Contents lists available at ScienceDirect Building and Environment journal homepage: www.elsevier.com/locate/buildenv Field performances of nanosized TiO 2 coated limestone for a self-cleaning building surface in an urban environment M. Lettieri, D. Colangiuli, M. Masieri, A. Calia * IBAM-CNR (Institute of Archaeological and Monumental Heritage), University Campus, Prov.le Lecce-Monteroni, 73100, Lecce, Italy ARTICLE INFO Keywords: Nanostructured TiO 2 coating Photocatalytic limestone surface Outdoor ageing Self-cleaning Surface soluble salts NPs deactivation ABSTRACT Over the last few years, photocatalytic titanium dioxide coatings have been explored in laboratory conditions to create building materials with self-cleaning and depolluting abilities. Assessing the performances of the pho- tocatalytic surfaces under real conditions may provide basic knowledge to evaluate the potential of real appli- cations of TiO 2 coatings in buildings. In this study, the performance of photocatalytic limestone surfaces obtained through coating with water- dispersed TiO 2 nanoparticles were investigated in an urban environment. Coated and uncoated samples were exposed to an urban site for one year. Before the exposure and periodically afterwards, optical microscopy observations, colour and contact angle measurements were performed on the sample surface. At the end of the exposure period, samples underwent a capillary water absorption test and self-cleaning efficiency was evaluated by a Rhodamine B photodegradation test. Ti and soluble fraction amounts on the sample surfaces were de- termined by X-ray Fluorescence (XRF) and ion chromatography, respectively. The overall results showed the TiO 2 coating was better able to preserve the surface colour properties early after exposure. After eight months, this effect was lost and the self-cleaning efficiency was reduced to negligible final rates. No clear wettability results and no meaningful capillary behaviour were recorded. A decrease of the photocatalytic activity was due to both partial titania loss and deactivation phenomena. The role of soluble salt ions either adsorbed from the environment or produced by the photocatalytic abatement of pollutants was recognized in the deactivation of the photocatalysts and their accumulation deserves attention for possible stone damage risk implications. 1. Introduction The urban built environment has notably grown worldwide and a progressive, increasing trend is expected in the future due to population growth. This contributes to the main concerns of the current societies, facing global environmental and energy challenges. High concentrations of people and activities in urban sites cause significant pollution levels that typically result in detrimental effects to the built heritage, in addition to risks to human health. It is well known that airborne contaminants derived from both natural and human sources are important causes of deterioration of the built surfaces [1,2] and compromise their preservation, thus leading to frequent main- tenance, with a significant consumption of resources. In addition, large amounts of building repairs cause high-energy consumption levels, with high costs and impacts on the environment. A progressive, significant increase in the energy supply is foreseen, for example, in developing countries, due to the air conditioning demand induced by higher ambient temperatures due to climatic change, which is further ampli- fied by the expected growth of the local population and income [3]. In this context, even more attention is being paid to new en- vironmentally friendly technologies and processes able to improve the performance of buildings and cities in order to contribute to sustainable built environments. Relevant options in this regard may be offered by the use of nanomaterials, such as titanium dioxide nanoparticles (TiO 2 NPs). Starting from the second half of the last century, the mechanisms that account for the heterogeneous photocatalysis promoted by TiO 2 NPs and for the self-cleaning activity in materials containing this metal oxide, have been described [4] and environmental applications have been progressively recorded in many fields [5]. In the construction sector, TiO 2 modified materials have been targeted for their photo- activated depolluting properties in urban environments, thanks to their ability to abate inorganic pollutants such as NO x , SO x [6], organic compounds, such as benzene, toluene and xylene (BTX) [7], volatile organic compounds (VOCs) emitted by exhausts, combustion gases [8], https://doi.org/10.1016/j.buildenv.2018.10.037 Received 11 July 2018; Received in revised form 29 September 2018; Accepted 19 October 2018 * Corresponding author. CNR-IBAM, University Campus, Prov.le Lecce Monteroni, 73100, Lecce, Italy. E-mail addresses: mt.lettieri@ibam.cnr.it (M. Lettieri), dinocolangiuli@hotmail.com (D. Colangiuli), m.masieri@ibam.cnr.it (M. Masieri), a.calia@ibam.cnr.it (A. Calia). Building and Environment 147 (2019) 506–516 Available online 22 October 2018 0360-1323/ © 2018 Elsevier Ltd. All rights reserved. T