Tech It Out: Self-cleaning concrete helps prolong buildings' life cycle
Updated 16:02, 02-Oct-2020
By Yang Zhao, Wang Yue

Over 2,000 years ago, the Romans invented what we now know today as cement-based concrete. It is one of the world's most crucial building materials, and is also the most prone to environmental risks.

With extensive exposure to air, rain and pollution, concrete structures tend to get dirty, discolored, and fractured. But scientists have found the future cement to help keep buildings safe. 

Xu Xin, a professor at the University of Science and Technology of China, said, the material's surface can absorb water. When people reduce cement's ability to retain water, it becomes hydrophobic.

Hydrophobic literally means "fear of water." Such surfaces exist in nature, from lotus leaves to insect wings. Their special structure and chemical make-up allow them to repel not only water, but also dust and pollutants. 

Scientists have been able to emulate these super-hydrophobic surfaces in the lab, for application to daily items like clothes and shoes. At the core of these products is a highly water-repellent silicone polymer, known as PDMS. 

"PDMS is an industrial product, which can be mass-produced and applied as a layer to protect surfaces," Xu told CGTN.

But how does PDMS polymer work to make an entire building water-repellent? It's not as easy as slathering it on walls, as you would with paint, because top layers would get worn away over time.

What if PDMS is mixed with cement?

"If you mix it with cement or completely cover a building with PDMS, there wouldn't be many problems, such as equal distribution. The aggregation of PDMS can lead to cracks instead," Xu added.

So how can we effectively get PDMS polymer into concrete? 

Professor Xu said the "perfect carrier" for this water-resistant substance is oil. But many of us know that oil and water don't mix. 

So how can this mixture happen to create self-cleaning concrete? 

Let me expound, with my favorite summer treat: ice cream – it's the perfect example of a marriage between oil and water, also known as emulsion. Through the use of emulsifiers, the process works by stabilizing a mixture of fluids to help them unite and stay together. Therefore, it's not entirely true that oil and water don't mix. They can. With a little bit of help.

What would happen after sufficient emulsification? Vast amounts of tiny oil droplets would emerge in the water. Xu said they could add cement and stir the mixture.

Now those countless oil droplets which carry hydrophobic particles are everywhere in the slurry. When it is dried and heated, the oil droplets will evaporate and leaves behind those tiny pores coated with hydrophobic polymer.

The end result is new lightweight and water-resistant concrete.

It can repel dust particles and liquids other than water, including milk, beer, soy sauce, coffee, and colored water. From the surface to the inside, the material keeps its super hydrophobic quality. Even after it's ground into powder.

The pore size of regular concrete is about one millimeter. Xu's team was able to make it 50 times smaller. This decrease in pore size enhances its capacity for thermal resistance and sound absorption.

Regular concrete undergoes a curing process for it to harden and gain strength, which involves days of keeping it moist with water. However, in the case of this new concrete it is impossible to add water into it once the super hydrophobic surface has been generated.

Researchers are now working to change this, so the material can go through the same curing process as regular concrete – in turn, making it stronger and more efficient.

Scientists are also looking to enhance the material from being self-cleaning to a medical level, where it can not only resist water and dirt but also bacteria. 

Damp surfaces are the perfect breeding ground for bacteria. If a surface is dry, there are less chances of contamination. Xu believes there's still much work to do if scientists want to expand the practical applications of this new concrete.