https://doi.org/10.14311/APP.2022.33.0015 Acta Polytechnica CTU Proceedings 33:15–19, 2022 © 2022 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague EFFECT OF TIO2 AND ZNO POWDER MIXTURES ON MECHANICAL AND PHOTOCATALYTIC PERFORMANCE OF HIGH PERFORMANCE CONCRETE Fouad Amora,∗, Lenka Ingrišováa, Zuzana Rácováa, Michal Baudysb, Petr Hájeka a Czech Technical University in Prague, Faculty of Civil Engineering, Department of Building Structures, Thákurova 2077/7, 166 29 Prague 6, Czech Republic b University of Chemistry and Technology Prague, Department of Inorganic Technology, Technická 5, 166 28 Prague 6, Czech Republic ∗ corresponding author: fouad.amor@fsv.cvut.cz Abstract. The development of new modified cement-based materials is increasingly becoming a necessity for improving the durability and surface performance of building materials. Titanium dioxide (TiO2) photocatalyst has been widely used in building materials science due to its ability to break down pollutants. Zinc oxide (ZnO) is often considered a substituent for TiO2 because of its photocatalytic and photoluminescent properties. A new inorganic nanocomposite photocatalyst, based on titanium and zinc oxides, is introduced in this work in order to study its compatibility with High Performance Concrete (HPC). This research aims to study the mechanical and photocatalytic behavior of mixtures based on nanoparticles in HPC. The study of the efficiency in the nitrogen oxides (NOx) degradation of modified HPC in TiO2 and ZnO with different percentages is studied. The studies have shown that the introduction of titanium dioxide in HPC presents a significant efficiency for the NOx degradation and a positive effect on the mechanical properties than zinc oxide, and thus represents potential contribution to sustainability of concrete structures. Keywords: High performance concrete, nanoparticles, photocatalysis, titanium dioxide, zinc oxide. 1. Introduction Atmospheric pollution is an alteration in the quality of the air, which is due to one or more gaseous sub- stances or particle materials present in concentrations and for a period of time sufficient to create a toxic or eco-toxic effect. The presence of these toxic gases in the atmosphere is due to industrial activity and road transport [1]. In this study, we are focused on emis- sions and reduction of NOx gases. The nitrogen ox- ides family, grouped under the formula NOx, mainly includes nitrogen moNOxide (NO) and nitrogen diox- ide (NO2). They can contribute to various environ- mental problems and have health consequences [2]. There are several methods to reduce NOx, in partic- ular photochemical oxidation by the use of photocat- alysts [3]. Among the many nanoparticles studied over the last ten years, titanium oxide (TiO2) and zinc oxide (ZnO) have aroused particular enthusiasm because of their many properties. Their photocatalytic proper- ties have allowed them to be used for self-cleaning, self-disinfection and environmental clean-up applica- tions [4–6]. TiO2 is currently the most efficient cat- alyst in this field thanks to its strong photocatalytic activity under UV irradiation. ZnO has a bandgap similar to that of TiO2, making it increasingly recog- nized as a suitable alternative to TiO2. ZnO is the subject of intensive research because of its remarkable properties which promote the development of photo- induced phenomena [7, 8]. The interest of this study is to mix these two oxides to form a nanocomposite which can be used to improve the photocatalytic ac- tivity of concrete. The photocatalytic activity of the TiO2-ZnO system is increasingly being studied with the aim of finding a more efficient material for photo- catalytic applications [9, 10]. S. Mayén-Hernández et al. [11] studied the photocatalytic activity of mixed oxides of TiO2 and ZnO with different Ti/Zn ratios, they concluded that the Ti/Zn ratio of 0.5 exhibited the best photocatalytic performance. Furthermore, the incorporation of mixed oxides into the cement ma- trix stabilizes the distribution of hydration products (C-S-H) and improves mechanical surface properties [12–14]. The objective of this work aims to evaluate the ef- fect of the inclusion of mixed oxides based on Zn and Ti on the hydration of High Performance Concrete by following the mechanical evolution and the photo- catalytic performances as a function of time. 2. Materials and methods 2.1. Materials The HPC has been developed and optimized in re- cent years at CTU Prague [15] and it is the self- 15 https://doi.org/10.14311/APP.2022.33.0015 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en F. Amor, L. Ingrišová, Z. Rácová et al. Acta Polytechnica CTU Proceedings Oxides SiO2 Al2O3 Fe2O3 CaO MgO SO3 K2O TiO2 P2O5 % 18.7 4.5 3.4 65.9 1.3 4.3 0.8 0.3 0.1 Table 1. Chemical analysis of CEM I 42.5 R (% by weight). Binder Samples Technical Sand % Quartz flour % Silica fume % Super- plasticizer % Water % Cement (C) % TiO2 % ZnO (Z) % R 41 13.92 7.47 1.25 7.31 29.05 0 0 CT1 41 13.92 7.47 1.25 7.31 28.05 1 0 CT2 41 13.92 7.47 1.25 7.31 27.05 2 0 CT3 41 13.92 7.47 1.25 7.31 26.05 3 0 CZ0.3 41 13.92 7.47 1.25 7.31 28.71 0 0.34 CZ0.6 41 13.92 7.47 1.25 7.31 28.38 0 0.67 CZ1 41 13.92 7.47 1.25 7.31 28.05 0 1 CT1Z0.6 41 13.92 7.47 1.25 7.31 27.38 1 0.67 CT2Z0.3 41 13.92 7.47 1.25 7.31 26.71 2 0.34 Table 2. Formulation of HPC samples. consolidating fine grain concrete without fibres. The components of the raw material used are: cement I 42.5 R, silica fume, technical sands with maximum grain size of 1.2 mm, silica flour, water and super- plasticizer. These materials come mainly from local sources. Table 1 illustrates the chemical composition of CEM I 42.5 R cement used for the hydration of HPC with the addition of the mixed oxides TiO2 and ZnO in proportions varying from 1 to 3% by weight. Titanium dioxide (TiO2) and zinc oxide (ZnO) used in this research were produced by the Czech manufacture Precheza. TiO2 powder consists of 99 wt% of pure Anatase and also some traces of sul- phates (0.7 wt%). The specific surface area is about 70-100 m2/g. 2.2. Formulation and experimental methods For the development of this study, 9 series of HPC mixtures were produced, each group consisting of 4 prisms. The concretes were mixed with different per- centages of the TiO2 and ZnO nanoparticles accord- ing to Table 2. After 28 days of hardening in water, the samples were dried for 24 hours at the room tem- perature. The flexural and compressive strength tests were carried out accordance with ČSN EN 12390-3 on pris- matic specimens 40×40×160 mm in accordance with the instructions of standard ČSN EN 12390-1 [16]. After 28 days of curing under water, each specimen is first subjected to a 3-point flexion test. Then, the two half-test pieces are tested in compression until they break. The compressive strength of a formula- tion is the arithmetic average of the four individual results obtained from four determinations made on a set of two prisms. If any of the four results vary by more than ±10 % from the average, that result is discarded and the arithmetic mean is calculated from the remaining three results. If any of the three results vary by more than ±10 % from their average, all of the results are rejected. The tests were carried out using a mechanical press of the BRIO HAANICE ® type with a capacity of 400 kN. Nanoparticles of ZnO and TiO2 were suspended in distilled water (2.5 g/dm3) and after 15 minutes of ul- trasonic pretreatment were prepared drop casted lay- ers on the soda lime glass (dimension 10×5 cm) con- taining 0.5 mg/cm2 TiO2 or ZnO respectively. Pho- tocatalytic activity was determined using standard ISO 22197-1 methodology (total flowrate 3dm3/min, initial concentration of NO 1 ppm, relative humidity 50%, UV intensity 1mW/cm2). The photocatalytic performances of TiO2 and ZnO nanoparticles were studied by NOx degradation un- der UV irradiation for 2 hours (Fig. 1). TiO2 showed a significant NOx degadation capacity which has reached 26%. On the other hand, ZnO was less reactive and had a value of 4%. Photocatalytic activity of concrete samples was de- termined in stirred flow through reactor. Due to the expected lower photocatalytic activity, total flowrate was decreased to 1dm3/min, initial concentration of NO was 1 ppm, relative humidy 50%, intensity of UV light 1 mw/cm2. 3. Results and discussion 3.1. Compressive and Flexural Strength Figures 2 and 3 showed the evolution of the addition of mineral powders on the compressive and flexural strength of HPC mixtures. The values of the com- pressive strength of concrete mixture Ref, CT1, CT2, 16 vol. 33/2022 TiO2 and ZnO in a High Performance Concrete Figure 1. NOx abatements for TiO2 and ZnO nanoparticles under UV irradiation. Figure 2. Compressive strength of HPC samples. CT3, CZ0.3, CZ0.6, CZ1, CT10.6 and CT2Z0.3 are respectively 124, 129.2, 116.5, 102.1, 103.8, 88.4, 84, 87.6 and 120.2 MPa. The addition of a small amount of TiO2 (1%) in the concrete increased its compres- sive strength compared to the reference concrete. On the other hand, from 2% of TiO2 the strength slightly decreased. The flexural strength value of unmodified concrete (Ref) was 14.5 MPa. As in the compressive strength measurements, the addition of 1 and 2 wt% TiO2 increased the flexural strength and decreased slightly for the CT3 to reach a value nearly equiv- alent to the Ref sample of 14.4 MPa. The highest values of the flexural strength were obtained for the samples with 1 and 2% by weight of TiO2 of the value of 16.2 MPa. The lower value of flexural strength was observed in the sample with 1% by weight of ZnO. In general, the substitution of cement by nanopar- ticles led to a slight decrease in mechanical strengths. Figure 3. Flexural strength of HPC samples. Unlike TiO2, the mechanical properties of concrete strongly depend on the amount of this nanoparticle used. According to Wang et al. [17], with increasing the dosage of TiO2 nanoparticles in cement mortars, the strength initially increased rapidly compared to the reference mortar until the dosage reached 2% by weight of TiO2, then there is a slowdown in the per- centage of that increase. The incorporation of nanoparticles into the cement matrix led to increases and slight decreases in com- pressive and flexural strengths except for samples containing only zinc oxide which showed a significant decrease. The retarding effect of zinc on the hydra- tion of cement, has a direct effect on the mechani- cal strength [18–20]. These results are not surprising since the added nanoparticles do not have the same performance as Portland CEM I cement. 17 F. Amor, L. Ingrišová, Z. Rácová et al. Acta Polytechnica CTU Proceedings Figure 4. NOx removal under UV irradiation over CT1, CT3, CZ1 and CT1Z0.6. 3.2. NOx decomposition The photocatalytic activity of HPC samples contain- ing TiO2 and ZnO was evaluated under UV irradi- ation by photodegradation of NOx (Figure 4). The maximum NO, NOx removal obtained was 8.8 and 6.7%, respectively, for CT3 sample. CT1Z0.6 sample removed an average of 0.1% of NOx and 1% of NO, however, CZ1 showed no photocatalytic reaction. On the other hand, the produced amount of toxic inter- mediate NO2 is very low, which is crucial in envi- ronmental application [21]. It is clear that concretes incorporating TiO2 have been shown to be superior to the photodegradation of NOx compared to those containing ZnO. B. Bica et al. [22] showed that with the same percentage of TiO2 and ZnO introduced in the concrete, the concretes incorporating the TiO2 presented a more degradation of the NOx for various test conditions and even with low percentages. These results are not surprising since the ZnO powder used in this work was not very reactive to photodegrada- tion. 4. Conclusions Different mixtures of HPC with the nanoparticles in substitution with cement have been prepared and their mechanical properties and capacities to degrade nitrogen oxides have been studied. The introduction of nanoparticles into the cement matrix resulted in increases and slight decreases in compressive and flexural strengths, except for sam- ples containing only zinc oxide which showed a re- markable decrease. The addition of TiO2 up to 2% by weight in the HPC did not decrease the mechani- cal properties but even increased the compressive and flexural strength. All samples showed photocatalytic degradation un- der UV irradiation, except for concrete containing 1% ZnO, achieving a higher NOx removal rate with a higher TiO2 dosage in concrete. The incorporation of zinc oxide alone into the cementitious matrix dete- riorates the properties of the concrete. 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