Diammonium hydrogenphosphate (DAP) water solutions are promising inorganic-mineral treatments for the consolidation of ornamental carbonatic stones and, in the last few years, DAP-based treatments are gaining increasing interest by the open literature. The core idea of the treatment is to transform part of the original stone material in insoluble newly-formed phosphate phases, through the reaction of the phosphate ions of the reagent with calcium ions of the substrate. At present, there is a growing quest for a deeper insight on their interaction with carbonatic stone matrixes, as well as the development of analytical protocol to characterize their diffusion and penetration profile within the matrix.
The research project of this PhD thesis aims to define the crystal-chemistry and distribution of reaction products within carbonatic substrates, exploring the effects induced by the crystallization to the stone matrix. Different multi-analytical approaches were used. Case by case, the combination of conventional and innovative analytical techniques allowed exploring the feasibility and potentiality of high resolution ATR-FTIR microspectroscopy, synchrotron radiation (SR) X-ray diffraction in transmittance and grazing geometry, and of SR micro-computed tomography to study inorganic-mineral treatments and their interactions with stone substrates.
Starting from the DAP reaction with calcite of two representative lithotypes, a compact metamorphic stone (Carrara marble) and a porous sedimentary one (Noto limestone), we investigated the crystallization of newly-formed phosphates, describing the variables that affect the reaction mechanism and the nucleation of the reaction products. The nucleation of different phosphate phases resulted influenced by the availability of free calcium ions, which, in turn, is influenced by several factors mutually interfering. The DAP molarity, the pH and the pH variation during the reaction, the lithotype and the application methods were found crucial factors in the formation of specific crystallites, determining their composition, crystallinity and localization.
The effects induced by the nucleation of phosphates to the stone substrate were evaluated in terms of micro-structural variations, modification of the porosity (pore size distribution, total open porosity, average of pore radius, specific surface area) and penetration depth. The composition and distribution of specific phases in the matrix, as well as the arrangement of the new phases into the voids, were investigated by combining different techniques, including qualitative and quantitative-morphometric imaging analysis.
On the basis of the experimental data collected on laboratory stone specimens, an applicative study was carried out on a decayed carved artefact (Angera stone), in order explore the effects of DAP consolidation when complex weathered stone matrixes are involved.
We expect that the experimental findings of this research activity will be transferred to the conservation practice, providing guidelines to restorers in order to perform treatments with improved performances, considering the peculiarity of specific case studies and the conservative history of stone artworks
