TiO2-Based Mortars for Rendering Building Envelopes: A Review of the Surface Finishing for Sustainability

Abstract

Building envelopes coated with TiO2-based mortars benefit from depolluting, antibiological and self-cleaning effects. Therefore, photocatalytic renders are allies in the quest for sustainability in the built environment, potentially combatting atmospheric pollution, enhancing durability and reducing maintenance needs. Surface finishing characteristics of the renders influence their photocatalytic efficiency and esthetic and functional properties. In this context, this study reviews the existing literature, focusing on proven surface-affecting parameters, the surface and color of TiO2-based mortars, to explore their impacts on photoactive behavior. The incorporation of TiO2 within an additional surface layer and its mixture into the mortar in bulk were observed for surface roughness. Mainly the addition of TiO2 during casting was identified in colored mortars. Generally, a moderate surface roughness led to better photoactivity; microroughness affected self-cleaning by facilitating dirt deposition. The interaction between the surface roughness and the photocatalytic layer affected the water contact angle, regarding superhydrophilicity or superhydrophobicity. The photoactivity of colored mortars with TiO2 depended on the color and amount of the added pigments, which influenced electron–hole recombination, physically occupied active sites or, on the other hand, led to a higher formation of reactive radicals. Surface finishing can thus be designed to enhance the photoactivity of TiO2-based mortars, which is fundamental for current climate concerns and emphasizes the need for life cycle assessments and environmental protection.