Reflective cement could help buildings stay cool in hot weather

A highly reflective cement could keep buildings cooler in hotter weather, alleviating the need for air-conditioning units. According to the researchers who developed the material, reflecting just 1% more of the sunlight that hits buildings and road surfaces worldwide would provide a planet-cooling effect equivalent to removing 44 billion tonnes of carbon dioxide from the atmosphere.

‘Traditional cement absorbs up to 70% of incident solar energy, which contributes to surface heating and energy consumption in buildings,’ says Guo Lu, from Southeast University in Nanjing, China. Reflective films and coatings on buildings’ surfaces can reduce solar absorption, but such materials often lack durability, insufficiently adhere to surfaces and have high costs.

Now, Lu and her colleagues believe they have tackled these issues with a new self-cooling cement. The researchers combine various calcium, aluminium and silicon-based minerals with water to create a paste, which is poured into a mould topped with a textured polymer film. Pumping the air out of the system creates microcavities on the material’s surface in which reflective crystals grow as the paste sets.

The cement is a strong reflector and emitter of infrared radiation, allowing the cement to remain cool. When exposed to peak daytime sunlight, the material’s surface stayed 5°C cooler than the surrounding air temperature of 38°C. Under the same conditions, commercial cement warmed up to 59°C.

The reflective cement is strong, stable across a range of pHs, and is adhesive – including to concrete, steel and ceramic. Edwin Chi-Yan Tso at City University of Hong Kong, who has previously worked on similar materials, believes that these properties offer ‘unprecedented practicality for real world-applications’ and ‘the research provides a blueprint for developing other cement materials for sustainable urbanisation.’

Lu’s team estimates that using the ‘supercool’ cement in place of traditional materials would reduce carbon emissions by up to nearly 2900kg per tonne of cement over the material’s 70-year life span.

Currently, the team can produce panels of the cement material, offering potential to coat existing buildings, with Lu explaining that they ‘have already built a production line with a company in China to make the raw materials of the cement, making use of existing infrastructure’. She adds that the team is now developing similar materials that can be both reflective in hotter months and retain heat in colder ones.