BBC: Although concrete is the world’s most widely used building material, it is prone to cracking. To make the material more durable, researchers at the Delft University of Technology in the Netherlands are experimenting with adding to the concrete mix a healing agent composed of calcium lactate and alkaline-resistant bacterial spores, which remain dormant in the concrete until rainwater works its way into the cracks. As the water activates the spores, they eat the calcium lactate and produce limestone. Although the process adds considerably to the initial cost of the concrete, the researchers expect it will actually save money in the long run.