I have written several blogs on the mystery of Roman concrete, and why it appears so much better than the concrete available to us today. Recently, studies have unlocked what scientists think are the reasons for this concrete supremacy of the Roman Empire.
The roof of the Pantheon in Rome is still the largest unsupported concrete dome in the world. Roman concrete is known to defy the centuries without losing much of its firmness. Indeed, it can even get stronger with age. What damage has occurred over the ages in, for example, the Colosseum in Rome, the Pont du Gard in southern Gaul, and the aqueduct in Segovia, has been caused mainly by stone robbery rather than structural failure. Even more surprising, Roman engineers developed concrete that cured under water, which we can’t do. Modern scientists have long wondered how the Roman engineers managed to do all this.
We have known for some time that Roman concrete used volcanic ash as part of their cement; cement is the powder that, when mixed with aggregate (gravel and ground-up rock) makes concrete. Recently, research has shown that they used a mixture of lime and volcanic ash.
I should add that this was described by Vitruvius but no-one, apparently, bothered to explore it until quite recently. Vitruvius was a Roman architect and engineer during the 1st century BC, known for his multi-volume work entitled De architectura.
Contemporary studies have shown that, unlike modern concrete, when water seeped into cracks in Roman concrete, it reacted with some of the volcanic minerals to create calcium aluminosilicate hydrates (C-A-S-H). CASH was not only the main binding material for making concrete, when water seeped in, the CASH flowed into the cracks making the concrete self-healing. Other studies have indicated that the lime used in Roman cement also helped fill in any cracks that appeared.
Interestingly, a study of a city wall in Privernum (today’s Priverno in central Italy) showed small lumps of lime in the concrete, which had not dissolved when the concrete was originally mixed. These lumps, either by design or luck, constituted a reservoir of “mending” agent buried in the concrete for when it was needed to fill cracks. Lucky, or ingenious, we just don’t know, but it was quite effective in self-healing Roman concrete.
Moreover, the distribution and shape of the clumps of lime suggest that it was added as quicklime rather than the slaked (hydrated) lime we use today. This would mean that Roman concrete was “hot” mixed, rather than the cold mixing process we use today. Although hot mixing does create expansion in the concrete, it does produce faster curing.
I could speculate here that this is the reason Roman concrete cured under water, allowing their engineers to create port structures that still exist under water today. I’m not an engineer, I hasten to add, just an observer, so this is speculation.
Roman concrete may therefore hold lessons for today’s builders. “Roman concrete”, says Didier Snoeck, a structural-design engineer at the Free University of Brussels, in Belgium, “shows that modern cement, of which the production emits huge amounts of CO2 (Greenhouse gas), is not indispensable for strong and durable concrete. We can’t”, he says, “Replace all Portland cement with volcanic material, due to the volumes involved, but we can do it partially. We can also use fly ashes, blast furnace slag (current waste products), limestone, and calcined clays instead.”
Studying Roman concrete could also help modern engineers develop recipes for more durable (maybe 2,000 years rather than the current approximate 100 years), self-healing concrete. That means less replacement, less maintenance, less CO2, less demolition and dust contamination, maybe less earthquake damage (after looking at the pictures of Turkey this week – many Roman concrete structures still exist in present-day Turkey after multiple earthquakes over the ages.
It sounds like a win/win to me.
The only downside is, we will have to drastically upgrade the quality of architects we allow to build these “Roman concrete” buildings. I would hate to think that some of the present architectural travesties might last 2,000 years.
Still, we have invented bulldozers, I suppose. That’s a comfort, at least.