Recently, the design of photocatalytic building materials for self-cleaning, self-disinfecting, and environmental pollution remediation has resulted in increasing interest. The advantage of using solar energy and rainwater as driving force has opened a new domain for environmentally friendly building materials. This work reports the fabrication of photocatalytic self-cleaning coatings with alternative building materials such as fly ash, sodium carbonate as an alkaline activator, and bismuth-based photocatalysts deposited on concrete. The photocatalytic materials were powders of Bi2O3, Bi2O2CO3, BiOI, BiVO4, BiPO4, which were synthetized by a precipitation method at room temperature without the use of surfactants or additives. The photocatalytic self-cleaning efficiency of the coatings was evaluated according to the ISO 10678:2010 norm by using aqueous methylene blue (MB) as a model pollutant in order to measure the photoactive surface’s ability to degrade dissolved organic molecules under light irradiation. The tendency of the photocatalytic self-cleaning coating activities was: Bi2O2CO3 (49%) > BiOI (30%) > BiVO4 (15%) > BiPO4 (14%) > Bi2O3 (5%) after 3 h of reaction. The best result was obtained when Bi2O2CO3 was incorporated into the coatings, and it was associated to its lowest crystallite (27 nm), flake-like morphology, and to its negatively charged surface (pHPZC = 11.5) at the pH of the experiments, that favored the adsorption of the cationic dye (MB) on its surface. The self-cleaning efficiency of the Bi2O2CO3 coating was investigated at different curing ages: 7, 14, and 28 days. It was observed a decrement in the photocatalytic activity with increasing the curing age. However, it was possible to restored the selfcleaning efficiency after washing the surface with water due to the removal of hydration products, which filled up the coating-pores forming diffusion barriers to both reactants and photons.

Introduction

The incorporation of photocatalysts in building materials to provide a self-cleaning effect offers a wide range of opportunities to ensure proper functionality maintenance over time [1–۳]. Furthermore, the self-cleaning effect on building materials provides the possibility of improving the air quality by removing air pollutants such as NOx, VOCs, and C7H8 [4–۶]. In this context, TiO2 is the most used photocatalyst material for self-cleaning applications. The fabrication of photocatalytic self-cleaning coatings with TiO2 has been demonstrated on different surfaces such as stucco [4], mortars [6], paints [6], and tiles [7]. However, since TiO2 only absorbs a small part of the light of the solar spectrum (<4%) its activation in outdoor conditions is limited. As an alternative, several efforts have been carried out in order to utilize the visible light from the solar spectrum, such as the modification of TiO2 with transition metals or non-metallic anionic species [5–۱۰]. In this context, Mosquera et al. synthesized composites based on SiO2- TiO2 with small amounts of precious metals (Ag and Au), which were added on stones [8,9]. They found a good synergy and a high self-cleaning efficiency between the semiconductor and the precious metals under visible light irradiation. Also, Cohen et al fabricated a Portland cement blended with Titanium Oxynitride (TiO2 xNy) nanoparticles with self-cleaning ability (40%) under visible light activation [10]. Alternatively, LiNbO3 and WO3/TiO2 have been coated on concrete for self-depolluting surface [11,12].