*5.2. INTERREG Project ECO2PROFIT*

The broad environmental sustainability project ECO2PROFIT dealt with the reduction of the emission of greenhouse gases and sustainable production of energy on industrial estates in the frontier area between Flanders and Holland. To reach these goals, several tangible demonstration projects were carried out on industrial sites in Belgium and the Netherlands. BRRC was involved in two such projects: "Den Hoek 3" in Wijnegem and "Duwijckpark" in Lier (both near Antwerp). Here, the regional development agency POM Antwerp was aiming to use a double layered concrete for the road construction, with recycled concrete aggregates in the bottom layer and photocatalytic materials (TiO2) in the top layer, using photoactive cements and/or coatings. That way, air purifying and CO2 reducing concrete roads could be built which are both innovating and energy efficient.

For these recently completed applications (2010–2011) BRRC was asked to set-up an elaborate testing program in the lab to help optimize the air purifying performance of the top layer, without interfering with other properties of the concrete (workability, strength, durability *etc.*). In the construction site of Wijnegem (Den Hoek 3), it was opted to use an exposed aggregates surface finish (with grain size between 0 and 6.3 mm) on the top layer for reasons of noise reduction and comfort of the road user. For the site in Lier (Duwijckpark) a brushed surface finishing was chosen to have more active cement at the surface. Indeed, the type of surface finishing and/or treatment of the pavement can have an effect on the photocatalytic efficiency, as shown in Figure 9 for three types of surface finishing: exposed aggregates, smooth (formwork side) and sawn surface. The results show that the exposed aggregates surface performs equally well as the smooth, formwork surface, but not as good as a sawn surface. This is the result of the combined action of less photoactive cement at the surface and a higher surface porosity (higher specific surface), two competing effects which in the end yield the final efficiency shown in Figure 9.

**Figure 9.** Effect of surface treatment on photocatalytic efficiency (only one type of "less" active product in mass).

For the application of photocatalytic materials in a concrete road (and in general for any other type of application) a fundamental choice can be made between: mixing in the mass (e.g., TiO2 in cement) and/or spraying on the surface (suspension of TiO2). The former has the advantage of a more durable action since the TiO2 will continuously be present, even after wearing of the top layer. On the other hand, the initial cost will be higher (higher TiO2 content, necessity for double layered concrete) and only the TiO2 at the surface will be active. In contrast, dispersing at the surface of a TiO2 solution will provide a more direct action, and a lower initial cost (e.g., "ordinary" cement). In this case however, the longevity of the photocatalytic action could be questioned because of loss of adhesion to the

surface in time. This fundamental choice was also investigated within the research program, together with the influence of several other parameters [28].

The effect of a curing compound for instance—generally applied to protect the young concrete against desiccation in Belgium and placed directly after concreting or after exposing the aggregates at the surface in case of denudation—is illustrated in Figure 10. From this, it appears the curing compound will initially inhibit the photocatalytic reaction, most likely because it is shielding off the "active" components from the pollutants in the air. Consequently, it is probable that the curing must disappear from the surface, *i.e*., under influence of traffic or weathering, before the TiO2 will reach its optimal air purifying performance. In case of a photocatalytic spray coating, this also means that it is best to apply the TiO2 dispersion some months after the curing compound to have the most durable effect. Alternatively, the exposed aggregates concrete can be covered with a plastic sheet to prevent dehydration (in case the concrete surface is denuded).

**Figure 10.** (**a**) Application of curing compound on fresh concrete, and (**b**) Effect of curing compound on photocatalytic efficiency (different samples A–D, with photocatalytic TiO2 in mass and/or applied as dispersion, with and without curing compound).

More detailed results of the laboratory research can be found in [28] and [29]. Based on the findings and the optimization of the concrete composition, a proper selection of photocatalytic materials and application techniques could be made, for the construction of double layered, photocatalytic concrete roads on the industrial zone "Den Hoek 3" in Wijnegem.

Double Layered Concrete at "Den Hoek 3" in Wijnegem

The concrete pavement of the industrial zone in Wijnegem has been constructed between the 15th and 18th of March 2011. The concrete was placed in two layers, wet-in-wet, with an interval time of approximately 1 hour. The bottom layer had a thickness of 180 mm, while the top layer was designed to be 50 mm. For the concrete of the bottom layer, 57% of the coarse aggregates were replaced by recycled concrete aggregates. For the top layer with TiO2, commercially available white cement with 4% TiO2 pre-mixed (by weight) was applied (CBR, Belgium, Heidelberg Cement Group). Two slip form pavers were used to place the concrete. As can be seen in Figure 11a, the color of the top layer is much lighter, due to the use of white cement and the presence of the TiO2 (about 0.8 wt% of the top layer).

More information on the concrete composition, the execution and the results obtained in the lab as well as on site can be found in [28] and [29]. Besides the photocatalytic concrete roads, photocatalytic pavement blocks were also used for the bicycle lanes, parking spaces and foot paths.

Since this was a completely new industrial zone, it was not possible to have measurements on site before putting the photocatalytic concrete in place. An overview of the project is given in Figure 12. Immediately after concreting, a retarding agent was sprayed on the surface to be able to wash out the top surface after 24 h, to create an exposed aggregates surface finish (see Figure 11b). In order to prevent dehydration of the concrete during the first days, some parts of the road have been treated with curing compound; the other zones were covered with a plastic sheet. This way, the influence of the curing compound on the short and long term photocatalytic efficiency could be investigated.

**Figure 11.** (**a**) Construction of double layered concrete pavement at industrial zone "Den Hoek 3" in Wijnegem; (**b**) Detail of exposed aggregates surface finish of the top layer.

**Figure 12.** (**a**) Situation plan of the new industrial zone "Den Hoek 3" in Wijnegem, Belgium (Google Maps); (**b**) "On site" testing of photocatalytic efficiency.

The photocatalytic efficiency of the top layer was measured in two ways: in the laboratory on cores taken from the surface at the places indicated in Figure 12a, and "on site" with a special measuring set-up, shown in Figure 12b. This "on site" test is developed to evaluate the photocatalytic properties of the concrete pavement over time and to compare the different sites (with and without curing, for example). It does not measure the overall purification of the air around the pavement but enables to measure the durability of the photocatalytic efficiency.

The set-up consists of a Plexiglas frame, screwed air-tight on the test surface (concrete pavement), and is covered with a UV-transparent glass lid. The input NO-concentration (1 ppmv), relative humidity (50% RH) and air flow (3 L/min) are taken similar to the laboratory set-up. However, the total area covered by the box is somewhat larger (700 × 300 mm²) to have a representative surface, and natural (varying!) sunlight is used in first instance to activate the surface. First results of the measurements on site are given in Figures 13 and 14, and were collected 5 months after the placement of the concrete (August 2011) at the places indicated in Figure 12a (points 1 and 2).

**Figure 14.** NO*x* concentration measured at the outlet for zone without curing compound, 5 months after concreting (point 1, August 2011).

First of all, the results shown in Figures 13 and 14 indicate a large influence of the relative humidity (red curves). The NO*x* abatement is lower when the relative humidity increases and higher when RH decreases again. The influence of the sun light intensity (measured through the UV intensity, light blue lines) is also visible, but on a different scale: variations over a shorter period of time do not influence the NO*<sup>x</sup>* concentration immediately; it is the average sun light intensity over a longer period that is determining the attained NO*x* abatement for the photocatalytic process.

Furthermore, the reduction in NO concentration is significantly lower for the zone with curing compound, indicating it is still (slightly) inhibiting the reaction: average reduction of 27% (with curing) *versus* 48% (without curing). Nevertheless, the effect of applying a curing compound on the fresh concrete (to protect against dehydration) seems to diminish over time. These results obtained on site (year 2011) are also in line with the results obtained in the laboratory, taking into account the difference in surface, relative humidity and light intensity [28].

In order to correctly compare the results between the lab and the field, the photocatalytic activity for NO*x* (= sum of NO and NO2) is expressed in terms of the photocatalytic deposition velocity in [m/h] under the assumption of a first order uptake kinetics and negligible transport limitations from the gas phase to the solid surface [30]:

$$k\_{\mathbb{R}} = \ln \left( \bigvee\_{\mathbb{C}\_l} \bigvee\_{\mathbb{C}\_l} \right) F\_A \tag{4}$$

where *c*0 and *ct* are the reactant concentration at the inlet and exit of the photo-reactor, respectively. In fact, this parameter refers to a first order reaction rate coefficient independent of the applied flow rate *F* and the active surface (*A*) to volume ratio of the used reactor (lab or on site). In the lab work [28], average values for the NO*<sup>x</sup>* deposition velocity *kR*,*NOx* of 0.25 and 0.70 m/h were obtained with and without curing compound respectively, which is in nice agreement with the results on site for 2011 (see further in Table 1).

The measurements on site are also repeated over time in order to see the influence of ageing and traffic on the photocatalytic efficiency. Recent measurements performed in the summer of 2012 for instance, are shown in Figure 15. Here, measurements were performed using an external UV-lamp (10 W/m²) as well as natural sunlight to activate the photocatalyst present in the pavement. It appears the activity under sun light is somewhat higher compared to the UV lamp only. This could be due to the fact that the applied TiO2 (in the active cement) is also partially active under visible light and/or is excited by the shorter UV wavelengths (UV-B, UV-C) present in the sun spectrum.

**Figure 15.** NO*x* concentration measured at the outlet for zone without curing compound, 17 months after concreting (point 3, August 2012).

On the other hand, the measurement of the UV-intensity comprised in the sun light could be erroneous because of the radiometer used here. This is only calibrated for specific UV-A lamps (between 300 and 400 nm) applied in the geometry of the lab set-up which differs substantially from these exterior tests. The activity observed under natural, varying sun light though, is still very interesting from the view point of practical application. The use of the external UV-lamp with constant light intensity in turn, allows making a more absolute comparison of the photocatalytic activity between different zones and for different times.

In any case, the results of Figure 15 reveal already that the efficiency of this kind of photocatalytic application (TiO2 integrated in the cement) appears to decrease in time: on average 34% NO-reduction (after 17 months) *versus* 48% (after 5 months). Possible causes could reside in the covering of the TiO2 at the surface by dirt, the detachment of the TiO2 from the surface or the deposition of products from chemical reactions which can take place at the surface.

In this respect, in October 2012 an aqueous TiO2 dispersion (Eoxolit® consisting of a mixture of two different types of TiO2 particles with a total concentration of 40 g/L TiO2) was also applied on the surface in some parts of the roads on the industrial zone in Wijnegem, as shown in Figure 16a, for the purpose of comparative measurements. In total four different zones were considered:


The photocatalytic dispersion was applied with a dose of approximately 1 L per 5 m² on a total of 800 m², followed by spraying of a hydrophobic product for optimal functioning of the coating (manufacturers' guidelines). Important to mention however, is the fact that at the time of application there was a severe pollution with soil and dirt at the surface of the pavement in some zones due to the presence of a grinding installation plant at the site. This most certainly had an impact on the efficiency of the TiO2 suspension (see further). Subsequently, provisional controls of the photocatalytic efficiency have been carried out to check the separate action of the two types of photoactive materials (mass and dispersion), and to further assess the durability of the air purifying performance. Most recent measurements on the site in Wijnegem were performed in the summer of 2013, at the measurement points (1–9) indicated in Figure 16b. All results obtained up till now (2011–2013) are summarized in Table 1.

**Figure 16.** (**a**) Application of photocatalytic dispersion on part of the roads at industrial zone "Den Hoek 3" (October 2012); (**b**) Localization of measurement points for "on site" testing (Google Maps).


**Table 1.** Summary of results in time for photocatalytic activity measured on site in Wijnegem.

First of all, when comparing the measurements on the surface of the pavement at points 1 and 3 (*cf.* Figure 16b) in 2013 with these of 2012, a very similar result can be noticed: a photocatalytic deposition velocity for NO*x* of *ca.* 0.2 m/h under UV light. This indicates that the decreasing trend in photocatalytic activity for the concrete with "active" cement (see evolution 2011–2012) seems to be stabilized in 2013.

Furthermore, the measured efficiency for points 1 and 3 (in 2013) appears to differ little or nothing with the one for points 6 and 9, with application of the photocatalytic coating (TiO2 dispersion) on the pavement surface. Here, the TiO2 dispersion did not produce a significant added value (yet) in terms of photocatalytic air purifying action. Only for point 4 (active cement with curing compound, after application of TiO2 dispersion) one can notice a strong improvement of the photocatalytic efficiency (deposition velocity of *ca.* 0.8 m/h for NO under sun light and nearly 0.3 m/h under UV ). Possibly, the pollution of the surface at the time of application has played an important part causing the adhesion of the coating to be far from optimal.

For points 7 and 8 (concrete without active cement, but with TiO2 dispersion on the surface), the activity is not significantly better either (or even less) compared to the "pure" concrete with active cement. In addition, point 8 (single layered concrete 0/20) reveals much smaller photocatalytic reactivity than point 7 (double layered concrete with top layer 0/6.3): deposition velocity of 0.08 m/h *versus* 0.15 m/h for NO reduction under UV. This probably has to do with the stronger adhesion of the coating on the surface of the finer (0/6.3) double layered concrete compared to the coarser (0/20) single layered concrete.

Finally, a measurement on site was also performed for the newly constructed pavements at the industrial zone in Lier, which have a different surface finishing as illustrated in Figure 17a. The results of this measurement, 20 months after construction, are shown in Figure 17b.

**Figure 17.** (**a**) Double layered photocatalytic concrete pavement with brushed surface finish at industrial zone "Duwijckpark" in Lier; (**b**) NO*x* concentration measured at the outlet for the site in Lier (active cement + curing compound), 20 months after concreting (August 2013).

In comparison with the measurements of Wijnegem in 2013 (see Table 1), a slightly lower photocatalytic reaction is observed in Lier, which among others is due to the use of a curing compound (for the brushed surface) and the lower TiO2 content (less cement used). However, if we make the comparison with the zone with curing compound in Wijnegem (*cf.* point 4 in zone 4) measured in 2012 (17 months after construction), a significantly better result under UV light is obtained in Lier: deposition velocity for NO of 0.14 m/h in Lier *versus* 0.06 m/h in Wijnegem. This higher activity probably has to do with the brushed surface finish instead of exposing the aggregates (*cf.* Figure 9). In any case, these measurements confirm the photocatalytic action 20 months after construction of the concrete pavement.
