Anomalous fluorescence in near-infrared Raman spectroscopy of cementitious materials

doi:10.1016/j.cemconres.2004.10.001

Cement and Concrete Research

Volume 35, Issue 8, August 2005, Pages 1620-1628

https://doi.org/10.1016/j.cemconres.2004.10.001 Get rights and content

Abstract

The results of near-infrared Fourier transform Raman spectral examination of cement minerals, several commercial Portland cements, and hydrating Portland cements are critically examined. It is shown that structured fluorescent effects dominate the observed spectra for C₃S, C₂S, and cement, and none of the bands generated by this mode of examination is a true Raman band. The apparent bands for the Portland cements are in different positions to those for the individual cement minerals. The fluorescence-derived spectra for different cements are similar to each other but vary enormously in intensity for different cements. Hydration progressively reduces the intensity of the bands, but does not generate bands at new locations. It is tentatively suggested that the fluorescence effect may be somehow associated with the status of the cement components as orthosilicates (i.e., composed of isolated silica tetrahedra. Samples of high-purity C₂S exhibit the fluorescence effect, but samples of CS (wollastonite) of similar purity do not. The latter are metasilicates (i.e., composed of linked chains of silica tetrahedra). They exhibit normal Raman Stokes and anti-Stokes bands.

Introduction

The potential of Raman spectroscopy as an analytical technique for cementitious materials was first demonstrated by Bensted [1], [2], [3], nearly three decades ago. Subsequently, Conjeaud and Boyer [4] successfully used a Raman microprobe to study individual crystals in clinker phases and Portland cements. Since this time, however, Raman spectroscopy has been used only sparingly to study such materials, probably in part because of strong fluorescence effects generated for many substances under both visible and near-infrared excitation modes.

It was generally considered that the development of Fourier transform (FT) Raman spectroscopy with near-infrared excitation in 1986 should eliminate the usual problems of fluorescence experienced in many dispersive Raman examinations by precluding electronic absorption of the incident radiation [5]. Unfortunately, FT Raman studies of cementitious materials with near-infrared excitation have generated ambiguous results. Dyer et al. [6] observed a “very unusual result concerning the intensity of the spectra” for near-infrared FT Raman spectra of cement minerals, and concluded that the origin of the bands was probably fluorescence rather than Raman scattering, but these authors were unable to identify the fluorescing species. Subsequently, Bonen et al. [7] published near-infrared FT Raman spectra of cement minerals, and allocated the bands observed to Raman scattering from various silicate vibrations rather than to fluorescence. A similar ambiguity exists in the literature for near-infrared FT Raman spectra of calcium phosphate minerals such as apatite [8], [9], [10], [11], [12].

One way of confirming that a given band exhibited in the usually determined (Stokes) Raman spectrum is truly a Raman band is to investigate the existence, or lack of it, of a corresponding band in the anti-Stokes Raman spectrum. The demonstration that an anti-Stokes Raman band exists corresponding to a given Stokes Raman band would confirm its identification as a true Raman band [13]. Another confirmation technique is to demonstrate that the band in question occurs at the same Raman shift wavenumber independent of the particular excitation wavelength used [13]. Neither of these checks has been previously applied to cement minerals or cements. Since most anti-Stokes bands are inherently of weak intensity, the second alternative is more generally applicable.

In the present work, we report Raman spectra obtained for both near-infrared and visible light excitation on cement components and on several different Portland cements. Progressively hydrating cement pastes were also examined, but only using near-infrared excitation. Our aim is to clarify the interpretation of FT Raman spectra of these materials, and to assess the general applicability of the technique to the study of cement systems.

Section snippets

Experimental

The major components of cements are the tricalcium and dicalcium silicates, alite, and belite, which have the nominal compositions Ca₃SiO₅ and Ca₂SiO₄ and are given the symbols C₃S and C₂S in cement nomenclature. Also present in cements are tricalcium aluminate (nominal composition Ca₃Al₂O₆, denoted C₃A), tetracalcium aluminate ferrite (nominal composition Ca₄Al₂Fe₂O₁₀, denoted C₄AF), and calcium sulfate, as well as numerous other minor phases. Synthetic preparations of pure C₃S, C₂S, C₃A, and C

Raman spectra observed for cements and individual cement minerals

Near infrared FT spectra derived from the cement minerals C₃S, C₂S, and C₃A, plotted as a function of Raman shift on both Stokes (>0 cm⁻¹) and anti-Stokes (<0 cm⁻¹) sides of the laser frequency, are shown in Fig. 1.

The Stokes spectrum of C₂S (Fig. 1a) exhibits strong bands at 805 and 730 cm⁻¹ and weaker broad bands in the regions from 1100 to 1200 cm⁻¹ and from 1500 to 1600 cm⁻¹. There are no hints in the anti-Stokes spectrum of corresponding anti-Stokes bands, even for the strongest Stokes

Conclusions

It had been widely reported that the use of near-infrared excitation (Nd:YAG laser operating at 1064 nm) for Raman spectroscopy is of great benefit for reducing undesirable fluorescence—a recurring problem when dispersive visible light excitation sources are used. However, for C₃S and C₂S, and also for cements, we have found that the reverse effect occurs. The spectral bands observed with such instrumentation are due entirely to structured fluorescence effects, and the true Raman bands are

Acknowledgments

We thank the University of London Intercollegiate Research Services (ULIRS) and Dr. Steven Firth at the University College (London, UK) for the visible light Raman spectra.

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Citation Excerpt :
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¹: Current address: Nalco, Energy Services Division, Cadland Road, Hardley, Hythe, Southampton SO45 3NP, UK.

²: Current address: Centre for Computational Science, Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK.

³: Current address: 23 Hadley Rd., Merrimac, MA 01860, USA.

View full text

Anomalous fluorescence in near-infrared Raman spectroscopy of cementitious materials

Abstract

Introduction

Section snippets

Experimental

Raman spectra observed for cements and individual cement minerals

Conclusions

Acknowledgments

Cem. Concr. Res.

Cem. Concr. Res.

Spectrochim. Acta

Cem. Concr. Res.

Biomatter

Spectrochim. Acta

The potential for Raman spectroscopy in cement chemistry

Cem. Technol.

Uses of Raman spectroscopy in cement chemistry

J. Am. Ceram. Soc.

FT-Raman spectroscopy: development and justification

Appl. Spectrosc.