Answer by Tim Simpson:
To be fair to Roman concrete, Luis Palas, the buildings in your picture of the Forum in Rome were not made of concrete, but marble. The building method was basically the stacking of one marble block on another, with simple cleats and pins holding things together. With time, perhaps earthquakes, theft for re-use and so on, such buildings suffer the fate shown.
Roman concrete, on the other hand, is a remarkable substance. It has a different composition than today’s Portland cement (concrete being the mixture of cement and aggregate, usually crushed rock of some sort). Roman cement was made differently than our cement, and lasts longer than modern concrete. Roman concrete is especially resistant to seawater.
As to modern concrete hardening with age – it seems to get harder for a period of several decades, then it begins to degrade. I’m not sure that the hardening is significant. Some concrete institute must have done along term study on this point.