Romans use what is called saltwater cement and we use portland cement. Both are the binder for concrete.
We can tell by analysis of roman concrete what the composition is but there is debate about the formula used. This is likely because like modern cement there is a large number of combinations and proportions that result in cement. Two things we know very well, they used a lot of pozzlin and they cured the mixture in forms in saltwater.
The romans pretty much used concrete to form bricks of concrete. Bricks of all sizes. The saltwater curing process required submerging the concrete in the saltwater so forms were needed, hence the bricks.
As far as quality and strength we really only see the most durable examples so what remains may be less typical than it seems. Also, it has had a long time for any residual curing to occur.
Modern portland cement and thereby modern concrete doesn’t need emersion in water much less saltwater. We just mix it in and pour it into any forms we’ve setup. As such, modern concrete is more versatile than what the Romans had. Also, we use reinforcement, which the Romans didn’t use, to over come the weak relative tensile strength of concrete so that we can make much more slender constructions.
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