The concrete contains limestone-producing bacteria, which are activated by corrosive rainwater working its way into the structure. The new material could potentially increase the service life of the concrete - with considerable cost savings as a result.Neat idea, but I'll bet something faster growing and more aggressive get the the lactate before the desired bacteria. Nature is perverse that way.
Bacterial spores and the nutrients they will need to feed on are added as granules into the concrete mix. But water is the missing ingredient required for the microbes to grow.
So the spores remain dormant until rainwater works its way into the cracks and activates them. The harmless bacteria - belonging to the Bacillus genus - then feed on the nutrients to produce limestone.
The bacterial food incorporated into the healing agent is calcium lactate - a component of milk. The microbes used in the granules are able to tolerate the highly alkaline environment of the concrete.
Found at Althouse.