1 Functional nano-hydroxyapatite for applications in 2 conservation of stony monuments of cultural heritage 3

Stony monuments have high artistic value and need continuously to be preserved from the 19 damages of time, in particular from the detrimental effects of the weathering. One of the new 20 environmentally-friendly (nano) materials for stone consolidation, particularly suitable for marble and 21 calcareous (limestone, sandstone) artifacts, is Ca10(PO4)6(OH)2 hydroxyapatite (HAp), which shows a 22 considerably lower dissolution rate and solubility compared to CaCO3 calcite, the building block of marble 23 materials, especially in acidic environments, thus having been proposed for the protection of calcareous 24 monuments against acidic rain corrosion. Promising results were obtained, but further optimization is 25 necessary as the treated layer is often incomplete, cracked and/or porous. Many parameters have to be 26 optimized to obtain a coherent and homogenous layer, and consequently to avoid the formation of 27 metastable, soluble phases instead of HAp: the role of the pH of the starting solution; the effect of organic 28 and inorganic additions, and in particular that of ethanol, that is reported to adsorb on calcite, hence possibly 29 favoring the growth of the HAp layer. The formation of HAp nanoparticles and the applications on stony 30 substrates were investigated by means of a multi-methodological approach, based on scanning electron 31 microscopy, x-ray diffraction, smalland/or wide-angle x-ray scattering, Fourier-transform Infrared 32 spectroscopy, completed by in situ measurements of laser-induced breakdown spectroscopy and acid attack 33 preliminary test on stony substrates. 34