Roman Concrete is “Hot Stuff”

Roman Concrete has always been a mystery. The Pantheon is home of the world’s largest unreinforced concrete dome, was completed in 128 AD, still stands. Same with the Colosseum which is over 2000 years old. The Roman Empire did not disintegrate. Yet modern concrete has a life expectancy of 100 years while in harsher climates can break down in as little as 50 years.

The question has always been about the Romans, who are considered such great engineers and builders; what is their secret sauce and why is ancient Rome still standing. Science has analyzed the cement for years an attributed its durability to pozzolanic volcanic ash, native to the region. But that doesn’t explain the self-healing properties where cracks repair themselves over time.

New findings by a research team at MIT say all of the above is true but the trick is in the mix. Rather than using Slaked Lime (calcium hydroxide), a mix of Quicklime and water, the Romans used Quicklime (Calcium oxide) when making concrete. Quicklime has a higher density than Slack Lime.

Researchers found lime clasts or chucks in Roman Concrete to lead to this theory. The mixing process of Quicklime, water and volcanic ash generated a heat hence the term "Hot Mix".   And when a crack develops and water seeps in there is a chemical reaction recrystallizing the cement and the cracks self-heal.

Other pros are Roman Cement does not release toxic gasses during production and its salt resistant so better near the coast. Disadvantages include longer drying times and lower strength than modern cement, although Roman Cement lasts for centuries.

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