New research sheds new light on why Roman concrete is so strong and water resistant, but let's not get carried away here.
The Guardian titles its post Why Roman concrete still stands strong while modern version decays. The Washington Post goes a step further and pens the dreadful Ancient Romans made world’s ‘most durable’ concrete. We might use it to stop rising seas -- as if we will build giant concrete walls around America to hold back the water. Both are writing about new research into Roman concrete, which obviously has lasted thousands of years.
Both articles are filled with engineering howlers. Nicola Davis of the Guardian writes:
The Roman recipe – a mix of volcanic ash, lime (calcium oxide), seawater and lumps of volcanic rock – held together piers, breakwaters and harbours. Moreover, in contrast to modern materials, the ancient water-based structures became stronger over time.
But all concrete becomes stronger over time. We’re demolishing buildings now where the concrete has not even totally cured. But Ben Guarino of The Washington Post goes off on a really weird tangent about how this might stop rising seas.
The Roman stuff is “an extraordinarily rich material in terms of scientific possibility,” said Philip Brune, a research scientist at DuPont Pioneer who has studied the engineering properties of Roman monuments. “It's the most durable building material in human history, and I say that as an engineer not prone to hyperbole.” By contrast, modern concrete exposed to saltwater corrodes within decades.
But modern concrete has reinforcing. Roman concrete does not. Mix salt water and steel or iron and you get corrosion, expansion and the destruction of the concrete. He quotes Roman concrete expert Marie Jackson:
Modern sea walls require steel reinforcements; a future in which “large relic walls of twisted steel” dot the coast would be “very troubling,” Jackson said. The Romans didn't use steel. Their reactive concrete was strong enough on its own.
Except that sea walls probably don’t require reinforcement if they are as thick as Roman ones, which are just big thick lumps of rock that resist the forces of nature by sheer mass. Concrete is strong in compression but weak in tension, which is where reinforcement comes in. Jackson's research found that Roman concrete resists cracking, but surely the main reason that Roman concrete survives is because of the lack of reinforcement.
Guarino of the Post suggests that one could build barriers around the country to hold back the oceans, made out of Roman-style concrete without reinforcing. That would require a lot of concrete, Roman or modern.
Both papers are reporting on new research by Marie Jackson and her team, which finds that seawater seeping into the concrete dissolves volcanic crystals with Tobermorite taking their place. These minerals, say the authors, helped to reinforce the concrete, preventing cracks from growing, with structures becoming stronger over time as the minerals grew. This builds on earlier research covered in TreeHugger where we noted:
Unlike modern concrete which actually shrinks, opening up tiny cracks that propagate and let moisture in, Roman concrete, made with volcanic ash instead of portland cement, is actually self-healing as a crystalline binder forms and prevents the concrete from cracking any further.
That was not quite accurate. Roman concrete is still made with lime and still subject to the lime cycle, where you cook limestone and it gives off carbon dioxide. As it sets, it absorbs carbon dioxide and over thousands of years gets stronger as it continues to harden. But the new research clarifies how the concrete was essentially self-healing, as explained in the Guardian:
But now Jackson and the team have made another discovery. “I went back to the concrete and found abundant tobermorite growing through the fabric of the concrete, often in association with phillipsite [another mineral],” she said. She said this revealed another process that was also at play. Over time, seawater that seeped through the concrete dissolved the volcanic crystals and glasses, with aluminous tobermorite and phillipsite crystallising in their place.
But about those sea walls...
I know Ben Guarino is a journalist trying to turn dull science into an exciting story. But to think that we could build big thick sea walls without reinforcement out of Roman concrete is nuts. We would have to dig up the country for aggregate and grind Italy and every other volcano in the world to dust to get enough Pozzolana. The CO2 generated by the lime production would probably sink us on its own.
The production of concrete, whether Roman or American, is one of the largest generators of CO2, which heats the planet and causes waters to rise. We would be cooking the planet to build sea walls to protect us from the effects of cooking the planet. How does this make sense?