engleski

Semi-quantitative analysis of the formation of a calcium oxalate protective layer for monumental limestone using combined μXRF/μXRPD

The aim of this study was to investigate the effects of the application of ammonium oxalate (AmOx) as to form a protective layer of calcium oxalate (CaOx) on the surface of cretaceous limestone samples. Solutions of different concentrations were employed, as well as various treatment times and application methods (poultice, immersion and brushing). In order to determine the thickness of the resulting CaOx layers, line scan measurements over cross-sections of treated samples have been performed using combined micro X-ray fluorescence and micro X-ray powder diffraction (μXRF/μXRPD) at the DIFFABS beamline, SOLEIL synchrotron, France. Signals were recorded in reflection geometry using a collimated X-ray beam of 12.5 x 7.5 μm2 (H x V) in size. The diffraction measurements allowed for the identification of calcium oxalate monohydrate (CaC2O4.H2O, whewellite) and calcium oxalate dihydrate (CaC2O4.2H2O, weddellite) as the two oxalate phases formed. The misalignment of the sample outer surface relative to the direction of the beam, caused a notable shift in the reflections of the crystalline phases This misalignment, however, was taken into account during the analysis. Weddellite was found only close to the treated surface (max. depth: 21 ± 6 μm), while whewellite was found over a larger depth in the sample (< 200 μm). The maximum depth at which CaOx formation took place was shown to be dependent on both the treatment time and the AmOx concentration used. Using a procedure for semi-quantitative XRPD analysis it was possible to determine both the thickness of the layer, as well as the relative amount of CaOx formed under different treatment conditions The obtained results showed that brushing treatment could be considered as an effective and economically more feasible alternative to poultice treatment for the creation of a protective CaOx layer on less porous large surfaces [1 ]. [1] D. Mudronja, F. Vanmeert, K. Hellemans, S. Fazinic, K. Janssens, D. Tibljas, M. Rogosic, S. Jakovljevic, to be published, Efficiency of applying ammonium oxalate for protection of monumental limestone by poultice, immersion and brushing methods.

microXRF, microXRPD, CaOX, monumental limestone

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