**2. Materials and Methods**

The main goal of the research was to increase the content of TiO2 on the exposed surface of the material and obtain its homogenous distribution over the entire surface of the element. The phenomenon of adsorption of solid particles on the outer layer of the SAP grains was utilized to achieve it. In molds (in which samples for air purification tests were prepared), a thin layer of SAP (either in non-saturated or hydrogel form) was placed and sprayed over with 5–7 g of TiO2 water dispersion. Next, the photocatalytic mortar of a proposed composition was poured into the mold (Figure 3).

**Figure 3.** Method of surface modification with a reverse layer order—after demolding, the bottom surface was considered as an exposed surface and tested in air purification tests.

The ability to purify the air from harmful gaseous pollutants is an essential aspect of photocatalytic composites, so the viability of each solution was assessed through the air purification tests from NO and NOx.

The cement used in this study—CEM II/A-S 52.5R (Ozar ˙ ów, Poland)—met the requirements of EN 197-1 [12]. The study used two types of fire-dried quartz sand aggregates of different granulations—0.1/0.5 and 0.5./1.2 (Łajski, Poland)—which met the requirements of EN 13139 [13]. Two types of titanium dioxide were used: TiO2 (A)—K7000 (Leverkusen, Germany) and TiO2 (B)—P25 (Shanghai, PRC) of properties in the powder state presented in [6] (Table 1). TiO2 (A) represented a VLA (visible light active) photocatalyst, while TiO2 (B) represented a UV-A (UV light active) photocatalyst. VLA photocatalysts are active in both UV and visible light. However, their market price is several times higher than for

UV-A photocatalysts. Due to this fact, a mix of two photocatalysts was used so that the obtained photocatalytic material would have the capability to purify the air in both UV and visible light, with its overall price reduced.

**Table 1.** The content of crystalline phases and the size of crystallites (measured via XRD), and specific surface area (measured via BET).


Titanium dioxide was introduced into the mortar as a dispersion made of a portion of mixing water and a PCE superplasticizer (SP). As TiO2 agglomerates in dry conditions, it was decided to use mechanical mixing and sonication to reduce its average grain size. Polyacrylic superabsorbent polymer (sodium acrylate and acrylic acid polymer) (Ludwigshafen, Germany) of properties described in [9] was used as a surface modification agent.

The water used in this study met the requirements of EN 1008 [14]. An additional mass of the PCE superplasticizer (SP) that met the EN 934-2 [15] requirements was added to mortars to modify their rheological properties (My´slenice, Poland). All described components were used to prepare a photocatalytic mortar. For each of the prepared series of samples, different surface modification techniques were applied (Table 2). The mass amount of dispersion was chosen so that the TiO2 content in the composite would be 12.5 kg/m3.

**Table 2.** Composition of the prepared mortar and information regarding the type of applied surface modification for each series of samples.


The designed mortars were prepared according to the mixing procedure described in EN 196-1 [16]. The consistency of the mortar was tested based on EN 1015-3 [17]. Due to the low viscosity of the prepared mortar, only free flow was measured. After 24 h, samples were demolded and cured in the curing chamber (temperature 20 ± 2 ◦C, relative humidity RH ≥ 95%) until further tests. The tensile and compressive strength of the hardened mortar after two days were tested according to EN 196-1 [16] (Table 3) on 40 mm × 40 mm × 160 mm samples. As the considered surface modification techniques did not include the modification of the material's composition in its entire volume (only within an approx. 1–2 mm layer of the sample), it was assumed that the strength characteristics of the material would remain unchanged compared to the reference series.

The air purification from the nitrogen oxides test was performed according to the procedure developed during the 'Technology for the production of innovative self-cleaning prefabricated facade and surface elements that improve air quality' TECHMATSTRATEG-III/0013/2019 project. Tests were performed on 40 mm × 140 mm × 160 mm samples made from the designed photocatalytic mortar after 28 days of curing.


**Table 3.** Average compressive, tensile strength and consistency of prepared mortar.

Before the test, the tested surface was cleaned of antiadhesive agent and other organic pollutants. It was sprinkled with distilled water, scrubbed, dried at 60 ◦C for two hours, and placed in the irradiation chamber for 16 h, with the test surface facing the light source, where the surface organic impurities were burned in UV-A radiation (irradiance 10 W/m2). After that, the surface was cleaned with distilled water and dried at 60 ◦C for 2 h. The air purification tests were performed during the earliest two hours after the last drying cycle.

The sample was then placed in the glass reaction chamber with the tested surface facing the light source. The temperature in the glass reaction chamber during the experiment was kept at 25 ± 3 ◦C and the relative humidity at 40 ± 5%. The gas flow was maintained at a constant value of 2 L/min.

This study consisted of several stages, including filling the reaction chamber with nitrogen oxides to achieve a concentration of 100 ± 5 ppb (parts per billion, 10−9), irradiating the sample with a UV-A light with an irradiance of 0.2 W/m2, and measuring the concentration of nitrogen oxides while maintaining the gas flow. Then, the concentration of nitrogen oxides after switching off the light source while maintaining the gas flow was measured. In the next step, the reaction chamber was emptied of nitrogen oxides. Consecutively, the same procedure was performed for visible light with an irradiance of 115 W/m<sup>2</sup> and for a combined presence of UV-A and visible light with an irradiance of 0.2 W/m2 and 115 W/m2, respectively.

The nitrogen oxide concentration in the performed tests was approximately 100 ppb to model the actual NOx concentration in Warsaw [6]. The UV-A and visible light irradiance were chosen as 0.2 and 115 W/m2 to model the UV index of 8 (summer solar radiation conditions in the Polish territory) and the autumn/winter irradiance of visible light.
