Durability and air-depolluting performance of lime-based mortars treated with water- and oil-repellent coatings Incorporating Nano-Zn₂TiO₄–ZnO and Nano-Bi₁₂ZnO₂₀–ZnO

Protecting porous mortar-based materials from moisture and environmental pollutants remains a major challenge in both heritage conservation and modern construction. This work presents multifunctional surface coatings that combine self-cleaning, photocatalytic, and water/oil-repellent properties to enhance the durability of lime and lime–cement mortars. The coatings were formulated by incorporating nano-heterostructured photocatalysts—Zn₂TiO₄–ZnO (TiZn) and Bi₁₂ZnO₂₀–ZnO (BiZn)—into two types of protective formulations: water-repellent (W) and combined water- and oil-repellent (O). Three non-ionic surfactants were additionally tested to improve photocatalyst dispersion. The coatings were applied to mortar substrates and evaluated for contact angle, photocatalytic NOx removal, and durability after artificial ageing. BiZn-W coatings achieved the best performance, reaching water contact angles of 121° and removing up to 25.9% of NOx, particularly on lime mortars. O-based formulations enhanced oleophobicity (up to 101° for sunflower oil) while retaining good photocatalytic activity. SEM and spectroscopy confirmed both photocatalyst functionality and coating degradation patterns after ageing. Despite surface damage, coatings retained significant activity, highlighting their potential for long-term protection. These results demonstrate a scalable strategy to create multifunctional, sustainable coatings for extending the service life of both historical and contemporary mortar-based structures.