**3. Results and Discussion**

The introduction of an additional layer of SAP in either non-saturated or saturated (hydrogel) form significantly affected the distribution of TiO2 on the surface of the mortar. An increase in the efficiency of air purification was observed for all light sources (Figure 4).

**Figure 4.** The efficiency of air purification from NO pollutants on a mortar sample formed in a mold with a layer of SAP hydrogel covered in TiO2 dispersion (SAP H) and reference series with no surface modification (REF). The test consisted of three parts: the study of the phenomenon of photocatalysis in UV-A light (first decrease in NO concentration), in visible light (second decrease in NO concentration), and in combined UV-A + visible light (third decrease in NO concentration). [ppb]—parts per billion, 10-9.

The photocatalytic mortar made with the SAP hydrogel layer showed the highest relative reduction of gaseous pollutants among the tested samples (Tables 4 and 5). The lowest coefficient of variation also characterized it. In the case of the photocatalysis intensity in visible light, compared to the reference series, the relative reduction of the NO concentration increased from 5.67% to 26.15%, and the relative decrease in NOx concentration increased from 3.05% to 24.12%. The relative NO and NOx concentration reductions increased to a slightly lower extent in the case of a layer made of non-saturated SAP. Spraying molds with dispersion alone also contributed to an increase in the relative reduction of the analyzed pollutants. However, this method was characterized by the highest coefficient of variation of all the tested series (from 18 to 37%). This observation suggests an uneven dispersion distribution on the surface of individual samples. The rheological properties of the dispersion itself are probably responsible for this phenomenon. When spraying the molds with a suspension with a viscosity similar to that of water, along with introducing the photocatalytic mortar into the mold, some of the dispersion previously placed flows out of the mold in an uncontrolled way, contributing to the uneven distribution of the photocatalyst on the surface.

**Table 4.** Relative reductions in NO concentrations in performed tests for all considered surface modification techniques.


**Table 5.** Relative reductions in NOx concentrations in performed tests for all considered surface modification techniques.


The proposed surface modification method with SAP significantly affected the efficiency of purifying the air from NO and NOx in visible light conditions. As one of the considered photocatalysts (K7000) is a VLA (visible light active) material, the SAP layer contributed to its better exposure to external radiation, increasing the overall efficiency of the material in purifying the air in conditions simulating common autumn/winter conditions.

Due to their properties, the SAP grains promote hydration in their direct vicinity [18]. This effect is linked with the kinetics of water desorption from the SAP structure—the formation of a thin water film on the outer layer of the SAP grain that acts as an environment of intense hydration/crystallization [9,19]. In the case of SAP covered with the TiO2 grains, this phenomenon allowed for the immobilization of the TiO2 grains in the nearsurface layer and its crystallization on the surface, increasing its exposure to external radiation (Figure 5). However, due to water entrapment in the mold due to its continuous desorption from SAP during hydration, the porosity of the tested surfaces significantly

increased. As the proposed surface modification method represents a first step in the development of surface modification methods for photocatalytic cementitious materials, in future iterations, the mold surface exposed to the SAP-TiO2 layer is going to be prepared as a semi-permeable/sponge-like layer to limit the influence of the excess water released from SAP on the surface porosity.

**Figure 5.** Micrograph of crystallized TiO2 on the surface of the photocatalytic mortar.
