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A LIVING cement for repairing cracks in buildings could help stop the
concrete facades of modern buildings crumbling away. Stuffed with bacteria, the
cement 鈥済rows鈥� to patch up cracks as they develop, the inventors say.

Cracks can form in concrete facades when strong sun or wind dry the outer
face of the concrete much faster than the inside. In wet conditions, these
cracks draw in water, which can freeze. This often leads to 鈥渟palling鈥濃�攊n
which large chunks of a facade fall off.

To repair such cracks, a team at the South Dakota School of Mines and
Technology in Rapid City made a cement by mixing together sand, calcium
chloride, sodium bicarbonate, urea and Bacillus pasteurii bacteria. When the
bugs come into contact with the urea, they break it down, producing ammonia and
carbon dioxide. The ammonia triggers a reaction between the calcium and
carbonate ions to form a strong mineral called calcite.

鈥淚t鈥檚 basically a bacterial sealant,鈥� says lead researcher Sookie Bang. Once
a crack is sealed, the bugs run out of oxygen and calcite formation stops. But
if the crack reopens, the bacteria reawaken and calcite formation restarts.

The technique is 鈥渨acky, but very appealing鈥�, says Rod Jones of the
University of Dundee鈥檚 Concrete Technology Unit. 鈥淚f this could prevent or
reduce spalling, it would certainly be useful,鈥� he says.