What is Roman concrete

On the trail of the secret of Roman concrete

Amazing material properties: While seawater wears down modern concrete structures over time, it strengthened the mysterious building material of the ancient port facilities: Roman concrete. Researchers have now gained insights into what lies behind this desirable property of the ancient material: The mysterious recipe leads to a material that, in contact with salt water, forms crystalline substances that make the marine structures increasingly resistant. This effect could help make today's types of cement and concrete more durable, say the researchers.

In ancient times, people were obviously well aware of the special properties of cast masonry. Around AD 79, the Roman scholar Pliny the Elder wrote: "A single mass of stone is created that is impenetrable to the waves and becomes stronger every day". He was not exaggerating: some Roman port structures withstood the surf for 2000 years and can still be admired today. You are now the focus of a research team led by Marie Jackson from the University of Utah in Salt Lake City.

The foundation of architecture - since ancient times

It is generally known that the ancient Romans first made cement from volcanic ash, lime and seawater for their building material. This combination led to the so-called Pozzolan reaction, named after the city of Pozzuoli in the Bay of Naples. The Romans then added volcanic rock to make concrete.
They used this cast masonry in the construction of many structures, such as the Pantheon and the Trajan's Markets in Rome, but also in massive marine structures.

To this day, concrete literally forms the foundation of architecture. The modern Portland cement concrete has excellent properties, but it is not particularly resistant to seawater: Modern concrete structures in the sea become crumbly and crumble over time. "The alkali-silica reaction is a major cause of the destruction of portland cement concrete structures," says Jackson. The scientist has therefore been researching the remarkable durability of Roman concrete for years. She and her colleagues have already been able to show that the marine concrete contains an exceptionally rare crystalline aluminum compound that appears to be responsible for the structural strength. For the current study, the researchers have now again examined drill cores from ancient port structures using modern analysis methods.

A lost recipe

They have now been able to demonstrate in detail that aluminum tobermorite crystals and a related mineral - phillipsite - have formed in the cement matrix. The amazing thing: Actually, high temperatures should have been necessary for the formation of these substances. But that was not the case in ancient manufacturing. Something else must have caused the minerals to grow at a low temperature after the concrete hardened. “Nobody produced tobermorite at 20 degrees Celsius - except for the Romans,” says Jackson.

Apparently penetrating seawater dissolved components of the volcanic ash in the concrete structures and then enabled the formation of Al-Tobermorite and Phillipsite. Their crystals are similar to those in volcanic rocks, explain the researchers. They have shapes that enormously strengthen the cement matrix and thus increase the resistance of the concrete to breakage.

But why not just use Roman concrete for modern marine structures? Another advantage would be that its manufacture produces fewer carbon dioxide emissions. "The recipe is lost," says Jackson. From ancient Roman texts and also from material analyzes it is not clear how the concrete is made. The researchers are therefore now working on a replacement recipe. The scientists emphasize that this new-old concrete could hardly outdo modern Portland cement in general: Roman concrete takes time to develop its strength from seawater and has a lower compressive strength than the modern version. For certain marine applications, however, it could again become a guarantee of long-term durability.

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© Wissenschaft.de - Martin Vieweg
3rd July 2017