How A Supposed Mistake Helped Roman Concrete Last Millennia

An important quality that employers look for in trade recruitment is a candidate’s focus on the little details and a key illustration of why this matters has recently explained how some early forms of concrete have lasted over 2000 years.

The mystery of Roman concrete has transfixed architects, historians and construction experts for hundreds of years, especially given that modern concrete mixes tend to require some form of reinforcement such as rebar and typically have a lifespan of 50 to 70 years.

However, the reason for the strength and enduring durability of Roman concrete could be the result of a material believed to be a mistaken impurity left inside the mixture.

Ever since Roman concrete has been studied, the prevailing theory is that volcanic ash works as a pozzolanic material to create a material that is more durable than commonly used, particularly for water-resistant concrete used in aqueducts.

This is in no small part the result of contemporary writers such as Vitruvius, the writer of Di Architectura, the Ten Books on Architecture. Vitruvius recommended pozzolana, a volcanic sand named after and found near Pozzuoli, and it was shipped across the empire for that purpose.

However, another ingredient was found in samples of Roman concrete that turned out to be even more critical. The distinctive “lime clasts” were dismissed as simply a consequence of sloppy mixing or poor raw materials, but this latest study suggests that they are more critical than was previously believed.

Using chemical mapping techniques and high-resolution imaging tools, the previous belief that Roman concrete was initially made using slaking (where lime is mixed with water to create a paste), was questioned in favour of the idea that lime was used in its more reactive quicklime form.

This means that Roman concrete was made using hot mixing techniques that as a consequence integrated reactive calcium carbonate crystals that could react and create a self-healing function.

This is why Roman concrete was said to get stronger as it gets older, which is the opposite of conventional concrete.