Low carbon building material using bacteria

Materials World magazine
,
29 Jan 2020

A material that can heal itself and reproduce has been engineered to offer a low-carbon option for the construction industry. Idha Valeur finds out how it works.

A bacteria-based structure that could replicate itself to effectively multiply has been created as a lower-cost, lower-carbon alternative to cementitious materials. Researchers from the University of Colorado Boulder (CU Boulder), USA, expect applications of the material to be building blocks, carbon-sequestering mortar, lightweight concrete and temporary disaster-relief shelters or roadways.

CU Boulder Assistant Professor, Wil V Srubar, told Materials World they believe it will be especially suitable in resource-scarce environments such as deserts or in the Arctic. Further, he explained that the structure is made with simple and readily available resources. ‘Our material is comprised of sand and a physically cross-linkable hydrogel. In this instance, we used gelatin as a model polymer, but any hydrogel with similar physical, mechanical, and non-toxicity properties would suffice, and a culture of photosynthetic Synechoccocus sp.7002 cyanobacteria strain.’

He added that the idea of using sources such as sunlight and carbon dioxide to make construction materials drove the research into this new method, as cost is always a factor when evaluating new approaches.

With less than 6% of worldwide CO2 emissions coming from cement production, concrete being the second most consumed material on Earth after water, the researchers were aiming to create an alternative that could lower emissions. ‘The living building materials are, in essence, “grown” using sunlight and carbon dioxide,’ Srubar said. ‘If more building products were produced this way, the built environment could transform from a carbon dioxide emitter into a carbon dioxide sink.’

Eight for one

To create the brick-esque cube, the team injected colonies of the cyanobacteria into the sand and gelatin solution. With some adjustments, the gelatin was mineralised by the calcium carbonate, binding the sand, resulting in a brick. ‘It is a lot like making rice crispy treats where you toughen the marshmallow by adding little bits of hard particles,’ Srubar said.

During strength testing, results showed that the material was close to achieving the same compressive rate of low-strength concrete and cement-based mortar. The team also discovered that the calcium carbonate producing bacteria increased the fracture toughness by more than 15%. ‘This is exciting because it shows that the mechanical properties of the brick were actually enhanced by enabling the bacteria to participate in the manufacturing.’

The team found that under humid conditions, not only was its strength comparable with modern mortar, but the material can reproduce.

By splitting one brick in half, each half could then grow into a new brick. ‘We use controlled increases in temperature and humidity to switch the material into a growth phase,’ Srubar said. ‘We add the other abiotic ingredients, e.g., sand, water, nutrients, and a little hydrogel, that enable the cyanobacteria to grow and mineralise into two new, full-size bricks. We did this a second and third time, demonstrating you could grow up to eight living bricks from one parent brick.’

While this process will never be able to compete with high-speed and large-capacity brick manufacture, it could be beneficial to small-scale projects focused on environmental aspects and producing the smallest carbon footprint possible. Research is now turning to optimising the formulations to get the most out of its mechanical properties, long-term viability and durability. Srubar expects the material to be commercially available in five-10 years.