Application of ammonium phosphate to marble. Investigation of newly-formed calcium phosphates with synchrotron light and high lateral resolution FTIR microspectroscopy

Inorganic-mineral treatments exhibit great potentialities for the consolidation of stone materials due to their high compatibility with the substrate. Their reaction mechanism is based on the diffusion, inside weathered stone matrixes, of water soluble precursors that, reacting with the substrate, supply a crystal network able to reconnect detached grain boundaries. The newly-formed crystalline phases are stable and due to their low solubility provide a passivating action toward atmospheric agents even in acid environment. Diammonium hydrogen phosphate [DAP, (NH4)2HPO4] has been recently suggested for the treatment of sedimentary and metamorphic carbonatic decayed stones. The reaction of DAP with calcite of the substrate involves a pseudomorphic replacement and favours the growth of calcium phosphates inside the porosity. Preliminary studies (Matteini et al., 2013; Possenti et al., 2016) show that the reaction at room temperature is non-stoichiometric and induces the formation of hydroxyapatite [HAP, Ca5(PO4)3(OH)] (Ni & Ratner, 2003) and other metastable phases. In this pilot study we characterized the complex assemblages of calcium phosphates formed after DAP treatments on Carrara marble specimens with a multi-analytical approach (scanning electron microscopy, vibrational spectroscopies and powder X-ray diffraction). A set of quarried and thermally decayed samples were treated by poultice and capillarity using DAP solutions at different molarities. Ground-breaking techniques such as X-ray diffraction with synchrotron light in transmitting geometry and high lateral resolution FTIR microspectroscopy were employed to overcome some of the analytical limits of conventional approach, assessing the overall composition of main and trace phases as well as their arrangement on the substrate. Preliminary findings show the formation of a shell-like layer around the calcite grains, composed by a mixture of crystalline and amorphous calcium phosphates; the formation of specific phases, their morphologies and the relative amount depend on the solution molarity and the treatment duration. Moreover, our data show a correlation between the kind of crystalline phase and its morphology and position within the shell structure. Matteini, M., Colombo, C., Botticelli, G., Casati, M., Conti, C., Negrotti, R., Possenti, E., Realini M. (2013): Ammonium phosphates to consolidate carbonatic stone materials: an inorganic-mineral treatment greatly promising. Proceedings of the Built Heritage 2013 Monitoring Conservation Management, 1278-1286. Ni, M. & Ratner, B.D. (2003): Nacre surface transformation to hydroxyapatite in a phosphate buffer solution. Biomaterials, 24, 4323- 4331. Possenti, E., Colombo, C., Bersani, D., Bertasa, M., Botteon, A., Conti, C., Lottici, P.P., Conti C. (2016): New insight on the interaction of diammonium hydrogenphosphate conservation treatment with carbonatic substrates: a multi-analytical approach. Microchem. J., 127, 79-86.