A New Preparation Method of Cement with Photocatalytic Activity

The studies of some mechanical properties and photocatalytic activity of new cements with photocatalytic activity are presented. The new building materials were obtained by addition of semi-product from titanium white production. Semi-product was calcined at 300 and 600 °C for one, three, and five hours and then this material was added to cement matrix in an amount of 1 and 3 wt.%. New materials were characterized by measuring the flexural and compressive strength and the initial and the final setting time. The photocatalytic activity was tested during NOx photooxidation. The cement with photocatalytic activity was also characterized by sulphur content measurements. The measurement of reflectance percentage of TiO2-loaded cements in comparison with pristine cement and TiO2 photocatalyst calcined at 600 °C were also performed. It should be emphasized that although in some cases, the addition of photocatalyst reduced the flexural and the compressive strength of the modified cements, these values were still within the norm PN-EN 197-1:2012. It was also found that the initial and the final setting time is connected with the crystal size of anatase, and the presence of larger crystals significantly delays of the setting time. This was probably caused by a water adsorption on the surface of anatase crystals.


Introduction
Recently, there has been a lot of interest in the use of photocatalysts in building materials. The addition of a nanosized photocatalyst into the building elements allowed inducing a specific functionality such as a photocatalytic, a superhydrophilic, and some antimicrobial properties. The combination of a sunlight utilization with the functionally engineered materials has contributed to the aesthetic durability of the building elements and reducing environmental pollution [1,2]. The photoinduced superhydrophilicity of the photocatalytic materials allows spreading out some water droplets, generating a thin film of water, which eliminates dust from the surfaces and limits the optical interferences on the photocatalytic glass windows [3]. The disinfecting activity of the photocatalytic materials consists of damaging the cellular membranes of the bacteria. The antibacterial and an antifungal action of the photocatalysis was applied to control biological growth on the building surfaces [1,4]. The photocatalytic degradation of the pollutants contributed to the self-cleaning of the building surfaces as well as a removal of air pollutants from other industrial sectors [5].
NO + ·OH → HNO 2 (4) NO 2 + ·OH → HNO 3 (6) NO The dominant materials in the construction industry have remained for years cement-based materials or composites. However, apart from their structural function, recently, the modern properties were added to by addition of steel fiber reinforcement, nano additives, or by the addition of self-compacting substances [13]. Photocatalytic cements were first prepared in Japan at the start of the 1990s [14]. Presently, there are conceived facades, streets, and the pavements from photocatalytic cements [15][16][17]. Thus far, TiO 2 containing cement-based materials was prepared using different techniques, which still faces a serious restriction.
Heterogeneous photocatalysis is a surface process. Therefore, nanomaterials are often immobilized on an appropriate building substrate. The photocatalytic coating for building materials were obtained by using dip-coating, spin coating, spraying, or brushing [6]. Feng et al. [18] prepared the photocatalytic TiO 2 /cement composite by a smear method through the floating emulsion of TiO 2 onto the pre-wet surface of the cement mortar. They showed that TiO 2 particles were dispersed equally on the surface of the cementitious material and the prepared mortar had a good cyclical photocatalytic performance. Baltes et al. [6] showed that the application of the dip-coating technique led to the high photocatalytic activity, but that low mechanical resistant layers were obtained. The methods using TiO 2 surface treatment for the cement-based materials appeared to be problematic due to the weak adhesion between Materials 2020, 13, 5540 3 of 12 the photocatalytic coating and the building material. The poor weathering resistance is evidence for this, especially in some aggressive outdoor environments [19]. A valuable resource to prevent the release of TiO 2 particles from the building surface is SiO 2 , owing to its pozzolanic activity with cement-based materials. Wang et al. [20] prepared SiO 2 /TiO 2 composites with different deposited densities of TiO 2 on SiO 2 spheres, which were used for the surface coating of a cement-based material. The authors showed good photocatalytic activity and the durability of the SiO 2 /TiO 2 composites. However, Mendoza et al. [21] indicated that despite SiO 2 action as an interlayer between TiO 2 and the substrate, it could not effectively stabilize TiO 2 coating.
Hernández-Rodríguez et al. [22] proposed a partial replacement of the cement with TiO 2 . The photocatalyst was only incorporated into a one-centimeter-thick surface layer on the cementitious specimens. A photocatalyst is also often embedded into the mass of cementitious material by simple intermixing of both substrates [23,24]. On the one hand, it leads to lower photocatalytic efficiency due to partial use of TiO 2 particles, which are active only when situated to the surface [19]. On the other hand, the effect of TiO 2 addition to cementitious mass leads not only to photocatalytic activity, but also improves the mechanical properties of cementitious materials. The enhancement of mechanical properties by filling the pores and interaction with the other components of cement were observed [6,22,25].
Despite the multiple examples of the photocatalytic cementitious materials, most of the products are not competitive enough and create difficulties during the production and applications in the real conditions [22]. There is, therefore, a need to discern new photocatalytic materials concerning new synthesis methods and new incorporation strategies into the building binders.
This paper aims to present the photocatalytic cementitious materials preparation from cement and semi-product of TiO 2 . The proposed method may lower the price of the photoactive cement. The obtained materials were analyzed in detail towards photocatalytic and some mechanical properties.

Materials
Portland cement CEM I 42.5 N (Holcim, Germany) as base material was used. Standard sand, according to the standard EN 196-1 "Methods of testing cement. Determination of strength" was used in the mixture. The semi-product from titanium white production supplied by Grupa Azoty Zakłady Chemiczne 'Police' S.A. (Police, Poland) was used as a starting material. This material was downloaded from installation for titanium white production by sulfate method; the material was taken from drum filters before calcination. The semi-product was calcined at 300 and 600 • C for 1, 3, and 5 h.

Specimens Preparation
The specimens 40 × 40 × 160 mm were produced according to the EN 196-1 standard with water to binder ratio w/b = 0.4 and cement to standard sand ratio 1:3. Cement was replaced by the photocatalyst in 1 and 3 wt.% of cement. For each type of mortars, 6 specimens were produced. Masses needed for the preparation of 2 types of specimens in Table 1 are presented. The standard mixer with stainless steel bowl with a capacity of 5 l according to EN 196-1 was used. First, water was poured into a bowl and cement was added. The casting molds containing fresh samples were wrapped with stretch film and stored in room conditions for 24 h. All specimens were demolded after 1 day and were cured in tap water for the next 27 days.

The Compressive and the Flexural Strength Measurements
After 28 days, specimens for the flexural and the compressive strength were tested. The flexural and the compressive strength measurements in accordance with the EN 196-1 standard were carried out. A standard testing machine (i.e., ToniNORM 2010.040, Toni/Technik, Berlin, Germany) was used.

The Initial and the Final Setting Time (Vicat Needle Test)
Vicat Apparatus (ToniSET COMPACT version 05/00, Berlin, Germany) was used to determine the setting time of cement paste. For each mortar type, 2 specimens were prepared. The mortar preparation and the setting time measurements were run according to the EN 196-3 standard. During measurements, specimens were cooled with 20 • C water. Water to binder ratio was w/b = 0.3. The time when the needle stops 6 mm from the base plate was recorded as the time for the initial setting. The final setting was defined as the time when the needle only made a 0.5-mm mark on the surface.

The Crystalline Structure, UV-Vis/DR Measurments, and Sulphur Content
The crystalline structure of the photocatalysts was characterized by X-ray powder diffraction (XRD) analysis (X'Pert PRO Philips diffractometer, Eindhoven, Netherlands) using CuK α radiation. The mean size of crystallite was calculated from full-width at half-maxima (FWHM) of corresponding X-ray diffraction peaks using Scherrer's formula, where λ is the wavelength of the X-ray radiation (λ = 1.54056 nm CuK α ), β is the full-width at half maximum (rad) and is the reflect angle. The width of the peak at half maximum was calculated after correction of the instrument error. The presented method was applied to estimate of change in the crystallite size of TiO 2 particles. The materials were characterized by the UV-VIS/DR (diffiuse-reflectance) technique using the Jasco V-530 (Tokyo, Japan) spectrophotometer equipped with the integrating sphere accessory for the diffuse reflectance spectra (BaSO 4 was used as a reference).
The content of sulphur (wt.%) in tested photocatalysts, as well as TiO 2 -modified cement were determined by means of CS230 elemental analyzer (Leco Co., St. Joseph, MI, USA). The BCS-CRM powder containing 1.48 wt.% of inorganic sulphur was used as a calibration standard.

NOx Decomposition
In our previous work [26,27], the NO gas (1.989 ppm ± 0.040 ppm, Air Liquid) was used as model pollution in photocatalytic tests. NOx removal was evaluated using the experimental installation ( Figure 1).

The Initial and the Final Setting Time (Vicat Needle Test)
Vicat Apparatus (ToniSET COMPACT version 05/00, Berlin, Germany) was used to determine the setting time of cement paste. For each mortar type, 2 specimens were prepared. The mortar preparation and the setting time measurements were run according to the EN 196-3 standard. During measurements, specimens were cooled with 20 °C water. Water to binder ratio was w/b = 0.3. The time when the needle stops 6 mm from the base plate was recorded as the time for the initial setting. The final setting was defined as the time when the needle only made a 0.5-mm mark on the surface.

The Crystalline Structure, UV-Vis/DR Measurments, and Sulphur Content.
The crystalline structure of the photocatalysts was characterized by X-ray powder diffraction (XRD) analysis (X'Pert PRO Philips diffractometer, Eindhoven, Netherlands) using CuKα radiation. The mean size of crystallite was calculated from full-width at half-maxima (FWHM) of corresponding X-ray diffraction peaks using Scherrer's formula, where λ is the wavelength of the X-ray radiation (λ = 1.54056 nm CuKα), β is the full-width at half maximum (rad) and is the reflect angle. The width of the peak at half maximum was calculated after correction of the instrument error. The presented method was applied to estimate of change in the crystallite size of TiO2 particles. The materials were characterized by the UV-VIS/DR (diffiuse-reflectance) technique using the Jasco V-530 (Tokyo, Japan) spectrophotometer equipped with the integrating sphere accessory for the diffuse reflectance spectra (BaSO4 was used as a reference).
The content of sulphur (wt.%) in tested photocatalysts, as well as TiO2-modified cement were determined by means of CS230 elemental analyzer (Leco Co., St. Joseph, MI, USA). The BCS-CRM powder containing 1.48 wt.% of inorganic sulphur was used as a calibration standard.

NOx Decomposition
In our previous work [26,27], the NO gas (1.989 ppm ± 0.040 ppm, Air Liquid) was used as model pollution in photocatalytic tests. NOx removal was evaluated using the experimental installation ( Figure 1). The studied cement plate (one at dimensions of 8 × 4 × 1 cm) was placed in the central part of the cylindrical reactor (Pyrex glass; Ø × H = 9 cm × 32 cm). The NO (II) was diluted with humidified synthetic air in ratio 1:1. The process was carried out continuously with a gas flow 500 cm 3   The studied cement plate (one at dimensions of 8 × 4 × 1 cm) was placed in the central part of the cylindrical reactor (Pyrex glass; Ø × H = 9 cm × 32 cm). The NO (II) was diluted with humidified synthetic air in ratio 1:1. The process was carried out continuously with a gas flow 500 cm 3 /min. At the beginning of the process, the dark conditions were maintained until NO concentration reached equilibrium (about 1 ppm during about 35 min). Then the 4 × 22 W UV lamps (Philips) were turned on for 30 min. The irradiation sources surrounded the rector and were characterized by the cumulative intensity of 100 W/m 2 UV and 4 W/m 2 Vis. The temperature of the whole system was stable at the level of 22 • C by using the thermostatic chamber. The NO and NO 2 concentrations were continuously measured in the outlet of the reactor using chemiluminescent NOx analyzer (T200, Teledyne).

The Compressive and the Flexural Strength
The compressive and the flexural strength of unmodified cement and cement with the addition of 1 and 3 wt.% of TiO 2 modified at 300 and 600 • C specimens were measured. The obtained results in Figure 2a,b are presented. The red line in all graphs represents the value of unmodified cement. The graphs also show standard deviations for the mean measurement values. As it can be seen in Figure 2a, the value of the compressive strength of unmodified cement amounted to 61.5 MPa. The highest value of the compressive strength was observed for specimens with 3 wt.% of TiO 2 calcined at 300 • C for 3 h, and amounted to 66.3 MPa. The lowest value of the compressive strength was found for the specimens with the addition of 3 wt.% TiO 2 calcined at 600 • C for 5 h, and amounted to 56.5 MPa. As it can be seen in Figure 2b, the value of the flexural strength of unmodified cement amounted to 8.1 MPa. The addition of 1 and 3 wt.% of TiO 2 decreased the flexural strength of specimens in almost all cases. The highest value of flexural strength was observed for a specimen with 1 wt.% of TiO 2 calcined at 300 • C for 3 h, and amounted to 8.37 MPa. The lowest value of the flexural strength was obtained using specimen with the addition of 3 wt.% of TiO 2 calcined at 300 • C for 3 h, and this value amounted to 7.36 MPa. It seems that the mechanical properties (the compressive and the flexural strength) of cements depended on the crystal size of anatase form of used titanium dioxide ( Figure 3). The crystal size of anatase obtained at 300 • C amounted to about 8 nm, and it can be seen that with the addition of this material the values of the compressive and the flexural strength was close to the value of these parameters for unmodified cement. When the crystal size of anatase increased, the compressive and the flexural strength decreased.
This behavior was especially visible in the case of addition of 3 wt.% of TiO 2 modified at 600 • C. While the anatase crystallite size increased with the increasing of calcination time, the decrease in the compressive and the flexural strength was observed. In Figure 3, the relationship between the size of anatase crystals of titanium dioxde and the compressive strength for cement with 1 and 3 wt.% of TiO 2 calcined at 600 • C for 1, 3, and 5 h is presented.
It should be emphasized that although in some cases, the addition of photocatalyst reduced the flexural and the compressive strength of the modified cements, these values were still within the norm PN-EN 197-1:2012.

The Initial and the Final Setting Time
The values of the initial and final setting time of tested specimens in Table 2 are presented. As it can be seen, in the case of 1 wt.%, addition of TiO 2 slightly extended the initial setting time in comparison to unmodified cement. In the case of TiO 2 modified at 300 • C regardless of the modification time, the initial setting was medium 30 min later, and the final setting time was around 50 min later than the initial setting time of unmodified cement. The initial setting time for TiO 2 modified at 600 • C was medium 40 min later, and the final setting time was around 70 min later than the initial setting time of unmodified cement. In the case of adding 1 wt.% of the photocatalyst, we did not observe any relationship between the initial and final setting time and the calcination time of TiO 2 at temperatures of 300 and 600 • C. almost all cases. The highest value of flexural strength was observed for a specimen with 1 wt.% of TiO2 calcined at 300 °C for 3 h, and amounted to 8.37 MPa. The lowest value of the flexural strength was obtained using specimen with the addition of 3 wt.% of TiO2 calcined at 300 °C for 3 h, and this value amounted to 7.36 MPa. It seems that the mechanical properties (the compressive and the flexural strength) of cements depended on the crystal size of anatase form of used titanium dioxide (Figure 3). The crystal size of anatase obtained at 300 °C amounted to about 8 nm, and it can be seen that with the addition of this material the values of the compressive and the flexural strength was close to the value of these parameters for unmodified cement. When the crystal size of anatase increased, the compressive and the flexural strength decreased.  It should be emphasized that although in some cases, the addition of photocatalyst reduced the flexural and the compressive strength of the modified cements, these values were still within the norm PN-EN 197-1:2012.

The Initial and the Final Setting Time
The values of the initial and final setting time of tested specimens in Table 2 are presented. As it can be seen, in the case of 1 wt.%, addition of TiO2 slightly extended the initial setting time in comparison to unmodified cement. In the case of TiO2 modified at 300 °C regardless of the modification time, the initial setting was medium 30 min later, and the final setting time was around 50 min later than the initial setting time of unmodified cement. The initial setting time for TiO2 modified at 600°C was medium 40 min later, and the final setting time was around 70 min later than the initial setting time of unmodified cement. In the case of adding 1 wt.% of the photocatalyst, we did not observe any relationship between the initial and final setting time and the calcination time of A different situation was observed when the photocatalyst was added in an amount of 3 wt.%. When this amount of photocatalyst was added to cement, it was possible to see the influence of calcination temperature of TiO 2 on the initial and the final setting time. For TiO 2 modified at 600 • C, it was impossible to give the medium value of initial and the final setting time because the calcination time influences the setting time. When the calcination time of photocatalyst increased from 1 to 5 h, the initial setting time also increased from 232 to 255 min and the final setting time increased from 315 to 366 min. Our earlier studies about the addition of nitrogen-modified TiO 2 to the CEM I 42.5 N [28] and studies presented by Hernández-Rodríguez et al. [22] about the addition of commercial TiO 2 P25 to CEM I 52.5 R showed that photocatalysts acted as a setting accelerator. In these studies, there was an opposite situation, and these photocatalysts were the setting retarder. One of the reasons for delaying the setting time could be the presence of sulfate species on the TiO 2 surface. The sulfate groups were present because this material was taken from the technological line of titanium white production by sulfate method. There are many publications describing the influence of sulfur on cement. Gies at al. [29] found that upon the absence of alkalis, increasing sulfate contents in belite-rich cement clinkers induced a significantly higher belite content, which is associated with a decrease in the alite content and, consequently, a reduction in the compressive strengths after 2 days of the resulting cements. The influence of SO 3 in clinker on the properties of cement might depend on the C 3 A content of the cement because SO 3 can react with C 3 A in the pore solution in cement in an early stage of ageing. However, few studies have focused on the heat of hydration and drying shrinkage of Portland cement with high-SO 3 clinker with different C 3 A contents and added gypsum [30].
To confirm the effect of sulfur on the initial and the final setting time of modified cement, the measurements of sulfur in photocatalysts were done. The obtained results in Table 3 are presented. Table 3. The mass% of sulphur in photocatalysts. Because the amount of added photocatalysts to cement amounted to 1 and 3 wt.%, and in the photocatalyst, the amount of sulfur reached minimally from 0.67 to maximally 2.48 wt.%. The amount of 0.06wt% of sulphur additional introduced to cement was too small to influence the initial and the final setting time, and there was no dependence between the initial and the final setting time and the amount of sulfur in photocatalysts. The only visible relationship during setting time concerned cements modified by the addition of 3 wt.% of photocatalysts modified at 600 • C for 1, 3, and 5 h.

Mass% of Sulfur
The observed changes could be connected with the changes in crystallographic structure of TiO 2 . In Figure 4, the XRD patterns of TiO 2 modified at 600 • C for 1, 3, and 5 h are presented.  In Figure 5, the calculated values of crystal size of anatase and rutile crystals of TiO2 modified at 300 and 600 °C for 1, 3, and 5 h is presented. As can be seen, the modification temperature influenced the crystal size of anatase. TiO2 modified at 300 °C had a smaller crystal size (around 8 nm) than TiO2 modified at 600 °C. In the case of TiO2 modified at 600 °C, it was even possible to see that the modification time influenced the crystal size of anatase. After 1 h of TiO2 calcination at 600 °C, the crystal size amounted to 24 nm, and after 5 h of calcination, the crystal size increased to 33 nm. The initial and the final setting time was connected with a crystal size of anatase, and the presence of larger crystals successfully delay the setting time. This probably caused by water adsorption on the anatase surface. Water adsorbed on the anatase surface was not available immediately for cement particles, and there was a reason for the delay of setting time.

NOx Decomposition
The absorption abilities, determined on the basis of the measurement of reflectance percentage, of TiO2-loaded cements in comparison with pristine cement and TiO2 photocatalyst calcined at 600 °C are presented in Figure 6. The character of the UV-Vis/DR spectra of modified cements was similar to the spectra of unmodified cement. The spectra of the photocatalyst were found to be typical for In Figure 5, the calculated values of crystal size of anatase and rutile crystals of TiO 2 modified at 300 and 600 • C for 1, 3, and 5 h is presented. As can be seen, the modification temperature influenced the crystal size of anatase. TiO 2 modified at 300 • C had a smaller crystal size (around 8 nm) than TiO 2 modified at 600 • C. In the case of TiO 2 modified at 600 • C, it was even possible to see that the modification time influenced the crystal size of anatase. After 1 h of TiO 2 calcination at 600 • C, the crystal size amounted to 24 nm, and after 5 h of calcination, the crystal size increased to 33 nm. The initial and the final setting time was connected with a crystal size of anatase, and the presence of larger crystals successfully delay the setting time. This probably caused by water adsorption on the anatase surface. Water adsorbed on the anatase surface was not available immediately for cement particles, and there was a reason for the delay of setting time.  In Figure 5, the calculated values of crystal size of anatase and rutile crystals of TiO2 modified at 300 and 600 °C for 1, 3, and 5 h is presented. As can be seen, the modification temperature influenced the crystal size of anatase. TiO2 modified at 300 °C had a smaller crystal size (around 8 nm) than TiO2 modified at 600 °C. In the case of TiO2 modified at 600 °C, it was even possible to see that the modification time influenced the crystal size of anatase. After 1 h of TiO2 calcination at 600 °C, the crystal size amounted to 24 nm, and after 5 h of calcination, the crystal size increased to 33 nm. The initial and the final setting time was connected with a crystal size of anatase, and the presence of larger crystals successfully delay the setting time. This probably caused by water adsorption on the anatase surface. Water adsorbed on the anatase surface was not available immediately for cement particles, and there was a reason for the delay of setting time.

NOx Decomposition
The absorption abilities, determined on the basis of the measurement of reflectance percentage, of TiO2-loaded cements in comparison with pristine cement and TiO2 photocatalyst calcined at 600 °C are presented in Figure 6. The character of the UV-Vis/DR spectra of modified cements was similar to the spectra of unmodified cement. The spectra of the photocatalyst were found to be typical for

NOx Decomposition
The absorption abilities, determined on the basis of the measurement of reflectance percentage, of TiO 2 -loaded cements in comparison with pristine cement and TiO 2 photocatalyst calcined at 600 • C are presented in Figure 6. The character of the UV-Vis/DR spectra of modified cements was similar to the spectra of unmodified cement. The spectra of the photocatalyst were found to be typical for white TiO 2 -based nanopowders. Despite the small quantities of TiO 2 photocatalyst added (1 and 3 wt.%), a characteristic band could be found, as in the case with a pristine photocatalyst. What is more, the color of the modified cement plates was lighter due to the addition of white TiO 2 , and the absorption of the radiation in the range of UV increased with the increase of the amount of added photocatalyst. The opposite observation was found for visible region.
Materials 2020, 13, x FOR PEER REVIEW 9 of 12 white TiO2-based nanopowders. Despite the small quantities of TiO2 photocatalyst added (1 and 3 wt.%), a characteristic band could be found, as in the case with a pristine photocatalyst. What is more, the color of the modified cement plates was lighter due to the addition of white TiO2, and the absorption of the radiation in the range of UV increased with the increase of the amount of added photocatalyst. The opposite observation was found for visible region. In Figure 7, the comparison of pure and modified cement under irradiation and the comparison of influence of dark and irradiation conditions during process of NOx decomposition on selected cement samples was presented. In Table 4 the photocatalytic activity of unmodified and modified cements is presented. The activity of obtained materials during NOx removal were tested. The reference sample, unmodified cement CEM I showed the removal of NOx on the level of about 6.3%.  In Figure 7, the comparison of pure and modified cement under irradiation and the comparison of influence of dark and irradiation conditions during process of NOx decomposition on selected cement samples was presented. In Table 4 the photocatalytic activity of unmodified and modified cements is presented. The activity of obtained materials during NOx removal were tested. The reference sample, unmodified cement CEM I showed the removal of NOx on the level of about 6.3%.
Materials 2020, 13, x FOR PEER REVIEW 9 of 12 white TiO2-based nanopowders. Despite the small quantities of TiO2 photocatalyst added (1 and 3 wt.%), a characteristic band could be found, as in the case with a pristine photocatalyst. What is more, the color of the modified cement plates was lighter due to the addition of white TiO2, and the absorption of the radiation in the range of UV increased with the increase of the amount of added photocatalyst. The opposite observation was found for visible region. In Figure 7, the comparison of pure and modified cement under irradiation and the comparison of influence of dark and irradiation conditions during process of NOx decomposition on selected cement samples was presented. In Table 4 the photocatalytic activity of unmodified and modified cements is presented. The activity of obtained materials during NOx removal were tested. The reference sample, unmodified cement CEM I showed the removal of NOx on the level of about 6.3%.   The same observation in relation to the blank sample was presented by Xu et al. [31]. They found that using reference cement composites without any TiO 2 , the NOx concentration slowly decreased by 6% during 15 min of irradiation. It is worth pointing out that in our studies, the photolysis of tested gas amounted to 1.3% under the same conditions and the same irradiation source. The application of TiO 2 in cement mortars involved the degradation of NOx on the photocatalytic path, which can be observed as the unambiguous decrease of NOx concentration directly after turning on the irradiation. The increasing temperature of calcination of TiO 2 loaded to the cement matrix caused the increase of the NOx degradation rate. A 3 wt.% addition of photocatalyst calcined at 300 • C for 3 h to cement caused that this material removed 10.9% of NO, while 3 wt.% addition of photocatalyst calcined at 600 • C for 3 h to cement caused this material to remove 25.3% of NO. There was a typical behavior with a higher amount of TiO 2 in the cement matrix to cause a higher amount of NO(II) removal. For example, when using 1 wt.% of TiO 2 calcined at 300 • C for 1 h as an additive to cement, 10.4% of NO was successfully oxidized, while utilizing 3 wt.% of the same photocatalyst caused 17.4% of NO removal.

Conclusions
The semi-product from the installation of titanium white production by sulfate method can be used after calcination as an additive to cement mortars to give the photocatalytic properties to these materials. All photocatalytic samples degraded NOx during irradiation time, achieving higher NOx removal rate with higher TiO 2 dosage in cement materials. Addition of the cement mortar sometimes slightly decreased and sometimes slightly increased the mechanical properties, but these values were still within the norm PN-EN 197-1:2012. Used semi-product after calcination at 300 and 600 • C included from 0.67 to 2.48 wt.% of sulphur, but this amount did not have an influence on initial and final setting time of obtained mortars. Obtained materials have photocatalytic activity, their activity was tested during NO (II) decomposition. Cement with 3 wt.% addition of TiO 2 calcined at 300 • C for five hours decomposed 17.4% of NO(II) under UV light irradiation. Funding: This research was funded by the Polish National Agency for Academic Exchange within the Bekker programme.