29Si Chemical Shift Anisotropies in Hydrated ... - ACS Publications

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Si Chemical Shift Anisotropies in Hydrated Calcium Silicates: A Computational Study

Pawel Rejmak,*,† Jorge S. Dolado,‡ Malcolm J. Stott,§ and Andrés Ayuela†,∥ †

Donostia International Physics Center (DIPC), p. Manuel de Lardizabal 4, Donostia-San Sebastián, Spain Tecnalia Research and Innovation, Geldo Edificio 700, 48160 Derio, Spain § Queen’s University, Kingston, ON K7L 3N6, Canada ∥ Centro de Física de Materiales CFM-MPC, Centro Mixto CSIC-UPV/EHU, Departamento de Física de Materiales, Facultad de Químicas, Universidad del País Vasco UPV-EHU, p. Manuel de Lardizabal 5, Donostia-San Sebastián, Spain ‡

S Supporting Information *

ABSTRACT: The 29Si chemical shift anisotropies are investigated for calcium silicate hydrates. The focus is on the naturally occurring minerals, jennite and 14 Å tobermorite and models derived from them to simulate calcium−silicate−hydrate gel, the main component of Portland cement. Our theoretical results show that the analysis of anisotropy and asymmetry of the 29Si chemical shift discriminates between different Si types, even if their isotropic chemical shifts are similar. Terminal and pairing silica tetrahedra are clearly distinguished and the chemical shift anisotropies set apart the Si tetrahedra that are hydroxylated. The chemical shift anisotropy measurements, although more challenging than the usual isotropic chemical shift experiments, could greatly improve our knowledge of not only cement materials, but silicate hydrates, in general.

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chemical shift anisotropy asymmetry η = (δxx − δyy)/CSA is also studied.12 Thus far, 29Si CSA has been measured for various crystalline samples using either the analysis of line shapes from static experiments,3,4 slow speed MAS5−7 single crystal NMR,8 or certain two-dimensional techniques.9,10 Although the measurement of 29Si CSA for amorphous systems is more complicated, results have been reported for various glassy materials9,11 using two-dimensional NMR techniques, such as magic-angle flipping and related techniques.13 It was shown that in many cases different Qn sites with very similar value of 29Si δiso can easily be distinguished by the large differences in CSA or by sign differences.3,5,7 These trends are commonly rationalized by patterns of long and short Si−O bonds in different types of Qn species.3,7 Such an interpretation invites verification by further theoretical studies. In this work, we focus on 29Si CSA in the calcium silicate hydrate (C−S−H) gel, the main component of hydrated Portland cement.14 Despite being the most widely manufactured material in the world, the nanostructure of this amorphous and complex solid is not yet fully understood.14−17 Several atomistic models of C−S−H gel were proposed on the basis of various experimental techniques (X-ray diffraction, electron microscopy, chromatographic analysis of the products

INTRODUCTION

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Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy remains one of the main tool in the characterization of complex silicates for over thirty years.1 29Si MAS NMR spectra follow the rule that isotropic chemical shift (δiso) decreases with the degree of condensation of SiO4 tetrahedra. According to this condensation degree, 29Si spectra are commonly denoted as Qn, where n is the number of linked tetrahedral, n = 0−4. The highest values of 29Si δiso are observed in orthosilicates for isolated tetrahedrons (Q0), and the lowest ones in tectosilicates for fully coordinated tetrahedra (Q4). The precise value of 29Si δiso depends on the structural details of the particular silicate and the measured ranges of δiso for different Qn sites partially overlap.1,2 Consequently, the interpretation of 29 Si NMR spectra and the assignment to a specific structural model can be uncertain, especially for poorly crystalline or amorphous silicates for which there are various nearest neighbor distances. More detailed insight into the structure of silicates is gained the measurements of the 29Si chemical shift anisotropy (CSA).3−11 The CSA parameter describes the largest deviation of diagonal elements the chemical shift tensor with respect to the averaged δiso = 1/3(δxx + δyy + δzz). Throughout this paper, we adopt the convention of ordering the diagonal elements of the chemical shift tensor so that |δzz − δiso| ≥ |δxx − δiso| ≥ |δyy − δiso| and define the CSA as δiso − δzz. In order to characterize the axiality of the chemical shift tensor, another parameter, the © XXXX American Chemical Society

Received: September 4, 2012 Revised: March 18, 2013

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dx.doi.org/10.1021/jp308787z | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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of trimethylsilylation, NMR)15,16 and they were recently reviewed in detail by Richardson.17 Most of these models of C−S−H gel are derived from the structure one of two C−S−H crystalline minerals, either jennite or 14 Å tobermorite.17 29Si MAS NMR is currently one of the most frequently used techniques for the characterization of C−S−H gel;16,18−20 see also ref 21 for recent reviews. Other magnetic nuclei present in C−S−H gel are also studied, for example 17O22or 43Ca,23,24 but these experiments are usually more challenging due to quadrupolar broadening or low natural abundance and are often less conclusive. Although 29Si MAS NMR measurements can be performed rather easily, partial overlapping of 29Si NMR signals from different Qn sites in C−S−H gel often hampers conclusive interpretation of experimental results. For example, it is a difficult task to distinguish clearly between the assumed component phases like jennite or tobermorite and to determine accurately mean silicate chain lengths from the relative intensity of Q2 and Q1 signals. One can expect that more information on the C−S−H gel can be gained from 29Si CSA measurements. Hansen et al. measured 29Si CSA for a number of cement-related crystalline minerals and suggested that CSA measurements can characterize disordered phases of cements.7 Recently, Rawal et al. performed CSA filtered one-dimensional NMR experiments on various cement materials, including C−S−H gel.19 They used a recoupling technique, which selectively suppresses signals from nuclei with larger CSA25 This resulted in improved resolution of Q1 resonances from otherwise overlapping Q2 signals. In this way, Rawal et al. were able to distinguish between signals with different sized CSA. However, as far as we know, no quantitative measured values of 29Si CSA or components of the shielding tensor for C−S−H gel have been reported. The objective of our work is to simulate 29Si CSA in calcium−silicate−hydrate (C−S−H) gel. In our preliminary investigation, we studied on the 29Si δiso spectra for several phases, and by comparison with available experimental data we found evidence supporting tobermorite-based models of C−S− H gel.26 In this paper, we aim to extend our insight into the nanostructure of C−S−H gel itself by the analysis of 29Si CSA. We are showing that 29Si CSA measurements allow for a more accurate determination of the Q1/Q2 ratio, as suggested by Hansen and co-workers on the basis of their experimental work.7 We also predict that the tensorial information in CSA will facilitate the identification of silanol groups in C−S−H gel. Note that this finding would also apply to other silicates hosting silanol groups. We hope that our study will encourage experiments to measure 29Si CSA for C−S−H gel and provide a solid theoretical basis for their interpretation.

Figure 1. Structural models used in this work: (a) jennite and (b) 14 Å tobermorite minerals with infinite silicate chains. Finite chain models of C−S−H gel are obtained by removing a certain number of bridging (SiO4) units: (c) pentameric models obtained from jennite structure (J5 model) and (d) dimeric models are obtained from tobermorite, employing protons to compensate dangling bonds (T2-H model). Qn stands for (SiO4) tetrahedra coordinated to n other tetrahedra; Q2 tetrahedra in bridging positions are denoted as Q2b; the presence of hydroxyl groups is marked with a “OH” index. Two different types of Ca ions (blue balls) are also seen, either in layered (L) or interlayer (I) positions.

bridging neutral SiO2 units are removed. The bridging tetrahedra in tobermorite models are protonated, so that the removal of a [SiO(OH)]+ group creates excess negative charge, which needs to be compensated by additional cations, such as 1/2 Ca2+, Ca(OH)+, or H+. All these charge compensation schemes are realized in our previous studies.26,29 In order to host pentamer species, the input unit cells for the jennite and tobermorite models were doubled along the b directions. The C−S−H gel structural models are provided in Supporting Information. Because of the large size of our models, up to 200 atoms per unit cell, we optimized structures using semiempirical core− shell potentials,30 as implemented in the GULP code,31 with parametrization thoroughly tested in the literature.32 Electronic structure calculations were performed with the Quantum Espresso code33 based on density functional theory and using the exchange-correlation functional of Perdew, Burke, and Ernzerhof.34 Norm-conserving pseudopotentials were used for the electron core interaction.35 The plane wave basis set had an energy cutoff of 80 Ry. For J models, the Γ-point was sufficient for the sampling of reciprocal space,whereas for T models two

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MODELS AND METHODS Our models of C−S−H gel were derived from the observed structures of jennite (J models)27 and 14 Å tobermorite (T models),28 see Figure 1. They were used previously by us to study other electronic26 and mechanical properties.29 It is currently commonly accepted that C−S−H gel resembles either jennite or tobermorite.14−17 The main difference from the gel is that these minerals contain endless silicate chains, whereas the C−S−H gel is built from finite silicate oligomers, consisting of (3n − 1) SiO4 units, where n = 1, 2,...14,17 Our models of C−S−H gel have finite chain species, dimers or pentamers, and they are constructed from the mineral structures by removing a number of the so-called bridging tetrahedra from the infinite chains, see Figure 1. In J models, B



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cations as the corresponding sites in T models. We found that Si δiso mainly depends on the degree of SiO4 condensation, which is in good agreement with experimental data. The exact values of δiso for Si atoms can be modulated, however, by other parameters such as how close are interlayer Ca cations or OH groups.26 29 Si NMR experiments on C−S−H gel revealed two main peaks in δiso spectra, one about −79 ppm assigned to Q1 sites, and the second about −85 ppm assigned to Q2 sites.22 A third peak at about −82 ppm is sometimes resolved and is frequently ascribed to Q2 sites with one AlO4 unit in their vicinity (i.e., Q2(1Al)). However, 29Si NMR lines at about −82 ppm were also detected for Al free C−S−H gel samples and assigned to Q2b sites,50 an assignment consistent with our previous results.26 Extra information can be obtained from the analysis of CSA. Figure 2 presents the calculated values of CSA plotted against

points along the shortest dimension of the unit cell were required. The magnetic shielding tensor for these periodic models was calculated using the gauge including projected augmented waves (GIPAW) method.36 GIPAW approach is a well-established method for predicting NMR parameters in solids; see ref 37 for a recent comprehensive review. In particular, GIPAW was widely applied in studies on silicates, both crystalline 24,38−44 and amorphous ones, such as glasses44,45 or even cement.26 To facilitate the direct comparison of our results with the experiments, the 29Si chemical shifts (δiso) were obtained from GIPAW magnetic shieldings using the same scaling formula as in ref 17 δGULP/GIPAW =

p δexp

− δGULP/GIPAW )

+

p q (δexp ) − δexp p q (σexp ) − σexp

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p (σGULP/GIPAW

(1)

α−quartz and β−belite were selected for the reference systems p and q. Each of these silicates has a single Si crystallographic site46 with a single value of δiso.1,47 The accurate prediction of δiso at DFT level usually depends on the suitable choice of reference system with a well-known δiso.48 The formula 1 is also suppressing systematic errors introduced by computing δiso at GULP optimized geometries, instead of DFT ones.26 In order to test the above formula 1, we have performed DFT calculations of the magnetic shielding tensor for several crystalline silicates for which the experimental 29Si δiso and CSA have been reported by Hansen et al.7 The comparison of theoretical and experimental quantities is presented in Supporting Information. The scaled δiso are close to the observed values. The individual components of the chemical shift tensor, and consequently CSA and η, are typically overestimated, but there is qualitative agreement with experiment in most cases. These numerical discrepancies are not surprising. In formula 1, we scale by δiso, which is the average of the tensor trace, whereas the individual tensor components in various silicates need not scale in the same way. Similar order of discrepancy (tens ppm) between experimental and GIPAW CSA was reported previously for sodium silicates, where the calculated and predicted δiso agreed very well.39 Moreover, CSA predicted for static structures, as in our case, may be overestimated if dynamical effects are neglected.,43,49 Nevertheless, we are able to reproduce the overall properties of 29Si CS tensor for different Qn species in C−S−H gel. Note that the main conclusions of this work are made on the basis of a large sample of Si atoms obtained from several C−S−H gel models. Thus, even if certain extreme values for specific Si atoms should be treated cautiously, the calculations should reproduce trends in 29Si CSA for cementitious materials.

Figure 2. Calculated 29Si chemical shift anisotropies (CSA, in ppm) plotted against 29Si isotropic chemical shifts (δiso, in ppm). The data for J and T models are marked with filled and empty symbols, respectively. Q1 denoted with circles, Q1OH with squares, Q2 with diamonds, Q2b with triangles, and Q2bOH with asterisks. Note that despite δiso of different Qn sites partially overlap, the Q1 sites can be generally distinguished by their negative CSA.

the calculated δiso. More details are given in the Supporting Information. Only Q1 species have a negative CSA that clearly differentiates them from other 29Si sites in the models. This finding agrees well with the experimental results of Hansen et al.7 A few of our calculated Q1 CSA have positive values. The CSA calculated for Q1OH like all types of Q2 sites are always positive. Rawal et al. in their CSA filtered 29Si NMR experiment observed the largest CSA in C−S−H gel for Q2 and Q2(1Al) sites.19 These experiments seem to agree with our results, because our calculated values of CSA for Q1 sites are usually smaller than those predicted for Q2, Q2b, and Q2bOH sites. Another useful quantity to analyze is the chemical shift anisotropy asymmetry parameter, η. Calculated results for η are presented in Figure 3. The values of η for Q1OH, Q2b, and Q2 sites are in the range of 0.4−1.0. The smallest η is predicted for Q2bOH sites that in almost all cases lie below 0.6. The predicted values of η for Q1 sites range from 0.2 to 1.0. The largest values of η reflect the large deviation from axial symmetry of electron densities for 29Si sites. Note that Hansen et al. also reported larger η values than expected for Q1 sites.7 The distortion of Q1 tetrahedra from idealized axial symmetry due, for example, to interactions with interlayer water molecules and Ca ions leads to large values of η. The anisotropy of the 29Si chemical shift tensor is usually explained in terms of different Si−O bond lengths within SiO4 tetrahedron.3,7 The smallest CSA is exhibited by Q0 and Q4

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RESULTS AND DISCUSSION We distinguish five main types of Si sites namely Q1, Q1OH, Q2b, Q2bOH and Q2, which can be identified in Figure 1. The presence of the hydroxyl group is denoted by the OH index and “b” denotes a Q2 unit in a bridging position. As we showed recently, the calculated δiso are ordered as Q1OH > Q1 > Q2b, Q2bOH > Q2, but the ranges of Qn partially overlap.26 Two main trends were found. First of all, the replacement of the compensating Ca2+/Ca(OH)+ ion in the vicinity of a Q1 site by a proton results in weaker shielding of the Q1OH hydroxylated sites. Second, the Q1 and Q2b sites in J models are typically less shielded as they are not as tightly bound to interlayer Ca C



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lengths within a given SiO4 tetrahedron. We know that the Si− O bond skewness should be nearly zero for tetrahedra with either all Si−O bond equal, as in Q0 or Q4 sites, or two short and two long ones, as in Q2 ones. Negative skewness should be found for tetrahedra with three long and one short bond, as expected for Q3 sites. Tetrahedra with three short and one long bond, as in Q1 species, should have positive skewness. However, Figure 4a shows that any correlation between CSA and Si−O bond skewness is quite weak. The chemical shift tensor depends in a complicated way on the electronic structure being a result of induced currents rather than simply the charge distribution. It would be useful to correlate the CSA and η with simple aspects of the electronic structure. After considering just the variations in Si−O bond lengths in the different species, we proceed by introducing the ion charges to the investigation. We now introduce a parameter that attempts to combine structural and electronic factors. First, we performed a Löwdin charge population analysis, as implemented in the Quantum Espresso code.51 The calculated atomic charges are summarized in Table 1. Three main types of O atoms can be distinguished

Figure 3. Calculated 29Si chemical shift asymmetry (η) plotted versus CSA (in ppm). The data for J and T models are denoted with filled and empty symbols, respectively. The symbols follow the notation of previous figure. Note that Q2bOH sites can be discriminated by positive CSA and small values of η.

species, which are the least distorted from ideal tetrahedra. The CSA values for Q1 and Q3 sites are respectively negative and positive because there are present three short and one long Si− O bonds or three long and one short. The Q2 sites with two long and two short Si−O bonds also have positive CSA with a magnitude larger than for Q1 and Q3 sites. These differences arise because symmetry lowers from approximately C3 for Q1 and Q3 sites to C2 for Q2 sites.19 We begin by checking this geometrical interpretation of 29Si CSA in terms of nearest neighbor distances. A common descriptor of distribution asymmetry is skewness (see Supporting Information). We show in Figure 4a the relationship between 29Si CSA and the skewness of Si−O bond

Table 1. Calculated Löwdin Charges for O and Si Atoms O charges Obridging Oterminal Ohydroxyl Si charges Q1 Q1OH Q2 Q2b Q2bOH

T models

J models

−0.32 − −0.41 −0.52 − −0.68 −0.33 − −0.35

−0.33 − 0.42 −0.64 − −0.72 −0.38 − 0.45

− − − − −

1.20 − 1.22 1.24 1.25 − 1.27 1.30

1.23 1.28 1.28 1.26 1.35

1.26 1.31 1.32 1.34 1.36

in our silicate oligomers: hydroxyl OH in Si−O−H groups, bridging Ob in Si−O−Si connectivity, and terminating Ot in dangling Si−O bonds. Although the values of atomic charges depend on the prescription used, the differences between calculated atomic charges for a given system indicate degrees of electronegativity. Large negative charges are found for Ot atoms indicating the higher polarization of the Si−Ot bonds with respect to bridging and hydroxyl bonds. The local dipole moment of a Si−O bond is characterized by the product of the oxygen charge and the corresponding Si−O bond length. The 29Si CSA is plotted against dipole skewness in Figure 4b. The dipole skewness of all Q1 sites is positive. The Q1 sites have three dangling Si−Ot bonds with strong ionic character and a single Si−Ob link, which is more covalent. This leads to a positive dipole skewness for all Q1 sites and a negative CSA for most of them. The four exceptions cannot be satisfactorily explained on the basis of structural data, however, all these Q1 sites with positive CSA have η close to unity (compare with Figure 3), which indicates their strong distortion from axial symmetry. On the other hand, for tensors with large η, small changes in computed δzz and δxx components may flip the sign of CSA, and we cannot exclude the possibility that positive CSA for several Q1 sites is just computational artifact. These rare exceptions aside, our results show clearly that only Q1 sites have negative CSA. Our theoretical results support the previous suggestion of Hansen and co-workers,7 based on their experimental results that 29Si CSA data could provide a much

Figure 4. 29Si CSA plotted versus (a) (Si−O) bond skewness and (b) (Si−O) local dipole skewness in (SiO4) tetrahedra. The data for J and T models are marked with filled and empty symbols, respectively. The symbols follows caption of Figure 2. (b) A semi−quantitative correlation between the electron properties of a given Qn tetrahedron and 29Si CSA: (i) Q1 sites have always positive (Si−O) local dipole skewness and typically negative CSA, (ii) (Si−O) local dipole skewness for Q1OH, Q2, and Q2b sites are close to 0, and (iii) (Si− O) local dipole skewness for Q2bOH sites is usually negative. Note that in cases (ii) and (iii), the CSA is always positive. D



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more accurate estimate of the Q1/Q2 ratio than the traditional deconvolution of 29Si δiso spectra. The remaining Si sites in any of the C−S−H gel models have positive CSA. The Q2 sites have dipole skewness close to zero due to a symmetric environment of the Si atom, consisting of two covalent Si−Ob and two ionic Si−Ot bonds. The dipole skewness for J models is very close to zero and is less scattered than for T models. This small dipole skewness could account for the small values of 29Si CSA for Q2 sites in J models (below 60 ppm), values that are even smaller than the absolute values of Q1 CSA in J models (50−80 ppm). This result for J models contradicts the available experimental data showing CSA to be larger for Q2 than for Q1 sites.19 We can tentatively consider this lack of agreement as a further evidence against the significant presence of jennite-like phases in mature C−S−H gel, as also discussed in our previous work.26 The positive CSA for Q1OH sites results from the similar Si− O bonding pattern as in Q2 sites. The Q1OH tetrahedra have two strongly polarized Si−Ot bonds and two more covalent, one Si−Ob and one Si−OH. We conclude that Q1OH sites, if present in C−S−H gel, could be clearly distinguished from other sites by the low δiso26 and the positive CSA. Finally, the dipole skewness of Q2bOH sites is negative. The bonding pattern for this site with three rather covalent bonds, namely two Si−Ob and one Si−OH, and with a highly polarized single Si−Ot bond results in axial-like symmetry. This symmetry is responsible for the rather small values of η, like those reported for Q3 sites.7 We suggest that Q2bOH sites can be distinguished from the Q2b and Q2 sites on the basis of a smaller η parameter. Unfortunately, we are not aware of any 29 Si CSA measurements for pure silicates containing Q2bOH and/or Q1OH sites in order to make direct comparisons. Since a number of minerals hosting these sites exists52 we believe that our results will be helpful in the NMR studies on such materials.

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ASSOCIATED CONTENT

S Supporting Information *

(1) Calculated NMR parameters for selected crystalline Ca silicates and their comparison with available experimental data, (2) calculated NMR parameters for tobermorite, jennite, and derived C−S−H gel models, (3) simulated XRD patterns for tobermorite, jennite, and derived C−S−H gel models, and (4) calculated Si−H distances for tobermorite, jennite, and C−S− H gel models. Calculated structures of C−S−H gel models attached as the .cif file. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. Phone: +34 943 01 88 26. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support of the Basque Departamento de Educación and the UPV/EHU (Grant IT-366-07), the Spanish Ministerio de Innovación, Ciencia y Tecnologı ́a (Grant Nos. TEC2007-68065-C03-03 and FIS2010-19609-C02-02), and the ETORTEK research program (NANO-IKER Grant No. IE11304) funded by the Basque Departamento de Industria and the Diputación Foral de Guipuzcoa. P.R. and M. S. gratefully acknowledge a grant and the hospitality of the Donostia International Physics Center, respectively.

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REFERENCES

(1) Lippmaa, E.; Mägi, M.; Samoson, A.; Engelhardt, E.; Grimmer, A.-R. Structural Studies of Silicates by Solid-State High Resolution 29Si NMR. J. Am. Chem. Soc. 1980, 102, 4889−4893. (2) Engelhardt, G. Silicon-29 NMR of Solid Silicates. Encyclopedia of Magnetic Resonance [Online]; John Wiley & Sons, Posted 15 March 2007; http://onlinelibrary.wiley.com/doi/10.1002/9780470034590. emrstm0506; accessed Mar 5, 2012. (3) Grimmer, A. R.; Peter, R.; Fechner, E.; Molgedey, G. HighResolution 29Si NMR in Solid Silicates: Correlations Between Shielding Tensor and Si−O Bond Length. Chem. Phys. Lett. 1981, 77, 331−335. (4) Stebbins, J. F.; Smyth, J. R.; Panero, W. R.; Frost, D. J. Forsterite, Hydrous and Anhydrous Wadsleyite and Ringwoodite (Mg2SiO4): 29Si NMR Results for Chemical Shift Anisotropy, Spin−Lattice Relaxation, and Mechanism of Hydration. Am. Mineral. 2009, 94, 905−915. (5) Smith, K. A.; Kirkpatrick, R. J.; Oldfield, E.; Henderson, D. M. High-Resolution Silicon-29 Nuclear Magnetic Resonance Spectroscopic Study of Rock-Forming Silicates. Am. Mineral. 1983, 68, 1206. (6) (a) Matijasic, A.; Lewis, A. R.; Marichal, C.; Delmotte, L.; Chézeau, J. M.; Patarain, J. Structural Characterization of the New Porous Sodium Silicate Mu-11 by 29Si and 23Na Solid-State NMR. Phys. Chem. Chem. Phys. 2000, 2, 2807−2813. (b) Fyfe, C. A; Skibsted, J.; Schwieger, W. Solid-State NMR Characterization of the Mineral Searlesite and its Detection in Complex Synthesis Mixtures. Inorg. Chem. 2001, 40, 5906−5912. (c) Clark, T. M.; Grandinetti, P. J.; Florian, P.; Stebbins, J. F. An 17O NMR Investigation of Crystalline Sodium Metasilicate: Implications for the Determination of Local Structure in Alkali Silicates. J. Phys. Chem. B 2001, 105, 12257−12265. (d) Brouwer, D. H. A Structure Refinement Strategy for NMR Crystallography: An Improved Crystal Structure of Silica-ZSM-12

CONCLUSIONS An earlier experimental study of various silicates, including amorphous glasses, showed that the measurement of chemical shift anisotropy, CSA, allowed identification of different Qn sites with less ambiguity than using 29Si δiso alone.5,7,11 Here we report first principles calculations of CSA for selected periodic models of calcium−silicate−hydrates (C−S−H) gel based on the structures of jennite and 14 Å tobermorite. We calculated that negative CSA is only found for Q1 sites, and usually this is the case. Therefore a negative CSA should distinguish Q1 sites from the other Qn sites. All types of Q2 sites, as well as Q1OH sites, have positive CSA. The Q1OH sites, if present in C−S−H gel with predominantly Ca charge compensating schemes, could be identified as the species with low field shifted δiso and positive CSA. Small values of CSA predicted for Q2 in J models seem to contradict available experimental data for CSA in C− S−H gel. This result indicates that mature C−S−H gel resembles mostly the structure of tobermorite. The Q2bOH sites have the smallest values of η, so that it might be possible to distinguish experimentally these sites from Q2 and Q2b ones. We also found that 29Si CSA correlates better with the asymmetry in the polarization of Si−O bonds, rather than with patterns in just the Si−O bond lengths as is often discussed.3,7 We expect this study to encourage measurement of 29Si CSA in cementitious materials and to prove helpful in the interpretion of experimental data. E



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