Learning from Ancient Roman Concrete to Improve Modern Masonry - Study the Past, If You Would Divine the Future
Around 27 BC, Marcus Agrippa, a decorated Roman general, grew bored defeating foes in battle (see: Battle of Actium, where Mark Antony and Cleopatra got their crowns handed to them), and decided to try his wings in the building industry. Visit Rome today and you can still see Agrippa’s first project, the Pantheon, standing erect and in decent shape despite centuries of sackings, storms, pollution, panhandlers, and Roman drivers.
The Pantheon, the world’s largest unreinforced concrete dome, still standing 2,000 years later, is by no means a singular phenomenon. The former Roman Empire is dotted with ancient marvels of constructional divine—which begs the question: in a world where nothing lasts forever, how were these remnant structures of the ancient world able to beat the odds?
Solving this mystery is important, both for mankind in general and for the producers and consumers of concrete products. In environmental terms, today’s way of producing concrete isn’t cutting it. Modern concrete requires the use of high proportions of energy-intensive Portland cement, causing 6% of global CO2 emissions. Ironically enough, our way of building the modern world is also destroying it.
Additionally, there’s the quality of modern concrete to consider—or lack thereof, to be more precise. Many modern concretes demonstrate poor resistance to weathering and deterioration, mostly due to factors like the corrosion of reinforcing steel, inadequate material selection, mix design formulation criteria, and variability in cement characteristics—all pitfalls that Roman concretes, we’ve since learned, have largely avoided.
To quote Robert Courland’s fantastic Concrete Planet, “The concrete Roman Senate House and Pantheon still stand after almost two millennia, but hardly any of the concrete structures that now exist are capable of enduring two centuries, and many will begin disintegrating after fifty years. We have built a disposable world using a short-lived material, the manufacture of which generates millions of tons of greenhouse gasses.”
How did the Romans produce concrete more durable than what we produce today? What was the secret of Agrippa and his contemporaries? Agrippa wasn’t the diary-keeping type, but a contemporary of his was. Marcus Vitruvius Pollio was a famed architect, engineer, author, and one might assume busy-body of the Roman Empire. His book, De Architectura, is a multi-volume work essentially chronicling his love affair with the local building resources—most notably Pozzolanic ash (“volcano ash” in English), which he reports as having a reactive capacity with hydrated lime (another natural resource). The ash’s high proportions of silica, he describes, react with calcium hydroxide and water to form highly-durable cementitious compounds. Compacting these compounds in formwork results in dense interparticle arrangements with minimal pore space. To summarize: nothing short of an act of God could erode this stuff.
Ingredients, however, are just half of the equation; process is also key, and this is where the Roman’s ingenious methods really come into play. Their method - geopolymer stabilization - utilized natural pozzolanic materials and alkali materials to produce highly durable cement-like binders. Compacting these compounds in formwork resulted in dense inter-particle arrangements with minimal pore space. The use of salt-water curing is also believed to have improved the properties of these novel concretes by promoting the irreversible binding of alkali cations. To give you some perspective, modern concretes act in the opposite way, where alkali cation and sulfate interactions typically produce detrimental effects and can contribute to premature deterioration.
In conclusion, the secret of Roman concrete is local resources and low-energy stabilization. Not only durable, but environmentally-friendly. Leave it to the Romans to be centuries ahead of their time.
Watershed Materials is working to build (pun intended) on the Roman standard. By implementing careful material selection and mix-design formulation processes, by employing processes of low moisture-content compaction and specialized curing, we’ve combined the Roman’s ancient methodology with the latest advances in masonry to create a solution to the environmental costs and endurance problems latent in common concrete.
We’re proud of what we’ve accomplished so far, but we’re just getting started. The National Science Foundation recently funded our research to further study the methods pioneered by the Romans. This research will significantly advance our understanding of geopolymer stabilization technologies with a focus on utilizing natural resources as the source of alumina and silica in binders. The result of this research will be structural, durable masonry using even less cement, possibly zero. Thousands of years later, we’re circling back to the building blocks of our past - long lasting structures built with locally sourced, environmentally friendly methods.
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