Concrete is one of the world’s most widely-used building materials, and it has an enormous carbon footprint – but researchers recently unearthed clues to a more environmentally-friendly concrete mixture used by the Romans 2,000 years ago. Paulo Monteiro of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), a professor of civil and environmental engineering at the University of California, Berkeley led the team that discovered a 2,000-year-old breakwater in the Mediterranean Sea that was produced using a fraction of the energy required to create modern concrete – and it’s more durable to boot.

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“Portland cement is the source of the “glue” that holds most modern concrete together,” Berkeley Lab explained in a recent press release. “But making it releases carbon from burning fuel, needed to heat a mix of limestone and clays to 1,450 degrees Celsius (2,642 degrees Fahrenheit) – and from the heated limestone (calcium carbonate) itself.”

The Romans perfected a mixture that used much less lime and cemented at 1,652˚ F or lower, according to the researchers, which means it requires less energy to make. In our current environment of escalating climate change due to rising carbon emissions, this ancient concrete recipe could dramatically slash the building industry’s overall emissions.

The Romans mixed lime and volcanic rock for regular concrete structures, while underwater structures were made with lime and volcanic ash that formed a mortar.

When this mix connected with seawater, a hot chemical reaction occurred that cemented the lime and ash mixture. The secret ingredient is aluminum-rich pozzolan ash and it turns out that oil-producing Saudi Arabia has a lot of it.

“For us, pozzolan is important for its practical applications,” says Monteiro. “It could replace 40 percent of the world’s demand for Portland cement. And there are sources of pozzolan all over the world. Saudi Arabia doesn’t have any fly ash, but it has mountains of pozzolan.”

+ Berkeley Lab