By Mette Oorthuijs
We have all seen the pictures of the massive reduction in air pollution above China, which is at least partly due to the COVID-19 induced lockdown. China is not alone, as many other countries that are under some form of lockdown have seen decreasing nitrogen dioxide (NO2) levels. Blue skies are now shining above huge concrete skyscrapers and the world is in awe (1). What people don’t realize is that concrete, our most popular building material, is emitting another pollutant in bulk. In the production of cement, concrete’s main element, the same amount of CO2 is emitted. As a result, concrete causes 5-10% of the total emissions of this greenhouse gas (2).
After the production process, the durability of concrete is challenged by its susceptibility to cracking, in which water can damage the material from inside. To address this problem, an innovative material was tested in 1995 (3): microorganisms! Why? Bacteria are capable of microbially induced calcium carbonate precipitation (MCIP) and this calcium carbonate (CaCO3) can be used as a filler.
This discovery propelled various laboratories to start experimenting with the addition of bacteria to the concrete mixture in advance (4). While promising as a self-sustaining material — cracks would fill as instantly as they arise — there were many problems to overcome (5). One of the biggest hurdles to cross was that bacteria would not stay alive for long, let alone yield enough CaCO3 to fill the cracks.
Almost all bacteria are capable of producing CaCO3 since it’s the byproduct of various common pathways. A very recent paper by Heveran et al. (2020) focused on robust photosynthetic cyanobacteria. They argue that ureolytic bacteria, the focus of most previous studies, are not likely to remain viable in the harsh environment of cement, even with improvements.
Will cyanobacteria save the day?
The cyanobacteria were mixed with sand, gel, and some minerals in molds to create a stone that is alive. The resulting material is strong and when compared to earlier studies, the viability of the organisms is high (4, 5). As if this is not breath-taking enough already, the group set out to explore another biological function that bacteria could add to the material: regeneration! Under the right circumstances, they found that the material can replicate itself. When the humidity is right and some additional sand and gel are added, one mother-stone can divide into two daughter-stones!
The idea of a living house is surreal, but we are unfortunately not quite there yet. For now, there is still a trade-off between the viability of the microorganisms and the toughness of the material. The material’s mechanical performance is the best when the humidity is low; however, decreasing the humidity of the material strongly decreases the viability of the microorganisms. Although these dry and nonviable stones could still be recycled for their abiotic components, overcoming this problem would result in a really strong material that keeps its biological functions!
What about all the concrete that we have already created?
A material that is loved for its durability poses a problem when you are trying to get rid of it, as it can take forever for concrete to erode back into sand (6). The answer might be found in the Netherlands! The TU Delft has partnered up with sustainable construction company Strukton to create a machine that can break up concrete in pieces that can be used for recycling directly on-site, as is explained in a short documentary on their science platform (7).
In a world where NO2 and CO2 are emitted every day, a material that not only reduces the need for new concrete but also removes CO2 from the air is truly inspiring. It’s fascinating to imagine what the future will bring us and inspiring to see what kind of role biologists can play in these living houses. The reduction in air pollution during the pandemic shows us that the way we pollute our planet’s atmosphere is not set in stone. Amongst other things, we need to re-evaluate our building materials. While our cities may continue to look somewhat the same and we will probably not live in flying houses soon, the production of those materials will need to change.
ORIGINAL ARTICLE: Heveran, C. M., Williams, S. L., Qiu, J., Artier, J., Hubler, M. H., Cook, S. M., … & Srubar III, W. V. (2020). Biomineralization and Successive Regeneration of Engineered Living Building Materials. Matter. https://www.sciencedirect.com/science/article/pii/S2590238519303911
About the writer
Mette is a second year student in the Biomolecular Sciences master at VU. After a bachelor in neuroscience, she has chosen this master to study the molecular basis that underlies every organ in the body.
- Gollapudi, U. K., Knutson, C. L., Bang, S. S., & Islam, M. R. (1995). A new method for controlling leaching through permeable channels. Chemosphere, 30(4), 695-705.
- Siddique, R., & Chahal, N. K. (2011). Effect of ureolytic bacteria on concrete properties. Construction and building materials, 25(10), 3791-3801.
- Jonkers, H. M., Thijssen, A., Muyzer, G., Copuroglu, O., & Schlangen, E. (2010). Application of bacteria as self-healing agent for the development of sustainable concrete. Ecological engineering, 36(2), 230-235.
Image credits: University of Colorado Boulder