The longevity of geopolymer-based radiative cooling coatings is questionable due to their hydrophilic nature. This study used four hydrophobic agents - sodium methyl silicate (SMS), polydimethylsiloxane (PDMS), polytetrafluoroethylene (PTFE), and triethoxyoctylsilane (TEOS) - to modify the surface hydrophobicity of geopolymer while maintaining its optical properties. The optimal agent contents were determined by optical performance and water contact angle: 5 % for SMS, 10 % for 50 cP viscosity PDMS, and a single-layer thickness for PTFE and TEOS (60 degrees C). The agents' working mechanisms were analyzed using FTIR and XRD characterization: SMS forms a hydrophobic alkylsilanol layer by reacting with the geopolymer's silanol group; PDMS lowers the geopolymer's surface energy with its hydrophobic methyl groups; PTFE's low electric polarizability is due to its fluorine content; and TEOS replaces the geopolymer surface's hydroxyl groups with hydrophobic octyl groups. The long-term durability of these modified coatings was evaluated through outdoor exposure tests, resulting in total solar reflectance losses of 0.74 %, 1.9 %, 3.9 %, and 0.05 % respectively. A slight reduction in the water contact angle confirmed their enduring hydrophobic characteristics. These modification methods open up possibilities for the practical use of hydrophobic geopolymer radiative cooling coatings.

Passive sub-ambient daytime radiative cooling (SDRC) is an energy-free method enabling heat dissipation away from buildings or infrastructures to cold outer space (similar to 3 K). However, although organic coatings (based on polymer matrices) have been proven to be a cost-effective means to achieve the SDRC effect by incorporating various functional fillers, organic coatings may have environmental concerns and aging problems. Herein, an inorganic phosphate activated geopolymer-based (PAGP) SDRC coating was synthesized using nano-silica (SiO2) particles and barium sulfate (BaSO4) sheets as the modifiers. The synthesized coating could achieve high infrared emissivity of 0.9634 and solar reflectance of 0.9471, which was also found to exhibit high-temperature resistance and optical stability up to 1000 degrees C due to its covalently bonded structure and electrically neutral system. The chemical composition, surface morphology, and elemental distribution of the coating were characterized by XRD, FTIR, SEM, BET, and EDS. The evolution of condensed structures and weight loss analysis during the high-temperature treatment was conducted by TGA and XRD to analyze the dehydration process and phase change during the heat process. The outdoor field tests showed a maximum sub-ambient temperature reduction of 3.8 degrees C by the developed coating without any energy input under direct sunlight in Hong Kong. Furthermore, the energy-saving performance of the PAGP-based SDRC coating is analyzed under different climatic conditions in China with the aid of EnergyPlus. Simulation results demonstrated that the PAGP-based SDRC coating can significantly save the cooling load in all five selected cities, which shows great potential in building energy conservation and carbon emission reduction.

Passive daytime radiative cooling has emerged as a promising green technology for the thermal management of buildings, vehicles, textiles, and electronics. Typically, both high solar reflectance and high thermal emissivity are prerequisites to achieve sufficient daytime cooling. However, colored radiative cooling materials are facing the dilemma of introducing visible light absorption, leading to challenges in balancing cooling and aesthetic demands. Here, three colored bilayer radiative cooling coatings, each comprised of a white base layer and a colored top layer with fluorescence enhancement are fabricated. Three phosphors (Sr2Si5N8:Eu2+, Y3Al5O12:Ce3+, and (Ba,Sr)SiO4:Eu2+) are employed with respective photoluminescence quantum yields (PLQYs) of 81%, 95.8%, and 91.0% as the colored pigment in the top layer. To mitigate the contradiction between coloration and solar reflectance, SiO2 microspheres are introduced into the top layer and utilize their Mie-resonance-based multiple scattering to increase the photoluminescent (PL) properties of the phosphors, which jointly boosts the effective solar reflectance (ESR) of the top layer. As a result, the three bilayer coatings exhibit soft colors while achieving subambient cooling with temperature drops of up to 1.5 degrees C. This fluorescence-enhancement strategy may pave the way for preparing highly efficient radiative cooling coatings with tunable colors. The colored bilayer radiative cooling coatings consist of a top layer containing SiO2 microspheres and phosphors and a TiO2-based white bottom layer, where the SiO2 microspheres can modify the electromagnetic environment of the phosphors and hence accelerate their spontaneous emission rate through Purcell effect. The field test shows that the colored coatings can achieve subambient cooling with temperature drops of up to 1.5 degrees C. image

Radiative cooling coating technology is gaining increasing attention in the context of mitigating urban heat island and tackling global climate change. This paper is concerned with the development of an inorganic geopolymer-based radiative cooling coating using red mud, incorporating varying sizes of TiO 2 and hollow glass beads. The developed colored coating demonstrates its radiative cooling capability in reducing the surface temperature by harnessing the potential of red mud. The physico-chemical properties, surface morphology, and element distribution of the coating were characterized by XRD, FTIR, TG-DAT, SEM and EDS. The coating exhibited lower UV (0.0844) and visible reflectance (0.5275) but high NIR reflectance (0.7536), as well as high infrared emissivity of 0.9455, enabling a red color with a CIE value of L* = 74.56, a* = 9.55, and b* =11.98, and a maximum temperature reduction of 7.3 degrees C under the climate of Hong Kong. Additionally, the skid resistance, abrasive resistance and adhesion strength of the proposed coating could meet the requirements of standards, indicating a great potential for footway applications.

Sub-ambient daytime radiative cooling coating (SDRCC) is an appealing thermal management technology with great potential for alleviating the global warming and urban heat island effect. Over the past few years, various polymeric SDRCCs have been developed. However, they may face problems of environmental aging under UV and moisture due to their organic nature. In this study, an ambient-cured inorganic geopolymer-based SDRCC was synthesized with the modification of barium sulfate (BaSO4) and nano-silica (SiO2) particles. The optical and physicochemical properties were systematically investigated. The chemical composition, functional groups, surface morphologies of the raw materials, and the formed geopolymer coating were characterized by XRD, FTIR, SEM, and EDS. The developed coating exhibits a high infrared emissivity of 0.9491 and solar reflectance of 97.6%. When exposed to direct sunlight, the coating's surface could cool down up to 8.9 degrees C below the ambient air temperature under Hong Kong's climate. In addition, the coating could retain well its performance under a variety of harsh environments, including high temperature, water immersion, mechanical wearing, and exposure to sunlight.

A Purcell factor of 2.6, obtained by matching the emission wavelength of phosphor with the Mie scattering resonance of TiO2 nanoparticles, and small stokes shift improve the effective solar reflectance of fluorescent cooling coatings by up to ∼4%.

Among the different existing techniques, the surface coating on steel provides an economical means to address the corrosion issue. In this study, a geopolymer anti-corrosion coating modified with in-situ reduced graphene oxide (RGO) was developed, and its physicochemical and electrochemical properties were investigated. Four different graphene oxide (GO) contents (i.e., 0.05%, 0.1%, 0.5% and 1% of the binder by weight) were studied. The GO content in the coating was optimized based on its corrosion protection ability, and the performance of GO modified geopolymer coated steel was compared to that of bare steel, cement coated steel and blank geopolymer coated steel when immersed in a 3.5% NaCl solution. It was observed that using a GO content of 0.1% can increase the corrosion resistance of geopolymer-coated steel by more than two orders of magnitude compared to bare carbon steel. Additionally, the physiochemical properties of different coatings were investigated, and the corrosion protection mechanism with RGO modification has been further elaborated.