Steel reinforcement (tension strength) and Portland cement.

Pure concrete can only be used for dome like structures as it only has compression strength.

And that steel reinforcement is why modern Portland cement concrete doesn't last as long. Eventually the steel rusts, which means it expands and cracks the concrete.

We also use a lot of asphalt concrete. Even less durable than Portland cement concrete, but really fast for making roads, and extremely recyclable. Just melt the asphalt, maybe add some new aggregates, and lay down a new road surface.

I am not a chemist, I may have this completely wrong, but:

Steel does not rust without cause.

It needs something like a drip on to a part of it to wash away the layer of oxide and create more. Electrical effects then spread the decay.

There are a lot of steel reinforced structures around from the 1920s that are doing very well with no visible signs of structural decay.

Here is one close to me form the nineteenth century. Everything else decayed. It burnt at least once. And it stands, still.

https://en.wikipedia.org/wiki/Cargill's_Castle

It will be interesting to see if continues for another few hundred years....