TiO 2 photocatalysis in cementitious systems: Insights into self-cleaning and depollution chemistry Andrea Folli a, ⁎, Claus Pade b , Tommy Bæk Hansen c , Tiziana De Marco d , Donald E. Macphee a, ⁎ a University of Aberdeen, Meston Building, Meston Walk, AB24 3UE, Aberdeen, Scotland, UK b DTI, Byggeri - Beton, Gregersensvej, Høje Tåstrup, Denmark c Aalborg Portland A/S, Product Technology, Sølystvej 18, 9220 Aalborg Ø, Denmark d CTG-Italcementi Group, Via Camozzi 124, Bergamo, 24121, Italy abstract article info Article history: Received 27 May 2010 Accepted 2 December 2011 Keywords: TiO 2 photocatalysis Self-cleaning Aesthetic durability Depollution Particle size The present work offers a general overview about application of titanium dioxide (or titania), TiO 2 , photoca- talysis to concrete technology in relation to enhanced aesthetic durability and depollution properties achieved by implementing TiO 2 into cement. Chemistry of degradation of Rhodamine B (RhB), a red dye used to assess self-cleaning performances of concretes containing TiO 2 , as well as oxidation of nitrogen oxides (NOx), gaseous atmospheric pollutants responsible for acid rains and photochemical smog, is investigated using two commercial titania samples in cement and mortar specimens: a microsized, m-TiO 2 (average particle size 153.7 nm ± 48.1 nm) and a nanosized, n-TiO 2 (average particle size 18.4 nm ± 5.0 nm). Experimental data on photocatalytic performances measured for the two samples are discussed in relation to photocatalyst properties and influence of the chemical environment of cement on titania particles. Impacts on applications in construction concrete are also discussed. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction The photochemistry of TiO 2 has become a subject of intense re- search since Fujishima and Honda [1] and Wrighton et al. [2] reported the photocatalytic splitting of water on TiO 2 and Sr-doped TiO 2 respectively in the 1970s. Applications of TiO 2 photocatalysts to construction materials began towards the end of the 1980s. Two im- portant effects related to the nature of photoactive TiO 2 coatings had by this time been discovered: a) the self-cleaning effect due to redox reactions promoted by sunlight (or in general, weak U.V. light) on the photocatalyst surface [3], and b) the photo-induced hydrophilicity [4,5] of the catalyst surface, which enhances the self-cleaning effect (inorganics causing dirt and stains on surfaces can be easily removed due to rainwater soaking between the adsorbed substance and the TiO 2 surface). Photocatalytic glasses provide an example of self- cleaning and anti-fogging (wetting) properties, e.g. Pilkington Active™ [6]. Recently, photocatalytic cementitious materials have been patented by Mitsubishi Corp.(NOxer™), and Italcementi SpA (TX-Aria™ and TX- Arca™) [7–10]. The application of TiO 2 photocatalysis to concrete aims to achieve two main goals, the self-cleaning effect discussed above and the depolluting effect due to the oxidation of nitrogen oxides (NO x ) in the atmosphere to NO 3 - , especially in street canyons where NO x concentrations can be considerable due to combustion engine exhausts. The great advantage provided by such products is that the only requirements, beyond TiO 2 in the construction material used, are sunlight, oxygen and water. 1.1. Self-cleaning and aesthetic durability Development of TiO 2 –cementitious binders providing self-cleaning has been carried out in order to enhance aesthetic durability of cemen- titious materials, particularly those based upon white cement. Although the use of such products is still restricted and limited, many buildings have been designed and constructed since 2000, to fulfil high aesthetic standards. Relevant examples are: Church “Dives in Misericordia”, Rome, ITALY, Music and Arts City Hall, Chambéry, FRANCE, Police Central Station , Bordeaux, FRANCE, Air France Building, Roissy-Charles de Gaulle Airport, FRANCE, Saint John's Court, Montacarlo, MONACO. In order to verify self-cleaning performances of photocatalytic cements/concretes, several tests involving organic substances have been set up and include the degradation of colour in dyes. Rhodamine B(N-[9-(2-Carboxyphenyl)-6-diethylamino-3H-xanthen-3-ylidene]-N- ethyl-ethanaminium chloride) is one of the most common dye tests and is adopted as an Italian standard (UNI 11259 (February 2008) [11]). Rhodamine B, RhB, degradation on TiO 2 surfaces has been ex- tensively studied in slurry systems (TiO 2 suspended in aqueous solu- tions of RhB) under different illumination conditions [12–16]. Results have shown efficient degradation of colour under both U.V. light and visible light. Under U.V. light illumination RhB is degraded by an ordinary TiO 2 -sensitised photoreaction [12,17]: a TiO 2 photocatalytic Cement and Concrete Research 42 (2012) 539–548 ⁎ Corresponding authors. E-mail addresses: a.folli@abdn.ac.uk (A. Folli), d.e.macphee@abdn.ac.uk (D.E. Macphee). 0008-8846/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconres.2011.12.001 Contents lists available at SciVerse ScienceDirect Cement and Concrete Research journal homepage: http://ees.elsevier.com/CEMCON/default.asp