Developing inorganic coatings with nano-TiO2 for heritage concrete and assessing their self-cleaning performance by a new laboratory test

Heritage concrete and cement mortars urgently need preventive treatments which slow down their deterioration, allow to avoid extensive demolition and reconstruction, and preserve the aesthetic significance of texturized surfaces. In this paper new inorganic treatments were developed, which combine the self-cleaning behaviour of nano-TiO2 and the beneficial effect of inorganic consolidants. In a preliminary phase, two consolidants were investigated for the incorporation of titania nanoparticles, namely diammonium phosphate and ethyl silicate (TEOS), and the latter one was selected as the most promising one. In the second phase, combined treatments with TEOS and titania nanoparticles were developed and applied by different techniques to cement mortars manufactured to mimic historic substrates, providing a novel and valuable solution for heritage concrete protection that is unprecedented in the literature. Besides evaluating the photoactivity of the treatments in terms of methylene blue discolouration, a novel test was developed in this study to assess in a more realistic way the actual ability of the treatments to boost the removal of soiling by rain. The combination of TEOS and nano-TiO2 into a single treatment has the potential to slow down carbonation and chloride diffusion, to help the removal of soiling and to provide the substrates with antimicrobial activity, hence it can be considered highly innovative. The results also showed that more realistic laboratory assessment procedures are necessary to predict the actual effectiveness of the treatments when applied to real substrates and exposed to real outdoor conditions. The novel test proposed and applied in this study may serve as a basis for the development of a new procedure of assessment of self-cleaning ability.