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For centuries, builders have been making concrete roughly the same way: by mixing hard materials like sand with binders, and hoping it stays fixed for a long time to come.

Now, an interdisciplinary team of researchers at the University of Colorado, Boulder, has created a rather different kind of concrete — one that is alive and can even reproduce.

Minerals in the new material are deposited by cyanobacteria, a class of microbes that capture energy through photosynthesis. The photosynthetic process absorbs carbon dioxide, in stark contrast to the production of regular concrete, which spews huge amounts of that greenhouse gas.

Photosynthetic bacteria also give the concrete another unusual feature: a green color. “It really does look like a Frankenstein material,” said Wil Srubar, a structural engineer and the head of the research project. (The green color fades as the material dries.)

Other researchers have worked on incorporating biology into concrete, especially concrete that can heal its own cracks. A major advantage of the new material, its creators say, is that instead of adding bacteria to regular concrete — an inhospitable environment — their process is oriented around bacteria: enlisting them to build the concrete, and keeping them alive so they make more later on.

The concrete, described in the journal Matter, “represents a new and exciting class of low-carbon, designer construction materials,” said Andrea Hamilton, a concrete expert at the University of Strathclyde.

To build the living concrete, the researchers first tried putting cyanobacteria in a mixture of warm water, sand and nutrients. The microbes absorbed light and began producing calcium carbonate, cementing the sand particles. But the process was slow — and DARPA, the U.S. Defense Advanced Research Projects Agency and the project’s funder, wanted the construction to go very quickly. Necessity birthed invention.

Srubar had worked with gelatin, which when dissolved in water and cooled, forms bonds between its molecules. It can be used at moderate temperatures that are gentle on bacteria. He suggested adding gelatin to strengthen the matrix being built by the cyanobacteria.

The researchers bought Knox brand gelatin and dissolved it in the solution with the bacteria. When they poured the mixture into molds and cooled it, the gelatin formed its bonds — “just like when you make Jell-O,” Srubar said. The gelatin provided more structure, and worked with the bacteria to help the living concrete grow stronger and faster.

After about a day, the mixture formed concrete blocks in the shape of whatever molds the group used. Individual 2-inch cubes were strong enough for a person to stand on, although the material is weak compared to most conventional concretes. Blocks about the size of a shoe box showed potential for doing real construction.

“The first time we made a big structure using this system, we didn’t know if it was going to work, scaling up from this little-bitty thing to this big brick,” said engineer Chelsea Heveran, a former postdoc with the group and the lead author of the study. “We took it out of the mold and held it — it was a beautiful, bright green.”

When the group brought samples to a meeting with DARPA officials, they were impressed, Srubar said: “Everyone wanted one on their desk.”

Stored in relatively dry air, the blocks reach their maximum strength over the course of days, and the bacteria gradually begin to die out. But even after a few weeks, the blocks are still alive; when again exposed to high temperature and humidity, many of the bacterial cells perk back up.

The group can take one block, cut it with a diamond-tipped saw, place half back in a warm beaker with more raw materials, pour it in a mold, and begin concrete formation anew. Each block could thus spawn three new generations.

The Department of Defense is interested in using the reproductive ability of these living building materials for construction in remote areas.

The blocks also have the advantage of being made from common materials. Most concrete requires virgin sand that comes from rivers, lakes and oceans, which is running short. The new material is not so picky. Srubar said, “We could use waste materials like ground glass or recycled concrete.”