Photocatalytic cement-based passive cooling composites: The synergy between radiative cooling and self-cleaning mechanisms

Abstract

Radiative cooling holds promise in combating climate change by reducing energy consumption. However, organic radiative coolers face fabrication complexities, weak environmental reliability, and limited self-cleaning capabilities. To overcome these challenges, we present autoclaved cement pastes, inorganic passive coolers with exceptional energy-free cooling and self-cleaning properties, achieved through a straightforward hydrothermal process. The inorganic cooler incorporating 28 wt% alumina exhibits impressive solar reflectivity (0.91), atmospheric window emissivity (0.91), radiative cooling power (−11.0 W/m2), and effective evaporative cooling (>8 h, 25 W/m2). The high solar reflectivity is attributed to tailored nano/micro particles and pores inducing strong Mie scattering within the solar spectral range (λ = 280–2500 nm). The theoretical cooling properties are validated by the outdoor on-site measurements, and the roles of moisture inside the cooler matrix are verified. Additionally, the cooler formulation featuring 14 wt% nanosilica and 14 wt% alumina demonstrates remarkable self-cleaning capabilities, eliminating approximately 68% of surface contaminants within 30 min. This self-cleaning performance stems from the evolution of C–S–H phases, leading to the formation of massive nano C-(A)-S-H "honeycombs" with "nanowires", enhancing mass adsorption and photocatalytic quantum effects at abundant active sites. This study would open new avenues for scalable, energy-free cement-bound cooling materials for building engineering and align with the UN's sustainable development goals.