China's Self-Cooling Cement Reflects 96.2% of Sunlight, Offering a New Way to Cool Buildings

A Cooler Surface Without Electricity

Researchers at Southeast University in China have developed cement that reflects 96.2% of sunlight and efficiently releases heat, allowing its surface to stay cooler than the surrounding air even in direct sunshine. In a rooftop test at Purdue University in Indiana, the material's surface measured 5.4°C (9.72°F) below the air temperature during peak sunlight, while conventional cement became much hotter.

The research, published in Science Advances, offers a possible way to reduce the electricity buildings need for air conditioning. Instead of using a powered cooling system, the cement combines two properties: it sends most incoming sunlight back away from the surface and releases heat as infrared radiation.

The material also addresses a familiar problem in cities. Conventional cement absorbs and stores heat from the sun, helping make buildings uncomfortable and contributing to the urban heat island effect. This is the tendency for cities to become warmer than surrounding rural areas because they have dense infrastructure and limited vegetation.

The new cement could be used on roofs and walls, in coatings and in structural components. Its developers describe a manufacturing process that can be expanded for wider production, alongside resistance to several forms of environmental damage. They also estimate that making it would cost about $5 less per tonne than making traditional Portland cement.

As of August 22, 2025, the work combines a demonstrated cooling effect with projections about energy savings and carbon emissions. The practical benefits for a particular building would depend on its climate, the condition of the material over time and how the cooling surface fits into the surrounding city.

How Tiny Crystals Change the Cement

The cement's cooling ability begins with changes much smaller than the features visible on a finished roof or wall. Researchers adjusted the chemical composition of its clinker particles and applied pressure during fabrication. The process encourages tiny crystals of a mineral called ettringite to assemble themselves within the material.

Those reflective crystals work with a network of hierarchical pores, meaning pores arranged at different scales. Together, the crystals and pores create a carefully engineered microscopic structure that scatters incoming sunlight. The researchers describe this structure as a metasurface, a surface designed to control how it interacts with light and heat.

The result is a material with two high performance measurements: solar reflectance of 96.2% and infrared emissivity of 96%. Solar reflectance describes how much sunlight a surface sends back instead of absorbing. Infrared emissivity describes how efficiently it releases heat as infrared radiation.

For a reader picturing an ordinary building, the simplest explanation is that the cement acts as both a mirror and a radiator. Its reflective structure reduces the solar energy entering the material, while its ability to emit infrared radiation helps heat leave the surface.

The two functions work together. Keeping sunlight out reduces the heat the cement must deal with, and releasing heat helps the surface remain cool. This combination is central to the effort to achieve temperatures below the surrounding air, including when the sun is shining directly on the building.

Where the Heat Goes

Radiative cooling allows a surface to release heat as infrared radiation through the Earth's atmosphere and toward space. The part of the atmosphere through which this radiation can escape is often called the atmospheric window. The new cement is designed to make effective use of that route.

Air conditioning needs electricity to provide cooling. Radiative cooling is passive: the cooling process requires no energy input. The cement's high sunlight reflectance and strong infrared emission help it release heat while limiting the amount it gains from the sun.

Clear skies and dry air provide more favorable conditions because the atmosphere is more transparent to the relevant infrared radiation. Humid or polluted conditions can reduce the cooling effect. This makes location an important part of deciding where the material would be most useful.

The ability to release heat also explains why the material's performance involves more than a bright surface. Its microscopic design aims to manage both the sunlight arriving at the building and the heat leaving it. Those processes determine whether the surface simply becomes less hot or can fall below the surrounding air temperature.

What the Rooftop Test Found

The Purdue University rooftop test provides the clearest practical result described for the cement. Under peak sunlight on a hot rooftop in Indiana, the surface reached a temperature 5.4°C below the ambient air. Conventional cement under the test conditions became much hotter.

Ambient air temperature means the temperature of the surrounding air. The reported difference therefore concerns the cement surface and the air around it. Keeping an exterior surface cooler can reduce the heat reaching a building, easing the demand placed on air conditioning and helping improve comfort indoors.

The material also maintained high strength and resistance to abrasion. Tests exposed it to corrosive liquids, ultraviolet radiation and repeated freezing and thawing, with the cement remaining stable. These properties matter for a material expected to face weather and physical wear as part of a building.

Architectural appearance is another consideration. The cement can be colored without a substantial loss of its reflective properties, allowing a wider range of uses than a material restricted to a single finish. Potential applications include coatings as well as structural components.

Longer use introduces a different challenge. Dust, dirt and weathering can reduce the reflective and emissive properties of cool roof materials. Maintenance and further material improvements will therefore matter alongside the strength and environmental resistance demonstrated in testing.

How It Compares With Other Cooling Methods

Reflective, evaporative and radiative cooling all aim to keep surfaces or buildings cooler, but they work through different processes. The new cement belongs to the radiative category while also making extensive use of sunlight reflection.

• Reflective cooling uses surfaces or coatings that bounce sunlight away, reducing heat absorption. Reflection alone does not actively release heat through the radiative cooling process described here, and its effectiveness can be more limited in extremely hot climates.
• Evaporative cooling uses the evaporation of water to cool a surface. It needs a water source, and its effectiveness falls in humid conditions.
• Radiative cooling releases heat as infrared radiation through the atmospheric window toward space. A surface designed for this process can provide cooling even in direct sunlight.

Field comparisons described for radiative cooling found stronger performance than reflective and evaporative methods, especially in hot, dry climates. A comparative study in Chongqing, China, found that radiative cooling surfaces achieved exterior temperatures 2°C below ambient air during working hours, outperforming the other two approaches.

The Chongqing result provides a useful comparison with the 5.4°C difference measured for the cement at Purdue. Both describe surfaces cooler than the surrounding air, but they concern different tests and conditions. Climate remains central to how much cooling a building can gain.

Energy Bills and the Cost of Installation

Studies of supercool roofs estimate annual savings of between 42.9 and 97.8 kilowatt hours of cooling electricity per square meter. A kilowatt hour is a unit used to measure electricity consumption. Reducing that consumption means less demand for powered cooling and potentially lower electricity bills.

The same studies estimate annual carbon emission reductions of 24.6 to 56.1 kilograms per square meter. Those figures describe the potential benefits of supercool roofs, while the cement offers a material that could help bring passive cooling into roofs, walls and other building surfaces.

Installation costs still matter. The extra expense of installing a supercool roof can be offset by its energy savings, with estimated payback periods as short as five years in hot cities. Payback means the time needed for savings to match the additional installation cost.

The cement's production economics offer another possible advantage. It can be manufactured at lower temperatures than traditional Portland cement, and its production cost is estimated to be about $5 less per tonne. A process described as scalable could support its use in both new construction and upgrades to existing buildings.

For building occupants, the intended benefit is concrete: cooler exterior surfaces, less need for air conditioning and more comfortable indoor conditions. For electricity systems, lower cooling demand could ease some of the pressure created by increasingly frequent and severe heatwaves.

Why the Carbon Calculation Goes Beyond Production

Cement manufacturing accounts for about 8% of global carbon dioxide emissions. Buildings are also a major part of the wider energy picture, responsible for roughly 40% of global energy consumption and 36% of carbon emissions. A cooling cement therefore connects two substantial sources of environmental pressure: construction materials and the energy used after construction.

The researchers used a life cycle assessment guided by machine learning to examine the material's potential carbon balance. A life cycle assessment follows environmental costs and savings over time, rather than looking only at what happens in the factory.

Their assessment suggests that widespread use of the cement could produce a net negative carbon emission profile over a building's lifespan. In this calculation, the carbon savings from reduced energy use could exceed the emissions generated during production.

The proposed benefit comes from avoiding emissions through lower cooling demand. A building using less electricity for air conditioning can accumulate savings over years of operation, potentially outweighing the carbon cost associated with making the cooling material.

The researchers' projection consequently gives the material a role beyond reducing summer electricity bills. Under the conditions examined in the assessment, it could help buildings move beyond carbon neutrality toward a negative carbon balance over their lifespan.

What Cities Must Consider

Roofs and walls covered with cooler materials could help reduce the heat that buildings release into surrounding streets. That could ease the urban heat island effect, improve urban comfort and reduce cooling demand at a time when heatwaves are putting pressure on infrastructure and public health systems.

City design still matters. Widespread adoption of cooling surfaces could alter local wind patterns, light pollution and the accumulation of pollutants. Urban planning would need to account for those effects while seeking the benefits of lower surface temperatures.

Oscar Brousse, a building scientist at University College London, described both the material's combined cooling functions and the need to understand how they perform over time:

The material increases both reflectivity and emissivity, so any energy captured or conducted to the material is emitted efficiently back. However, further testing is needed to fully understand its long-term performance in real-world conditions.

Durability, cleanliness and local weather would all affect performance in daily use. A surface exposed to dust and pollution may need maintenance to retain its cooling properties, while humid conditions can reduce the amount of heat that escapes through the atmosphere.

Further testing is needed to understand performance over long periods in ordinary building conditions. The research describes promising cooling and durability results, alongside a need to examine how the material behaves through continued exposure and use.

A Building Material With Wider Uses

The cement is part of a wider movement toward passive cooling and buildings that use less energy. Advances in nanophotonics, which deals with light at very small scales, and metamaterials, materials whose engineered structures control their behavior, have supported the development of supercool roofs and roofs that adapt to temperature.

These technologies can keep surfaces below ambient air temperature under direct sunlight. Their intended benefits include lower building cooling demand, reduced carbon dioxide emissions, better urban air quality and greater resilience to climate change.

China's rapid urbanization and industrialization have increased both energy use and emissions. A material that combines ordinary construction uses with passive cooling could contribute to the country's carbon neutrality goals, while also serving cities elsewhere that face extreme heat and high energy costs.

The researchers describe a material with strong sunlight reflection, efficient heat release, resistance to damage and a potentially favorable production cost. Turning those properties into dependable savings for buildings will depend on climate, maintenance and careful use within the urban environment.

Key Points

• Southeast University researchers developed cement with 96.2% solar reflectance and 96% infrared emissivity.
• A Purdue University rooftop test recorded a surface temperature 5.4°C (9.72°F) below surrounding air during peak sunlight.
• Ettringite crystals and pores at different scales help the material reflect sunlight and release heat without electricity.
• The cement remained stable after exposure to corrosive liquids, ultraviolet radiation and freezing and thawing, and can be colored with little loss of reflectivity.
• Production at lower temperatures could make it about $5 per tonne cheaper than traditional Portland cement.
• A life cycle assessment suggests that energy savings could outweigh production emissions over a building's lifespan.
• Climate, urban design, dirt and weathering will affect performance, and further testing over longer periods is needed.