What is living concrete

The cuboid looks like a granola bar, but is a stone made of concrete. Not the usual mouse-gray mixture of sand, gravel and cement, but a greenish-yellow variant interspersed with grains that still contains life. The grains of sand in it are held together by a glue made from cyanobacteria and their metabolic products.

"Our concrete is particularly climate-friendly and you can let it grow if necessary," says Wil Srubar of the University of Colorado, Boulder, in the USA. Like plants, cyanobacteria, also known as blue-green algae, use light and carbon dioxide to multiply. "Instead of emitting carbon dioxide like the building materials industry, we use it," emphasizes Srubar. The bacteria are also very robust, can survive in the concrete for several weeks and provide for offspring directly on the construction site.

Fresh bacterial concrete has a strong green color; Srubar and his team recently published the ingredients in the trade journal Matter presents: bacteria, water, nutrients, gelatine and salts, which are also contained in baking powder and mineral water. Mixed together at around 40 degrees Celsius, the microbes multiply and produce calcium carbonate, which is also the main ingredient in conventional cement. If the liquid bacterial mix is ​​mixed with sand, poured into a mold, cooled and dried for several days, a solid stone is created. The gelatin speeds up the process and makes the concrete stronger.

To multiply a concrete block, the researchers cut it in half, heated each half in beakers with the ingredients from the basic recipe, but without adding more bacteria, added sand and poured the mix into two molds. Seven days later they pulled more stones from these descendants.

The material is suitable as street paving and for facades

Although concrete is a rather inhospitable environment for bacteria, they survived for several weeks. After 30 days at refrigerator temperature and humidity over 50 percent, about ten percent were still alive. According to the study, this is significantly more than previously reported for similar bacteria-concrete compositions. The bacterial concrete is less resilient than most conventional types of concrete, but is suitable, for example, as street paving or facade material.

It is not the first attempt to make concrete more environmentally friendly with the help of bacteria. The Dutch startup Green Basilisk, for example, gives concrete self-healing powers by adding bacteria and nutrients in order to increase the lifespan of the building material. The North Carolina-based company Bio-Mason, on the other hand, produces tiles from concrete, which, like the one from Colorado, is held together exclusively by bacterially produced calcium carbonate. However, instead of being fed with sunlight and carbon dioxide, these bacteria are primarily fed urea, which is currently largely obtained from natural gas.

It is unclear whether and when the concrete made from cyanobacteria will conquer the first construction sites. "We are still at the beginning of the research," says Srubar. Among other things, his team is working on improving the mechanical properties of the material and keeping the bacteria alive longer. The researchers also want to test concrete recipes with ground glass instead of sand and alternatives to gelatine. Srubar is not afraid of acceptance problems, even if some people may get scared of bacteria.