Keywords

2.1 Introduction

According to the World Health Organization (WHO), air pollution possesses the sixth place among the leading causes of death globally [1]. Traffic, industry, and agriculture are the main responsible sources that are emitting air pollutants, including nitrogen oxides (NO×), sulfur dioxide (SO2), carbon monoxide (CO), and particulate matters (PMs). NO× and PMs are to a certain extent related to nearly 450,000 premature deaths in Europe [2]. Particularly, in the urban atmosphere, NO× count as one of the most common gaseous pollutants, which are very harmful to human health. Adverse environmental effects (like acid rain and ozone depletion) can be caused toward the NO× Earth’s atmosphere release [3].

However, the reduction of atmosphere pollutants cannot only be mitigated by active reduction of the emissions but also needs to be mitigated by novel technologies and materials. During the last two decades, novel multifunctional building materials (belonging to the so-called passive reduction techniques) have been designed by embedding photocatalysts, in a large number of surfaces, employed to reduce (oxidize) the pollutants via photocatalytic reaction (PCR) [4, 5]. Photocatalysts, for example, TiO2, are using UV light to convert adsorbed water and oxygen into highly reactive radicals (OH, O2−) which oxidize adsorbed pollutants. For example, NO× is oxidized to NO3−, which stays on the surface until rainwater may successfully clean it [6]. Another effect of PCR is the hydrophilic behavior of water on irradiated surfaces. In combination with the oxidation process of the PCR, organic and inorganic substances are prevented from sticking to the surface, which leads to their self-cleaning effect. Self-cleaning of these materials will also prevent blocking of active sites of the catalyst by pollutants (i.e., “urban grime”), prolonging its photocatalytic activity for many years [7, 8]. These two fundamental surface photochemical properties of photocatalysts have been extensively studied in a number of publications [9,10,11].

The application of TiO2 photocatalysts to the construction materials started toward the end of the 1980s [8]. To improve the practical applications of TiO2 photocatalysts, among several supporting materials, the cement-based materials are of great interest due to their strong binding property and porous structures. In recent years, several photocatalytic cementitious materials have been developed and patented by researchers all over the world [12,13,14,15]. The great advantage of developing photocatalytic cementitious materials is that the only requirements, beyond TiO2 in the construction material used, are sunlight, oxygen, and water [16]. Besides, immobilizing the powdered photocatalysts into the cement matrix provides surfaces that can be reached by light irradiation. The irradiation of the surface with solar/UV light with a wavelength <400 nm leads to a photo-induced oxidation of the compounds absorbed on cement surface caused by TiO2 [17].

There are generally two kinds of methods to combine TiO2 with cementitious composites: (i) incorporation into the cement matrix during mixing process and (ii) coating TiO2 on the surface of cement specimens by using spray pyrolysis techniques [18, 19]. However, many researches have shown that these methods still have critical problems for long-term applications like agglomeration of TiO2 particles in the cement matrices, blockage of TiO2 particles by cement hydrates, and adhesion and abrasion between the TiO2 and cement surface, resulting in low photocatalytic efficiencies [8, 20,21,22]. Above all, by mixing the TiO2 into the cement matrices, most of TiO2 will be placed deeply inside the cement hydration products, so that little UV light can pass through the composite to activate the TiO2.

Adjusting the microstructure of cement matrix and introducing pores into the matrix could be an effective method to build up one interconnected structure for TiO2 photocatalysis and to improve the dispersion and exposure degrees of TiO2 particles in cement matrix [23]. In this direction, highly porous TiO2-modified cement-based foams present a micro- to meso-structure, which can be beneficial for photocatalytic efficiency and TiO2 utilization. A continuous uniform pore structure can allow the UV light to go deeper inside into the porous foam to activate TiO2. Therefore, the corresponding foam composite enhances the reactants and light diffusion depth, resulting in better photo-cleaning performance [24,25,26,27].

In this study, the effects of the photocatalytic performance of highly porous TiO2 cementitious foams are evaluated. The study is thus also investigating the interaction between TiO2 powders, the cementitious hosting environment, and their consequences on the conventional properties of the material, such as rheology, thermal and mechanical resistance, or durability. In this sense, the proposed highly porous cementitious composite represents a novel breakthrough insulation system that will replace available insulation materials with multifunctional energy-saving and air-cleaning performances. Current activities at the WiB TU Darmstadt deal with developing a novel translucent PCM-TIO2-glass fiber-reinforced cementitious foam as further extension of the foam to possibly enhance photocatalyst components for better air cleaning in combination with energy saving. It is important to highlight that insulation materials are not directly exposed to external environment. To achieve a multifunctional behavior (insulator and photocatalyst) for the highly porous TiO2 cementitious foams, UV exposure is necessary, and also air flowing should be allowed. For this purpose, a translucent external plaster will be needed in practical applications.

2.2 Materials and Methods

This section deals with the materials and related preparation of the TiO2-foamed concrete samples. Then, methods for testing the NO× reduction efficiency, through photocatalytic activity, and for characterizing thermal and mechanical properties are shown.

2.2.1 Materials

Raw Material

For the cement paste, an ordinary Portland cement (i.e., type 52.5 by the Heidelberg Cement AG, Germany) together with two types of chemical additives, namely, Melamine resin sulfonate as stabilizer (i.e., Sika ST 3, ρ = 1070 kg/m3) and chloride-free hardening accelerator (i.e., DARASET 304, ρ = 3000 kg/m3), were used. In addition, a protein-based foaming agent (i.e., Pantapor FA by Ha-Be Betonchemie GmbH & Co. KG, Germany) was chosen in order to reach higher stability with more favorable thermal and mechanical characteristics [28, 29]. TiO2 photocatalyst (ρ = 3900 kg/m3) KRONO Clean-7050 was provided by the KRONOS International, Inc.

2.2.2 Mixture Design and Sample Preparation

Foamed concrete samples were prepared with three different porosities, 54%, 77%, and 88% (meaning the ratio between the aqueous foam volume to 1 m3), and three different amounts of TiO2 powder of 0 V.-%, 5.0 V.-%, and 10.0 V.-% in substitution of the used Portland cement, as shown in Table 2.1. Aqueous foam with the density of 60 kg/m3 was prepared by using a pre-foaming method. A stabilizer was used to prevent aggregate sedimentation in the fresh state of the cement paste as well as surface bleeding. The hardening accelerator causes an acceleration of the reactions of the cement and thus of the hardening process of the fresh paste. This results in the acceleration of the foam structure formation and the improvement of the pore size stability.

Table 2.1 Overview of mixture proportions in 1 m3 cementitious foam with three different TiO2 contents and three different porosities
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Production of the Porous TiO2-Cementitious Foam

The preparation and mixing of the TiO2 cementitious foam can generally be divided into three steps: (i) preparation of the TiO2 cement paste, (ii) production of the aqueous foam, and (iii) mixing the generated foam thoroughly with the TiO2 cement paste mixture. Figure 2.1 shows the basic principle of foam production.

Fig. 2.1
A diagram. Foaming agent goes into the foam generator with air and water. From the generator it goes to the mixer. The colloidal mixer cement paste, which is made from cement and stabilizer, T I O 2 and hardening accelerator, connects to the mixer.

Production of TiO2 mineral foam – mixing scheme

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The TiO2 cementitious foam was prepared at 20 °C and following the proportions listed in Table 2.1. In order to prepare the foam with homogeneous pores, water and the half amount of Portland cement were firstly poured into a colloidal mixer. After stirring them for 60 s at a speed of approximately 1300 U/min, the other half of the cement was poured into the colloidal mixer followed by another 60 s stirring. Next, the stabilizer was added and mixed at a low speed. After 30 s, TiO2 particles were slowly dispersed into the fresh paste for 30 s. Finally, the hardening accelerators were poured into the mixture for 30 s as well. Lastly, the speed was increased to about 5000 U/min for another 30 s until complete homogenization was achieved.

The aqueous foam was produced using a foam generator (Fig. 2.2) to the aimed 60 kg/m3 density. The generator is equipped with a proportional mixer to add a fixed amount of foaming agent (i.e., 2%) to the water. Using compressed air flowing into the fluid, the kinetic energy is converted into surface energy. The generated cementitious foam was then thoroughly mixed with the paste mixture and poured into a mold (600 mm3) and left for one/two days in an air-conditioned room (20 °C). Then, samples were fully covered with polyethylene film and cured at 20 °C until 28 days. After that, all specimens were oven dried at 45 °C until constant weight was achieved.

Fig. 2.2
A photo of the foam generator with the following parts labeled. The Air inlet, water inlet, foaming agent, aqueous foam and foam generator.

Experimental setup for the foam generator at the TU-Darmstadt

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For each mixture, two samples (9.9 cm × 4.9 cm × 2.0 cm) were prepared for the photocatalytic tests. The experimental program was then completed for evaluating physical (e.g., specific weights and dry densities), thermal, and mechanical properties of the considered cementitious foams. These latter have been omitted in this paper for the sake of brevity.

2.2.3 Methods

Photocatalytic Tests

Experiments on the considered TiO2 cementitious foams were carried out in collaboration with the Atmospheric and Environmental Research (AER) Institute of the University of Wuppertal. For the evaluation of air purification efficiency, titanium dioxide powder (TiO2, Kronos) added with 5% and 10% in substitution of the cement powder was considered. The reactivity of NO× (NO+NO2) degradation and the side products, nitrous acid (HONO) and formaldehyde (HCHO), were investigated by measuring the photocatalytic deposition rate.

Prior to the experiments, samples were rinsed with high purity water, dried, and activated for several days with a fluorescent lamp at 33 W m−2 UVA light. This pretreatment serves to oxidize possible organic impurities by the self-cleaning effect of photocatalysis to achieve higher activity and better reproducibility of the results.

The photocatalytic activity of the samples was evaluated close to the reaction conditions specified in the ISO 22197-1 (2007), that is, 10 W m−2 UVA, 40% relative humidity, and 20 °C in a temperature-controlled Photo Flux reactor; see Fig. 2.3. NO and NO2 were used as reactants.

Fig. 2.3
3 photos. A photo of open-pore cementitious. A rectangular Flux reactor for the determination of photocatalytic activity. Adjustable diode array with intensity control, which is inside a box.

Left: Open-pore cementitious foam specimens with 5% and 10% TiO2 content (77% porosity). Middle: Flux reactor for the determination of photocatalytic activity. Right: Adjustable diode array with intensity control (QUMA-Elektonik & Analytik, Wuppertal) as UVA light source (λmax = 370 nm; Δλ ± 20 nm)

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A chemiluminescence meter (Ansyco AC 31M with molybdenum converter, 2 ppb detection limit) was used to determine the concentration of nitrogen oxide (NO× = NO + NO2). The reactant concentration (NO+NO2) was adjusted from the ISO standard of 1 ppm to approximately 50 ppb and 100 ppb in respect of the typical environmental concentration. Inside the reactor, the distance between the specimen and the glass is about 1–2 mm (contrary to the suggested ISO value of 5 mm) to guarantee a turbulent airflow. The chemiluminescence meter was previously calibrated with NO gas for each test.

NO2 concentration was calculated from the difference between the NO2 signal of the chemiluminescence device (NOy-NO) and the measured HONO concentration. For this purpose, a highly sensitive and selective HONO-LOPAP measuring device with a detection limit of 5 ppt was used (see Heland et al. (2001) [30], Kleffmann et al. (2002) [31]). Formaldehyde (HCHO) was determined with a sensitive formaldehyde monitor (AL 4021) from the company AERO LASER (detection limit 50 ppt), which is based on the so-called Hantzsch method. Both the formaldehyde and the LOPAP measuring device were regularly calibrated. Figure 2.4 illustrates a scheme of the experimental setup. Bottles with NO or NO2 gases are diluted together with synthetic air. A flow controller (Brooks thermal mass flow controller) is used to control the desired concentration of the pollutant gas, either NO or NO2 in synthetic air.

Fig. 2.4
A diagram of N O or N O 2 and air connected to mass flow controller, leading to the humidifier. This is connected to the I S O reactor, sample and bypass her. The outcome has reference sensors N O 2, H O N O, and H C H O.

Illustration of the experimental setup

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The mixed gas flows through a humidified coil to the NO×, HONO, and HCHO reference sensors. This process is called a bypass, whereby the gas flow is not yet connected to the reactor. This allows the reference sensors to be calibrated. The gas stream can also be connected to the reactor to analyze the photocatalytic performance.

For the samples, the self-emissions of NO, NO2, HONO, and HCHO were first investigated. For this purpose, after daily calibration of the NO× measuring instrument (zero, span, zero), humidified (40% r.h.) synthetic air was passed over the samples, and their self-emissions were measured first in the dark and then under UVA irradiation (10 W m−2) and finally again in the dark. Then, the reactions with the reactants ((1) NO2 and (2) NO) were carried out at two concentrations each of approximately 50 ppb and 150 ppb, respectively. A measurement includes the following sequence: zero/bypass, NO×/bypass, NO×/reactor dark, and NO×/reactor UV.

Mechanical and Thermal Tests

Mechanical and thermal tests are also performed on the considered foam mixtures. On one hand, the mechanical tests of the samples are tested according to the procedures described in EN 196-1. For each mixture, nine prisms (40 mm × 40 mm × 160 mm) are tested under both three-point bending and compression. The obtained peak stress of compressive strengths ranged between 0.22 and 0.28 MPa. On the other hand, thermal conductivities were measured through the guarded hot plate method [32] for the considered mixture on three temperature levels (i.e., 10, 25, and 40 °C). Subsequently, the thermal conductivity of the dry material at 10 °C is extrapolated. These tests are carried out according to both DIN EN 12667:2001 and ISO 8302:1991-08.

2.3 Results and Discussion

2.3.1 Photocatalytic Activity

The evaluation of photocatalytic activity (νPhoto) was made by deposition velocity:

$$ {\nu}_{\textrm{Photo}}=\frac{\ln \left(\frac{c_{t=0}}{c_t}\right)\cdot {J}_{\textrm{gas}}}{S} $$
(2.1)

being ct=0 and ct the reactant concentration [ppb] at the input and output, respectively, Jgas the total flux [cm3/s], and S the active surface [cm2]. The photocatalytic deposition velocity (νPhoto) in [cm/s] does not contain transport resistances because of the turbulent flow conditions. The deposition velocity refers to the total degradation between input and output values. More details of the different kinetic evaluations to ISO 22197-1 can be found in [33].

Besides the kinetic evaluation, the formation of the side products was investigated. The emission flux of formaldehyde EHCHO [molecules/(cm2 × s)] was calculated with the volume flux (ФØgas, cm3/s, at 298 K, 1 atm), the active surface (S, [cm2]), and the HCHO concentration (ppb):

$$ {E}_{\textrm{HCHO}}=\frac{\left({\left[\textrm{HCHO}\right]}_{\textrm{Photo}}-{\left[\textrm{HCHO}\right]}_{\textrm{Dunkel}}\right)\cdot 2.46\times {10}^{10}{\textrm{cm}}^{-3}{\textrm{ppb}}^{-1}\cdot {\Phi}_{\textrm{gas}}^{\phi }}{S} $$
(2.2)

In the following, the photocatalytic results of the f-77-5% and f-77-10% mixtures have been reported. They represent the optimum combination of acceptable mechanical and thermal performances together with excellent NO× cleaning.

Results on NO×

In the dark, the samples show no activity against NO. Contrarily with NO2, they showed a clear reaction under dark conditions which can be caused by the reaction of NO2 with water to HONO and nitrate, that is, 2 NO2 + H2O → HONO + HNO3. This could also explain the formation of HONO (however, quite low) under the same dark conditions. On the other hand, under UV irradiation, the samples show significant photocatalytic activity with both NO and NO2. The observed NO and NO2 degradation rates of both specimens under UV irradiation were ν (NO) Photo = 0.48 cm/s and ν (NO2) Photo = 0.55 cm/s, respectively, which clearly exceeded the limit for the deposition velocity of ≥0.1 cm/s proposed in the literature (see, for example, Ifang et al. (2014) [33]), at which an improvement in air quality can still be expected. More details on the photocatalytic activity of the samples and the photocatalytic deposition rates can be seen in Fig. 2.5 and Table 2.2.

Fig. 2.5
2 graphs with the concentration time curve that have random move across time in hours and minutes, for N O , N O 2, H O N O, and H C H O.

Concentration-time curve for the reaction of NO2 or NO for the sample (a) with 5% TiO2 and (b) with 10% TiO2. The bars drawn at the top indicate the times for zero (white), NO×- bypass (light gray), reactor dark (dark gray), and reactor + light (yellow). At the beginning of the experiment (blank), the humidity was lower than toward the end

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Table 2.2 Photocatalytic deposition velocities [cm s−1] of the investigated samples against NO, NO2, and HONO. The data refer to relative humidity of 40% and radiation intensity of 10 W m−2
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As shown in Table 2.2, the degradation of NO× of the specimen with 5% TiO2 exceeded the degradation performance of the specimen with 10% TiO2. This could be explained by the better foam structure of the specimen achieved with the 5% amount of TiO2. The hydrophilic behavior of water in contact with TiO2 could be a reason for the damage of the foam structure during casting, which is much more evident in the 10% TiO2 concrete foams.

The reaction of NO to nitrate is proceeding by the intermediate step of NO2: NO → NO2 → HNO3/nitrate. The intermediate NO2 was observed in the experiments with around 10–33% which can be explained by the short residence time in the reactor (~0. 3 s). Due to that, no complete degradation to nitrate can take place.

It is worth mentioning that values of 0.50 cm/s ca. of deposition velocities for NO and NO2 were observed for concrete samples in [34] and values in the range of 0.03–0.60 cm/s were achieved by Boonen et al. (2017) [35] for different cementitious materials on pavement blocks. Contrarily, values of up to 1.06 cm/s of deposition velocities for NO and NO2 can be observed in this experimental campaign, shown to be the high potential of the tested TiO2 porous cementitious composites.

Results on Nitrous Acid (HONO)

Due to the alkaline cement, the formation of HONO during active photodegradation of NO2 is around ~0.3 %, which is significantly below the limit value of 5% required (see Ifang et al. (2014) [33]). Also, the HONO formation of ~0.4 % during dark conditions is significantly below the theoretical expected value (50 %), which can be explained by the very strong adsorption of HONO on the alkaline cement surface.

Results on Formaldehyde (HCHO)

The formation of HCHO emissions from the samples cannot be neglected. This is due to the much more harmful impacts of HCHO than the nitrogen oxides on health. The average formation of HCHO can be considered similar to the deposition flux of NO2; see Table 2.3. This formation can also come from the photocatalytic degradation of organic binders or additives which could be, for example, the foam agent in this case.

Table 2.3 Photocatalytic HCHO emission flux [molecules cm−2 s−1] as well as equivalent concentration of NO2
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To verify if there is a decrease in HCHO formation with time, further long-term investigations on the specimen or the additives themselves should be conducted.

2.4 Conclusions

This chapter presented the preliminary results of an experimental study which investigated the photocatalytic performance of highly porous cementitious foams enhanced with TiO2 powders. Several porous structure ratios (i.e., 54%, 77%, and 88%) together with TiO2 powder fractions (i.e., 0, 5, and 10% in substitution of cement powders) were pretested and analyzed. The results confirmed that the photocatalytic activity measured in terms of NO and NO2 degradation, under UV irradiations, reached excellent performances when compared to TiO2 concrete substrate data, available in scientific literature. Moreover, the generation of other air pollutants (like nitrous acid, HONO, and formaldehyde, HCHO) was quite low which makes this new material a very appealing multifunctional composite for photocatalyst air cleaning and building envelope/energy-saving solutions in the field of the construction and building materials. Further investigations are however required to study the possible side effects generated by the other pollutants (i.e., HONO and HCHO) when this material is employed.