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A single stone mass, impregnable to the waves and stronger every day,” that is how the Ancient Roman author Pliny the Elder described Roman maritime concrete structures. While modern marine concrete structures cannot even survive a few decades, Roman piers build 2000 years ago still stand, stronger now than when they were first constructed.

Harbours were the most important physical infrastructure supporting the Roman Empire, both militarily and economically. With the Meditteranean Sea being ‘Our Sea’ to the Romans, it was the core of the Empire in more ways than one. Now a new study has appeared, trying to uncover the secrets of this long-lasting concrete.

The author of this new study is University of Utah geologist Marie Jackson. Her interest in Roman concrete began during a sabbatical year in Rome. Studying first tufts, then volcanic ash deposits, she soon became fascinated with the role these deposits played in the durability of Roman concrete.

So what makes Roman concrete so resilient? According to Jackson, one factor is the mineral intergrowths between the aggregate and the mortar, which prevents cracks from lengthening. In a study of drill cores of Roman harbour concrete from the ROMACONS-project (2002-2009), Jackson and colleagues found “aluminous tobermorite”, an exceptionally rare mineral, in the marine mortar. The mineral crystals formed in lime particles at somewhat elevated temperatures, through pozzolanic reaction.

The presence of the mineral surprised Jackson, as “it is very difficult to make.” So Jackson returned to the ROMACONS drill cores, newly examining them using a variety of methods. What they found, is that aluminous tobermorite and a related zeolite mineral, phillipsite, formed in pumice particles and pores in the cementing matrix. From previous research, it was clear that those minerals must have been caused to grow at low temperature long after the concrete had hardened. This became the great riddle to solve for Jackson:

No one has produced tobermorite at 20 degrees Celsius. Oh – except the Romans! As geologists, we know that rocks change. Change is a constant for earth materials. So how does change influence the durability of Roman structures?

The team concluded the secret was seawater. When seawater percolated through the concrete in breakwaters and in piers, it dissolved components of the volcanic ash used in the concrete. This allowed new minerals to grow, aluminous tobermorite and phillipsite in particular. Because tobermorite has a silica-rich composition, it reinforced the cementing matrix. Its interlocking plates increasing the concrete’s resistance to brittle fracture.

Normally, this rust-like process would be a bad thing for modern materials. As Jackson puts it:

We’re looking at a system that’s contrary to everything one would not want in cement-based (ie ‘modern’) concrete. We’re looking at a system that thrives in open chemical exchange with seawater.

Nevertheless, Roman concrete is extremely durable in maritime applications. So why isn’t it used more often? For a start, the recipe was completely lost. Even after extensive study of ancient Roman texts, the precise methods for mixing the mortar, needed in order to be able to fully recreate the concrete, has not yet been found. Jackson says the Romans were fortunate too:

They observed that volcanic ash grew cements to produce the tuff. We don’t have those rock in a lot of the world, sosubstitutions would have to be made.

Jackson is now working with geological engineer Tom Adams to develop a recipe using materials from the Western U.S. It is hoped this recipe will recreate the properties of Roman concrete. However, despite its durability, it took time to develop strength from seawater, while featuring less compressive strength than typical modern cement. But while the ‘new’ Roman concrete is unlikely to see widespread use, it could be useful in niche application.

For example, Jackson has proposed the New Roman-concrete for a proposed tidal lagoon to be built in Swansea, UK. Since the lagoon would need to operate for 120 years in order to recoup the costs of construction, New Roman-concrete might be the way to go. For that to be able to happen, Jackson intends to continue the work of Pliny and other Roman scholars to answer the remaining questions about the long-term chemical reactions in the aggregate materials.

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