That ubiquitous building material concrete has been with us since Roman times. It may be ugly when combined with poor design, but concrete is one of the most practical building materials available. More than seven cubic kilometres of the stuff is produced each year, and it could soon see a doubling in lifetime thanks to nanotechnology.
Concrete is a robust material, but like everything else it degrades over time. For example, the penetration into concrete of chloride and sulphate ions from road salt, sea water and soils causes structural damage that leads to cracks. Less porous concretes with higher than normal density will improve material lifetime, but they are still prone to cracking.
Engineers at the National Institute of Standards and Technology (NIST) in Maryland have taken a different approach, and found that ion infiltration can be reduced with the help of nanoscale viscosity enhancers. Dale Bentz and his colleagues reasoned that altering the viscosity of concrete solution would be preferable to changing the size and density of its pores. “Swimming through a pool of honey takes longer than making it through a pool of water,” says Bentz.
The size of the additive molecule is critical when it comes to reducing ion diffusion speed. Larger molecules such as cellulose ether and xanthan gum will increase viscosity, just as they do in foodstuffs, but they do not cut diffusion rates. Molecules less than 100 nanometres across, on the other hand, will slow diffusion, and, after studying a variety of possibilities, the NIST researchers focused on the compound silver nitrate.
Silver nitrate and other additives can be blended directly into concrete solution with current techniques, but better performance can be achieved when the additives are mixed into the concrete by saturating an absorbent, lightweight sand. Materials other than silver nitrate are a possibility, and research into these is ongoing. A patent has been filed on the NIST technique.
This may not on the face of it seem like a high-profile scientific breakthrough, but the implications of a doubling of concrete lifetime are huge. Maintenance costs will be reduced, as will the number of catastrophic failures.
What the researchers fail to mention is that their discovery could have major environmental benefits. The production and use of concrete leaves a large carbon footprint, and this is why ecologically minded builders are returning to materials such as lime screed. While this works well for foundations and floors in domestic houses, it is not feasible on an industrial scale. Doubling the lifetime of concrete via nanotechnology is therefore a green development, as well as one that will save money and human lives.
Further reading: “Viscosity Enhancers Reducing Diffusion in Concrete Technology”, Bentz et al., Concrete International 31, 31–36, (2009).
Figure: The barely visible blue-green area at the top of this X-ray image of concrete with the NIST nanoadditive shows that very few chloride ions (in green) penetrate into the concrete (source: NIST).