Photocatalysis applied to concrete products

1  Influence of pigments on the degradation process
Enhanced aesthetic and architectural requirements in inner-city areas on the paving of public places demand the application of colored concrete paving blocks. The application of pigments helps to integrate a concrete paving within an already existing architectural situation. This meets the public perceptions regarding the aesthetic quality of public places but also causes problems when photocatalytic materials are applied. Problems mainly result from the increased water demand of the mixtures when fine pigment particles are added.

For a demonstration project in the Dutch city of Hengelo (ca. 1000 m2), the influence of two different red pigments on the degradation of photocatalytic paving blocks was investigated. The main results of this investigation are shown in Figure 1. The direct comparison shows that the degradation rate of the mix containing 3  % TiO2 (no pigment, 2  % P1, and 2  % P2) is not influenced by the added pigments. The values of the degradation rates vary in the same order of magnitude as expected for the scattering of the measuring data. Also, higher contents of TiO2 (5  % and 10  %) associated with higher pigment contents (5  % P1) do not verifiably reduce the degradation rates. Solely the workability of the fresh concrete mix was reduced significantly as the content of fine particles (TiO2 and red pigment) was increased.

2    Prevention of algae growth
The staining of concrete due to the growth of green algae poses a problem when concrete is exposed to shady and permanently humid conditions. This biological staining is typical for roof tiles and façades facing north or in the direct neighborhood of the outskirts of a forest. Paved areas without direct sunlight are also affected by this phenomenon. For roof tiles and façades, the consequences are reduced aesthetics but in the case of concrete paving blocks, safety issues are also affected as the paving blocks become slippery when they are wet.

Besides the oxidative destruction of a wide range of organic and inorganic compounds, biological species (bacteria, algae and molds) can also be decomposed by means of UV-A in the presence of TiO2. The decomposition of green algae (Cladophora) was successfully shown on TiO2 coated glass beads [1]. Therefore, the decomposing mechanism of TiO2 in its anatase modification is also expected for concrete paving blocks.

To carry out the test on inhibiting the growth of green algae, a concrete paving block containing TiO2 in its functional top-layer and a blank sample were placed at a location with humid conditions and low direct sunlight where the staining of an already existing paving by algae is a typical problem (Fig. 2). After a short period of time, the reference sample (a) of Figure 2 was covered with algae on its lateral side as well as on its top side. This process also occurred on the lateral side of the concrete paving block containing TiO2, but here only up to the level of the core mix which did not contain photocatalytic active TiO2. The remaining lateral side of the functional top-layer as well as the top side of the paving block is not covered by any algae as here the photocatalytic material prevents fouling by algae. This exposition of concrete paving blocks at a location, which is not suitable for the degradation of NO caused by high humidity and low natural light, showed another potential of photocatalytic products regarding the prevention of undesirable staining due to algae growth.

3    Durability
The deterioration of the degradation ability of the photocatalytic paving block describes both, (i) the abatement of the paving block’s performance immediately after exposure and (ii) the recovery of the NOx removal performance after every cleaning cycle.

The first mentioned abatement (i) of the degradation per­formance is hardly referred to in literature [2] and was also not quantified during in-house experiments. Explanations for this phenomenon could not be found in the relevant literature.Therefore, it is not in the scope of the present paper. All results presented in this report refer to the state after this initial loss
of performance. Therefore, no further decrease of performance is expected. The second and for practical application more relevant deterioration process is the recovery of the NOx removal performance. The basic principle of this process is shown in Figure 3. NOx removal performance will decrease during the operation caused by the agglomeration of the reaction products on the active surface. Reaction products and intermediates produced during the photocatalytic oxidation process will occupy the active sites of the catalyst surface and result in the deactivation of the catalyst. The deactivation of the catalyst can be reversed by removing the reaction products from the surface. Therefore, no special treatment is required. A washing process by means of rain has been proven to be efficient enough. After removing the reaction products, the performance of the catalyst is fully recovered and the NOx removal performance will again reach the original value (Fig. 3).

Durability should be understood as the maintenance of all properties of the entire expected lifecycle of a system including the impact of the corresponding environmental influences.  From the perspective of durability, it seems to be of interest to assess how long the PCO can be maintained at the surface,
i.  e. how long the active catalyst is available at the surface.

As mentioned in [3] there are two different fundamental procedures for the application of powder-like photocatalysts on minerally-bound material surfaces. These are the bulk mixing of the powder with other mixture constituents, which results in a distribution of the catalyst in the product volume, and secondly, the application of coatings on the surface. Regarding the first procedure, no durability problems are expected, as any abrasion caused loss of catalyst at the surface is accompanied by the emergence of new catalyst from deeper layers. Therefore, the coating systems are in the focus of the presented research.

In order to demonstrate the stability of the coatings, the samples introduced in chapter 3 of [3] were selected and subjected to a defined physical treatment. These samples are coated with a water-catalyst suspension (5  % TiO2 concentration) not containing any binder. However, the application technique of the suspension was varied. One group of samples was coated in already hardened state (hardened substrate, 28 days) whereas for the other group of samples the catalyst suspension was sprayed on the fresh mortar surface (fresh-in-fresh). This way a better fixation of the catalyst on the surface was demonstrated [3] but also lower degradation rates were measured compared to the hardened substrate samples.

For an analysis of the environmental influences on the degradation efficiency, the above-mentioned samples were subjected to an accelerated aging procedure. This treatment incorporated ten cycles of which one cycle was characterized by a water saturation step, a freezing period and a subsequent washing. ­After an initial saturation of the samples with water they were exposed to a temperature of – 45 °C for 16  h. Subsequently, the samples were immersed into water of room temperature for quick thawing and further saturation. This step takes 32  h. Then the samples were flushed with a hard water jet for 5 min. This marks the end of the first cycle and with the exposure of the samples to frost another cycle is started.

In the following, the experimental results of the samples treated with the above-described procedure are explained. The applied measurement procedure is again identical to the one explained in [4]. As we can see from Figure 4, the degradation rates of the hardened substrate samples decreases with an average of 78  % whereas the fresh substrate samples do not experience a measurable difference before and after the accelerated aging treatment. After the aging process the degradation rates of both sample types appear to be in the same range. This is in good agreement with the microscopic findings presented in [3]. Here, it was found that the coating on the hardened substrate results in a thick catalyst layer on the surface, which hardly penetrates deeper layers. A clear interface between mortar and coating was found. This surface seems to be removed by the accelerated treatment. Analyzing the fresh substrate samples, it was observed that a large part of the suspension was absorbed by the outer surface layers during hydration. Therefore, despite equal TiO2 concentrations, the efficiency of these samples was not as high as for the hardened substrate samples. However, due to the better fixation of TiO2 particles on the surface, these types of coating applications appear not to be vulnerable for the experienced accelerated aging procedure. It can be concluded that the spraying technique with binder-free systems on fresh concrete or mortar surfaces is a cheap and durable way of applying photo­catalytic degradation ability. The relatively low degradation rates of the fresh substrate samples can be increased by increasing the catalyst concentration of the suspension used. Some tentative results on degradation rates of fresh substrate samples presented in [3] (Fig. 11) indicate an almost linear relationship between catalyst concentration and the obtained degradation rate. Even a considerable increase of the catalyst dosage still would result in clearly more efficient systems compared to samples containing the catalyst mixed in bulk.

The following facts illustrate this point more clearly. It is assumed that a representative photocatalytic paving stone, with a top layer thickness of 8 mm contains about 4.6 g TiO2 per stone. Such a stone was measured with a degradation rate of 12  % under standard conditions (1 ppmv pollutant concentration, volumetric flow of 3 l/min and 50  % RH). A comparison of degradation rate [%] per mass of TiO2 used per stone
results in a factor of about 3. In the case of the coated fresh substrate samples, the amount of TiO2 per stone amounts to 0.10–0.15 g. These samples achieved average degradation rates of 15  % so that their efficiency ratio is about 160, i.  e. more than 50 times higher compared to the paving stone. This comparison is not exactly fair as most of the TiO2 of the bulk mixed sample is inactive in deeper layers but note that also for the fresh substrate samples a large quantity of TiO2 suspension was absorbed by the mortar during hydration. This fact is reflected in the notably decreased performance of fresh substrate samples compared to hardened substrate samples sprayed with suspensions of equal concentration [3].

4    Modeling of the degradation process
This section is dedicated to the modeling of the reaction process taking place in the reactor. In order to perform modeling work, one active paving stone from [4] was taken and subjected to a number of measurements under varying pollutant concentrations and volumetric flow rates (Table 1), while maintaining the light intensity of 10 W/m2 and the relative humidity of 50  %. The PCO in this case is a two-step process with first the diffusion of NO to the concrete surface and the subsequent conversion to NO2 and NO3–. So the process consists of two transfer steps, the mass transfer from gas to wall and the conversion at the reactive surface.

In [5] it is demonstrated that not the diffusion but the conversion is the rate-limiting step. For the prevailing photocatalytic gas-solid surface reaction, only adsorbed NO can be oxidized. In the past therefore the Langmuir-Hinshelwood rate model has been widely used, e.  g. by [6, 7] as well as [8], and will also be applied again here. According to this model, the disappearance rate of reactant reads:

rNO =    kKdCg(1)
          l + KdCg
with k as the reaction rate constant (mg/m3s), Kd as the adsorption equilibrium constant (m3/mg) and Cg the NO concentration (in mg NO per m3 gas) in the inlet gas flow. The NO balance equation now reads:

vair  dCg = – rNO = –    kKdCg(2)
       dx                     l + KdCg
Supposing that Cg = Cg,in and considering the reactor geometry, integration of Eq. (2) yields:

                lnCg,in
                    Cg,out
1 +   1                      =            L           =       Vreactor(3)
k    kKd (Cg,in – Cg,out)     vair (Cg,in – Cg,out)    Q (Cg,in – Cg,out)

as Vreactor = LBh and Q = vairBh, again, Cg,out = Cg(x = L). 

In Table 1 this Cg,out of the experiments with the paving stones is summarized. The inlet concentration Cg,in had values of
0.1, 0.3, 0.5 and 1 ppmv NO (equivalent to 0.135, 0.404, 0.674 and 1.347 mg/m3), and the flow rate Q was 1, 3 and 5 l/min.
In Figure 5, y = Vreactor/Q(Cg,in–Cg,out) is set out versus x = ln(Cg,in/Cg,out)/(Cg,in–Cg,out), and the data fit with the line
y = (1.19 s)x+2.37 m3s/mg. The intersection with the ordinate corresponds to 1/k, so that k = 0.42 mg/m3s, and the slope to 1/kKd, so that Kd = 2.00 m3/mg. With the obtained values of k and Kd the conversion rate and diffusion rate can be compared.

The air-purification results obtained with the photocatalysis reactor show that a successful decomposition of NOx along photocatalytic concrete paving stone surfaces is feasible by using PCO in the presence of UV light. For this research both experimental and modeling work has been conducted. With the derivation of a reaction model, based on Langmuir-
Hinshelwood kinetics, now the treatment performance of certain air-purifying concrete products for decomposition of gaseous pollutants (NO) can be predicted. Furthermore, now the unique characterization of photocatalytic concrete products is possible by the derivation of its conversion and adsorption rate constants. The derived model also confirms that in the considered reactor the conversion of NOx is the rate-determining step in the photocatalytic oxidation of NOx. Furthermore, for the first time a mathematical expression is proposed, describing the performance of photocatalytic active concrete products.

5    Conclusion
A negative influence of two different red pigments on the degradation rate of the photocatalytic paving blocks tested cannot be shown by the present results of the tests carried out on in-house mixtures. It seems that there is no significant influence of red pigments on the obtainable degradation rates but there is a remarkable influence of pigments on the workability of the fresh concrete mixture caused by the high specific surface of these fine particles. Another characteristic of great interest for practical application is the inhibition of algae growth. This is of special practical use for surfaces with high risk of algae growth as both the safety and the appearance of these products will be maintained.

Furthermore, the durability of binder-free coatings is analyzed. The results of these tests show that the coating sprayed on fresh mortar substrate (fresh-in-fresh) showed the highest resistance during the accelerated aging test (simulation of winter climate) and no decline of the degradation rates was detectable. However, samples coated with TiO2 on the hardened substrate showed a remarkable decline of the degradation rates of approx. 80  %. To conclude, the application of binder-free aqueous suspensions on fresh mortar or concrete surfaces is an appropriate method for coating surfaces in a durable and cost-efficient way. The fresh-in-fresh coating results not only in a financial advantage (less TiO2 is needed) but also a more efficient and direct use of the TiO2 at the surface is possible. The simple application of this method can be realized with little device-related efforts.

Based on the numerous measurements of this research, the ­degradation process can be formulated by an ­appropriate ­model. Therefore, the Langmuir-Hinshelwood model for hetero­geneous catalysis turns out to be suitable. This model contains relevant parameters of the degradation process such as pollutant concentration and volumetric flow and is currently extended for the influence of irradiance and relative humidity. The measurements are in good agreement with the predictions of the model.

Acknowledgment
The authors wish to express their sincere thanks to the European Commission (I-Stone Project, Proposal No. 515762-2) and the following sponsors of the research group: Bouwdienst Rijkswaterstaat, Rokramix, Betoncentrale Twenthe, Graniet-Import Benelux, Kijlstra Beton, Struyk Verwo Groep, Hülskens, Insulinde, Dusseldorp Groep, Eerland Recycling, Enci, Provincie Overijssel, Rijkswaterstaat Directie Zeeland, A&G Maasvlakte, BTE, Alvon Bouwsystemen, and v. d. Bosch Beton (chronological order of joining).