Photocatalytic hydroxyapatite-titania nanocomposites for preventive conservation of marble

Soiling of external surfaces is an increasing issue for conservation of architectural heritage. Providing architectural surfaces with self-cleaning ability is one of the most promising routes of preventive conservation. To this aim, several methods have been proposed in the literature, based on the use of photocatalytic TiO2nanoparticles, either directly applied onto the surfaces or incorporated in protective coatings. However, when nano-TiO2is directly applied onto architectural surfaces, the particles are easily removed by rain. When TiO2is incorporated in polymeric coatings, durability issues arise as well, because the photoactivity of TiO2nanoparticles can promote degradation of the polymer. Here, we present an innovative alternative method, based on combination of TiO2nanoparticles and hydroxyapatite (HAP). The incorporation of nano-TiO2into an HAP coating protects the nanoparticles from leaching by rain, thanks to the chemical bonding between TiO2and HAP, without diminishing their photoactivity. As a result, marble treated with HAP-TiO2composites exhibits high self-cleaning ability and high durability, with results superior to those achieve by direct application of nano-TiO2onto the surface, as frequently performed on site.