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Does it turn into rusted out rubble in 100 years like the reinforced concrete we use today? I remember reading an article about the longevity of Roman concrete. The best they could come up with was the use of volcanic ash and much slower curing in salt water.
You are correct that reinforced concrete will corrode, generally the service life of reinforced concrete is around 50 years if no special measures are taken. This is because the initial alkalinity of the concrete, which protects the steel, will be neutralized over time. This is the same for modern and for Roman concrete. But many methods are available both to extend life, and to monitor the health of reinforced concr…
Roman concrete is far more durable than modern concrete, reinforced or otherwise. Roman concrete self-heals cracks.
They think they've figured out why, quicklime: https://news.mit.edu/2023/roman-concrete-durability-lime-cas...
The theory is that the Romans hot-mixed their concrete with quicklime. As soon as tiny cracks start to form within the concrete, the cracks travel to lime clasts. The lime then reacts with water, creating a calcium-saturated solution. That solution then recrystallizes as calcium carbonate, sealing the cracks and strengthening the concrete.
The concrete is cheap. They are going to commercialize a modern formulation. One to watch.