QUANTITATIVE EVALUATION OF THE CONSOLIDATING EFFECTS INDUCED BY INORGANIC MINERAL TREATMENTS APPLIED ON POROUS CARBONATE STONE SURFACES
Carbonate stones have been extensively used in cultural heritage (CH) throughout history due to their abundance and workability for many historical monuments and artworks. However, these stone materials are decaying at a noticeable rate, particularly under the influence of pollutants like sulfur oxides and CO2, compounded by climate change. The heterogeneity of carbonate stones, including texture and porosity, further complicates their conservation, often leading to unpredictable decay processes. Moreover, previous conservation efforts, frequently lacking in standardized diagnostic assessments, can create complex challenges for future interventions. This means that stone conservation research is of crucial importance. This research focuses on two inorganic treatments for carbonate stone materials: ammonium oxalate (AmOx) and diammonium hydrogen phosphate (DAP) water solutions. AmOx generates a calcium oxalate (CaOx) layer on the stone surface, preventing further weathering. DAP forms calcium phosphate (CaP), addressing the loss of cohesion and increasing the strength of a decayed matrix by binding detached grains. Despite their efficacy, challenges remain in optimizing treatment penetration, unambiguous results, and long-term effectiveness. Given that the stone conservation field is constantly evolving, this dissertation explores a novel sequential treatment approach combining AmOx and DAP, aiming to harness the protective surface layer of CaOx and the deeper consolidation effects of CaP. While initial studies show promise, results are extremely variable. A comprehensive understanding of the single treatments' interactions with the stone’s substrate is crucial. Specifically, this research focuses on the kinetics, crystallization effects on porosity, and mineralogical transformations within the calcite matrix during and after the reaction with the single solutions. The second objective of this study is to identify advanced, high-resolution imaging techniques to characterize these conservation processes better. Emphasis is placed on synchrotron-based imaging, including synchrotron 4D X-ray computed microtomography (SR-μCT), micro-X-ray diffraction (SR-μXRD), and micro-X-ray fluorescence (SR-μXRF). These high-resolution techniques are complemented by laboratory methods for initial assessments. The first manuscript published in the framework of this research activity investigates the crystallization kinetics of DAP-treated in porous Noto limestone using SR-μCT to track porosity changes over time, revealing the dynamic microstructural evolution during treatment. This time-resolved (4D) analysis unveils the complex nucleation processes and porosity evolution during treatment. The second paper introduces a combined SR-μXRD and SR-μXRF approach to map the spatial distribution and composition of new phases formed by AmOx and DAP treatments. This method overcomes challenges posed by the heterogeneous nature of the treated stone, allowing for precise localization and characterization of crystalline and poorly crystalline CaOx and CaP phases within the calcite matrix. Finally, the third paper extends the research described in Article #2, combining SR-μXRD and SR-μXRF mapping to visualize the direct effect of AmOx and DAP reaction with the matrix, providing insights into the reaction site of different macroareas, i.e., areas of unique phases crystallization as a function of depth in each treatment. The Rietveld method and Pearson VII peak analysis were used to study the microstructural changes in the calcite substrate in the reaction sites. The paper deals with the difficulty of understanding the effect of dissolution and precipitation processes. The third paper also extends these methods to sequential AmOx-DAP treatments, exploring and quantifying the macro- and microstructural changes across spatial and depth-dependent reaction sites. This study uncovers the impact of application methods, substrate composition, and the interaction of the two treatments on the effects of the consolidation. This body of work offers valuable insights into the performance and properties of treated materials and the pros and cons of various imaging techniques used to examine these properties. The general discussion addresses the research implications of the study’s objectives, critically analyzing the data within the field context. Any unexpected or contradictory findings are explored and explained where possible, with suggestions for re-evaluating the results. This comprehensive discussion aims to raise awareness of the thesis topic's challenges, inspire new ideas, and provide a foundation for future X-ray imaging research. The experimental findings are expected to contribute to conservation practice, providing guidelines for restorers to perform treatments with improved performance, considering the specifics of each case study and the conservative history of the stone artwork.
Settore GEOS-01/D - Georisorse minerarie e applicazioni mineralogico-petrografiche per l'ambiente e per i beni culturali
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