What exactly is self-cleaning concrete, and how does it work?

The Jubilee Church in Rome is coated in self-cleaning cement. Image: Psidium

In recent months, a new building material that claims to both clean itself and filter pollutants out of the air around has been popping up on new buildings and infrastructure. It's one of those advances in construction technology that does actually seem, well, really good. 

As a result of its self-cleaning abilities, the concrete keeps its colour for far longer than traditional building materials, so doesn't need to be replaced so often; but it can also reduce general air pollution. Which, to be honest, sounds like a bit of a win-win. But how exactly does the magic work? And is there a catch?

How was it invented?

The technology was actually invented pretty much by accident, by Luigi Cassar, an Italian chemist at cement manufacturer Italcementi. He was trying to create a construction material which would keep a bright white colour even in polluted conditions, and hit upon a method called "photocatalysis", which uses the sun's energy to zap away dirt. 

To his surprise, when the air around the treated concrete was tested, it contained up to 80 per cent less nitrous oxide: the concrete was cleaning the air as well as itself.

How does it work? (Warning: science.)

When we clean stuff, we tend to use a substance which can break down dirt so it can be washed from the object's surface, plus a bit of energy to make sure that reaction happens. When you scrub a plate, for example, you use soap and water, plus your own elbow grease, to remove dirt. 


On the surface of self-cleaning cement, the cleaning happens without any scrubbing involved. The secret? The power of the sun.

When light and heat strikes the concrete's surface, catalysts (usually titanium oxides) use that energy to break down the dirt into molecules like oxygen, water, carbon dioxide, nitrates, and sulphates. Gases float away, while liquids or solids are left on surface to be washed away by rain. 

Through a similar process, concrete can also break down pollutants in the air around it: if a pollutant strikes the surface, the titanium oxide reacts with it in the same way.

This diagram shows a nitrogen oxide hitting the surface and being converted into a nitrate:

So what's the catch?

Other scientists have dug into the theory behind self-cleaning cement and found a few problems.

1. Eagle-eyed readers might have already worked this one out: if those new substances left on the surface of the cement are "washed away", where exactly do they go? Unfortunately, the answer is probably "into groundwater, and then rivers and lakes". This is bad news when it comes to nitrates, which cause algae blooms and in turn deplete the body of water’s oxygen levels. 

2. Researchers from Indiana University found that, while the cement does what it says on the tin in specific lab conditions, it reacts quite differently if the humidity or level of pollution is lower. In fact, they found that, in lower pollution levels, the titanium dioxide would catalyse a reaction with ammonia which actually increases nitrogen oxides in the atmosphere.

3. The kilns used to make cement actually give off large amounts of nitrogen oxides and sulphur dioxide, which means the cement would have to be pretty effective (despite the limitations outlined above) to result in a net decrease in atmospheric nitrogen.

So in summary: yes, these compounds do a good job of keeping buildings clean and white. But only time will tell whether they're 100 per cent brilliant for the environment.


 

 
 
 
 

To beat rising temperatures, Vienna launches a network of 'Cool Streets'

A Vienna resident cools off at one of the city's new Cool Streets installations. (Courtesy Christian Fürthner/Mobilitätsagentur Wien)

Over the past several months, Austria has recorded its highest unemployment rate since World War II, thanks to the economic aftermath of the Covid-19 pandemic. With no job or a suddenly smaller income – not to mention the continued threat of the virus – many Viennese will opt for a staycation this summer.  

At the same time, last year, Austria’s capital experienced 39 days with temperatures of over 30°C (86°F), one of its hottest summers in history according to the Central Institute for Meteorology and Geodynamics.

Climate experts expect a similarly sizzling 2020 season, and city officials are now doubling down on efforts to combat the heat by launching a “Cool Streets” initiative as well as a new, state-of-the-art cooling park.

“As the city councilwoman in charge of climate, it is my job to ensure local cooling,” Vienna’s deputy mayor Birgit Hebein proclaimed at the opening of one of 22 new “Cool Streets” on 22 June.

“In Austria, there are already more heat deaths than traffic fatalities,” she added.

Hebein was referring to the 766 people the Austrian Agency for Health and Food Security included in its 2018 heat-associated mortality statistics. The number was up by 31% compared to 2017, and in contrast to the 409 people who died in traffic collisions the same year.

The project includes 18 temporary Cool Streets located across the city, plus four roads that will be redesigned permanently and designated as “Cool Streets Plus”.

“The Plus version includes the planting of trees. Brighter surfaces, which reflect less heat, replace asphalt in addition to the installation of shadow or water elements,” said Kathrin Ivancsits, spokeswoman for the city-owned bureau Mobilitätsagentur, which is coordinating the project.


Vienna's seasonal Cool Streets provide shady places to rest and are closed to cars. (Petra Loho for CityMetric)

In addition to mobile shade dispensers and seating possibilities amid more greenery provided by potted plants, each street features a steel column offering drinking water and spray cooling. The temporary Cool Streets will also remain car-free until 20 September.

A sensor in the granite base releases drinking water and pushes it through 34 nozzles whenever the outside temperature reaches 25°C (77°F) . As soon as the ambient temperature drops to 23°C (73°F), the sensor, which operates from 10 a.m. to 8 p.m., turns off the water supply.

The sensors were included in part to allay concerns about legionella, a pathogenic bacteria that can reproduce in water.  

“When the spray stops, the system drains, and therefore no microbial contamination can develop,” said Dr. Hans-Peter Hutter, deputy head of the Department of Environmental Health at the Center for Public Health at Medical University Vienna, in a televised interview.

Hutter also assured the public that there is no increased risk of a Covid-19 infection from the spray as long as people adhere to the one-meter social distance requirement.


But Samer Bagaeen of the University of Kent's School of Architecture and Planning notes that air cooling systems, like the ones used in Germany at abattoirs, have been found recently to be a risk factor for Covid-19 outbreaks.

“The same could be said for spay devices,” he warned.

Vienna’s district councils selected the 22 Cool Street locations with the help of the city’s Urban Heat Vulnerability Index. The map shows where most people suffer from heat by evaluating temperature data, green and water-related infrastructure, and demographic data.

“Urban heat islands can occur when cities replace the natural land cover with dense concentrations of pavement, buildings, and other surfaces that absorb and retain heat,” as the US Environmental Protection Agency states.


A rendering of Vienna's planned park featuring a Coolspot, which is scheduled to open in August. Click to expand.
(Courtesy Carla Lo Landscape Architecture)

Vienna’s sixth district, Mariahilf, is such an area. The construction of the capital’s first “Cooling Park”, a €1 million project covering the 10,600 square-metre Esterházypark, is designed to provide relief. 

Green4Cities, a centre of excellence for green infrastructure in urban areas, designed the park’s main attraction, the “Coolspot”. The nearly 3.40-metre high steel trellis holds three rings equipped with spray nozzles. Textile shading slats, tensioned with steel cables, cover them.

The effects of evaporation and evapotranspiration create a cooler microclimate around the 30 square-metre seating area, alongside other spray spots selectively scattered across the park.

The high-pressure spray also deposits tiny droplets on plant and tree leaves, which stimulates them to sweat even more. All together, these collective measures help to cool their surroundings by up to six degrees.

The landscape architect Carla Lo and her team planned what she calls the “low-tech” park components. “Plants are an essential design element of the Cooling Park,” Lo says. “By unsealing the [soil], we can add new grass, herbaceous beds, and more climate-resistant trees to the existing cultivation”.

Light-coloured, natural stone punctuated by grass seams replaces the old concrete surfaces, and wooden benches meander throughout the park.

Living near the park and yearning for an urban escape close by, Lo says she’s motivated to ensure the park is completed by mid-August.

“If we don't do anything, Vienna will be another eight degrees Celsius hotter in 2050 than it already is,” Hebein said.

Vienna recently came in first in the World's 10 Greenest Cities Index by the consulting agency Resonance.

“There is no one size fits all on how cities respond to urban heat,” says the University of Kent’s Bagaeen, who points out that Vienna was one of the first European cities to set up an Urban Heat Islands Strategic Plan in 2015.

In the short term, prognoses on the city’s future development may be more difficult: Vienna votes this autumn.

Petra Loho is a journalist and photographer based in Austria.