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Correlation between Composition and Mechanical Properties of Calcium-Silicate-Hydrates Identified by Infrared Spectroscopy and Density Functional Theory Mohammadreza Izadifar, Franz Königer, Andreas Gerdes, Christof Wöll, and Peter Thissen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11920 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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The Journal of Physical Chemistry

Correlation between Composition and Mechanical Properties of Calcium-Silicate-Hydrates Identified by Infrared Spectroscopy and Density Functional Theory Mohammadreza Izadifar, Franz Königer, Andreas Gerdes, Christof Wöll, and Peter Thissen* Karlsruhe Institute of Technology (KIT), Institute of Functional Interfaces (IFG), Hermannvon-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

ABSTRACT: Building and construction industry are at the same time the backbone and the driving force of our modern society. Nearly all our today's technical infrastructure is based on cement-based materials. Detailed, spectroscopic investigations of model reactions on well‐defined mineral substrates under UHV‐conditions are largely lacking, thus prohibiting a validation of theoretical methods. Even simple chemical processes are only poorly understood. As a result, the rational design of anti‐corrosion strategies is virtually impossible. In this manuscript, we have worked on eight different calcium-silicate (CS) or calcium-silicate-hydrate (CSH) phases, namely Tobermorite 14 Å, Tobermorite 11 Å, Tobermorite 9 Å, Wollastonite, Jaffeite, Jennite, γ-C2S, and α-C2SH. These representative model phases are calculated with the help of Density Functional Theory (DFT) modeling method. Initially, we take care of the mechanical properties of the material. Our results revealed that Jaffeite, γ-C2S, and α-C2SH have a linear bulk modulus due to the monomer structure of silicate tetrahedra. Tobermorite 14 Å and γ-C2S present the lowest and highest bulk modulus, respectively. In the second part, the optimized geometries allow for the precise calculation of vibrational eigenmodes and frequencies by the force‐constant (FC) approach. The proportions of C/S and H/C are major criteria for the classification of the calculated wavenumber of ν(Si-O) for all phases in our model system. However, γ-C2S and α-C2SH have an equal C/S ratio. As a result of the different H/C ratios, wavenumbers were found for four characteristic vibration modes. The contribution of the C/S ratio for Tobermorite 11 Å in comparison to Jennite is another aspect to be considered for phases with equal H/C ratios. Due to the higher H/C ratio, Jennite exhibits a higher wavenumber than Tobermorite 11 Å.

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1. INTRODUCTION Concrete is a man-made material that is used in a wide variety of applications. It is a simple and important material extensively used for the construction of buildings, bridges, roads, public infrastructures, and dams. Annually, more than 20 billion tons of concrete are produced worldwide, corresponding to 5 to 10% of the production of anthropogenic carbon dioxide1. A longer effective lifetime of concrete components and infrastructure durability are required in order to enhance the service life. For both, reduction of environmental impacts2 and increasing the service life of concrete, the reaction of cement with water on the molecular level must be examined in detail1, 3-8. Failure of concrete-based structures may be due to many reasons, such as corrosion of reinforcements, thermal cycling, and permanent dynamic stresses caused by frequently passing vehicles. However, interaction of calcium-silicate (CS) phase with water is considered the most important aspect9. Calcium-silicate-hydrate (CSH)10 gel is one of the most indispensable binding phases in cement-based material and considered to be the main component of concrete11, which contains more than 60% of hydration products12. In other words, the cement paste is one of the most widely used construction materials, the mechanical behavior of which is determined by the properties of C-S-H gels10, 12, 13. Recent studies about calcium-silicate-hydrate (CSH) phases highlight the significance of cement based materials for mankind. Morshedifard et al.14 recently employed an incremental stress marching approach to survey time-dependent behavior of calcium-silicate-hydrate (CSH) phases through molecular dynamics simulation (MD). Bauchy et al.13 reported the fracture toughness of calcium-silicatehydrate (CSH) model, made of 501 atoms by molecular dynamics using the LAMMPS package. In another study, Bauchy et al.15 analyzed the structure of a realistic simulated model of C-SH, compared the latter, to crystalline tobermorite, a natural analogue of C-S-H, and to an artificial ideal glass. The impact of chemical compositions on the eﬀective interactions between C-S-H nanolayer in aqueous solution has been studied by Masoumi et al.16. As the mineral composite is extremely porous, better understanding of its surface chemistry and its reaction with water is required to prevent corrosion of the infrastructure over time. The metal-proton exchange reaction (MPER) is the best-known of all possible corrosion reactions of C-S-H phases9, 17. Initially, corrosion takes place by breaking up the silicate chain in the surface region of concrete when a concrete component containing of calcium-silicate-hydrate (CSH) phases is exposed to an aqueous environment. The MPER causes leaching of Ca into the environment and formation of surface OH groups by proton exchange between the water and mineral surface, as is demonstrated in equation 19, 18-20. Ca3Si3O9 + 2 * H+ + 2 * OH-

Ca2H2Si3O9 + Ca(OH)2

(1)

So far, the atomic characterization of the calcium-silicate-hydrate (CSH) phases has been primarily carried out by Nuclear Magnetic Resonance (NMR) spectroscopy. Rejmak et al. studied the NMR spectra of 29Si in (C-S-H) gel through computation of the isotropic shielding of silicon atoms within the density functional theory21. The results demonstrate that the 29Si chemical shifts are dependent not only on the degree of condensation of the (SiO4) units, as commonly assumed, but also on the local arrangement of the charge compensating H and Ca cations. In another study, 29Si Nuclear Magnetic Resonance was exerted in order to investigate 2 ACS Paragon Plus Environment

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the effects of temperature and the addition of reactive silica on the silicate structure of amorphous calcium-silicate-hydrate (CSH)22. The results show that by increasing both, the temperature of formation and the addition of reactive silica, the degree of silicate polymerization in the C-S-H increases as well. Cong et al. also accomplished a comprehensive investigation of single-phase calcium-silicate-hydrate (CSH) with known compositions using the combined capabilities of 29Si magic angle spinning (MAS) Nuclear Magnetic Resonance, powder X-ray diffraction (XRD), and chemical analysis of the solution and solid23. They have reported that 1) calcium-silicate-hydrate (CSH) shows diversity and continuity in both structure and composition and forms a continuous structural series. 2) Phases-pure calcium-silicatehydrate (CSH) have C/S ratios between 0.6-1.54. 3) Ca-OH and Si-OH bonds both forming in calcium-silicate-hydrate (CSH) simultaneously. In the future, the community can take advantage from another technique, namely Fourier Transform-Infrared Spectroscopy (FTIR), which is investigated here with the help of Density Functional Theory (DFT). In general, one of the biggest problems of building materials was to correlate single peaks measured in FTIR spectra with certain vibration modes on the atomistic level.



2. METHODOLOGY A. Theoretical Considerations. In this paper, the Density Functional Theory (DFT) was employed as defined in the Vienna ab initio simulation package (VASP)24-27 for the calculation of the electronic structure. The Perdew-Brucke-Ernzerhof (PBE) functional was employed to describe the electron exchange and correlation energy within the generalized gradient approximation (GGA)29. The projector-augmented wave (PAW) method30 and pseudopotential were used to describe the electron-ion interaction. A plane-wave up to kinetic energy cut-off of 360 eV was defined for the DFT calculations. The Brillouin zone sampling was carried out with 1 x 1 x 1 meshes of Monkhorst-Pack k-points31. The optimization of the atomic coordinates (and unit cell size/shape for the bulk materials) was performed through a conjugate gradient technique, which utilizes the total energy and the Hellmann–Feynman forces on the atoms (and stresses on the unit cell). The structures were considered to be fully relaxed when the forces on the atoms were smaller than 0.01 eV Å−1. Vibrational eigenmodes and frequencies were calculated by the force‐constant (FC) approach28. We also used open-source XcrySDen software, which is a crystalline and molecular structure visualization program to display the crystalline structure and electron densities 32.


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3. RESULTS AND DISSCUSION For a comprehensive survey, we selected eight hydrated phases as model systems compatible with the phases created during the hydration of cement: Tobermorite 14 Å33-35, 11 Å34, 36-38, and 9 Å34, Wollastonite11, 17, 34, 39, 40, Jaffeite34, 41, Jennite34, 42, 43, γ-C2S34, and α-C2SH34, 44. Table 1: Characterizations of selected calcium-silicate (CS) and calcium-silicate-hydrate (CSH) phases during the hydration of cement. Chemical name

Formula

C/S ratio

H/C ratio

Number of water molecules

Tobermorite 14 Tobermorite 11 Tobermorite 9 Wollastonite Jennite

Ca8Si12O47H28 Ca8Si12O43H20 Ca8Si12O33H2 CaSiO3 Ca9Si6O32H22 Ca2SiO5H2 Ca2SiO4 Ca6Si2O13H6

0.67 0.67 0.67 1 1.5 2 2 3

3.5 2.5 0.25 2.44 1 1

7 5 -

α-C2SH γ-C2S Jaffeite

Table 1 illustrates the composition of the calcium-silicate (CS) and calcium-silicate-hydrate (CSH) phases. The structures of the phases mentioned above are shown in figure 1. The increasing C/S ratio induces two simultaneous working mechanisms: (1) the uptake of calcium in the interlayer and (2) the depolymerisation of the silicate chains which result in general changes of the CS and C-S-H structure. Of all phases characterized, Tobermorite 14 Å was considered the most hydrated phase of the group. In contrast, Jaffeite is the less hydrated phase.



Figure 1: Optimized unit cell side views of Tobermorite 9, 11, 14 Å, Wollastonite, Jennite, Jaffeite, γ-C2S, and α-C2SH, relaxed by means of first principles calculation. The silicate tetrahedra chains were found in Tobermorite 9, 11, 14 Å, Wollastonite and Jennite. The monomer structure of silicate tetrahedra was observed in Jaffeite, γ-C2S and α-C2SH. Ten total energies were computed by making the unit cell of each phase larger or smaller. Linking the computed total energies resulted in an excellent E-V curve which really fits to the Birch–Murnaghan equation of state plotted in46. To determine the bulk modulus, P-V diagrams were computed for all phases, as is shown in figure 2a. Horizontal axis represents coefficients from the range of 0.9 to 1.1, which were applied to alter the volume of relaxed unit cell of 6 ACS Paragon Plus Environment

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calcium-silicate (CS) and calcium-silicate-hydrate (CSH) phases. Thus, V0 is the original volume of relaxed unit cell for coefficient of 1.0 and V is the volume of relaxed unit cell for each corresponding coefficient. The trend of the diagram must be linear for isotropic solids found in the Jaffeite, γ-C2S and α-C2SH phases. Jaffeite, γ-C2S and α-C2SH were found to be the most isotropic phases due to the monomer structure of silicate tetrahedra. In fact, monomer structure of silicate tetrahedral represents four bonding of Si-O without forming a chain of silicate by linking to the neighboring silicate. To determine the compressibility of the phases as a function of the C/S ratio, the existence of water molecules inside the structure of each phase were identified as one of the fundamental parameters. The bulk modulus of all phases were computed by Density Functional Theory (DFT). Equation 2 indicates the bulk modulus (K) is as follows: K= -V0 [ P/(Vn-V0) ] (2) Where V0 is defined as the orginal volume of unit cell. Vn is determined as the volume of unit cell at the stationary point. P is defined as the change of pressure. The trends of the bulk modulus for all phases are illustrated in figure 2b. Manzano et al.47 have earlier measured the bulk modulus of Tobermorite 9, 11, 14 Å, and Jaffeite as are plotted with the pink points in figure 2b. They reported that the C/S ratio of Tobermorite 9, 14 Å equal to 0.8 with the bulk modulus of 68 and 46 Gpa, respectively. As it is earlier mentioned, as the C/S ratio increases, the bulk modulus increases as well. Results represented that the bulk modulus of Tobermorite 9 and 14 Å increased by 36% and 43%. In comparison with our results with the C/S ratio of 0.67, respectively. The bulk moduls of Tobermorite 11 Å (C/S ratio of 0.67) and Jennite are almost identical with our results.

Figure 2: (a) shows the pressure of calcium-silicate (CS) and calcium-silicate-hydrate (CSH) phases calculated by means of first principles calculation. Jaffeite, γ-C2S and α-C2SH have a linear bulk modulus in contrast to the rest of phases. (b) shows Tobermorite 14 Å and γ-C2S with the lowest and highest bulk modulus, respectively.



Recently, IR spectra of synthetic calcium-silicate-hydrate (CSH) were analyzed. Walker et al. studied the FTIR spectra of samples containing various C/S ratios from 0.4 to 2. Calciumsilicate-hydrate (CSH) gel was prepared at room temperature over a period of 112 weeks and FTIR spectra were measured for representative samples. Afterwards, samples were equilibrated for more than 64 weeks. Walker et al.48 have stated about preparation of samples, which reached to a steady state after 64 weeks. However, we feel that this is not true, because the samples were pressed into KBr pellets at a ratio of 1 mg of sample to 100 mg KBr, which prevents chemical reaction. Although wavenumbers change slightly between 800 and 400 cm1 for different C/S ratios from 0.4 to 2, it is not clear what kind of phases and vibration modes were considered. On the other hand, there is no shifting between the wavenumbers of 3800 and 3400 cm-1, but some new peaks appeared for C/S ratios of 2.0, 1.8, and 1.7. In other words, the material has changed and new phases were created. Figure 3a illustrates an initial model of calcium-silicate (CS) phase as a fundamental structure of cement before contacting with water. Figure 3b represents the partial calcium-silicate (CS) and calcium-silicate-hydrate (CSH) phases; such as Jaffeite (1), γ-C2S (2), α-C2SH (3), and Jennite (4) after 12 hours and before complete hydration time. The number of phases increase as time goes on. At the end, the most indispensable phases of Jaffeite (1), γ-C2S (2), α-C2SH (3), Jennite (4), Wollastonite (5), and Tobermorite 9 (6), 11 (7), and 14 Å (8) after hydration are depicted in figure 3c. The increase in the number of hydrated phases is time-dependent. In general, one of the biggest problems of building materials is to correlate peaks measured in FTIR spectra with certain vibration modes on the atomistic level.

Figure 3: (a) A three-level time-dependent model for the process of phase formation during the hydration of cement. Figures (b) and (c) illustrate the progressive creation of phases over the time. In the end, the shape of created phases is changed and more pores appear among them. Figure 4 depicts the calculated IR spectra of calcium-silicate (CS) and calcium-silicate-hydrate (CSH) phases. Based on the atomistic information, we can discuss the existence or even shift of the vibration mode e.g. as a function of the C/S and H/C ratios. Obviously, the wavenumber shifts and decreases as the C/S ratio increases. Four representative vibration modes identified in all model systems are shown in figure 5.


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Figure 4: IR spectra of calcium-silicate (CS) and calcium-silicate-hydrate (CSH) phases selected as model systems.

Figure 5: Representation of the wavenumber of ν(Si-O) in silicate tetrahedra chains and monomer structure of silicate tetrahedra for four specific vibration modes. Figures (a) and (b) illustrate the stretching for asymmetrical and symmetrical modes, respectively. Figures (c) and (d) show the deformation for asymmetrical and symmetrical modes, respectively.



Figure 6 depicts the wavenumber of ν(Si-O) in silicate tetrahedra chains and the monomer structure of silicate tetrahedra for four specific vibration modes as a function of different C/S and H/C ratios. Obviously, this vibration mode does not simply shift as a function of the C/S ratio, but depends on many more factors, such as carbonation and the H/C ratio. We found that when the proportion of C/S increases, the carbonate can be distinguished. In the study of Walker et al.48, the samples equilibrated for 64 weeks exhibited carbonation at a C/S ratio ranging from 1.2 to 2. In other words, the kinetics of carbonation clearly depends on the C/S ratio. Consequently, we calculated the wavenumbers of all phases in our model system. Two main factors were found to influence the wavenumber of ν(Si-O) in silicate tetrahedra chains and the monomer structure of silicate tetrahedra for four specific vibration modes: (1) hydrogen bonding and (2) the existence of calcium atoms. Consequently, we classified and illustrated the calculated wavenumber of ν(Si-O) as a function of the H/C and C/S ratios. Figure 6 does not reveal any simple global trend for the wavenumber of ν(Si-O) in silicate tetrahedra chains and the monomer structure of silicate tetrahedra for all vibration modes, but suggests trends depending on the C/S and H/C ratios. Although C/S ratios are identical for γ-C2S and α-C2SH, various frequencies were calculated. As shown in figure 6, the reason is that the wavenumber increases from γ-C2S to α-C2SH with increasing H/C ratio. In other words, when the H/C ratio increases, while the C/S ratio remains constant, the wavenumber of ν(Si-O) increases as well. Moreover, the wavenumber decreases from Tobermorite 9 Å to Wollastonite, because the H/C ratio decreases as well. Another aspect to be considered is the influence of the C/S ratio at an almost constant C/H ratio for Tobermorite 11 Å in comparison to Jennite. Tobermorite 11 Å has a lower wavenumber than Jennite due to the lower C/S ratio, as was mentioned above. In comparison to Wollastonite, the influence of the C/S ratio on Jennite exceeds that of the H/C ratio. That is why the wavenumber measured for Jennite is higher than that of Wollastonite due to the influence of the C/S ratio.


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Figure 6: The wavenumber of ν(Si-O) in silicate tetrahedra chains and the monomer structure of silicate tetrahedra for four specific vibration modes. Figures (a) and (b) indicate the wavenumber for asymmetrical and symmetrical stretching modes as a function of different C/S and H/C ratios. Figures (c) and (d) illustrate the wavenumber for asymmetrical and symmetrical bending modes as a function of different C/S and H/C ratios.

As the next step, studies based on experimental methods, which plan to synthesize C-S and CS-H phases in order to verify the calculations in figure 6.



4. Conclusions In this study, the Density Functional Theory (DFT) was employed to investigate the composition and mechanical properties of eight calcium-silicate (CS) and calcium-silicatehydrate (CSH) phases, which are formed during the hydration of cement. The bulk modulus of all phases were computed and Jaffeite, γ-C2S, and α-C2SH were combined in isotropic models. The monomer structure of silicate tetrahedra were found in Jaffeite, γ-C2S, and α-C2SH. To determine the wavenumber of ν(Si-O) in silicate tetrahedra chains and the monomer structure of silicate tetrahedra, four specific vibration modes were considered for the computation of the wavenumber of ν(Si-O) based on the two criteria of the H/C ratio and C/S ratio. The wavenumbers were found to decrease from Tobermorite 9 Å to Wollastonite because of the decreasing H/C ratio. While the C/S ratios are identical for γ-C2S and α-C2SH, the wavenumbers of α-C2SH exceed that of γ-C2S. The wavenumbers of ν(Si-O) increase with increasing H/C ratio. At an almost constant H/C ratio, Tobermorite 11 Å has lower wavenumbers than Jennite due to the lower C/S ratio. In general, one of the biggest problems of building materials was to correlate single peaks measured in FTIR spectra with certain vibration modes on the atomistic level. CS and CSH phases do not reveal any simple global trend for the wavenumber of ν(Si-O) in silicate tetrahedra chains, but suggests trends depending on the C/S and H/C ratios. The main reason are structure-dependent changes of the silicate tetrahedra for all vibration modes, which appear as a function of the C/S and H/C ratios.

AUTHOR INFORMATION Corresponding Author *Dr. Peter Thissen. Phone: +49 721608-28223. Email: [email protected]. Homepage: http://www.ifg.kit.edu/english/379.php.

ORCID: Peter Thissen 0000-0001-7072-4109

ACKNOWLEDGMENTS We gratefully appreciate financial support from the DFG. The authors highly acknowledge supply of computational resources by the Texas Advanced Computing Center (TACC).


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