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Hydration Mechanism of Reactive and Passive Dicalcium Silicate Polymorphs from Molecular Simulations Qianqian Wang,† Hegoi Manzano,*,‡ Yanhua Guo,† Iñigo Lopez-Arbeloa,‡ and Xiaodong Shen*,† †

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China ‡ Molecular Spectroscopy Laboratory, Department of Physical Chemistry, University of the Basque Country, UPV/EHU, 48080 Bilbao, Spain S Supporting Information *

ABSTRACT: Belite (C2S, CCaO, SSiO2) based cements are promising low-CO2 substitutes of ordinary Portland cement. The main drawback is their low hydration rates, which makes them unpractical for construction. Yet more disconcerting is the different reactivity between polymorphs of belite: while β-C2S reacts slowly with water, γ-C2S is almost inert. Due to the demand of improving C2S reactivity, in this work we aim to understand the hydration mechanism of belite polymorphs by density functional theory and molecular dynamics simulations methods. We calculated the low-index cleavage energies, and the thermodynamic equilibrium structures were constructed through Wulff shape constructing method. We built the adsorption energy surface (AES) maps and found out the transition state structures for (chemi)sorption of water molecules. Finally molecular dynamics were employed to simulate the reactions taking place during 2 ns at room temperature. We found that water dissociation consists of three steps, rotation, dissociation, and diffusion, with different energy barriers. Considering the AES, DFT energy barriers, and the molecular dynamics simulations, the number of reactive sites is the key aspect that controls hydration; even though water reacts preferentially in γ-C2S surfaces over in β-C2S in terms of energy, a considerably lower number of reactive points in γ-C2S would limit the surface hydration and dissolution. Belite is an orthosilicate mineral with isolated SiO44− tetrahedra and Ca2+ atoms as counterions as shown in Figure 1, with several polymorphs: α-, αH′-, αL′-, β-, and γ-C2S. The so-called β-form is the most common one in the traditional cement manufacturing process and appears also as natural mineral called larnite, with structural resemblances to the arcanite mineral family. It exhibits a much higher reaction rate with water than the γ-form, which belongs to the olivine mineral group and is almost inert in contact with water. However, a reactive version of the γ-polymorph has been long pursued, since its low synthesis temperature (less than 773 K in contrast to 963 K of β-belite) makes it more interesting from economic and environmental points of view. In addition, both belite polymorphs have great potential for CO2 capture and sequestration, which is influenced by their surface hydration state.11−13 The hydration degree of the γ-form has also been found relevant on the anisotropic propagation of the seismic waves in Earth’s upper mantle.14 In this work, we investigate the surface properties of β- and γC2S and how water interacts with them, aiming to determine

1. INTRODUCTION The hydration and dissolution of mineral surfaces have important environmental implications, for instance, in CO2 sequestration,1 nuclear waste immobilization,2 or geochemical and biological processes.3 The dissolution of cement phases to recombine and form the cement paste is another example of mineral dissolution with great environmental implications, due to the widespread use of cement as building material. The manufacture of ordinary cement entails significant CO 2 emission, reaching 5% of worldwide emissions, coming mainly from CaCO3 decomposition and the intensive fuel combustion.4 Cements based on dicalcium silicate, also known as belite, have been proposed as a possible low CO2 alternatives due to the lower calcium content and energy consumption during the manufacturing process compared with ordinary cements. However, there is an important drawback: the low hydration rate of belite, which increases the construction time to unpractical limits.5 Despite the considerable experimental efforts conducted over the years,6−10 little success has been achieved in the acceleration of belite dissolution. Therefore, molecular simulations could be a valuable tool to understand the hydration mechanism of belite, in order to devise alternatives to accelerate its dissolutions. © XXXX American Chemical Society

Received: June 2, 2015 Revised: July 21, 2015

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DOI: 10.1021/acs.jpcc.5b05257 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

moments due to the slab surfaces and to the presence of the water molecule. ReaxFF21 reactive force field was employed to calculate the water adsorption energy surfaces (AESs) and the molecular dynamics simulations, due to its low computational cost compared with DFT. The force field parameters are a combination of the Si−O/H22 and Ca−O/H23 sets, merged and tested to reproduce the structure and mechanical properties of calcium silicate hydrates.24. This force field has been previously used to investigate mechanical properties of the amorphous C−S−H gel under tensile 25 and shear strain,26 as well as its structure−properties relationships for a wide range of chemical compositions.27 More recently, the force field has also been proven to reproduce elasticity, surface properties, and water dissociation on calcium silicate crystals.28 The simulations were performed using the Reax/c version implemented in the LAMMPS code (version 01-Feb14).29 The energy minimizations were done using a conjugate gradient method, with an energy and force cutoff tolerance of 1.0 e−5 kcal/mol. The molecular dynamics were performed in the NVT ensemble, using the velocity Verlet integration scheme with a 0.2 fs time step, and a Nose−Hoover thermostat to control the temperature with a 20 fs constant.

Figure 1. Representation of the optimized unit cell of (a) β-C2S and (b) γ-C2S crystals. Calcium atoms are represented as green spheres, the oxygen atoms as red spheres, and the silicon atoms as orange spheres. The silicate tetrahedral [SiO4]4− groups are depicted as blue tetrahedra.

3. RESULTS AND DISCUSSION A. Bulk Dicalcium Silicate Polymorphs. First, the β- and γ-C2S bulk structures were optimized, relaxing both the atomic coordinates and lattice parameters. The relaxed unit cell parameters are listed in Table 1, together with the experimental values30,31 and data from previous simulations.32,33,35 The relaxed crystal structures are shown in Figure 1. Our DFT results are in great agreement with the lattice parameters obtained from X-ray diffraction, just showing less than 1.5% expansion as usual for GGA exchange−correlation functional.36 The results show the same accuracy as previously reported from DFT simulations with plane-waves including dispersive terms.32,33 ReaxFF relaxed unit cells also show a great agreement with the experimental and DFT values. We also computed the elastic tensor of both C2S polymorphs from strain−stress relationships. From the elastic tensor, we derived the bulk (K) and shear (G) moduli using the Voight− Hill−Reuss definition, and the anisotropic Young modulus (see ref 34 for more detailed expressions). The results obtained by ReaxFF for β-C2S are in good agreement with experimental data and previous computational studies. To our knowledge, the elastic properties of γ-C2S have not been previously reported. According to our simulations, the Young modulus of γ-C2S is ∼20% smaller than the one of β-C2S. B. Anhydrate Surfaces and Wulff Shapes. Once we have a proper description of the bulk crystals, we turn to compute

the origin of their diverse reactivity, using a combination of reactive force field (ReaxFF) and density functional theory (DFT) simulations. First, we study the surface energies for lower index crystal orientations and construct their equilibrium shapes. Second, the adsorption energies of water on a selected surface for each polymorph are carefully studied, constructing the water adsorption energy surfaces (AES). Then, the water dissociation energy barriers15,16 in the most energetically favorable adsorption locations are computed, using DFT simulations. Finally, the water dissociation dynamics are followed at finite temperatures using molecular dynamics to confirm the suggested hydration mechanism.

2. COMPUTATIONAL DETAILS The DFT calculations were performed in DMol3 package.17 The exchange and correlation effects were described in the generalized gradient approximation (GGA) 18 with the Perdew−Burke−Ernzerhof (PBE)19 functional. A double numerical plus polarization (DNP) basis set was generated applying an orbital cutoff of 5.5 Å. The Monkhorst−Pack kpoint meshes20 in the first Brillouin zone of the β-C2S and γC2S slab models were set to 1 × 2 × 1 and 1 × 1 × 1, respectively. An external potential to the vacuum region of the slabs was added when necessary to avoid macroscopic dipole

Table 1. Unit Cell Parameters and Elastic Properties for β- and γ-C2S β-C2S this work, DFT/ReaxFF a (Å) b (Å) c (Å) β (deg) K (GPa) G (GPa) E (GPa)

5.58/5.50 6.33/6.57 9.41/9.30 94.53/94.59 125.8 59 153.1

expt30 5.50 6.75 9.30 94.59

130−140

γ-C2S ref 32, 34, 35

this work, DFT/ReaxFF

expt31

ref 33, 35

5.50, 5.57 6.81, 6.80 9.36, 9.36 94.88, 94.71 111 53.1 137.9

5.21/5.13 11.37/11.17 6.83/6.33

5.08 11.23 6.76

5.17, 5.11 1.43, 11.33 6.88, 6.80

B

95.56 48.58 124.7 DOI: 10.1021/acs.jpcc.5b05257 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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crystal orientations. On the other hand, β-C2S is less symmetric, and the structure of the various cleavages is lower, so their cleavage energy difference is less marked. This effect is reflected in the equilibrium shapes of the crystals, reconstructed using the Wulff method38−40 as shown in Figure 2a,b. The Wulff structure is a method for graphically plotting the surface energy. The surface energy is proportional to the norm of each crystallographic orientation vector. A plane perpendicular to each vector is then drawn passing through the end point of the vector. The inner envelope of all these planes forms the Wulff construction with the thermodynamically equilibrium surfaces. The shape for γ-C2S is more regular than the shape of β-C2S, which we attribute to the higher symmetry of the former. C. Adsorption Energy Surfaces for β-C2S(100) and γC2S(010). In order to understand the hydration of dicalcium silicate polymorphs, we investigate the adsorption energies of water molecules in the mineral surfaces. We chose the lowest energy surface, that is, the (010) for γ-C2S and the (100) surface for β-C2S, despite it being the second lowest energy, because the four types of oxygen atoms and the two types of calcium atoms that can be defined by symmetry are exposed to the vacuum. Due to the lower symmetry of the crystals compared with the other phases, for example, the metallic oxides, it is hard to select a priori the most stable water adsorption points. An intensive sampling of possible configurations using DFT methods would be too expensive from a computational point of view. Hence we decided first to create an adsorption energy surface map (AES) on the whole surface using ReaxFF force field as a first step to identify the relevant adsorption sites and in this way to narrow the number of DFT simulations to be performed. The AES was constructed following the methodology explained in ref 28. One water molecule at a time was placed at about 1.5 Å over the surface in a grid of points spaced by 0.1 Å, and the water adsorption energies, Eads, were calculated as

cleavage energies of β- and γ-C2S low-index surfaces. Multiple surface terminations were tested for each cleavage; the most stable ones were chosen following the energy minimum principle. The constructed slabs models were superstructures larger than 10 × 10 Å in the periodic directions and at least six Ca−SiO4 layers were used perpendicular to the surface, resulting in thicknesses equal to or larger than 15 Å with a 14 Å vacuum slab as shown in Figure 2c. During the relaxation

Figure 2. Wulff shapes for (a) β-C2S and (b) γ-C2S. The color scale indicates the cleavage energy of the surface in (eV). (c) Example of one slab model to study surface energies.

process, the central Ca−SiO4 layers in the slab were fixed. The cleavage energies can then be calculated from Uslab − nUbulk (1) 2A where Uslab is the energy of the relaxed slab model, Ubulk is the energy of the bulk, A is the surface area, and n is the number of unit cells of the bulk in the perpendicular direction used to build the slab model. The cleavage energies (listed in Table 2) of most γ-C2S surfaces are higher than those of β-C2S. These results may look Ucleavage =

Eads = Ewater + slab − Ewater − Eslab

where Ewater+slab is the total energy for the optimized slab with adsorbed H2O, and Ewater and Eslab are the energies of an isolated water molecule and slab, respectively. At each point of the grid, 12 initial water configurations were tried to avoid artifacts due to the initial orientations, and the most favorable adsorption energy from those 12 tested orientations was used to create the adsorption energy surface (AES) map. Figure 3 shows the AESs projected onto the β-C2S(100) and γ-C2S(010) surfaces, with the energies presented in a color scale. In both surfaces the adsorption energies range goes from 0 to about −1.6 eV, yet in γ-C2S the energies tend to be higher, which indicates a more favorable water adsorption. As for the cleavage energies, this result is counterintuitive, since the water adsorption is more favorable in the polymorph with lower reactivity. The crystals symmetries are also reflected in the AES. The β-C2S(100) AES is irregular, and the favorable water adsorption sites are distributed in regions between calcium atoms. The γ-C2S(010) presents a very symmetric AES, with a single large favorable region, also between calcium atoms. D. Chemisorption at the Favorable Locations. We turn to investigate in detail, using DFT calculations, the water adsorption and dissociation in some points selected by checking the previously computed AES results. Different water molecular orientations and potential water dissociation locations were tried. All the configurations were relaxed using the same parameters as in the slab model relaxation process.

Table 2. Cleavage Energies for the Low-Index Surface for βC2S in J/m2 β-C2S γ-C2S

(100)

(010)

(001)

(110)

(101)

(011)

(111)

0.85 1.72

1.00 1.13

1.05 1.87

1.37 2.25

0.76 1.47

1.03 1.42

1.20 1.52

(2)

counterintuitive, as they imply that the phase with higher surface stability reacts faster with water. However, from the cohesive energy calculated in the published results,35 β C2S has lower cohesive energy, which corresponds with the lower cleavage energies. Moreover, previous DFT results confirmed this trend: the (010) cleavage energy of γ-C2S computed here is in good agreement with previous simulations,33,37 and other authors32 reported similar or even lower surface cleavage values for β-C2S. It is also interesting to note that the diversity between cleavage energies is considerably higher for the γpolymorph than for the β one. In β-C2S, the lower surface energy, which corresponds to the (100) cleavage, is about 0.5 J/ m2 lower than the highest one (110), while in γ-C2S, the highest energy surface (110) is twice the lowest one (010). The different trends in surface energies, more homogeneous in β-C2S and more disparate in γ-C2S, are attributed to the symmetry of the polymorphs. On the one hand, γ-C2S has a more symmetric crystalline arrangement, which may favor some C


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configuration may explain its higher adsorption value, but further simulation would be necessary to confirm this point. Starting from these points, we investigated the energy barriers involved in water dissociation using DFT simulations. We computed the energy of intermediate configurations extrapolated from the stable adsorption configuration to the dissociated one, fixing the position of the water oxygen atom and relaxing the rest of the system. The energy evolution during the dissociation for a model case is given in Figure 5

Figure 3. Surface structures and adsorption energy surface (AES) maps for (a) β-C2S(100) and (b) γ-C2S(010). Different colors referring to different adsorption energies as indicated by the scale bars. The atoms are represented as in Figure 1.

Nine adsorption points (labeled β1 to β9) on the β-C2S(100) surface and three (γ1 to γ3) on γ-C2S 010) surface were obtained. Here we will give a general description of the adsorption energies and configurations, while more detailed information for all the points is included in the Supporting Information. The adsorbed water molecules on the β-C2S surfaces are all in “side” configurations (the typical configurations are represented in Figure 4): the water oxygen atom (Ow) is

Figure 5. Reaction paths and barriers for water dissociation from the initial state (IS) to the final state (FS). The red, blue, and pink arrows marked on figures represent the rotation, dissociation, and diffusion barriers, respectively.

(the Supporting Information includes a similar figure for all the remaining cases, together with detailed information of all the energy barriers). Depending on the initial configuration, the dissociation may involve up to three steps: first, a rotation of the molecule to reach an optimal orientation for the reaction; second, dissociation of the water molecule into a hydroxyl pair; third, a surface diffusion of both products toward equilibrium positions. Table 3 summarizes the highest and lowest energy barriers of water dissociation and the final chemisorption energy. The

Figure 4. Typical water adsorption configurations, flat and side configurations, are represented in top (a) and front (b) view, respectively.

Table 3. Energy Barriers and Water Chemisorption Energy on the β-C2S and γ-C2S Surfacesa

coordinated to a surface Ca atom, with one hydrogen atom (Hw) forming a hydrogen bond with the nearest surface oxygen atom and the other Hw pointing toward the vacuum. The adsorption energies range from −0.78 to −1.24 eV, matching well with the ReaxFF values. We did not find any significant relationship between the water adsorption configurations, bond lengths, angles, and corresponding adsorption energies. On the γ-C2S(010) surface, two different water adsorption configurations were found: the “side” configuration already described for β-C2S and a “flat” one in which both hydrogen atoms of the water molecule are coordinated to surface oxygen atoms, forming two hydrogen bonds. The adsorption energies are between −1.15 and −1.55 eV, and the “flat” configuration is energetically more favorable than the “side” one. In a previous work, Keriset et al.33 obtained adsorption energies for γC2S(010) ranging from −1.26 to −1.53 eV, which was in very good agreement with our results. However, contrary to our case, they found that the side configuration is more stable than the flat one. The formation of two hydrogen bonds in the flat

final dissociation configuration

starting point

highest (lowest) energy barrier (meV)

water chemisorption energy (eV)

Fβ1 Fβ2 Fβ3 Fβ4 Fβ5 Fγ

β1 β2−β3−β4 β6 β8 β9 γ1−γ2−γ3

56 136(33) 24 49 432 483(0)

−1.29 −1.17 −1.26 −1.07 −1.22 −1.77

a

For reactions that start with different configurations and lead to the same final state, the highest and lowest energy barriers are given.

rotation and dissociation barriers present the same energy range, between 0 and 483 meV, and the surface diffusion is barrier-free or its energy barrier is smaller than 15 meV. Furthermore, we found that the dissociation in γ-belite can be barrierless. Using transition-state theory, we can do a rough estimation for the reaction rate constant of water dissociation:41 D


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Figure 6. Top view of the hydroxylation surfaces ((a) β-C2S(100) and (b) γ-C2S(010)) with the cumulative representation of the hydroxylation points (shown as blue bubbles) during the 2 ns of simulation. (c) Number of dissociated water molecular per surface areas during the first 2 ns.

k=

kBT −ΔU * /(kBT ) e ℏ

The reactive sites match those predicted by the AES and DFT results and depict clearly the larger number of reactive points in the β-C2S polymorph. The number of dissociated water molecular per surface area during the first 2 ns is calculated and plotted as shown in Figure 6c. In the very early steps, the water dissociation in γ-C2S is faster than that in β-C2S, as expected from the energy barriers found by DFT (see Section D). However, after 65 ps, the trend is reversed and the degree of reactions increases in β-C2S, since there are more sites available for water dissociation. After 1 ns, about 1.5 water molecules/nm2 have reacted on the γC2S surface, and the kinetics reach a quasi-steady regime. At the same time, 2.6 water molecules/nm2 dissociate on the β-C2S surface, and the increase at ∼1.7 ns suggests that the difference could be even higher for longer simulation times.

(3)

where kB and ℏ are the Boltzmann and Planck constants respectively, ΔU* is the potential energy barrier, and T is the temperature. Using the limiting cases, we obtained a rate constant that ranged from spontaneous dissociation (in the case of barrier-free) to 48 ms−1 for the highest energy barrier process (0.483 eV). These values suggested that, at room temperature, water molecules would have already reacted in most of the possible surface locations in a macroscopic time scale. It is important to notice that the data in Table 3 is organized as a function of the final state, that is, we group the adsorption points that upon dissociation lead to the same final arrangement of the hydroxyl pair. Here, we define as reactive points the locations in which water can be dissociated. This way, a remarkable feature related to the surface symmetry, illustrated in Figure 5, pops out. In β-belite, different initial configurations lead to at least five different hydroxyl pair configurations, while in γ-belite all the tested adsorption sites lead to a single dissociated state (see the Supporting Information for a detailed picture of the final dissociation states). According to these results, the theoretical quantities of reactive points for water dissociation per nm2 are roughly 7.2 and 2.9 for β- and γ-polymorphs, respectively. Hence, the higher symmetry of the γ-C2S polymorph may be the reason behind its lower reactivity, since they are fewer points susceptible to chemical attack that may decrease the overall hydration rate of the crystal compared with the β-C2S polymorph. E. Dynamics of water dissociation on the surfaces. Based on the above results, there is a larger number of possible reactive points in the β-C2S than in the γ-C2S; hence, we suggest that despite the water dissociation being energetically more favorable in the latter, the lower amount of reactive sites may be responsible for its lower reactivity with water. To further confirm this hypothesis, we performed ReaxFF molecular dynamics simulations in systems containing 10 water molecules per nm2 placed above the studied surfaces. This amount of water is in excess with respect to the predicted quantities of chemisorption sites, which guarantees that the surface can react until it is fully saturated by hydroxyl pairs. Figure 6a,b shows the cumulative trajectories of H atoms from the reacted water molecules on the β-C2S(100) and γC2S(010) surfaces during the first 2 ns, or in other words, all the hydroxyl locations after water dissociation. All the predicted reaction points are reacted and occupied by the hydroxyl pairs.

4. CONCLUSION Dicalcium silicate, or belite, is a candidate mineral to produce environmentally friendly cements with low CO2 emissions and energy consumption. However, its low hydration rate restricts the practical application in construction. Therefore, it is important to understand the hydration of this mineral, in order to design routes to accelerate it. In this work, we used a combination of DFT and reactive force field simulations to shed light on the hydration mechanism of belite and the differences in reactivity between the γ and β polymorphs. Our DFT results indicate that the belite cleavage energies are lower for the β-C2S polymorph than for γ-C2S one. The constructed Wulff shape of the latter is more symmetric, which reflects the higher symmetry of this polymorph. The water adsorption energies obtained by ReaxFF and DFT for γ-C2S surfaces are generally higher than those for the β-form, and the dissociation process entails lower energy barriers. All these results indicate that the γ-C2S surfaces are less stable than the βC2S ones in vacuum and that water adsorption and dissociation are energetically favorable, which is opposite to the experimental dissolution rates. However, we found a considerably lower number of reactive sites on the γ-C2S surface (almost 2.5 times lower) compared with those in β-C2S due to its more symmetric structure. Therefore, we suggest that less water will react on γ-C2S, and this will be the key point behind its lower dissolution rate. Molecular dynamics simulations on water dissociation at 300 K confirm our hypothesis. At very short simulation times water dissociates faster on γ-C2S, yet the reactions slow as the few reactive sites are saturated. In contrast, β-C2S keeps on reacting for a longer time since the surface has more reactive points. E


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Despite that more surfaces and the role of structural and chemical defects should be studied for a complete understanding of belite hydration, this work suggests that the difference in reactivity between C2S polymorphs originates in their crystalline symmetry. The fewer reactive sites on γ-C2S compared with β-C2S would hamper the ligand-exchange reactions between the crystal and the solution, decreasing the ionic desorption rate from the crystal.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05257. Adsorption energies and configurations for water, stable chemisorption energy for H+ and OH−, and final water dissociation configurations on all the locations on βC2S(100) and γ-C2S(010), Reaction paths and barriers for water dissociation, and constructed adsorption energy surfaces on the top of β-C2S(100) and γ-C2S(010) surfaces calculated by 1, 2, 4, 8, and 12 initial orientations of the water molecular (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The National Basic Research Program of China (Grant 2009CB623100), the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, Grant IRT1146), and the Graduate Education Innovation Project in Jiangsu Province (Grant CXZZ13_0424), as well as the NANOGUNE 2014 project (Grant IE14-293) from the Basque Country Government ETORTEK program provided funding for this work. We are thankful for the discussion from Dr. Feng Li and Eduardo Duque-Redondo. Thanks for the funds and projects from our governments and department. H.M. acknowledges the “Juan de la Cierva” postdoctoral contract from the Spanish Ministerio de Industria y Competitividad.

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