Concrete is the most common building material in the world. We produce billions of tons a year, but at a huge environmental cost. The quarrying and processing necessary to produce concrete is estimated to produce from 4% to 8% of total CO2 emissions. It’s an incredibly useful material though, and frankly, working with it is one of my favorite parts of building. The stuff is magical—it pours into forms like pancake batter, and a couple of hours later you can walk on it.
Apart from those two aspects, we tend to take the stuff for granted.
Concrete has been around in various forms for a long time. The Romans used it in monolithic placements, and the Minoans and others as mortar before them. Roman concrete has a reputation even today, as evidenced by the 2,000-year-old concrete Pantheon dome. Roman concrete isn’t chemically much different from what we use today. It’s a blend of aggregates (gravel down to sand), a pozzolan, water, and calcined lime. The pozzolan and the lime react with water to form a cement that binds the aggregate together.
Aggregate is easy to understand. Rock is blasted from the earth, crushed, graded for size, and washed. Sand is mined. The pozzolan and calcined lime are the mysterious components.
What the heck is a pozzolan?
Pozzolans are aluminosilicate minerals, essentially compounds of aluminum, silicon, and oxygen. Many occur naturally. The name comes from Pozzuoli, a city located in a volcanic caldera along Italy’s west coast. Certain types of volcanic ash are pozzolans. The pozzolan in Roman concrete is volcanic ash.
Clay and shale are pozzolans. Diatomaceous earth is a pozzolan. Pozzolans can be artificially made, too. Ash from rice hulls is a pozzolan. Blast furnace slag and fly ash from burning coal are pozzolans, and are…