From single to sequential consolidation treatments of carbonate stones. Mineralogical and structural variations by synchrotron μXRF-μXRD 2D mapping

This study explores the crystallization of new phases and structural changes of calcite induced by single and sequential ammonium oxalate (AmOx) and di-ammonium hydrogen phosphate (DAP) inorganic mineral treatments on Noto limestone, a UNESCO World Heritage biocalcarenite. These conservation treatments, commonly used in Cultural Heritage stone conservation, lead to the formation of calcium oxalate (CaOx) and calcium phosphate (CaP) phases. Utilizing synchrotron-based micro-X-ray fluorescence (µXRF) and micro-X-ray diffraction (µXRD) 2D mapping at high lateral resolution, we identified distinct macroareas- characterized by spatial distribution of specific newly formed phase assemblages-depending on the treatment type and application sequence. Reaction sites within these macroareas are recognized and make it possible to explore structural changes induced in calcite by treatments. Structural refinement of calcite in the reaction sites, using both the Rietveld method and peak shape analysis, reveals, for the first time, the crystallization of calcite of new formation, whose features vary as a function of the treatment application. Such a research aims to pave the way to a new investigation modality of the crystallization processes triggered by single and sequential inorganic treatments, and opens new research perspectives and practical applications for the conservation of decayed stone materials of cultural heritage that have been restored earlier.