Superhydrophobic surfaces for the sustainable maintenance of building materials and stone-built heritage: The challenges, opportunities and perspectives

Bio-inspired superhydrophobic surfaces have demonstrated great potential for functional applications across a wide range of fields, including the surface maintenance of building materials. In the outdoor environment, the degradation of building materials, such as concretes, stones, bricks, tiles and mortars, poses severe structural, functional and aesthetic risks to the entire construction, raising growing concerns worldwide. Superhydrophobic surfaces are ideal multifunctional protective coatings, owing to the inhibition of liquid adhesion/penetration, spontaneous surface self-cleaning and hindering the adhesion of bacterial cells to surfaces. Yet, despite the appealing multi-functionalities and the large number of materials reported in recent years, several drawbacks that hamper wide production and application remain unresolved, e.g., poor chemical/mechanical/weathering durability, low transparency, insufficient antimicrobial effect in humid environments, toxic and environmentally unfriendly raw materials upon fabrication. In this review, the key bottlenecks identified after tentative applications are summarized underlying the underpinning mechanisms in depth. The newly proposed emerging strategies for addressing the specific limitations are then categorized and discussed in detail. Additionally, taking into account the physicochemical properties of building materials, the particular requirements concerning stone-built heritage conservation and the outdoor environment, the feasibility and the pros and cons of novel strategies are critically reviewed, outlining the future prospects of the field.