Abstract
In Heritage Science, the evaluation of stone consolidation treatments by investigating the nature of in situ newly formed products and their penetration depth within the consolidated matrix is a grand challenge. A number of analytical methods have been proposed, but, currently, most of them are not able to supply a full overview of the spatial, structural and compositional information of the newly formed crystalline and amorphous phases with a submicrometric lateral resolution. Here, we examined, the capabilities of synchrotron radiation (SR)-based two-dimensional X-ray absorption near-edge structure (2D-XANES) spectroscopy at Ca K-edge for determining the structural and compositional properties of the compounds formed after the application of a calcium acetoacetate-based consolidant on a porous carbonatic stone (limestone) and for investigating their stratigraphic distribution at the submicrometric scale length. We evaluated advantages and drawbacks of three Ca K-edge 2D-XANES-based approaches: (i) transmission mode full-field-XANES (FF-XANES) imaging; (ii) micro-X-ray fluorescence (μ-XRF) mapping above the Ca K-edge combined with the acquisition of XRF mode μ-XANES spectra at a limited number of spots; (iii) full-spectral µ-XANES (FS µ-XANES) mapping in XRF mode and its variant called selectively induced X-ray emission spectroscopy (SIXES) mapping. Overall, Ca K-edge 2D-XANES spectroscopy provided accurate qualitative and semi-quantitative information on the newly formed calcium carbonates (i.e., amorphous calcium carbonate, vaterite and calcite) and their stratigraphic distribution at the submicrometric scale, thus opening a new scenario to study the carbonatation process of calcium-based consolidants in limestones.
Highlights
In Heritage Science, the evaluation of stone consolidation treatments by investigating the nature of in situ newly formed products and their penetration depth within the consolidated matrix is a grand challenge
We evaluated three different Ca K-edge 2D-X-ray absorption near-edge structure (XANES)-based approaches: (i) transmission mode full-field (FF)-XANES imaging; (ii) μ-XRF mapping just above the Ca K-edge combined with the collection of XRF mode μ-XANES spectra at a limited number of locations; (iii) full-spectral (FS) μ-XANES mapping in XRF mode
We started by performing a bulk characterization of the CFW powder by XRD and FT-IR spectroscopy complemented with single point XRF mode Ca K-edge macro-XANES measurements (Fig. 1)
Summary
In Heritage Science, the evaluation of stone consolidation treatments by investigating the nature of in situ newly formed products and their penetration depth within the consolidated matrix is a grand challenge. Examples are the new classes of calcium- and silicon-based products, developed to guarantee high chemical and physical compatibility with the stone substrate and designed either to produce inorganic Ca-based phases in carbonate stones or to form new cross-linked silica network in silicate s tones[4] It follows that the in situ characterization of newly formed crystalline and amorphous phases in the stone matrix necessitates analytical spatially-resolved approaches that enable to overcome the limits of conventional (bulk) techniques (such as SEM–EDX, XRD, FT-IR, NMR), which may suffer from low specificity and/or sensitivity, destructiveness and limited lateral r esolution[5,6,7]. Synchrotron radiation (SR)-based μ-XRD technique complements the range of the micro-analytical tools available for these applications, providing highly specific identification of the crystalline consolidant phases and their localization at a comparable lateral resolution[11,12,13]
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