Like many microbiologists, Sookie Bang studies bacteria. But while other researchers might focus on ways to manipulate bacterial genes or combat disease, Bang, of the South Dakota School of Mines and Technology, has found a somewhat more concrete use for bacteria. She and Shannon Stocks-Fischer, a graduate student in chemical engineering, are using bacteria to make cement.
Many common types of bacteria use urea, the main component of urine, as their source of nitrogen. They break down urea, creating carbon dioxide and ammonia. The ammonia reacts with water to form ammonium hydroxide, which makes any nearby calcium precipitate out as calcium carbonate crystals, or limestone. It’s a very slow process in nature, says Stocks-Fischer, so we tried to speed things up a little.
The researchers separately mixed two ordinary soil bacteria, Bacillus pasteurii and Sporosarcina ureae, with sand and placed each into a syringe. Into the syringes they slowly dripped a urea-based nutrient broth that also contained calcium chloride. As the bacteria went to work metabolizing the urea, calcium carbonate crystals began to form around the bacteria, filling in the gaps between each grain of sand. Within a few days, the top half inch of sand in the syringes had solidified, stopping the flow of the broth. When the nutrients ran out, the bacteria died off, leaving limestone behind.
Unlike conventional sealants, the bacteria fill up fissures in cement from the inside out, completely meshing with the existing material. Bang and Stocks-Fischer hope their bacterial builders can be used to seal up cracks and fissures in concrete buildings and other structures. So far they’ve used the bacteria to fix cracks in concrete blocks in the lab, and they’re working on scaling up the process by encasing thousands of bacteria in separate gel beads for easy application.
Bang and Stocks-Fischer have their eyes on nearby Mount Rushmore as a potential recipient of a bacterial facial. The wind and weather wreak havoc on Mount Rushmore year after year, says Stocks-Fischer. The cracks constantly have to be filled in with silicone sealer, sand, and all sorts of things.
