Emerging Materials – Shining a Light on Photocatalytic Concrete’s Air-Purifying Potential.

De-polluting, self-cleaning and self-disinfecting. Photocatalytic concrete is an innovative application of materials science that aims to leverage the power of light and chemistry to enhance concrete’s performance and environmental impact in an urban setting. 

Photocatalytic concrete paving blocks being laid in NeckarPark, Stuttgart. Credit: Adolf Blatt

The Problem with Air Pollution

Clean air is a basic requirement for human health. Particularly in urban settings, poor air quality is an issue that continues to worsen. It is the largest environmental risk to public health in the UK.

Air pollution is known to cause cardiovascular disease, lung cancer, asthma, chronic bronchitis and has even been linked to increased rates of diabetes and dementia. It is estimated that long-term exposure to man-made air pollution in the UK has an effect equivalent to 28,000-36,000 deaths annually.

Nitrogen oxides (NOx) are some of the most common pollutants in our atmosphere and are the primary pollutants emitted by the transport sector. They’re some of the most important emissions to reduce to resolve public health concerns. 

Concrete’s Role in Tackling Air Pollution

Concrete is the most widely used construction material on earth. The material’s versatility and integration into our daily surroundings make it ideally placed for ecological adaptations but also assigns a wider social responsibility for its design and manufacture to be sustainably implemented. 

There is huge scope for eco-positively engineered concrete. This has been a major focus for the construction industry, with many low-carbon or recycled alternatives being developed (e.g. Hempcrete, Olivine concrete, waste composite concrete). Photocatalytic concrete is another sustainable alternative to traditionally manufactured concrete, this time tackling air pollution. 

Photocatalytic concrete is a form of concrete that incorporates TiO₂ (titanium dioxide). Initially developed for the practicality of its self-cleaning properties, it was discovered that this concrete could also purify air and even disinfect its own surface. TiO₂ is a photocatalyst; its particles react with ultraviolet light causing them to produce highly reactive oxygen species such as hydroxyl radicals that serve some practical and environmental benefits when on the surface of concrete.

The Practical and Environmental Benefits of Photocatalytic Concrete:

  • Pollutant Degradation and Air Purification: Hydroxyl radicals break down various pollutants in the surrounding air and contribute to the oxidation and decomposition of NOx (Nitrogen oxides) and volatile organic compounds, converting them into water, CO₂, nitrates and sulfates. The air purification efficacy of this concrete varies significantly with the application type (e.g. pavement, wall, interior), location and exposure to sunlight. However, with appropriate application, nitrous oxide levels in the surrounding air have been found to reduce by 45%.  
  • Self-Cleaning: The pollutant degradation effect acts as a cleaning mechanism by breaking down dirt on the material’s surface. TiOalso becomes superhydrophilic (attracted to water) under UV radiation. This creates a thin layer of moisture on the surface of the concrete, which acts as a transparent protector causing dirt and contaminants to wash off the surface. The combined effects reduce material surface discolouration and dirt accumulation, diminishing the need for cleaning maintenance or the use of harmful chemicals. 
  • Self-Disinfecting: the Photocatalyst reaction also inhibits and reduces algae and bacteria growth. Adding titanium dioxide to cement has been found to reduce algae growth by over 60% and increase E. coli inactivity. 
Concrete samples coated with Rhodamine B before and after 6h of UV radiation. The sample on the right has 0% TiO2, sample on the left has 5%.

There are 3 main methods of producing photocatalytic concrete. The first involves adding TiO₂ directly to the cement mixture. The second is coating the aggregate with a thin photocatalyst film before adding cement. The third is by coating the final product with a layer of TiO₂ paint.  

It must be noted that whilst there are several effective photocatalytic compounds other than titanium dioxide, it has considerable advantages over the others in being; safe, stable, relatively inexpensive, most effective and compatible with concrete.

This concrete’s air purifying ability is most effective in wet climates or with periodic cleaning, as the photocatalyst reaction will reduce in intensity if there is any particulate accumulation on its surface. Further research into the development of photocatalytic concrete will yield improved durability of the reaction and will likely happen in the years to come. 

Photocatalytic Concrete in Action

Various applications have been considered and are already used for this emerging material. Many of these applications take advantage of the self-cleaning mechanism. One popular use is for concrete paving blocks, they stay clean for long periods with minimal maintenance. Another convenient use has been for white buildings which would have otherwise discoloured rapidly. 

Pristine white walls maintain themselves. Jubilee Church, Rome | Copyrighted architecture: Richard Meier

Environmentally directed applications are also becoming more common since the discovery of this material’s air purifying capabilities. The range of different applications is nearly endless regarding air purification, as so many places could benefit. Air purification is always a bonus in an urban setting, from uses in tunnels to bridges to roadsides or housing facades. 

 A trial by researchers at the KICT (Korea Institute of Civil Engineering and Building Technology) in Goyang very recently applied photocatalytic concrete to the interior of the Banpo underground road tunnel in Seoul. The concrete could fulfil its air-purifying purpose by installing UV lights inside the tunnel to trigger the catalytic reaction without the need for natural light. The trial was successful, observing a decrease in nitrous oxide levels by 18% in only 24 hours. 

“Construction technology using photocatalysts can have an immediate effect on reducing fine particulate matter in the nation’s living environment. We plan to build a system of cooperation with local governments and public corporations to expand trial demonstrations to other sites to achieve commercialisation and distribution with practical effects,” said Dr Jong-Won Kwark, head researcher on the project. 

A view of Banpo underground road tunnel, Seoul | Source KICT

Photocatalytic paint is an increasingly popular way to render concrete eco-friendly, particularly for smaller construction projects. The convenience of simply applying this paint is very appealing to developers and it can be applied to already low-carbon alternatives to concrete, such as Seratech.

Graphenstone, which produces a wide range of ecologically adapted paints, has released a natural photocatalytic variant with graphene technology, AmbientPro+Premium. This paint not only helps clean the material surface but absorbs CO₂ and other pollutants, all whilst improving energy savings due to its reflective properties and thermal emittance.

As the status of air quality declines in major cities around the world and pollution-related health issues continue to rise, the need for environmentally positive technology and sustainably adapted materials becomes more urgent. There is little doubt that in years to come, many photocatalytic products will flood the market and contribute to practical urban air purification measures.  

Learn More on Firstplanit

Click here to read about Olivine, an abundant, carbon-sequestering material that may help to drastically cut the carbon emissions of concrete. 

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