Impregnable to the waves and every day stronger

Sunday, July 16th, 2017

Pliny the Elder, in his Natural History, described Rome’s concrete as “impregnable to the waves and every day stronger” — which, it turns out, was literally true:

Writing in the journal American Mineralogist, Jackson and colleagues describe how they analysed concrete cores from Roman piers, breakwaters and harbours.

Previous work had revealed lime particles within the cores that surprisingly contained the mineral aluminous tobermorite — a rare substance that is hard to make.

The mineral, said Jackson, formed early in the history of the concrete, as the lime, seawater and volcanic ash of the mortar reacted together in a way that generated heat.

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.

These minerals, say the authors, helped to reinforce the concrete, preventing cracks from growing, with structures becoming stronger over time as the minerals grew.

By contrast, modern concrete, based on Portland cement, is not supposed to change after it hardens — meaning any reactions with the material cause damage.


  1. Bob Sykes says:

    As a retired civil/sanitary engineer, I found this story fascinating. I had long thought that the durability of Roman concrete was due to the lack of a strong annual freeze/thaw cycle in most of the Empire. It is especially interesting that Roman concrete resists sulfate attack, which Portland cement concretes are prone to. Now the problem will be how to get civil engineers, who are notoriously conservative in all things, to adopt a major change in concrete technology.

  2. Ross says:

    Pozzolonic soils (rich in volcanic ash) are a big deal. In addition to the harbor thanks to the King of Judea, check especially The Pantheon (Hadrian, I think?). That latter is a high point of Roman engineering — 2000+ years old, no rebar, still strong.

    Our modern quickset stuff lasts 50–100 years, maximum, and rapidly turns to crap after that.

    The minimum we need to do: coated rebar, or some other means of managing oxidation.

  3. Ross says:

    Bob Sykes, you might like Concrete Planet by Robert Courland. It’s a historical treatment; some great stories in there, well-researched, and quite competently touches current technical issues for the layman.

  4. Name says:

    i think commercial construction has been using epoxy coated bar for decades…

  5. Name says:

    “impregnable to the waves and every day stronger”

    In 20 years, you will all be speaking Latin!

  6. Kirk says:

    Basalt rebar shows some considerable promise, as well.

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