More adventures of bacillus pasteurii - mending concrete

The efficiency with which bacillus pasteurii manufactures calcium carbonate and glues things together makes for ways in which our microbial friend can assist us beyond just preventing earthquake damage and sculpting deserts (see the previous post) and here's, literally, a concrete example. But I should point out here that the image above is quite unrealistic, simply a Sandglass photoshop special.

The recipe for basic concrete is simple and has been around for a long time. The ancient Egyptians knew how to make it (there is a lively debate as to whether the pyramids, at least in part, are made of concrete), and the Romans perfected the formula. The fundamental ingredients are around 75 percent sand and gravel, 15 percent water, and 10 percent cement. The cement, cooked from materials such as limestone and clay, is the chemical glue; the hardening of concrete is not simply due to drying, but involves complex chemical reactions. The physical characteristics of the sand, its size and shape, influence the properties of the concrete, but because of the importance of chemistry, the composition of the sand and the other ingredients is critical. The wrong impurities will ruin the quality of the concrete.

The global demand for concrete is massive: after water, concrete is the most consumed material on Earth. Every year, the equivalent of more than 400 million dump trucks of concrete is transported to construction sites. Every man, woman, and child on the planet “consumes” around forty times their own weight in concrete per year. Which is, of course, an average—for residents of the Western world, it’s much more, despite the fact that around half the world’s concrete production and consumption today is accounted for by China.

But concrete is vulnerable to deterioration, corrosion, and cracks, and the consequent damage and loss of strength requires immensely expensive remediation and repair. So, is there a way in which incipient microscopic fractures in concrete can self-heal? The answer, thanks to our microbial friend, would seem to be yes, through the process referred to as biomineralisation, microbiologically induced calcite precipitation (MICP). Around the world, research groups are working and collaborating on different approaches to turning this into reality, but the principle is quite simple. The chemistry of concrete is alkaline, but fortunately, bacillus pasteurii tolerates this - it simply needs nutrition, space to work, and a little encouragement. A team led by Henk Jonkers at the Delft University of Technology in the Netherlands are well on the way to accomplishing this, through actually mixing living bacteria, along with calcium lactate, an organic compound that such bacteria convert to calcium carbonate, into the concrete to begin with. They found that, as the concrete cured, incipient cracks were effectively sealed with calcium carbonate. There remain problems of keeping the microbes alive for a long period of time, and retaining sufficient microscopic space in the concrete for them to have room to maneuver, but progress is being made (the addition of clay to the mix provides a safe haven). And, importantly, the more effective this process proves, the less concrete we will have to manufacture and the lower the associated carbon dioxide emissions from making cement.

As a recent article in The Economist concluded, "If the process can be scaled up, it may be prove that the best way to preserve concrete is to infect it."

[for details on the work of Henk Jonkers, see http://www.tudelft.nl/live/pagina.jsp?id=8691221d-ebab-4841-97cb-1cfacad3a4bc&lang=nl, a recent conference presentation, a link to the abstract of the group's most recent paper here, and The Economist at http://www.economist.com/science/displaystory.cfm?story_id=13570058. For work at the South Dakota School of Mines and Technology, see http://adsabs.harvard.edu/abs/2001SPIE.4234..168R, at the S. V. National Institute of Technology in India, http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1285418, and for non-bacterial approaches, http://www.eurekalert.org/pub_releases/2009-04/uom-scf042209.php.]