ASSESSING THE SELF-CLEANING BEHAVIOUR OF INORGANIC TREATMENTS FOR STONE

The increasing amounts of particulate matter and gaseous nitrogen oxides in polluted urban areas affects a variety of materials in heritage buildings and sculptures exposed outdoor, by causing soiling deposition and deterioration. To fight this problem, the use of nanoparticles of photocatalytic titanium dioxide (TiO2) was proposed several years ago as a strategy to provide architectural surfaces with self-cleaning behaviour. Very promising results were obtained in many laboratory studies, where nano-TiO2 applied over different substrates was found to successfully reduce nitrogen oxides concentration in air and to deteriorate dyes of different organic compounds applied over the substrates. However, many studies remained at a laboratory stage, as some limitations were highlighted for these treatments, such as the short-term retention of TiO2 over the substrates, with limited effectiveness and possible release of harmful titania nanoparticles in the environment due to rain washout. Moreover, the onsite self-cleaning performance of these treatments in real heritage buildings, which involve different materials, roughness, exposure, and climatic conditions, is far from being assessed. In this study, the performance of an inorganic treatment based on the combination of ethyl silicate and titania nanoparticles applied to porous limestone was investigated. This combination is expected to guarantee long-term adhesion of the nanoparticles over the substrate without significantly affecting the photocatalytic ability of TiO2. After assessing the compatibility of treatments in terms of colour and physical properties, their self-cleaning performance was assessed in realistic conditions that aim at overcoming the limitations of current tests. First, a purposely developed artificial soiling was applied over the treated and untreated substrates, then the self-cleaning behaviour was investigated, by spraying artificial rain also reproducing the impact pressure of real one over vertical façade. The results contribute to a better understanding of the behaviour of TiO2 as a protective treatment and may help in developing durable and effective surface treatments.