reaction process, H+ ions do not associate with the bridging oxygen atoms that exhibit a hydrophobic nature.

Furthermore, as shown in Fig. 4b, at low Ca/Si ratio (1.1 to 1.3), the number of Ca-OH groups is almost same as that of the Si-OH groups, and the Ca-OH groups occupy a larger percentage at high Ca/Si ratio. The discrepancy between Si-OH and Ca-OH groups can be explained by different reaction mechanisms, which are illustrated in Fig. 4c. In the C-S-H gel with a low Ca/Si ratio, water molecules dissociate into H+ and OH-, the H+ ions diffuse to associate with the ONB atoms in the silicate chains, while the OH- ions form Ca-OH bonds with the neighboring interlayer calcium atoms. This results in the same number of Si-OH and Ca-OH connections. On the other hand, at a high Ca/Si ratio, H+ ions in the water molecules not only associate with the ONB atoms in the defective silicate chains, but they also connect with the dangling oxygen atoms in the calcium sheet due to the elimination of the dimmer structures. The latter reaction only produces the Ca-OH groups. In this respect, Ca-OH bonds can be both observed in the interlayer region and the surface of the calcium silicate sheet so that the percentage of Ca-OH bonds significantly increases.

It is worth noting that there are both Si-OH and Ca-OH in the simulated C-S-H gels with different Ca/Si ratios. These hydroxyls are important to determine the local structure of the C-S-H gel. As exhibited in Fig. 4b, the simulated Ca-OH/Ca ratio continues to increase with the Ca/Si ratio, which is consistent with the trend obtained



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from the inelastic neutral scattering (IENS)28. However, the Ca-OH/Ca ratios from experiments are somewhat smaller than those from the current simulation. The H2O/Si ratios of the C-S-H gel prepared for the IENS testing range from 0.95 to 1.34 is quite smaller than the water content in our simulation, and may result in the lower percentage of Ca-OH bonds.

The hydrolytic reaction also has great influence on the dynamic properties of water molecules in the C-S-H gels. The mean square displacement 3, a parameter to estimate the dynamic properties of water molecules, is defined by Eq. (1),

MSD(t) =< |࢘࢏ (t) − ࢘࢏ (0)|ଶ >

(1)

where ri(t) represents the position of atom i at time t and MSD takes into account the 3-dimensional coordinates. A large MSD value at time t indicates that the atoms diffuse rapidly and are displaced far away from the original position. Fig. 5 shows that the diffusion rate of atoms is ranked in the following order: Os > Oh >Ow. Different MSD curves characterize the chemical bonding and the physically associated water molecules, as mentioned in the C-S-H models proposed by Powers and Brownyard 29, and Feldman and Sereda

30

. The hydrolytic reaction transforms some of the surface

adsorbed water molecules into the “immobile” hydroxylation layers. In the transition layer, hydroxyl groups, resembling the dynamic nature of the solid calcium silicate skeletons, are strongly restricted and cannot diffuse freely. Because of the restrictions, the only available motions for the hydroxyl bonds are vibrations and rotations at fixed positions so that the MSD values for hydroxyl are quite low.


2

1

Os Oh Ow

0.1

0.01

0.1

1

10

100

Time (Ps) Fig.5 Mean square displacement of Os, Ow and Oh atoms; Ow represents oxygen atom in the water molecule; Oh is O in the Ca-OH and Si-OH groups; Os is the O in the silicate chains without hydroxylation.

3.2 H-bonds network 3

RDF O-H (ab. units)

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Mean square displacement (Å )

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Ow-Hw Os-Hw Oh-Hw O-Hw

2

1

0 2

4

6

8

10

Distance (Å)

Fig.6 RDF of O-H for the water confined in the C-S-H gel (Ca/Si=1.7) and its components Ow-H, Os-H and Oh-H.

In the nano-pores of the C-S-H gels, the H-bond is an important chemical connection bridging the neighboring calcium silicate sheets. The structuring of the confined water molecules is the result of H-bond connections between the water molecules and the



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calcium silicate sheet, and among water molecules. After hydrolytic reaction, some of the water molecules are transformed to Si-OH and Ca-OH, so the radial distribution function (RDF) of O-H can be further decomposed into three types: Ow-Hw, Oh-Hw and Os-Hw. These RDF curves can reflect the local structure of water molecules and characterize the H-bond distribution in the nano-pores. As shown in Fig.6, the first peak value of Oh-Hw occurs at 1.65 Å, which is shorter than the value 2.12 Å for Ow-Hw. The locations of the first peaks, representing the H-bond lengths, imply that the H-bond connections between the water and hydroxyl groups have stronger bonding strength. Additionally, the small RDF intensity plateau from 1.62 to 2.44 Å of the Os-Hw can be translated as a small number of H-bonds forming between the water molecules and bridging oxygen atoms in the silicate chains.

In the medium range from 2.5 to 5 Å, the difference between the three RDF curves is more apparent: triple peaks of Oh-Hw are located at 3.14, 3.84 and 4.9 Å; a single peak of Ow-Hw with larger intensity occurs only at 2.94 Å. Triple peaks mainly result from the fact that the water molecules are geometrically constrained by the surrounding silicate chains, and the spatial correlation between Oh and Hw can be maintained in the medium range. Compared with the constraining effect from the silicate chains, the restraining influence from the neighboring water molecules is much weaker and the arrangement of water molecules in the medium range shows poor order.

The average H-bonds number per water molecule is calculated as a function of the Ca/Si ratio. The H-bond formation requires two conditions: the distance between two water neighbors DHO should be less than 2.45 Å and the angle ߠ between the OH vector and OO vector should be less than 30° 31. As shown in Fig.7, with increasing Ca/Si ratio, the average H-bond number gradually increases from 2.27 to 2.71, and is mainly attributed to the richness of the water molecules in the C-S-H gel with high Ca/Si ratios. According to the accepting and donating roles, and the different types of oxygen atoms, the H-bonds are further decomposed into five parts. On average, while the number of H-bonds donating to the Os atoms progressively decreases from 1.28 to


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0.33, the number of H-bonds donating to and being accepted by the Oh atoms increases to 0.51 and 0.85, respectively. On the one hand, the long silicate chains translate into short dimmer structures with missing Os atoms as the Ca/Si ratio increases. On the other hand, some of the Ow and Os atoms become Oh atoms after the hydrolytic reaction, which extensively occurs at high Ca/Si ratios. Both effects result in increasing Oh numbers and decreasing Os numbers, and influence the corresponding H-bonds of Ow-Os and Ow-Oh. In addition, the amount of Ow-Ow connections also increases from 0.266 to 0.516, and is mainly contributed by the interaction between neighboring molecules in the third layer, as illustrated in the atomic profile. In previous simulations by the empirical force field CSHFF, 2.34 H-bonds connected by silicate chains and neighboring water molecules were calculated by using Kumar et al’s method10,32. At a Ca/Si ratio of 1.7, the H-bond intensity simulated by reactive force field increases to about 21.7% as compared with that from the empirical force field. CSHFF cannot describe the water reactions, so no hydroxyl groups exist in the nano-pores. However, since 44.7% of the water molecules dissociate into hydroxyl groups and form chemical bonds in the current simulation, the hydroxylation layer deeply embedded in the calcium silicate sheets densify the H-bond network, which is evidenced by the shorter Oh-H bonding in the RDF curve.

Average H-bond number/water

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


3.0 2.5 Ow-a-Ow Ow-a-Oh Ow-d-Ow Ow-d-Oh Ow-d-Os Total

2.0 1.5 1.0 0.5 0.0 1.0

1.2

1.4

1.6

1.8

2.0

Ca/Si ratio

Fig.7 Average H-bond number evolution with Ca/Si ranging from 1.1 to 2.0. Ow-a-Ow : water molecules that accept H-bonds from water molecules. Ow-a-Oh: water molecules that accept H-bonds from Oh atoms; Ow-d-Ow: water molecules that donate



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

H-bonds to water molecules; Ow-d-Oh: water molecules that donate H-bonds to Oh atoms; Ow-d-Os: water molecules that donate H-bonds to Os atoms.

The number of H-bonds of different types varies with the Ca/Si ratio, which influences the interactions between the neighboring calcium silicate sheets. As schematically illustrated in Fig. 8, at low Ca/Si ratios, Os-Hw bonds can be distributed across the interlayer region and directly bridge the neighboring calcium silicate sheets. With increasing water content, water molecules, in the interlayer region, transform from single layer to multi-layer packing. In the case of a single layer, the water molecules can bridge the neighboring calcium silicate sheets by both Hw-Os and Hw-Ow, while in the case of multi-layer, water indirectly connects the surface water molecules by Hw-Ow bonds. In this way, the connection between neighboring calcium silicate sheets is screened by the increasing number of water layers.

Fig.8 Schematic diagrams of the interlayer H-bond connections.


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Page 21 of 42

3.3 Mechanical properties

-16 x y z

Stress (GPa)

-12

-8

-4

0 0.0

0.2

0.4

0.6

0.8

Strain (Å/Å)

(a)

-16 x y z -12

Stress (GPa)

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


-8

-4

0 0.0

0.2

0.4

0.6

Strain (Å/Å) (b)


0.8


-16

x y z

Stress (GPa)

-12

-8

-4

0 0.0

0.2

0.4

0.6

0.8

Strain (Å/Å) (c)

-16

x y z

-12

Stress(GPa)

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 22 of 42

-8

-4

0 0.0

0.2

0.4

0.6

0.8

Strain (Å/Å) (d)

Fig.9 Stress-strain relation of layered structures tensioned along x, y and z direction at strain rate 0.08/ps. The C-S-H gel with Ca/Si ratio (a) 1.1 (b) 1.2 (c) 1.4 (d) 1.7. The stress-strain relation at strain rate of 0.008/ps is presented in the supplementary material.

The stress-strain curve characterizes the mechanical behavior of the layered structure during the tensile process and helps gain insights into the constitutive relation between stress and strain. Different stress-strain relations for tensile loading in the x, y and z directions indicate the anisotropic nature of the layered structures. The relations for C-S-H gel with a low Ca/Si ratio of 1.1 are first analyzed. In the x direction, as


Page 23 of 42

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


shown in Fig.9a, during the tensile process, the stress initially increases linearly in the elastic stage and subsequently slowly increases to a maximum value of 13 GPa at a strain around 0.17 Å/Å. After the maximum value, the stress firstly drops to 6.5 GPa at strain 0.24 Å/Å and then the stress slowly decreases as the strain reaches 0.8 Å/Å. In the post-failure stage, the evolution of stress in the y direction is categorized into three stages: first the stress decrease by around 1 GPa as strain varies from 0.15 to 0.21 Å/Å; subsequently, the stress steps into a plateau until the strain reaches 0.4 Å/Å; finally stress decreases in a slow rate. In the post-failure region, the three-stage “ladder-like” stress-strain relation, a symbol of good plasticity, indicates some structural rearrangement in the calcium silicate sheets. It is of interest to note the molecular structural evolution at the stress turning point for a strain around 0.21 Å/Å, and is discussed in the following section. The ladder-like stress-strain feature can also be observed in the cases of Ca/Si=1.2 and 1.4 (Fig. 9b and 9c), while at a Ca/Si ratio 1.7 (Fig. 9d), the stress directly decreases in the post-failure regime. In addition, the C-S-H gel is more likely to be stretched broken in the z direction and the strain at the fracture state is 0.4 Å/Å, indicating that the interlayer structure has a more brittle nature. Furthermore, the stress-strain relation is influenced by the loading rate. The loading rate effect is discussed in detail in the supplementary material.

Uniaxial tension testing was applied to ten C-S-H samples in the x, y and z directions to investigate the Ca/Si ratio influence. The tensile strength and Young’s modulus, calculated from the stress-strain curves, are plotted versus Ca/Si ratio in Fig.10. All the C-S-H samples show anisotropic mechanical behavior: the XY plane has larger cohesive force and stiffness than those along the interlayer direction for all Ca/Si ratios. As shown in Fig. 10a, in the y direction, with increasing Ca/Si ratios, the tensile strength gradually reduces from 14.5 GPa to less than 7 GPa. In Fig.10b, Young’s modulus decreases from 113.7 GPa to around 62.5 GPa as Ca/Si varies from 1.1 to 1.7, and is maintained at around 62 GPa as Ca/Si increases from 1.7 to 2.0. The strength weakening trend can also be observed in the tensile samples in the x direction: the tensile strength decreases from 13 GPa to 5.7 GPa accompanied by Young’s modulus



reduction from 94 to 50 GPa. In regard to the properties along the interlayer direction, as shown in Fig.10, in the z direction the tensile strength decreases from 6.68 GPa to less than 2.7 GPa, with Young’s modulus reducing from 52 GPa to 33 GPa. It should be noted that previous experimental and computational investigation on the mechanical properties of C-S-H crystal shows similar trend: as the calcium content increases, the Young’s modulus of the C-S-H crystal reduces

33,34,35

. Additionally, as

compared with that in the XY plane, the weaker stiffness in the z directions is more in line with the results from recent nano-indentation tests on synthesized C-S-H gels 36, where the Ca/Si molar ratio of C-S-H increases from 0.7 to 2.1 and the elastic modulus achieved experimentally reduced from 27 to 20 GPa. Because nano-indentation cannot eliminate the influence of the nano-porosity that significantly weakens the mechanical properties, the experimental results demonstrate lower values than those obtained from our simulation. 120

Young's modulus (GPa)

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 24 of 42

x y z

100

80

60

40 1.2

1.4

1.6

1.8

Ca/Si (ratio)

(a)


2.0

Page 25 of 42

15

Tensile strength (GPa)

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


x y z

12

9

6

3 1.2

1.4

1.6

1.8

2.0

Ca/Si ratio

(b) Fig.10 (a) Young’s modulus and (b) Tensile strength evolution with Ca/Si ratio

The mechanical behavior variation with Ca/Si ratio can be interpreted as the result of the silicate morphology and the structure of the interlayer water molecules. On the one hand, due to the de-polymerization role of the Ca atoms, the mean silicate chain length becomes shorter and the Si-OB-Si bonds are significantly substituted by the Si-ONB-Ca bonds. A previous first principle study on the tobermorite structure demonstrated that the Si-O bond energy is larger than that of the Ca-O bond in the C-S-H mineral analogue

37

. Due to the strength contribution from the long silicate

chains, the mechanical properties of the C-S-H sample with low Ca/Si ratio are better than those with high concentrations. However, in the C-S-H structure, short silicate chains, such as dimmers or monomers, occupy a predominant percentage, extremely weakening the tension strength and ductility. Murray’s

22

study stated that the tensile

strength of C-S-H can dramatically decrease to one third of the original value, while the infinite silicate chains are completely converted to dimmers. Analogously, in other calcium silicate composition systems, the silicate chain length also plays a significant role in strength development. On the other hand, at high Ca/Si ratios, increasing numbers of water molecules penetrate into the cavities of the calcium silicate sheet, substituting ionic-covalent bonds with the unstable H-bonds. The diffusion of the interlayer water molecules results in the frequent breakage and formation of the



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 42

H-bonds, which to a great extent reduces the stability of the C-S-H gel. Recently, a combinatorial molecular optimization method has been employed to investigate the structural and mechanical properties correlation with Ca/Si ratio

38

. The atomic

simulation results showed that the modulus to hardness ratio is closely related with Ca/Si ratio, the medium range silicon-oxygen and calcium-oxygen environments. In this study, the CSH-FF 39, the empirical force field developed for the cement system, was utilized to study the mechanical performance of C-S-H gel. When the Ca/Si ratio increases from 1 to 2.2, the Young’s modulus parallel to calcium silicate layer reduces from 99 GPa to 63 GPa and modulus perpendicular to the layer decreases from 73 GPa to 45.5 GPa. The elastic modulus calculated by the CSH-FF matches well with current simulation result by the reactive force field. Furthermore, different from the empirical force field, ReaxFF can upscale the connectivity of the chemical bonds continuously by using bond order analysis, which allow for the bond breakage and formation during the simulation. Therefore, the chemical reactions can be calculated during the large deformation of the C-S-H gels. Q species evolution and water dissociation reaction will be discussed in the following section.

3.4 Molecular structure evolution


Page 27 of 42

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


(a)

(b)

Fig.11. molecular structure evolution of C-S-H gel with Ca/Si ratio (a) 1.1 (b) 1.7 tensioned along y direction. From top to bottom: the C-S-H gel at strain 0.0, 0.2, 0.4 and 0.6 Å/Å. The yellow sticks are the silicate chains; green balls are calcium atoms; red lines are the water molecules.

The configurations of the Ca, Si, O and H atoms from strain of 0 Å/Å to 0.6 Å/Å are shown in Fig.11a and Fig. 11b for preliminary qualitative illustration of the damage process of C-S-H gel with low and high Ca/Si ratio, respectively. For the C-S-H gel with low Ca/Si ratios, the siloxane bonds grafted in the calcium sheet, subjected to tensile loading, are elongated in the elastic region. The Si-O-Si angle is stretched open to take up the strain. In the yield region, there is a gradual breakage of the siloxane bonds that allows transformation of the morphology. It is seen in Fig.11a that at strain



0.4Å/Å, the layered crystal phase gradually changes to an amorphous one due to bond breakage and reconnection. More importantly, it can be observed that some distorted silicate chains are entwined, which enhances the interlayer connections. In the tensile period, small cracks are created but the local structure rearrangement slows down the crack propagation, as shown in the configuration at strain 0.6 Å/Å. The local structure rearrangement, enhancing the plasticity of C-S-H gel in post-failure stage can explain the “ladder-like” stress-strain relation discussed in the previous section. On the other hand, deformation of the C-S-H gel with high Ca/Si ratios demonstrates a different failure mechanism. As shown in Fig.11b, at a strain 0.2 Å/Å, the Ca-O bonds elongate and distort, and the calcium octahedron carries up the tensile loading at the initial stage. Because the dimmer structures are distributed separately, the silicate chains cannot provide sufficient mechanical contribution by stretching the Si-O-Si bonds. At the failure stage, the cracks grow and coalesce rapidly through the region with defective silicate chains, as shown in Fig.11b, which describes the strain states from 0.4 Å/Å to 0.6 Å/Å, resulting in a continuous decreasing of the stress.

Q species evolution under tensile loading

In order to investigate the morphological evolution quantitatively, the change of Qn percentages is calculated to describe the structural variation of the silicate skeleton during the tensile process, as shown in Fig.12. 100

Q species percentage (%)

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 28 of 42

Q0 Q1 Q2 Q3 Q4

Ca/Si=1.1 80

60

40

20

0 0.0

0.2

0.4

0.6

Tensile strain (Å/Å)


0.8

Page 29 of 42

(a)

Q species percentage (%)

100

80

Q0 Q1 Q2 Q3 Q4

Ca/Si=1.2

60

40

20

0 0.0

0.2

0.4

0.6

0.8

Tensile strain (Å/Å)

(b) 100

Ca/Si=1.4 Q percentage (%)

80

Q0 Q1 Q2 Q3 Q4

60

40

20

0 0.0

0.2

0.4

0.6

0.8

0.6

0.8

Strain (Å/Å)

(c) 100

80

Q species (%)

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


Q0 Q1 Q2

60

40

20

0 0.0

0.2

0.4

Strain ( Å/Å) (d)



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 42

Fig.12 Q species evolution of C-S-H gel with Ca/Si ratio 1.1, 1.2, 1.4 and 1.7. Q species evolution for every Ca/Si ratio is presented in supplementary material.

As in the silicate composite structure, the connectivity factor, Qn (n=0, 1, 2, 3, 4), is an important parameter in estimating the silicate connection, where n is defined as the number of connected neighboring silicate tetrahedrons. Q0 is the monomer; Q1 represents the dimmer structure (two connected silicate tetrahedrons); Q2 is the long chain; Q3 is the branch structure; Q4 is the network structure 40. The long silicate chain in the C-S-H gel with low Ca/Si ratio varies greatly during the tension process. As shown in Fig.12a, the Q species evolution can be clearly distinguished in three stages as the strain increases along the y direction. Initially, the percentage of Q species remains unchanged during the first 0.15 Å/Å strain period, at which stage, the silicate chain length is elongated and Si-O-Si angle is stretched open to carry the loading. Subsequently, when the strain exceeds 0.15 Å/Å, Q2 begins to decrease, and Q1 and Q0 increase. This means that some parts of the silicate chains are stretched broken, resulting in the morphological transformation from long silicate chains to dimmer structures. Meanwhile, the Q3 species grows at a strain of 0.3 Å/Å, implying the formation of the branch structures. Some silicate chains that were stretched broken can reconnect to form a new silicate skeleton. The newly connected branches can bridge the neighboring calcium silicate layers and form the three dimensional structure. It is worth noting that the structural rearrangement significantly improves the mechanical performance. By previous ab initio calculation 37, the presence of the Q3 species can improve the interlayer stiffness of tobermorite

41

by forming a hinge

mechanism. It explains why the stress has a plateau after the initial drop in the stress-strain relation curve in Fig. 12b. Finally, the ratio of Q2 slightly increases and Q1 decreases, as the strain reaches 0.46 Å/Å, from which the stress continues to be reduced. It can be observed in Fig. 12b and 12c that as the Ca/Si ratio increases from 1.1 to 1.4, the changed percentage of the Q2 species reduces from 33% to less than 5%, indicating weaker mechanical contributions from the defective silicate chains.


Page 31 of 42

However, when the Ca/Si ratio exceeds 1.4 or the mean silicate chain length is reduced to 4, the Q species does not change, implying that there is no de-polymerization during tension along the y direction for the C-S-H gel. Hence, the mean silicate chain length (MCL) is a very important parameter, which is closely related with the mechanical performance of C-S-H gel. The tensile deformation of the C-S-H gel coincides with the polymerization of the silicate chains. In the current study, as the MCL decreases to 4, in the post failure regime, the ladder-like stress-strain behavior gradually disappears and the silicate skeleton does not deform to resist tensile loading. Defective silicate chains, isolated distributed in the C-S-H gel, have not been drawn broken. Due to the missing bridging silicate tetrahedrons, the mechanical contribution from the Si-O bonds is significantly reduced. In this respect, the tensile strength and stiffness of the C-S-H gel in the y direction is weakened to a great extent.

Hydrolytic reaction under tensile loading

500

Number of hydroxyls

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


SiOH CaOH water

Ca/Si=1.1

400

300

200 0.0

0.2

0.4

0.6

Tensile strain (Å/Å) (a)


0.8


550

Number of hydroxyls

500

SiOH CaOH water

Ca/Si=1.2

450 400 350 300 250 0.0

0.2

0.4

0.6

0.8

Tensile strain (Å/Å) (b)

Number of hydroxyls

700

SiOH CaOH water

Ca/Si=1.4

600

500

400 0.0

0.2

0.4

0.6

0.8

Tensile strain (Å/Å) (c)

900

Number of hydroxyls

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 32 of 42

800

water Si-OH Ca-OH

Ca/Si=1.7

700

600

500

400 0.0

0.2

0.4

0.6

0.8

Strain (Å/Å) (d)

Fig.13 Number of Ca-OH, Si-OH and H2O evolution with tensile strain (a) Ca/Si=1.1


Page 33 of 42

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


(b) Ca/Si=1.2 (c) Ca/Si=1.4 (d) Ca/Si=1.7. The evolution for the number of hydroxyls for every Ca/Si ratio is presented in the supplementary material

In addition to de-polymerization of the silicate chains, the water molecules’ dissociation can also be observed during the tensile process. As shown in Fig.11b, the intrusive water molecules attack the Si-O-Si bonds, and many Si-OH groups and Ca-OH bonds are formed at the end of the crack tips, which further weakens the calcium silicate structure in the loading resistance. Hence, the number of water molecules and Si-OH and Ca-OH groups was recorded during the tensile process in order to quantitatively analyze the water dissociation process.

As shown in Fig.13, the number of water molecules continuously decreases and the number of Ca-OH and Si-OH groups increases significantly during the tensile process, implying that hydrolytic reactions extensively occur. It should be noted that as the strain is less than 0.05 Å/Å, the hydrolytic reaction rate is quite slow and only a few water molecules decompose into hydroxyls. Thermodynamically, according to the theory of Zhu et al

42

, increasing the stress in the structure can reduce the energy

barrier for activating the hydrolytic reaction. Below 0.05 Å/Å, about 30% of the failure strain, the hydrolysis reaction is thermodynamically unfavorable. When the Ca/Si ratio increases from 1.1 to 1.7, the evolution of hydroxyl groups demonstrates different features. As shown in Fig. 13a, at Ca/Si=1.1, the number of Ca-OH and Si-OH groups first increase simultaneously, but as strain reaches around 0.2 Å/Å, the number of Si-OH groups becomes higher than that of Ca-OH. The two-stage evolution of the hydroxyl groups also occurs in the case of Ca/Si=1.2. However, as the Ca/Si ratio is larger than 1.4, as shown in Fig. 13d, the variation of the Si-OH groups is almost the same as those of Ca-OH during the whole tensile process. The discrepancy in the hydroxyl groups at high and low Ca/Si ratios can be interpreted by the different reaction mechanisms of C-S-H gel in carrying tensile loading.



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

(a)

Page 34 of 42

(b)

Fig. 14 Hydrolytic reaction pathway for (a) breakage of Si-O-Ca bond (b) de-polymerization of silicate chains.

The reaction mechanism of water dissociation can be categorized into two types: the first one is caused by the breakage of Ca-O-Si bonds and the second one is induced by de-polymerization of the silicate chains. At the beginning of the yield stage, the ionic


Page 35 of 42

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


connection, such as the Ca-O bond, is easily stretched broken so that the first reaction occurs. The first reaction mechanism of water dissociation is listed in the following sequence, as illustrated in Fig.14a: the Ca-O bond is initially stretched; water adsorbs with silicon, forming a Si-water connection; water dissociates to the hydroxyl group; the free dissociated OH group bonds with the other Ca, forming Ca-OH; the Ca-O bond breaks. The hydrolytic products of the first reaction are Ca-OH and Si-OH groups in the same amounts.

On the other hand, with increasing tensile strain, the silicate chains are stretched broken and the second reaction occurs. The sequence for the second mechanism is illustrated in Fig. 14b: water diffuses and is adsorbed near the neighboring silicon atoms; bond formation occurs between the water and silicon atoms, and a five-fold silicate structure forms; water dissociation; proton transfers and Si-OB bond breakage. In the dissociation process, the water molecules lead to the formation of a penta-coordinated silicon structure, which is energetically unfavorable. The unstable structure subsequently relaxes and dissociates with the OB atoms. In this way, the water molecules attack the tensile siloxane bonds and accelerate the separation between the neighboring silicate species. Interestingly, the reaction is reversible for the oligomerization of two silicate monomers 40: Si (OH ) 4 + Si (OH ) 4  →( HO) 3 SiOSi(OH ) 3 + H 2O

monomers de-protonate; ionized monomers form penta-coordinate silicon; unstable bond breakage and water formation. It should be noted that the hydrolytic product of the second reaction is only the Si-OH group. Therefore, the reaction pathway can explain the hydroxyl differences between C-S-H gels with high and low Ca/Si ratios. At low Ca/Si ratios, the Ca-O-Si and Si-O-Si bond breakage occurs in sequence, which leads to two-stage water dissociation. On the other hand, at high Ca/Si ratios, the silicate chains are not de-polymerized and only water dissociation, induced by the Ca-O breakage, results in the same amounts of Si-OH and Ca-OH.



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

4. Conclusions

In light of the molecular dynamics by the reactive force field, the structures, dynamics and mechanical properties of C-S-H gels with different Ca/Si ratios have been investigated. Several conclusions can be drawn from this study as follows:

(1) With increasing Ca/Si ratio, the silicate chain length gradually decreases and the more defective silicate chains contribute to the larger number of ONB atoms. The hydrophilic nature of ONB atoms leads to the water adsorption and dissociation. The ONB atoms are rich in the defective silicate chains in the C-S-H gel and result in the high reactivity for the confined water molecules. Water molecules dissociate and form Ca-OH and Si-OH with the surrounding calcium silicate skeleton. Due to the presence of dangling oxygen atoms, Ca-OH bonding becomes dominant in the C-S-H gel at high Ca/Si ratios. (2) Dynamically, water molecules, rich in the interlayer region, frequently collide with the calcium silicate backbone, which weakens the stability of the C-S-H gel. However, the hydroxyl groups, embedded in the calcium silicate sheet, exhibit lower diffusion ability. (3) Differing Young’s modulus and tensile strength values along the three directions, obtained by uniaxial tension testing, indicate anisotropic mechanical performance. The high stiffness and cohesive force along the y direction is mainly contributed by the silicate chains, while the weakest z direction behavior is attributed to the frequently broken H-bonds network. (4) With increasing Ca/Si ratio, the mechanical performance of C-S-H gel is weakened by reduction of the mean silicate chain length and the increasing number of interlayer water molecules. (5) The reactive force field combines both the mechanical response and chemical response during the large tensile deformation process. On the one hand, silicate


Page 36 of 42

Page 37 of 42

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


chains, acting in a skeletal role in the layered structure, de-polymerize thereby enhancing the loading resistance. On the other hand, water molecules, attacking the Si-O and Ca-O bonds, dissociate into hydroxyls, which are detrimental to cohesive force development.

Acknowledgements

Financially support from Shandong Provincial Natural Science Foundation, China under grant 2014ZRB01AE4, the China Ministry of Science and Technology under grant 2015CB655104 and Major International Joint Research Project under Grant 51420105015 are gratefully acknowledged.

Supporting Information Available: The supporting information includes the supplementary illustration of the molecular structure for the CSH model, the tensile loading rate and the chemical reactions during large deformation. This material is available free of charge via the Internet at http://pubs.acs.org.

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Phys. Chem. C 2008, 112, 16893-16897. (2) Krasilnikov, O.; Rodrigues, C.; Bezrukov, S. Single Polymer Molecules in a Protein Nanopore in the Limit of a Strong Polymer-Pore Attraction. Phys. Rev. Lett. 2006, 97, 018301. (3) Kerisit, S.; Liu, C. X. Molecular Simulation of Water and Ion Diffusion in Nanosized Mineral Fractures. Environ. Sci. Technol. 2009, 43, 777-782. (4) Greener, J.; Peemoeller, H.; Choi, C; Holly, R.; Reardon, E. J.; Hansson, C. M.; Pintar, M. M. Monitoring of Hydration of White Cement Paste with Proton NMR



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Table of Contents: hydrolytic reaction of water molecules confined in the C-S-H gel underwent tensile loading


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Reactive Molecular Simulation on Water Confined in the Nanopores of

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