PERFORMANCE AND PERMANENCE OF TIO2-BASED SURFACE TREATMENTS FOR ARCHITECTURAL HERITAGE: SOME EXPERIMENTAL FINDINGS FROM ON-SITE AND LABORATORY TESTING

The possibility of providing historic façades with self-cleaning ability in urban polluted environments by means of treatments based on photocatalytic nano-TiO2 dispersions, has recently received growing attention. The potential impact of these treatments for the protection of heritage buildings is evidenced by the high number of papers where the performance of TiO2-based nanocoatings on stone (mainly marble, travertine and limestone), mortar and brick were investigated by laboratory tests. The results seem encouraging, even if the nature of the treatments, the kind of substrate and the methods used for assessing the coatings’ performance differ greatly from one study to the other, thus making the results difficult to compare or even contradictory. Several aqueous titania nanodispersions are already available in the market and some applications of these treatments as trial testing in real heritage buildings are known, but information about their performance (colour change, self-cleaning ability, etc.) on real substrates and in real outdoor environments are still very scarce. In particular, the long-term permanence of TiO2 nanoparticles on outdoor exposed surfaces, also in relation with strategies for promoting the adhesion between nanoparticles and substrate, has also not been fully elucidated yet. In the present paper, some experimental findings collected during last years from on-site and laboratory testing campaigns are reported, as a contribution towards a better assessment of the behaviour of TiO2 treatments when exposed to real and accelerated environmental conditions.