Biogenic SiO₂ Nanoparticles Derived from Diatomaceous Biosludge for Sub-ambient Radiative Cooling Geopolymer Coatings
J. Environ. Nanotechnol., Volume 15, No 1 (March 2026) pp. 643-657
Abstract
The development of non-polymeric, environmentally resilient radiative cooling materials is critical for reducing passive heat loads in buildings. This study reports the novel use of biogenic SiO₂ nanoparticles derived from diatomaceous biosludge as multifunctional modifiers for geopolymer-based radiative cooling coatings. The biosludge-derived silica was processed into nanoscale SiO₂ and functionalized using 3-aminopropyltriethoxysilane (APTES) to improve dispersion and interfacial compatibility within the geopolymer matrix. Three systems were investigated: a plain geopolymer, a geopolymer with unfunctionalized biogenic nano-SiO₂, and a geopolymer with APTES-functionalized biogenic nano-SiO₂. Characterization (XRD, FTIR, SEM/EDX, TGA, UV–Vis–NIR, and infrared emissivity) revealed that biogenic nano-SiO₂ actively participated in geopolymer gel formation, refining the pore structure and improving thermal stability. APTES-functionalized biogenic SiO₂ produced the most pronounced improvements, yielding a densely crosslinked aluminosilicate matrix with enhanced broadband solar reflectance (~36%), near-unity infrared emissivity (8–13 μm), and the highest compressive strength (32.4 MPa). Outdoor field measurements confirmed that the APTES-functionalized coating consistently achieved 2–3 °C sub-ambient daytime cooling under direct solar exposure, whereas the plain geopolymer exhibited above-ambient temperatures. These results demonstrate that waste-derived, surface-functionalized biogenic silica provides a sustainable, scalable route for tuning the structural and optical properties of geopolymers for passive thermal management.
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