Years ago, I read that the Romans made cement that is substantilly stronger than modern portland cement. It was called “pozzoilailana”, and was a mixture of sand, lime, clay and volcanic ash. Supposedly, it gets stronger with time-and Roman structures built of this cement are stronger than a modern structure. Is this true? And, is this pozzoiliana still made?
Given the amount of scientific research involved in optimizing the performance of modern cement I would tend to doubt this claim.
See Roman Cement
All concrete gets stronger with time, including the modern stuff.
[nitpick]
Concrete is made out of cement and some other stuff. The words ‘concrete’ and ‘cement’ aren’t interchangeable.
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ETA: We also have reinforced concrete, which wasn’t invented until the 1800’s. This enables the concrete to withstand some amount of tensile forces, which unreinforced concrete cannot take. In that respect it is much stronger than Roman concrete.
Polymer Concrete will last longer than human civilization.
Why yes, I do work in the manufacture of said polymers 
Most cities don’t like it because it means the union road-workers won’t have to replace it every ten years.
:rolleyes:
Yeah, that’s the same reason they don’t build roads out of diamonds.
I hope readers understand what you’re saying: the concrete isn’t affected by the reinforcement, but the thing being built is. I think the OP is asking about the aggregate/cement component.
The concrete in reinforced concrete can’t take any more tensile force than the concrete in unreinforced concrete - in the former the steel takes the tensile forces.
Right, I should have phrased it better. The concrete structures will take tensile forces better.
There may be some blurring of what we’d call “concrete” in the fiber-reinforced kind, but, as you say, that doesn’t seem to be the OP’s question.
Pozzolans are still used, by the way.
Wow … how? What does that mean? What’s the difference between “gets stronger with time” and “is stronger after a proper set”?
I remember reading that the use of ash in modern concrete is a relatively new addition, and that it made for much stronger concrete, but that in Roman times, ash was usually used in concrete. This could be part of what the OP is remembering.
As concrete ages, its strength increases. A 5-day strength is going to be less than a 21-day strength, for example. Here is an example graph.
Here is an article describing concrete strength and explaining how hardening is different from setting.
That same site has a brief history of cement.
Wikipedia has some good information on the use of fly ash in cement:
(From the “fly ash” article).
I think your mixing up a myth that probably had some truth to it.
The Romans were great engineers. Great at trying all kinds of stuff, figuring out what worked and what didnt, refining processes and then FOLLOWING that process.
Through experimenting and testing, the Romans figured out how to make damn good (for the time) concrete. They then followed the recipe.
When the Roman Empire fell, engineering works then fell to the backwards heathens of the time. Since they didnt know the “secrets” for making good concrete, they probably couldnt make the stuff as well as the Romans did. They also probably didnt engineer their structures as well as the Romans did, leading to failures too. You cant just eyeball an engineered structure or concrete and obviously tell how to make them right.
So, what you are probably thinking of is the idea that the Romans made concrete and concrete structures that could not be replicated for many centuries afterwards.
Just an educated guess on my part.
Oh, another possibility. Their good shit might be better than our cheap shit, though very unlikely to be better than our good shit and certainly not better than our high tech shit. The Romans, being Romans, probably used their good shit on big projects. They were probably the original “do it right the first time” ers. Us, we usually use the cheap shit if we think we can get away with it.
I think the Air Force uses something similar for quickly patching shallow holes in runways (for bigger holes, quick patches are made by filling the hole with crushed rock, the chunks of pavement that used to reside where the hole now is, and a big sheet of aluminum or fiberglass, but that’s not relevant to the OP).
And, of course, the presumably many examples of shoddy Roman construction haven’t survived to the present day, so all we have left is the “do it right” work.
Don’t ever let my old high school latin teacher hear you say that 