NO x de-pollution by hardened concrete and the influence of activated charcoal additions M. Horgnies ⁎, I. Dubois-Brugger, E.M. Gartner Lafarge Centre de Recherche, 95 rue du Montmurier, 38291 Saint Quentin Fallavier, France abstract article info Article history: Received 16 April 2012 Accepted 21 June 2012 Keywords: Adsorption (C) Concrete (E) Characterization (B) Microstructure (B) NO x de-pollution The atmospheric pollution by nitrogen oxides affects the health of millions of people located in urban areas. Our work demonstrates that, even in the absence of photocatalysts, concrete walls can strongly absorb NO 2 and limit the intensity of pollution peaks. Our results show that concretes can continuously absorb a signif- icant fraction of NO 2 from the surrounding atmosphere over long periods, probably due to a reaction with strongly alkaline cement hydrates. Moreover, the introduction of a small amount of activated charcoal into the concrete mix can significantly enhance and prolong this NO 2 absorption without greatly increasing total porosity or decreasing strength. We hypothesize that the NO 2 is adsorbed irreversibly by a neutraliza- tion reaction with the alkaline aqueous solution covering the surfaces of the hydrates and the activated char- coal. Simple calculations suggest that walls made of activated charcoal concrete could ameliorate the problem of NO 2 pollution peaks in road tunnels and parking garages. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction Environmental pollution is an important issue with respect to sustain- able development [1,2]. Nitrogen oxides (NO x ) are common toxic gaseous pollutants produced by combustion processes, and especially by automo- tive traffic. They affect the health of hundreds of millions of human beings, especially in urban areas. The term “NO x ” represents two separate molec- ular species: nitrogen dioxide (NO 2 ) and nitric oxide (NO). According to the World Health Organization (WHO), NO 2 is the more toxic of these two gases [3]. It is also a precursor of ozone, which is the other major toxic atmospheric pollutant in urban areas [4,5]. These hazardous gases readily diffuse into buildings [6,7] and contaminate tunnels and under- ground car parks [8]. It has been suggested that NO x pollution in cities or on roads could be treated by photocatalysts such as titanium dioxide (TiO 2 ) and zinc oxide (ZnO) [9–11]. Under UV-irradiation, these particles create hydroxyl rad- icals that can oxidize NO to NO 2 , the latter gas being potentially more readily removed from the atmosphere by mineralization after a series of reactions [12,13]. Photocatalytic reactions of this type were first dis- covered in Japan [14] in the 1970s and they have been widely studied ever since as an approach to treating both indoor and outdoor air pollu- tion [15]. Pigments based on the anatase form of titanium dioxide have been incorporated into concretes to create so-called “photocatalytic” or “self-cleaning” walls and pavements, such as those described by Beeldens and Van Gemert [16], Ballari et al. [17] and by Maury Ramirez et al. [18]. The idea of using this approach to reduce NO x pollution has been dis- cussed by Poon and Cheung [19], Hassan et al. [20] and Guerrini [21]. However, the potential of these photocatalytic systems to de-pollute the outdoor atmosphere has not been clearly demonstrated [22,23]. Folli et al. [24] recently observed apparently photocatalytic removal of both NO and NO 2 from gases passed over white cement mortars containing TiO 2 powders. They observed no detectable increase in NO 2 corresponding to the observed decrease in NO, which is different from what is observed with pure TiO 2 powders. They interpreted this as being due to the stabili- zation of both nitrite and nitrate ions due to the high pH of the cement matrix. Despite much recent interest in photocatalysis, the reaction between NO x and ordinary concretes has not been studied in detail, although it is known that NO 2 can easily dissolve in basic aqueous solutions [25] to produce nitrites and nitrates. Only a few authors (Spicer et al. [26], Wade et al. [27], Gröntoft and Raychaudhuri [28], Yamanaka [29] and M. Nicolas [30]) have dealt with the question of how adsorption or ab- sorption of pollutants by residential materials can influence the indoor atmosphere. We believe that the preliminary study described in this paper is the first specifically devoted to the quantification of NO x absorp- tion by ordinary concretes. In addition, we will describe tests that confirm our contention [31] that the incorporation of activated charcoal into a fresh concrete mix can greatly enhance the absorption rate of NO 2 by the hardened product. Activated charcoals and other activated carbons are among the best known of industrial adsorbents. They are widely used to decontaminate water by adsorbing organic residues such as pesticides [32], or to Cement and Concrete Research 42 (2012) 1348–1355 ⁎ Corresponding author. Tel.: +33 474823340. E-mail address: matthieu.horgnies@lafarge.com (M. Horgnies). 0008-8846/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconres.2012.06.007 Contents lists available at SciVerse ScienceDirect Cement and Concrete Research journal homepage: http://ees.elsevier.com/CEMCON/default.asp