If you haven’t heard of “pozzolans,” then you are missing out on a concrete idea that can make projects greener.  Thousands of years ago the Greeks and Romans built structures that are still standing today by incorporating pozzolans into the mix - literally into the mix.

The Romans used lime and a fine volcanic ash (pozzolana) to create cement that would harden underwater. These builders also figured out that by using stepped rings of concrete, where each had a lower density than the one below, they could then create large domes like the one at the Pantheon. That one is about 142 feet in diameter.

During the last century in the U.S., pozzolans were incorporated into the Hoover Dam, the Los Angeles aqueduct, the San Francisco-Oakland Bay Bridge and the Hungry Horse Dam in Montana, just to name a few. This cement also showed up in the tunnel between the UK and France, in France’s Puylaurent Dam and in the Commerzbank Tower in Frankfurt, Germany.

Today, everyone is getting on the pozzolans bandwagon, but not everyone has natural ones like volcanic ash, pumice, tuff, diatomaceous earth, and opaline shale just lying around. So, people are now using the waste products that are generated when coal is burned while making electricity. It’s typically called fly ash.

They are also using rice husk, brick dust, calcined kaolin, condensed silica fume and waste from blast furnaces. These are all defined as fine-grained materials that react with lime resulting in cement. The result is that you can use less Portland cement. Depending upon the pozzolans, that reduction can range from 15 to 60 percent. When you consider that the embodied energy of concrete is estimated to be 3 kWh per pound, and 90 percent of that is in the making of Portland cement, you can see how reducing the amount of Portland used can be a green thing.

There are also the added advantages of the concrete being lighter, more plastic and easier to work, and that it requires less water resulting in lower bleeding and lower heating.

Suppose you want to explore using pozzolans in the next or current project. Here’s a resource to get you started.