Shedding light onto the spectra of lime: Raman and luminescence bands of CaO, Ca(OH)₂ and CaCO₃

Abstract

In microscopy studies of 19 th -century cement stone, we found free lime in the form of darkened spherical structures, as they were described in the literature already. When trying to determine their phase composition by Raman spectroscopy, we encountered contradictive assignments in literature spectra of the lime phases CaO, Ca(OH) 2 and CaCO 3 and observed strong spectral features that have been ignored or erroneously assigned so far. In this study we present Raman spectra of pure lime phases and of a naturally grown calcite crystal, burnt limestone (quick lime, mainly CaO), aged slaked lime putty (mainly Ca(OH) 2 ), and carbonated lime putty (mainly CaCO 3 ). Based on the results, we shed light mainly onto these two questions: (1) Does CaO have a Raman spectrum? (2) Which features in the spectra are luminescence bands that could be (and already have been) misinterpreted as Raman bands? We proof our assignment of luminescence bands in lime phases by using three different laser wavelengths for excitation, and give hypotheses on the origin of the luminescence as well as practical advices on how to identify these misleading features in Raman spectra. This article is mainly addressed to users of Raman spectroscopy in different fields of material analysis who might not be aware of the presence of interfering bands in their spectra.

Portlandite crystal: Bulk, bilayer, and monolayer structures

“…The high-frequency peak A 1g matches well the previously reported experimental and theoretical data, however, E 2g shows slight differences in peak position. For example, Padanyi et al [50] observed an E 2g peak at around 260 cm −1 while reports from Schmid et al [51] agrees well with our results. Accordingly, observed variations in the E 2g peak might be linked to differences in the phase or to hidden phases introduced during the growth.…”

Raman scattering or fluorescence emission? Raman spectroscopy study on lime-based building and conservation materials

“…Schmid and Dariz [14] have suggested in their detailed report on spectral profile of lime materials excited by various laser lines that this phenomena occurs due to the generation of point defects by thermal and pressure conditions of lime burning, e.g. oxygen radical anions in the crystal lattice.…”

“…However, one can also observe laser-induced fluorescence features of strong intensity when using a near-infrared line, which may be mistakenly identified as Raman scattering. This effect has been observed during collection of NIR-Raman spectra by several groups studying different materials, mostly minerals and inorganic materials [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Dyer and co-workers have found fluorescence bands in cement materials [1], while others noticed their presence in materials containing calcium phosphate obtained synthetically and in the form of a biomaterial [2][3][4][5][6].…”

“…Other materials for which such bands have been identified, belong to groups of alumina [8], lanthanide salts [9][10][11][12] and almandine minerals [13]. Recently, Schmid and Dariz have discussed in detail luminescence features in spectra of lime phases acquired with the laser excitation at 633 and 785 nm [14]. They have concluded that luminescence bands, which interfere with Raman bands depending on excitation wavelength, probably occur due to the presence of rare earth elements, point defects or radical ions and actually their origin must be explained individually for a given material.…”

Water pressure enhanced sintering of alkaline-earth perovskite ceramics

“…Note, the traces of SrO can be also detected by the presence of electronic transitions in the Raman spectrum arising from 4f ion interband luminescence emissions, excited by the laser beam (Fig. 3a) [31]. Such features are not observed on high dense ceramic pellets, free of any traces of second phases.…”