Ecogeek:

Sustainable Brick Made from Sand, Bacteria and Pee

An excerpt:

These Better Bricks are created through a chain of chemical reactions known as microbial-induced calcite precipitation. Once all the ingredients are combined, the bacteria serves as a glue that binds the sand together, creating a brick that is as tough as a fired-clay brick or even marble and requires no baking to achieve that strength.
If Better Bricks replaced all traditionally-fired bricks, 800 million tons of CO2 emissions would be eliminated each year.