4D Imaging Synchrotron X-ray Microtomography to Investigate the Dynamic of the Consolidation Process due to Inorganic Treatments on Calcareous Stone

A burning goal in Conservation Science is to study the diffusion of inorganic-mineral treatments inside stone materials and to understand how these treatments modify the microstructure of the substrate. Diammonium hydrogenphosphate (DAP, (NH4)2HPO4) solutions are commonly used in stone conservation to restore the mechanical and microstructural features of decayed carbonatic matrix (Matteini et al., 2011). These treatments act in a water-based solution and partially transform the original minerals of the carbonatic substrate in newly formed crystalline phases by a dissolution and recrystallization reaction nucleating on calcite grains (Sassoni, 2018). The kinetics of the reaction and the induced effects on the stone microstructural features are ruled by several variables i.e., treatment methodology, solution molarity, free ions availability, specific surface area, microstructural characteristics of the lithotype (Possenti et al., 2016). Therefore, the direct effects induced by the solution on the stone microstructure (e.g., pore morphology, connectivity, etc.) are still poorly understood and no information is available on the interaction between the calcite crystal and the DAP solutions during the reaction process (Possenti et al., 2019). Here, for the first-time, time-resolved (4D) high-resolution synchrotron X-ray microtomography was used to obtain real-time quantitative information on the structural evolutions induced by the treatment process on the Noto limestone, a porous carbonatic stone used as building stone since antiquity (Val di Noto - Siracusa, UNESCO’s World Heritage List). The experimental setup developed allowed us to consider the intrinsic microstructural heterogeneity of the Noto limestone and, at the same time, to unambiguously attribute any specific microstructural variations during the treatment to the peculiar effect of the DAP treatments. With the aid of image analysis, microstructural modifications in terms of boundary morphology, porosity (both the total porosity and its components), interconnection and pore size distribution have been demonstrated in different stadia of the consolidation process. The quantitative analysis of the VOIs pointed out differences in the consolidation dynamic asserting the influence of molarity was asserted. This study paves the way to a deeper understanding of consolidation mechanisms and the effects induced to the 3D microstructural features of porous materials by inorganic treatment. Above all, this study supports the application of inorganic-mineral treatments in conservation worksites based on advanced knowledge of the dynamics of the consolidation process and effects induced by DAP conservation treatments.