Scientists discovered the reason that allows Roman concrete structures to still stand strong

There are Roman concrete breakwaters built more than 1,500 years ago that still stand strong, maybe even stronger now than they were back then. A team of scientists from USA, China and Italy have discovered the recipe of the Roman concrete: a mix of volcanic ash, lime (calcium oxide), seawater and lumps of volcanic rock. Their findings could help today's builders construct durable marine structures.

There are Roman concrete breakwaters built more than 1,500 years ago that still stand strong, maybe even stronger now than they were back then. A team of scientists from USA, China and Italy have discovered the recipe of the Roman concrete: a mix of volcanic ash, lime (calcium oxide), seawater and lumps of volcanic rock. Their findings are published in the journal American Mineralogist. Even though the exact proportions of the mixture are still to be discovered, these ingredients held together piers, breakwaters and harbors for a long period of time and modern cement tends (made out of sand and gravel) cannot measure up to their durability. “They spent a tremendous amount of work [on developing] this – they were very, very intelligent people,” said Marie Jackson, a geologist at the University of Utah and co-author of a study into Roman structures.

It seems that what made such a great difference is the seawater, which reacted with the volcanic materials in the cement, creating new minerals that reinforced the concrete and prevented it from cracking. The researchers saw the first indications in 2009, when they discovered aluminous tobermorite, a very rare and hard to make mineral, in the marine mortar of drill cores taken from around Italy. As the investigation continued and by studying the drill cores again, they found several other minerals formed in the cement, including zeolite and phillipsite.  Jackson said: “I think [the research] opens up a completely new perspective for how concrete can be made – that what we consider corrosion processes can actually produce extremely beneficial mineral cement and lead to continued resilience, in fact, enhanced perhaps resilience over time.”

As the ingredients used in the original Roman recipe are not readily available in most regions of the world, the team hopes to develop a replacement cement using materials from the western US. 'Romans were fortunate in the type of rock they had to work with. They observed that volcanic ash grew cements. We don't have those rocks in a lot of the world, so there would have to be substitutions made’, says Jackson. “There are many applications but further work is needed to create those mixes. […] The challenge is to develop methods that use common volcanic products – and that is actually what we are doing right now”, she adds.

The discovery of aluminous tobermorite in the marine mortar of drill cores was researchers’ first indications in 2009