Structural-Thermal Decoupled Cement-Based Aerogels with Negative Poisson's Ratio for Passive Radiative Cooling

Passive radiative cooling (PRC) holds immense promise for sustainable building thermal management but faces a critical material paradox: high-performance optical coolers are typically mechanically fragile, while robust structural materials lack spectral selectivity. Herein, a structural-thermal decoupling strategy is proposed to resolve this conflict by developing a bio-inspired, mechanically robust, and spectrally selective cement-based cooling aerogel (CCA). Central to this design is the identification of synthesized ettringite (AFt), a fundamental cement hydration product-not only as a rigid mechanical skeleton but also as an intrinsic infrared emitter derived from its abundant sulfate and hydroxyl groups. Mimicking the hierarchical organic-inorganic interplay in natural structural composites, we construct a directionally aligned honeycomb architecture by chemically bridging rigid, infrared-active AFt nanocrystals with a tough poly(vinyl alcohol)/para-aramid nanofiber ligament network. This anisotropic design enables the CCA to reach a compressive stress of 43.25 MPa at 80% strain along the alignment direction and an ultralow thermal conductivity of 0.031 W m-1 K-1 perpendicular to the alignment. Crucially, the material exhibits a strain-dependent negative Poisson's ratio (-0.165) under compression, conferring exceptional structural resilience and impact resistance. Synergistically, the multiscale pore scattering and intrinsic molecular vibrations grant the CCA a high solar reflectance of 0.91 and a mid-infrared emissivity of 0.96. In an outdoor model-house test, the CCA roof reduced the indoor temperature by up to 8.5 °C compared with the bare cement-roof control. Coupled with robust environmental durability, this work establishes a paradigm for transforming fragile photonic coolers into resilient.