Materials scientists are experimenting with the possibility of creating self-healing concrete using biomineralisation.
The process would involve putting spores of bacteria — which naturally produce calcium carbonate — in the concrete mix, which would be activated upon the formation of cracks.
‘Concrete is basically weak in tension and strong in compression,’ said collaborator Prof Paramita Mondal of the University of Illinois at Urbana-Champaign. ‘People have tried to deal with the problem over the years in a variety of ways. The most common solution has been to use steel rebar to reinforce concrete, but it still cracks.’
The team noted that, in nature, bacteria that form calcium carbonate are known to influence the rock-formation process of carbonate rocks and sediments such as limestone. The challenge was finding one that would be active in concrete’s environment of high alkalinity and low oxygen.
The team first tested B. pasteurii — a non-pathogenic micro-organism commonly found in soil — in the lab, showing that it could deposit limestone minerals under the right conditions.
‘Then we made a cement specimen and applied the bacteria with food,’ said Mondal. ‘We saw the same kind of deposition. We did a chemical analysis of it, and it is the same calcium carbonate that’s forming.’
Eventually, the team hopes to prove that, after introducing these micro-organisms into concrete during mixing, they will form spores, or hibernate, in the highly alkaline condition inside the concrete. Once a crack occurs, the pH level at the cracked surface will drop owing to the exposure of surface to air.
The combination of the pH drop and a flow of oxygen and carbon dioxide at the crack face will activate the micro-organisms and will provide the conditions favourable for growth. The micro-organisms will deposit calcium carbonate and, as the crack fills up, the supply of oxygen and carbon dioxide will be interrupted, causing the micro-organisms to hibernate again, ensuring the continual effectiveness of the micro-organisms in filling up cracks at the same location.