Consolidation of bio-calcarenite stone by treatment based on diammonium hydrogenphosphate and calcium hydroxide nanoparticles

In the past decades, interest in using nanomaterials for preservation of valuable cultural heritage items rapidly increased. In the present study, a method for the consolidation of weathered calcareous stone (i.e. Lecce Stone) by diammonium hydrogenphosphate and calcium hydroxide nanoparticles, which provide hydroxyapatite, is evaluated. Stone specimens were artificially weathered by several wetting-drying and freezing-thawing cycles, then they were treated with Ca(OH)2 nanoparticles in alcoholic dispersion and diammonium hydrogenphosphate (DAP) in water solution. The mineralization process affording hydroxyapatite was carried out in situ at room temperature, by mimicking the growth mechanism of bone. Ca(OH)2 nanoparticles were synthesized and characterized by X-ray powder diffractometry (XRPD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and dynamic light scattering (DLS). The progression of apatite in stone substrate was studied by using XRPD. Treatment efficacy was evaluated by measuring physical-structural properties such as water capillary absorption, permeability to water vapour, chromatic variation, porosity, surface cohesion and the resistance to salt crystallization. Morphological and microstructural features as well as distribution of the deposited phase on the surface and into the stone substrate were examined by SEM-EDS experiments. Experimental measurements suggest that the neoformed hydroxyapatite is highly concentrated on the stone surface with a consequent increase of the surface cohesion, while its diffused presence in depth induces also a moderate enhancement of resistance to salt crystallization. This work confirms that biomimic formation of hydroxyapatite represents a very promising method for the conservation of calcareous materials such as Lecce Stone.