FIRST-PRINCIPLES STUDY OF WATER ADSORPTION AND DISSOCIATION ON Β-C2S (100) SURFACE

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1 FIRST-PRINCIPLES STUDY OF WATER ADSORPTION AND DISSOCIATION ON Β-C2S (100) SURFACE Qianqian Wang(1), Yanhua Guo(1), Hegoi Manzano(2), Iñigo Lopez-Arbeloa(2), Xiaodong Shen(1) and Feng Li(1) (1) State Key Laboratory of Materials-Oriented Chemical Engineering, College of Materials Science and Engineering, Nanjing Tech University, Nanjing, , China (2) Molecular Spectroscopy Laboratory, Department of Physical Chemistry, University of the Basque Country, UPV/EHU, 48080, Bilbao, Spain Abstract Belite-rich cements with enhanced reactivity have been long pursued, due to the benefits of the low sintering temperature and CaCO 3 demand compared with alite-rich cements. In this work, the physi- and quimisorption of water molecules on the β-c 2 S (100) surface have been studied using the first-principles calculations based on Density Functional Theory. Energy and structures have been investigated, as well as energy barriers and reaction paths for water dissociation using the Nudged Elastic Band method. Our results indicated that the first step of hydration is molecular adsorption with a binding energy between to -1.24eV. It is followed by dissociation of the adsorbed H 2 O molecule to form a hydroxyl pair, with binding energy between and -1.29eV. The reaction barriers and reaction paths for the H 2 O molecule from chemical adsorption to dissociation on oxygen and calcium atoms are significantly distinct, and the activation energy of O1 type oxygen atoms (labeled by their symmetry) is the lowest. This is consistent with the published results that O1 is the active oxygen points in the β-c 2 S bulk. With a water dissociation energy barrier of 4-414meV on β- C 2 S (100) surface, the exothermic reaction can take place spontaneously, as corresponds to the fact that β-c 2 S reacts with water at normal temperature. 1. INTRODUCTION Cement reacts with water and binds with sand and stones to transform into highly functional solid at room temperature. Furthermore, it is very cheap in comparison with other materials [1]. However, the manufacture of cement entails a serials of environmental problems, especially the CO 2 emissions, coming mainly from the CaCO 3 decomposition and the fuel combustion [2]. Belite-rich cements have been proposed as an alternative due to the lower CaCO 3 demand and energy consumption during calcinations and grinding compared with alite-rich cements. However, hydration rate and early age mechanical properties of belite-rich cements is lower than OPC, which impose restrictions on their usage [3]. Therefore, it is imperative to understand the origin of belite hydration and activate the belite phase [4]. 252

2 Considerably efforts have been conducted for several decades to enhance the hydration rate of belite. Most of these attempts to activate the belite are i) stabilizing the high temperature polymorphism of belite [5]; ii) promote the lattice distortion[6-8]; iii) adding chemical admixtures[9, 10] ; and iv) altering the specimen-preparing method, [11, 12]. Although the hydration rate of belite is increased by these strategies, only partial success is achieved [13]. Moreover, there is little understanding how C 2 S reacts with water due to experimental limitations. Atomistic simulations could be of great importance in the understanding of clinker hydration. Manzano [13] and Durgun [14] used first-principles calculations based on density functional theory (DFT) to study the impact of chemical substitutions in alite and belite to suggest appropriate routes to modify their reactivity. Wang et al. [15] studied the reactive sites in β-c 2 S, finding active oxygen atoms in β-c 2 S which are more susceptible to electrophilic attack than the rest. On the basis of these works, we further investigated how water molecules interact with the β-c 2 S surface. By means of the first-principles calculations, we calculated belite surface energies for lower index orientations. Then the water adsorption and the reaction barriers for dissociation [16, 17] on the (100) surface of β-c 2 S were calculated. 2. COMPUTATIONAL DETAILS All calculations were performed within density functional theory (DFT) framework as implemented in DMol 3 program package[18]. The exchange and correlation effects were described by the generalized gradient approximation (GGA) [19] with the Perdew-Burke- Ernzerhof (PBE) [20] functional. The Double Numerical plus polarization (DNP) basis set was generated by applying an orbital cutoff of 5.5 Å. The Monkhorst-Pack k-point meshes [21] in the first Brillouin zone of the β-c 2 S slab model were set to An external potential to the vacuum region of the slab model was added due to periodic slab dipole moments and polar adsorbates (water molecule) on slabs. First, the β-c 2 S bulk structure is relaxed (both atomic coordinates and lattice parameters) which is shown in Fig. 1(a). The optimized lattice parameters are: a=5.58 Å, b=6.84 Å, c=9.41 Å, β=94.52, which are in good agreement with the experimental results [22]. All the low-index surfaces are cleaved on the optimized bulk structure considering multiple surface terminations and the most stable surface was chosen following the energy minimum principle. All the slab models for the low-index surfaces are superstructures larger than Å in the periodic directions. The thickness of the vacuum slab was set to be 14 Å, as shown in Fig. 1(c),and at least 6 layers of Ca-SiO 4 were used perpendicular to the surface. During the relaxation process, two Ca-SiO 4 layers in the slab center were fixed at the bulk positions. Adsorption energies, E ads, are calculated by the equation: E ads E ads a s s a E ads a s E s a (1) where E ads a s s a is the total energy for the optimized slab with adsorbates (H 2 O in this paper). E ads a s is the energy of the isolated adsorbate and E s a is the energy for the bare surface. Possible transition states (TSs) were calculated using the NEB method. 253

3 3. RESULTS AND DISCUSSION 3.1 The clean surfaces of β-c 2 S The surface energy, E s, is defined as: E s where E s a is the energy of the relaxed slab model, E is the energy for the relaxed bulk, A is the surface area and N is the number of the [Ca 2 SiO 4 ] in the bulk compared with the slab model (6 in this work). The surface energies for the low-index surface are listed in Table 1. Table 1: The surface energies for the low-index surface for β-c 2 S Surface (100) (010) (001) (110) (101) (011) (111) Energy (J/m 2 ) The surface energies for β-c 2 S are slightly smaller than those of Forsterite (Mg 2 SiO 4 ), from 1.22 to 2.18 J/m 2 [23]. In order to investigate the reaction between different types of Calcium and Oxygen pairs reacted and water, we chose the (100) surface because of the first layer included all the four types of oxygen atoms and the two types of calcium atoms. 3.2 Water adsorption on the (100) surface of β-c 2 S The optimized water molecule was placed onto the (100) surface in 16 different locations uniformly placed on the surface and with different initial orientations as shown in Fig. 1(d). After relaxation, we obtained 9 stable adsorption configurations with water molecules adsorbed similarly on the surface: the oxygen of water (O w ) is coordinated with a Ca atom on the surface and the hydrogen of water molecule (H w ) points to a surface oxygen atom, forming an hydrogen bond. The adsorption energy and configurations structure parameters for the 9 stable adsorption configurations are shown in Table 2. It is curiously to note that the calcium from the surface coordinated with O w displaces outward to the vacuum. (2) Figure 1: The optimized bulk (a) and (100) surface for β-c 2 S. (b) is the top view and (c) is the front view for (100) surface with one H 2 O molecule.(d) are the water configurations put onto the surface manually for test. 254

4 From the results, the adsorption energy is from -0.78eV to ev for the 9 stable adsorption configurations. The differences are mainly due to the adsorption on different oxygen and calcium pairs and the adsorption pairs on the surface are selective. The adsorption energy for water on calcio-olivine (γ-belite) ranges from to ev [24], slightly higher than our results on β-belite. Table 2: Adsorption energy and configurations for water on the β-c 2 S (100) surface No. Adsorption Adsorption Bond distance (Å) Bond angle points energy (ev) Ca-O w H w O-H w Hw-O w H w H w -O w -H w 1 Ca1 O Ca2 O Ca2 O Ca2 O Ca1 O Ca2 O Ca1 O Ca2 O Ca1 O Water dissociation on the (100) surface of β-c 2 S The 9 configurations of adsorbed water molecules on the surface are employed as initial points to find reaction paths using NEB method. The 5th and 7th stable water adsorption configurations did not lead us to water dissociation. The quimisorption energies for the remaining reactions are listed in Tab. 3, and the reaction paths in Fig. 3. From the reaction path, we conclude that the reaction processes consist in an initial rotation of the molecule towards an optimal orientation for the reaction, the dissociation of the water molecule into a hydroxyl pair, and in some cases a final surface diffusion of both products, mostly H +. Figure 2. The adsorption ((a) and (b)) and dissociation ((c) and (d)) configurations for the 6th stable state are shown here. (a) and (c) are the top views; (b) and (d) are the front views. 255

5 Table 3: The stable adsorption energy for H + and OH - and reaction barriers for the 7 dissociation paths of water No. Dissociation points Water quimisorption energy (ev) Rotation barrier Dissociation Barrier Diffusion Barrier (1) ΔH 1 Ca1 O Ca2 O Ca2 O Ca2 O Ca2 O Ca2 O Ca1 O (1) H E E ads Figure 3: Reaction paths and barriers for water dissociation. The left side of the points are the IS (Initial states) and the right side is FS (Final states). The red, blue and pink arrows marked on figures represent the rotation, dissociation, and diffusion barriers respectively. The reaction barriers for water dissociation on the Ca2 and O1 pair are remarkably lower compared with other Ca 2+ and O 2- ion pairs. The rotation and dissociation energy barrier are only 24 mev and 4 mev respectively, both of which are the lowest barriers compared with the other reaction paths. Furthermore, the 6th reaction path presents also the highest H r among 256

6 these reactions. Therefore, the water dissociation on the Ca1 and O1 pair is the most favorable reaction point from both kinetic and thermodynamic points of view. Previously published results [15] based on bulk investigation of C 2 S already pointed out that O1 was the most active point for electrophilic attack, confirming our results. With a water dissociation energy barrier 4-414meV on β-c 2 S (100) surface, the exothermic reaction can take place spontaneously, which is consistent with the fact that β-c 2 S reacts with water at normal temperature. The water quimisorption energy to give H + and OH - ion are in the range of those on T-C 3 S (100) surface the value of which is from to ev [25]. 4. CONCLUSION In summary, the surface energies for all the low index crystal surface of β-c 2 S are calculated with first-principles calculations. The most stable surface is (101) surface, with the highest symmetry compared with the other low-index surfaces. Water adsorption energies on different types of calcium and oxygen pairs on the (100) surface are significantly different, ranging from -0.78eV to ev depending on the Ca 2+ and O 2- implied. The adsorption configurations for water on different calcium and oxygen pairs are almost the same, with the water oxygen atom coordinated to surface Ca ions and one hydrogen bond between the water and a silicate oxygen atom. Water dissociation process on the surface can be divided into three steps: rotation, dissociation and diffusion process. The lowest barrier during the rotation and dissociation process was on the Ca2 and O1 pair among the other calcium and oxygen pairs. This is consistent with the published results that O1 is the active points in β-c 2 S with high energy level for the O1-2p orbital. ACKNOWLEDGEMENTS The author gave thanks to the supports of the National Basic Research Program of China (2009CB623100). A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, IRT1146), and Graduate Education Innovation Project in Jiangsu Province (CXZZ13_0424) and the ETORTEK program from the Basque Country Government should also be appreciated. H.M. acknowledges the "Juan de la Cierva" postdoctoral contracts from the Spanish Ministerio de Industria y Competitividad. REFERENCES [1] Scrivener, K. L., Kirkpatrick, R.J., 'Innovation in use and research on cementitious material', Cem. Concr. Res., 38 (2) (2008) [2] Ma, B., Ma, M., Shen, X. D., Li, X. R., 'Compatibility between a polycarboxylate superplasticizer and the belite-rich sulfoaluminate cement: Setting time and the hydration properties', Constr. Build. Mater.,51 (2014) [3] Martín-Sedeño, M.C., Cuberos, A.J.M., De la Torre, A., Alvarez-Pinazo G., Ordonez M.L., Gateshki, M., Aranda, M.G.A., 'Aluminum-rich belite sulfoaluminate cements: Clinkering and early age hydration', Cem. Concr. Res., 40(3) (2010) [4] Cuberos, A.J.M., De la Torre, A., Martín-Sedeño, M.C., Moreno-Real L., Merlini, M., Ordonez L. M., Aranda, M.G.A., 'Phase development in conventional and active belite cement pastes by Rietveld analysis and chemical constraints', Cem. Concr. Res., 39(10) (2009) [5] Ghosh, S., Rao, P. Bhaskara, Paul, A. K., Raina, K., 'The chemistry of dicalcium silicate mineral', J. Mater. Sci., 14(7) (1979)

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