**1. Introduction**

Air pollution caused by human activity is one of the most worrying problems in urban areas nowadays [1,2]. Due to an increase in the concentration of harmful gaseous pollutants, mainly nitric oxides, ozone, and others, methods to reduce their presence need to be developed. As urban areas occupy significant areas of land, any active air purification system would require a vast amount of energy to operate and would contribute to a reduction in the quality of life in its vicinity (noise generated by high-efficiency air turbines). Moreover, most pollutants are produced due to the activity of transport systems within city boundaries, contributing to the differences in their concentrations over different areas (residential areas, roads, and other transport pavements, etc.) [3]. Therefore, different solutions to the problem have been developed over the years, with one of them being the use of photocatalytic materials embedded into cementitious materials—a passive system that requires only the presence of solar radiation to operate and purify the air from pollutants [4,5]. As a significant part of urban areas is covered with different types of pavements, mostly made of various cementitious materials and exposed to solar radiation, such a passive system would not require any changes to the existing infrastructure other than replacing non-photocatalytic surfaces with photocatalytic ones.

For a photocatalytic reaction to occur, a photocatalytic material (a semiconductor—for example, titanium dioxide) needs to be irradiated (for instance, with solar radiation). Due to energy introduction, electrons (e−) from the semiconductor shift from the valence band (vb) to the conduction band (cb), leading to the formation of electron holes h+ (Equation (1)). The resulting pairs of charges initiate a reduction–oxidation process (Figure 1). In the presence of adsorbed oxygen and water (strong oxidants) it is possible to decompose a wide range of air pollutants due to the formation of hydroxyl radicals (OH•) and superoxide radicals (*O*•− <sup>2</sup> ) (Equations (2) and (3)) [6].

$$TiO\_2 \stackrel{hv}{\rightarrow} TiO\_2 + h\_{vb}^+ + e\_{cb}^- \tag{1}$$

**Citation:** Kalinowski, M.; Woyciechowski, P.; Jackiewicz-Rek, W. Surface Modification of Photocatalytic Cementitious Composites with Polyacrylic Superabsorbent Polymers (SAP). *Mater. Proc.* **2023**, *13*, 23. https://doi.org/10.3390/ materproc2023013023

Academic Editors: Katarzyna Mróz, Tomasz Tracz, Tomasz Zdeb and Izabela Hager

Published: 15 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Faculty of Civil Engineering, Warsaw University of Technology, 00-637 Warsaw, Poland

**<sup>\*</sup>** Correspondence: maciej.kalinowski@pw.edu.pl

$$(H\_2O)\_{ads} + h^+ \to H^+ + OH^\bullet \tag{2}$$

$$(O\_2)\_{ads} + \varepsilon^- \to O\_2^{\bullet-} \tag{3}$$

**Figure 1.** General mechanism of the photocatalytic reaction.

As shown, titanium dioxide grains, or other photocatalyst grains, must be exposed to UV/solar radiation for a photocatalytic reaction to occur. To accomplish this goal, cementitious materials incorporating that type of addition usually consist of two layers one exposed to the external environment containing photocatalytic material and the other with no (or close to no) photocatalytic material. As photocatalysis occurs only in a nearsurface material layer, dividing the composite into two parts reduces its overall cost without impacting its properties regarding air purification [7].

Due to its fine granulation (starting at 10 nm), titanium dioxide functions as nuclei for hydration products during the hydration of the binder. Due to this, the TiO2 grains are usually covered with hydration products, limiting the effectiveness of the photocatalytic reaction of the hardened material (the TiO2 grains must be exposed for it to occur) [8].

To improve the effectiveness of air purification from gaseous pollutants, the authors decided to investigate new methods of the surface modification of photocatalytic materials using polyacrylic superabsorbent polymers (SAP). Superabsorbent polymers can be described as cross-linked hydrogel networks which absorb water due to high osmotic pressure caused by the accumulation of ions within their structure [9]. Water absorption into the polymer network causes the SAP grains to increase their volume, pushing ions apart. With its network stretched out, the osmotic pressure decreases. Hydrogel obtained in that way can later desorb water stored in its volume under external pressure or due to changes in the properties of the desorption environment [10]. For that reason, SAP are usually used as an internal curing agent in cementitious materials, preventing the cement matrix's self-desiccation and promoting the hydration of the binder.

As the SAP hydrogel is introduced into the cement matrix, it desorbs water due to the high electrochemical activity of the environment. This process is prolonged (water desorbs from SAP during more than 30 days from its introduction into cementitious material) compared to water absorption time (usually 5–15 min in a water environment) as water desorption occurs during hydration and is limited by water pressure within the pore network and its microstructure [11].

The SAP also facilitates hydration in the direct vicinity of its grains. This effect was linked to the course of water desorption from the SAP grains. As water desorbs from SAP, it creates a thin film over the volume of a shrinking SAP hydrogel grain, which acts as

an environment of intense hydration of the binder [9]. This phenomenon can be used to introduce various nanoparticles into cementitious material with the view of increasing their homogenous distribution within the cement matrix (Figure 2).

**Figure 2.** SAP hydrogel grain covered with nano-grains of titanium dioxide used in the study.
