**6. Conclusions and Perspectives**

Photocatalytic (TiO2 containing) paving materials with the potential of reducing air pollution by traffic are being used more frequently on site in horizontal as well as in vertical applications, also in Belgium. Laboratory results indicate a good efficiency towards the abatement of NO*x* in the air by using these innovative materials. The durability of the photocatalytic action also remains mostly intact, though regular cleaning (by rain) of the surface is necessary. The relative humidity (RH) is an important parameter, which may reduce the efficiency on site. If the RH is too high, the water will be adsorbed at the surface and prevent the reaction with the pollutants.

The translation from the laboratory results to the "on-site efficiency" is still a difficult and critical factor, because of the great number of parameters involved. Hence, there is still a need for large scale applications to demonstrate the effectiveness of photocatalytic materials in "real life" and evaluate the durability of the air purifying action, such as the European Life+ project PhotoPAQ and the industrial zones "Den Hoek 3" in Wijnegem en "Duwijckpark" in Lier. These recent applications in Belgium show already some interesting results.

It seems the use of photocatalytic cement-based coatings inside road tunnels is not mature for application on a large scale yet. From the experience gained during the Leopold II tunnel campaigns in Brussels, recommendations for the proper use of these innovative materials can be made though, such as:



For the double layered photocatalytic concrete pavements using active cement, an efficiency comparable to the one measured in the laboratory is obtained initially; though it seems to decrease somewhat in time due to dirt build-up and other deposits on the surface, the air purifying action has stabilized after more than two years (2011–2013). Application of a curing compound—to protect the fresh concrete against desiccation—initially strongly reduces the photocatalytic activity and also has an impact on the long term. Use of a plastic sheet to protect the young concrete is therefore recommended. Furthermore, the exposed aggregates technique is not ideal for the photocatalytic efficiency since in this case a lot of aggregates are present at the surface and the TiO2 is only present in the paste. The application of a brushed surface finish could lead to a better result.

Use of a photocatalytic coating (TiO2 dispersion) on the surface of the concrete pavement does not produce an added value for the air purifying action compared to mixing in the mass, despite the good results in the laboratory. This probably has to do with the loss of adhesion in time and the filthiness of the surface at the time of application. Possibly, the coating is partially washed away with the dirt. In addition, better results are obtained on the finer, double layered concrete (0/6.3) than for the coarser, single layered concrete (0/20) which could be due to the better adhesion of the coating on the surface.

Durability of the photocatalytic action in time (for products mixed in the mass and/or applied on the surface) and optimization of the adhesion of photo-active coatings on the concrete surface, are topics that need to be investigated further.

Finally, the best results will be achieved by modeling the environment, validating the models by measurements on site, followed by an implementation of the different influencing parameters to assess the real life effect. One must bear in mind that photocatalytic applications are not effective everywhere; "good" contact between the airborne pollutants and the active surface is crucial and factors such as wind speed and direction, street configuration and pollution sources all play a very important role.
