Zinc oxide in photocatalytic cementitious materials for environmental remediation and self-cleaning: Performance, limitations, and critical research gaps

Zinc oxide (ZnO) is a promising functional additive for cementitious materials due to its photocatalytic and antimicrobial properties. While numerous studies have reported positive outcomes in terms of pollutant degradation efficiency, antimicrobial performance, and, in some cases, mechanical enhancement, the practical application of ZnO in construction materials remains limited. This review critically examines the current state of the art in ZnO-modified cementitious materials (ZnO-CM), focusing particularly on their role in environmental remediation through photocatalytic and antimicrobial activity. The analysis focuses on the interaction with the cement matrix, functional performance, long-term stability, and the main barriers to practical application. A bibliometric analysis was also conducted to identify research trends and emerging directions in this field. The available evidence suggests that the photocatalytic performance of ZnO-CM is influenced by a combination of factors, including particle design, synthesis route, morphology, dosage, incorporation strategy, interaction with the cement matrix, environmental exposure and testing methodology, rather than being solely governed by the intrinsic properties of ZnO. Several limitations have been identified, including alkaline instability, photocorrosion, progressive encapsulation within hydration products, surface ageing, and methodological variability among studies. Additionally, the potential release of Zn²⁺ ions raises concerns about long-term environmental safety. Overall, this review identifies the key physicochemical factors that govern ZnO functionality in cementitious materials and outlines the main research needs related to durability, environmental safety, and their practical implementation.