Characterization of calcium silicate hydrates, C-S-H phases, and hydrated Portland cements by solid-state NMR spectroscopy

Activity: Talk or presentation typesLecture and oral contribution

See relations at Aarhus University

Jørgen Skibsted - Lecturer

  • Interdisciplinary Nanoscience Center
  • Department of Chemistry

 

Jørgen Skibsted

Instrument Centre for Solid-State NMR Spectroscopy and Interdisciplinary Nanoscience Center (iNANO), Department of Chemistry, Aarhus University, DK-8000 Aarhus C, Denmark.

     Solid-state NMR spectroscopy has become an important research tool for characterization and structural analysis of Portland cement pastes and hydrated cement blends including supplementary cementitious materials [1,2]. High-resolution in NMR spectra of powdered, dry samples is principally achieved by magic-angle spinning (MAS), 1H high-power decoupling, and high static magnetic fields. Under these conditions information about connectivities and local structure of different NMR-spin nuclei may be achieved from double- and triple-resonance experiments employing simultaneous radio-frequency radiation of different spin nuclei.             

     MAS NMR has proven particularly useful in the structural analysis of C-S-H phases either in synthesized model samples or more importantly in hydrated Portland cements. The absence of long-range order in these phases prevents a detailed structural characterization by X-ray diffraction techniques while the NMR chemical shifts reflect the local structural order. C-S-H phases can in principle be studied by all its constituent elements (1H, 17O, 29Si, and 43Ca), however, the natural abundance for 17O (0.037%) and 43Ca (0.145%) is so low that isotopic enrichment is generally required.

     29Si MAS NMR can provide important information about the chains of silicate tetrahedra in C-S-Hs, since the 29Si chemical shifts reflect the degree of condensation of SiO4 units and the peak intensities allow a quantitative determination of the different silicate species. Furthermore, it has been shown that incorporation of AlO4 tetrahedra in the bridging sites of the silicate chains results in a high-frequency shift of the Q2 resonance, allowing an indirect determination of the average Al/Si ratio for the alumino-silicate chains of the C-S-H from deconvolutions of the 29Si MAS NMR spectra [3]. The validity of the assignment of the Q2(1Al) resonance has been further supported by the mutual detection of Al in the alumino-silicate chains by 29Si and 27Al MAS NMR in a study of the C-S-H in a series of hydrated Portland cements [4] and most recently by a 29Si{27Al} REAPDOR NMR investigation of the C-S-H in a synthetic slag glass which will be presented in the lecture. The latter technique utilizes 29Si-27Al dipolar couplings to dephase resonances from 29Si sites with 27Al in their nearest vicinity, thereby providing a direct proof for the presence of Q2(1Al) sites. 29Si-29Si dipolar couplings have been utilized in a study of synthesized C-S-H samples (enriched in 29Si) using a 29Si homonuclear correlation NMR experiment which provides direct evidence for connectivities (Si-O-Si bonds) between different silicate species [5]. Thus, these spectra revealed resonances from dimeric units (Q1-Q1) dreierketten chains (Q1-Q2, Q2-Q2), and the linkage of two silicate chains in the interlayer space (Q2-Q3, Q3-Q3) as also found in the structure of 11-Å tobermorite.

     The identification of different silicate species in cement pastes by 29Si NMR is often based solely on the isotropic chemical shifts and their dependencies on the degree of condensation (Qn, n = 0,1,2,3,4) and Al for Si substitution (Qn(mAl); m ≤ n). However, in complex mixtures including several different silicate species a severe overlap of resonances may occur, preventing a straight-forward interpretation of the 29Si NMR spectra. For example, the chemical shift for the unique resonance from the sorosilicate jaffeite, Ca6(Si2O7)(OH)6, δiso(29Si) = -82.8 ppm, is very similar to the Q2(1Al) resonance from C-S-Hs which may affect the analysis of autoclaved cements where jaffeite may be formed. However, in this case it can be utilized that the linear Si-O-Si bond in the dimeric silicate unit of jaffeite results in a unique 29Si chemical shift anisotropy (CSA) tensor which allows an identification of this phase in mixtures including C-S-Hs. Generally, it is found that the shift anisotropy (δσ) and the associated asymmetry parameter (ησ), both characterizing the CSA tensor, provides an improved reflection of the electronic/geometric environments for the SiO4 tetrahedra [6], which will be illustrated for a series of calcium silicate hydrates. A severe overlap of 29Si NMR resonances may also occur in Portland cements including aluminium-rich SCMs which may lead to the formation of strätlingite (2CaOAl2O3SiO28H2O). The 29Si MAS NMR spectrum of strätlingite is dominated by a Q3(2Al) resonance at -86.4 ppm but minor resonances at -90.8, -82.1, and -79.9 ppm from Q3(1Al), Q2(1Al), and Q2(2Al) sites, respectively, are also observed, reflecting disorder in the double-layer structure of SiO4 and AlO4 tetrahedra. However, an unambiguous detection of even small amounts of strätlingite can be achieved by 27Al MAS NMR where the Q3 AlO4 site results in a narrow resonance at δiso(27Al) = 63.3 ppm with a small quadrupole coupling (CQ = 1.5 MHz, ηQ = 0.58) [7].

     Finally, it will be illustrated that 27Al MAS NMR represents a unique tool to study the small amount of aluminate phases in Portland cements, cement - SCM blends, and of aluminium incorporated in C-S-Hs. For example, this approach has led to the observation of aluminium in the interlayers of the C-S-H structure in precipitated C-S-H samples [8] and of a new aluminate species in hydrated Portland cements, the third aluminate hydrate [9], which typically constitutes the equivalent of 0.4 wt.% Al2O3 in hydrated cement pastes. The latter phase has been assigned to an amorphous/disorded aluminate hydroxide, produced either as a separate phase or as a nanostructured surface precipitate on the grain boundary of the C-S-H phase [9].

 

[1]     J. Skibsted, M.D. Andersen, H.J. Jakobsen, Applications of solid-state Nuclear Magnetic Resonance (NMR) in studies of Portland cement-based materials, Zement Kalk Gips 60 (2007) No. 6, 70 - 83.

[2]     J. Skibsted, C. Hall, Characterization of cement minerals, cements and their reaction products at the atomic and nano scale, Cem. Concr. Res. 38 (2008) 205 - 225.

[3]     I.G. Richardson, A.R. Brough, R. Brydson, G.W. Groves, C.M. Dobson, Location of aluminium in substituted calcium silicate hydrate (C-S-H) gels as determined by 29Si and 27Al NMR and EELS, J. Am. Ceram. Soc. 76 (1993) 2285 - 2288.

[4]     M.D. Andersen, H.J. Jakobsen, J. Skibsted, Incorporation of aluminium in the calcium silicate hydrate (C-S-H) phase of hydrated Portland cements: A high-field 27Al and 29Si MAS NMR investigation, Inorg. Chem. 42 (2003) 2280 - 2287.

[5]     F. Brunet, P. Bertani, T. Charpentier, A. Nonat, J. Virlet, Application of 29Si homonuclear and 1H - 29Si heteronuclear NMR correlation to structural studies of calcium silicate hydrates, J. Phys. Chem. B 108 (2004) 15494 - 15502.

[6]     M.R. Hansen, H.J. Jakobsen, J. Skibsted, 29Si Chemical shift anisotropies on calcium silicates from high-field 29Si MAS NMR spectroscopy, Inorg. Chem. 42 (2003) 2368 - 2377.

[7]     T.T. Tran, H.J. Jakobsen, J. Skibsted, Disorder in the double tetrahedral layers of strätlingite (2CaO·Al2O3·SiO2·8H2O) studied by 27Al and 29Si MAS NMR spectroscopy, (manuscript in preparation).

[8]     G.K. Sun, J.F. Young, R.J. Kirkpatrick, The role of Al in C-S-H: NMR, XRD, and compositional results for precipitated samples, Cem. Concr. Res. 36 (2006) 18 - 29.

[9]     M.D. Andersen, H.J. Jakobsen, J. Skibsted, A new aluminium-hydrate species in hydrated Portland cements characterized by 27Al and 29Si MAS NMR spectroscopy, Cem. Concr. Res. 36 (2006) 3 - 17.

19 Jun 2009

Event (Conference)

TitleThe Fred Glasser Cement Science Symposium
Date19/06/200919/06/2009
CityUniversity of Aberdeen, Scotland
CountryUnited Kingdom

ID: 16632855