Sustainable Solar-Reflective Ceramic Engobes Based on Secondary Raw Materials

The ceramic tile industry is increasingly required to reduce its environmental impact while maintaining high technological and aesthetic standards. In this context, the use of secondary raw materials (SRMs) represents a promising strategy to decrease the consumption of virgin resources and the energy demand associated with conventional frit production. At the same time, solar-reflective engobes can contribute to passive cooling by limiting solar heat absorption and mitigating the urban heat island effect. In this study, white solar-reflective engobes were developed by incorporating at least 8 wt.% of SRMs, including various recycled glass streams, ceramic wastes, and yttria-stabilized zirconia residues. The results demonstrate that optimized formulations achieve high solar reflectance values (up to 0.79) while maintaining the technological and aesthetic requirements of industrial ceramic tiles. Recycled glasses act as effective fluxing agents, whereas waste zirconia enhances optical performance due to its strong light-scattering capability. The most promising formulations were validated at the industrial scale, confirming their applicability under real production conditions. Overall, the developed engobes represent a scalable alternative to traditional frit-based systems, enabling reduced resource consumption and supporting the development of energy-efficient ceramic surfaces.