How ancient Roman concrete grows stronger

Pliny the Elder, the 1st century scholar and naval commander, wrote proudly of Roman harbour concrete, describing it as “impregnable to the waves and every day stronger”. The fact that many Roman harbour works still stand today, after millennia of exposure to the sea, shows that Pliny’s confidence was justified.

Now a group of scientists from the US and China have studied ancient Roman concrete, which was made from volcanic rock and lime (calcium oxide), to understand the secrets of its longevity. The results, published in American Mineralogist, could provide us with tips to make a modern material to build seawalls or waste-containment walls that will last another 2000 years.

Using techniques such as X-ray micro-diffraction and Raman spectroscopy (a laser technique that examines molecular vibrations) to examine samples of the Roman concrete, the team was able to identify crystals of a rare mineral called aluminous tobermorite, along with a more common porous material called phillipsite. These crystals are thought to have grown as seawater seeped into the concrete structure, dissolving minerals from the volcanic rock and replacing them with needle and plate-like structures reinforcing the concrete.

“Contrary to the principles of modern cement-based concrete,” says Marie Jackson, the study’s lead author, “the Romans created a rock-like concrete that thrives in open chemical exchange with seawater.”

Modern concrete relies on cement to bind together “aggregates”; particles made from sand or crushed rock. Any chemical changes to these aggregates over time can cause the concrete to expand and weaken. In contrast, the Roman concrete is bound together and strengthened by the aluminum-tobermorite minerals, which are deposited over millennia.

The production of synthetic tobermorite is a difficult process, and has not been achieved in modern times without using high temperatures. Methods to produce tobermorite-containing concretes at ambient temperatures could have a big impact, and Jackson’s group is exploring this idea.

There could also be environmental benefits to Roman concrete. The production of modern Portland cement accounts for about 5% of global CO2 emissions, as limestone and other additives are heated to 1450°C in a kiln. Learning a lesson from the ancient Romans could help us to reduce this impact.

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