Self-healing concrete relies on enzyme-driven mechanism

SOPHIA CHEN: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics of biomaterials, quantum materials, sustainability, artificial intelligence, perovskites, and robotics. My name is Sophia Chen. We live in a world made out of concrete, as Nima Rahbar well knows. It’s in sidewalks, building foundations, parking lots, skyscrapers. 

NIMA RAHBAR: Concrete is most used engineered material in the world after water, in terms of volume.

CHEN The material is ubiquitous. And Rahbar, a civil engineer at Worcester Polytechnic Institute, is also very aware of how much that concrete contributes to the global carbon footprint. A 2020 study found that concrete is responsible for 8% of human-made greenhouse gases. This includes concrete used not only to construct new buildings, but also to repair existing structures. Rahbar is working to reduce concrete’s carbon footprint. His team has devised a way to reduce the repairs that concrete requires – by designing concrete able to heal its own cracks. Here’s how it works. Rahbar’s team has combined a cement-like material, which binds together the aggregates in concrete, with an enzyme known as carbonic anhydrase. The enzyme catalyzes calcium in the cement to react with carbon dioxide from the air to form crystals of calcite, which repairs the crack. They demonstrated the material could heal millimeter-wide cracks.

RAHBAR You spray it and you blow CO2, and honestly, in a matter of hours, and the crystals will grow very quickly.

CHEN Their material departs from previous designs of self-healing concrete from the 2000s that used bacteria rather than enzymes to self-heal. Similar to what happens in Rahbar’s material, the bacteria in the concrete grew calcite crystals on their surface to close narrow cracks. While the idea was creative, it presented problems. The bacteria repaired cracks very slowly, bridging about 400 microns in about a month. In addition, it’s a hard sell to builders to use “bacteria” in their structures because they connote something unhealthy, although Rahbar says that the health effects of such bacteria in buildings is not known. Rahbar’s enzyme, on the other hand, heals cracks ten times as wide in a matter of hours. He also says it’s safe to use, as you can find carbonic anhydrase in the human body.

RAHBAR It actually is in your gut, so there's no health effect.

CHEN To test their material, they compressed samples of it and compared them to control cement samples. In one test, they made cube-shaped samples, some of which had millimeter-scale holes drilled into them as a defect. They repaired the specimens with two different methods, one with a conventional cement paste and the other with the enzyme. With the enzyme repair, they could reach 85 percent the compressive strength of the undamaged sample, whereas the conventional repair achieved only 73 percent. 

RAHBAR For the purpose of waterproofing and sort of plugging the pores and holes, this works really nicely.

CHEN Rahbar envisions builders applying the material perhaps as a spray to their buildings. Self-healing capability would lengthen the lifetime of the concrete, which would decrease its carbon footprint. However, Rahbar points out a limitation of the material. Enzymes degrade over time, which means that in order for the concrete to stay self-healing, the enzyme would need to be re-applied every year or so. This work was published in a recent issue of Applied Materials Today (10.1016/j.apmt.2021.101035).  My name is Sophia Chen from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.