Key Takeaways & Executive Findings
- •• • CCP-30 achieves solar reflectance of ~93% in dry state and ~89% when wetted, with emissivity ~95% in the atmospheric window (8–13 μm), ensuring robust radiative cooling performance under humid conditions. • • Field tests in Singapore (RH ~80%) demonstrated a ~5°C lower surface temperature on CCP-coated facades compared to commercial radiative cooling paint, and an ~8°C reduction on a proximate black absorber, directly mitigating urban heat island effects. • • Building energy simulations indicated 30–40% more savings in air conditioning electricity consumption compared to commercial counterparts, with stable performance across varying weather conditions. • • The paint's cementitious matrix, reinforced with PVA and LiCl, inhibits plastic shrinkage and promotes continued hydration, yielding a dense, robust microstructure that ensures long-term durability and broad substrate adhesion, addressing the swelling and degradation issues of hydrogels.
Abstract
Passive radiative cooling dissipates heat through the atmospheric transparency window (8–13 μm) into cold outer space, offering energy-free building cooling. However, its performance degrades substantially in humid environments; for instance, in Singapore, where average relative humidity is ~80%, achievable cooling power can be as low as ~20 W/m², far below the theoretical maximum of ~150 W/m² under dry conditions. Restricted sky view factor on building facades further curtails efficiency. Evaporative cooling, leveraging water's high latent heat of vaporization (~2256 J/g), provides an omnidirectional heat dissipation pathway but porous materials like hydrogels suffer from swelling, poor adhesion, and structural degradation. Here, we report a cement-based integrated cooling paint (CCP) that synergistically combines radiative and evaporative cooling. The paint utilizes a calcium silicate hydrate (C-S-H) porous network matrix with barium sulfate nanoparticles, polyvinyl alcohol (PVA), and lithium chloride (LiCl). The optimized formulation (CCP-30) achieves high solar reflectance of ~93% in the dry state and maintains ~89% reflectance when wetted. Its high emissivity (~95%) within the atmospheric window ensures efficient radiative heat dissipation. PVA and LiCl inhibit plastic shrinkage and promote continued hydration, yielding a denser, robust microstructure. The interconnected porous structure and hygroscopic components enable passive water capture from rainfall and ambient moisture, driving sustained evaporative cooling. Field tests in Singapore showed a ~5°C lower surface temperature on CCP-coated facades compared to commercial radiative cooling paint, and an ~8°C reduction on a proximate black absorber, indicating mitigation of local heat island effects. Building energy simulations indicated 30–40% more savings in air conditioning electricity consumption. The paint is prepared via a simple one-pot method compatible with standard production, indicating excellent commercialization potential.
1. Introduction
Passive radiative cooling has emerged as a promising energy-free strategy for building thermal management, yet its real-world deployment faces a critical bottleneck: performance degradation in humid climates. In tropical regions such as Singapore, where relative humidity hovers around 80%, the achievable cooling power plummets to approximately 20 W/m²—a mere fraction of the theoretical maximum of ~150 W/m² under dry conditions. Additionally, the restricted sky view factor on vertical building facades severely limits radiative heat dissipation, rendering conventional radiative cooling coatings ineffective for high-rise urban environments. Evaporative cooling, which exploits water's high latent heat of vaporization (~2256 J/g), offers an omnidirectional heat dissipation pathway that could theoretically compensate for these radiative limitations. However, porous materials like hydrogels used for evaporative cooling suffer from swelling, poor adhesion, and structural degradation during repeated hydration-dehydration cycles, hindering their long-term reliability. The challenge, therefore, is to develop a coating that synergistically integrates both cooling mechanisms while maintaining mechanical robustness, adhesion, and ease of application.
This study addresses that bottleneck by introducing a cement-based integrated cooling paint (CCP) that leverages a calcium silicate hydrate (C-S-H) porous network as its matrix. By incorporating barium sulfate nanoparticles, polyvinyl alcohol (PVA), and lithium chloride (LiCl), the paint achieves high solar reflectance (~93% dry, ~89% wet) and high emissivity (~95%) within the atmospheric window. The cementitious matrix provides inherent mechanical strength and adhesion, while PVA and LiCl mitigate plastic shrinkage and promote continued hydration, ensuring durability. The interconnected porous structure, combined with hygroscopic components, enables passive water capture from rainfall and ambient moisture, driving sustained evaporative cooling. This dual-mode cooling approach not only overcomes the humidity-induced degradation of radiative cooling but also addresses the structural instability of hydrogels, offering a scalable, commercially viable solution for energy-efficient building cooling in humid urban environments.
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Qisheng Liu, Jinping Qu, Xiang Lu (2026). A 'Cooling Balm' for Humid Cities: Cement-Based Integrated Cooling Paint for Passive Radiative and Evaporative Cooling. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3907-0
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Frequently Asked Questions
What is the long-term durability of the cement-based cooling paint under repeated hydration-dehydration cycles, and how does it compare to hydrogel-based coatings?
The cement-based paint (CCP) is designed to overcome the swelling and structural degradation seen in hydrogels. The introduction of PVA and LiCl inhibits plastic shrinkage during cement curing and promotes continued hydration, resulting in a denser, more robust microstructure. Field tests in Singapore demonstrated stable performance across varying weather conditions, with no reported degradation over the test period. The cementitious matrix provides inherent mechanical strength and adhesion, ensuring long-term reliability.
How does the cooling performance of CCP in humid conditions (e.g., 80% RH) compare quantitatively to commercial radiative cooling paints?
In field tests in Singapore (average RH ~80%), a building facade coated with CCP showed a ~5°C lower surface temperature compared to one coated with a highly reflective commercial radiative cooling paint. Additionally, CCP lowered the temperature of a proximate black absorber by ~8°C, demonstrating superior performance in mitigating local heat island effects. The paint maintains ~89% solar reflectance even when wetted, whereas many radiative cooling paints lose reflectance upon moisture absorption.
What is the cost and scalability of the one-pot synthesis method for industrial production?
The paint is prepared using a simple one-pot method compatible with standard paint production specifications, indicating excellent commercialization potential. The raw materials—cement, barium sulfate nanoparticles, PVA, and LiCl—are low-cost and readily available. The process does not require specialized equipment, making it scalable to existing paint manufacturing lines. While exact cost figures are not provided, the use of commodity materials and standard processing suggests cost parity with or advantage over commercial radiative cooling paints.
How does the paint's mechanical adhesion to various substrates (e.g., concrete, metal, glass) perform under environmental stress?
The cementitious matrix formed by spontaneous hydration of cementitious materials ensures broad substrate adhesion. The addition of PVA and LiCl promotes continued hydration, leading to a denser microstructure that enhances mechanical integrity. While specific adhesion tests to different substrates are not detailed, the robust structure formed by cement hydration is known to bond well to porous building materials like concrete and masonry. For non-porous substrates, surface preparation may be required, but the paint's design targets building facades, which are typically cementitious or painted surfaces.
What is the mechanism by which the paint maintains high solar reflectance when wetted, and how does this affect evaporative cooling performance?
The high solar reflectance in the wet state (~89%) stems from the persistent synergistic scattering effect between barium sulfate nanoparticles and the porous C-S-H matrix, which remains effective even when pores are filled with water. The interconnected porous structure, combined with hygroscopic components (LiCl and PVA), enables passive water capture from rainfall and ambient moisture. This water storage provides the driving force for sustained evaporative cooling, which operates alongside radiative cooling. The dual-mode approach ensures that even if radiative cooling is reduced due to humidity, evaporative cooling compensates, maintaining overall cooling performance.
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