Scalable Hydrogel‐Programmed Porous Photonic Cement for Passive Daytime Cooling

Cement is among the most widely produced materials on Earth, yet its strong solar absorption causes building envelopes to act as heat sources, increasing cooling energy demand. Here, we structurally program cement hydration to transform conventional cementitious materials into passive cooling interfaces. Through hydrogel-confined mineralization of cement clinker, we construct a robust, hierarchically porous calcium-silicate-hydrate (C-S-H) network that integrates broadband light scattering, vapor transport and thermal insulation. By regulating the composition and crystallinity of the hydrates, the material achieves ∼95% solar reflectance and ∼98% infrared emissivity without the use of additional photonic fillers. The hygroscopic nature of the network further enables sustained evaporative cooling under humid or cloudy conditions where radiative cooling alone becomes inefficient. Life-cycle analysis indicates that this strategy provides a pathway to transform high-emission cement into carbon-mitigating materials integrated within opaque building envelopes, enabling the scalable deployment of daytime radiative cooling across the built environment.