Main image: Photocatalytic Oxidation of NOx under Visible Light on Asphalt-Pavement Surface. Courtesy of ASCE Library
Photocatalytically active construction products can support air pollution control strategies in urban areas in the long term. During production, it is necessary to use titanium dioxides that are optimally adjusted to the cement.
Photocatalytically concrete works like any other concrete, but has a different chemical makeup. The titanium dioxide additive helps break down pollutants and smog in the air. It is also used as a preserver on buildings to help coatings keep their colour and to assist in keeping buildings clean.
Breaking down pollutants in the air
Nitrogen oxides (NOX) are one of the most critical groups of air pollutants in the vicinity of major urban roads. One of the options to reduce the concentration of these pollutants in the air we breathe is to create photocatalytically active surfaces in appropriate locations. High-energy UV radiation triggers the formation of highly reactive compounds on the surface of such photocatalysts. These compounds have been shown to mineralise nitric oxide (NO) and nitrogen dioxide (NO2) present in the air, thus removing these substances from the atmosphere.
Bryan Perrie, MD of The Concrete Institute, comments: “When nitrous oxides are present with volatile organic compounds (VOCs), ozone formation takes place, which amplifies the impact of the fine articulate matter. Concrete pavements and structures in the immediate vicinity can assist with reducing the effect of vehicle emissions in a city. They can even achieve an air-purifying effect if titanium dioxide is used.”
Titanium dioxide acts as a catalyst to convert harmful compounds into nitrates when exposed to UV light. These nitrates settle on surfaces and are washed away by the rain. Specially formulated cements that contain nanoparticles of titanium dioxide can be used, or the titanium oxide can be added to the cement.
Bryan says that various research projects have demonstrated the effects of photocatalytic cement. “Reductions of 30 – 95% were measured in laboratory tests where nitrogen oxide converted as a result of a single contact between the air and the photocatalytic material. In real-life applications, the efficiency of photocatalytic materials will depend on a number of factors such as light intensity, relative humidity, the amount and type of pollution in the area, and so forth.”
Increases in number of patents
The technical interest in applying the principle of photocatalysis has grown exponentially over the past few years, which can be seen in the rise in the number of patents. Still, this isn’t a widely used application in South Africa.
“Photocatalytic concrete has been used in Europe and the US, but a lot of awareness is needed for this solution to be more broadly accepted locally,” says Bryan.
The Concrete Society of the UK has confirmed that photocatalytic concrete can improve both external appearance and the environment through self-cleaning and reducing pollution in the surrounding air. The principle of exploiting photocatalysis in cement-based products is now well established and the performance of photocatalytic concrete surfaces has been proven both in the laboratory and in full-scale practical trials and applications.
Many cities across the globe exceed the limits for air-borne pollution. The use of concrete with photocatalytic surfaces to reduce NOx levels in the surrounding air would assist local authorities in their initiatives to comply with the new limits.
Recommendations to support the wider use of photocatalytic concrete:
The Concrete Society of the UK gives the following recommendations to support the wider use of photocatalytic concrete and exploit its significant environmental benefits:
• Establish realistic photocatalytic performance levels for self-cleaning and de-pollution;
• Produce guidelines for specification, production and compliance testing of completed structures;
• Investigate ways to minimise the additional cost of producing concrete with photocatalytic surfaces.
• Increase awareness of the environmental benefits among concrete professionals, architects and other specifiers, end-users and the public.
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