Ancient Roman concrete gained strength from seawater, study finds

While modern concrete structures crack and crumble within decades of being submerged in the ocean, Ancient Roman concrete structures – such as the Pantheon and Trajan’s Markets in Rome – are still standing today, having endured millennia. Some of these are stronger now than they were when first constructed.

During previous studies of ancient architectural concrete in Rome, Professor Marie Jackson of the University of Utah and her colleagues identified that mineral intergrowths between the concrete’s rocky aggregate and mortar appeared to prevent cracks from spreading through the structures.

Ancient Roman concrete was made by mixing volcanic ash with seawater and lime to create a mortar, to which an aggregate of chunks of volcanic rock was added. This mixture produced a pozzolanic reaction, which induces cementing in the mixture.

Modern Portland cement concrete is mixed with sand and gravel and is intended to be inert. Reactions with this cement can form new gels which expand and cause the concrete to crack.

“This alkali-silica reaction occurs throughout the world and it’s one of the main causes of destruction of Portland cement concrete structures,” said Professor Jackson.

In their most recent study, the researchers examined ancient marine concrete and discovered that an exceptionally rare mineral called Al-tobermorite had formed in the cementing matrix. While Al-tobermorite can form in lime through the pozzolanic reaction, this process would have been short-lived.

The researchers concluded that something else must have caused the minerals to grow at low temperatures after the concrete had hardened.

“As geologists, we know that rocks change,” said Professor Jackson. “Change is a constant for earth materials. So how does change influence the durability of Roman structures?”

The researchers concluded that seawater seeping through the concrete had dissolved components of the volcanic ash in the mixture and allowed new, rare minerals to grow from the filtered fluids.

The “platy” crystalline shapes interlock, increasing the concrete’s resistance to fracture.

“We’re looking at a system that’s contrary to everything one would not want in cement-based concrete,” said Professor Jackson. “We’re looking at a system that thrives in open chemical exchange with seawater.”

This precise “recipe” for Roman concrete has been lost over the millennia and it is still not known exactly how the marine mortar can be fully recreated. Even if it could, the amount of time it takes to strengthen from sea water could make it unfeasible to rely upon for future marine constructions.

Professor Jackson and her colleagues hope, however, that their past and future studies could contribute towards developing new types of concrete that could survive for far longer in the sea than current options.

“The Romans were concerned with this,” said Professor Jackson. “If we’re going to build in the sea, we should be concerned with it, too.”