As you probably gathered from your references, concrete curing is a continuous process - every time a stray water molecule binds with some cement the concrete w

As you probably gathered from your references, concrete curing is a continuous process - every time a stray water molecule binds with some cement the concrete will get a little bit stronger. We use 28 days to call the concrete 'cured' because it's a reasonable time on construction schedules (why it's in an even number of weeks) and hits a nice fat part of the exponential strength curve. It's a bit like asking when a pile of plutonium is done decaying. What will happen is that at some point, the strength gains are so marginal that various degradation processes will cause the concrete to start losing overall strength. But - the idea that the concrete is curing and will be finished in 50 years is fairly goofy. No one is doing high-temperature long-duration concrete curing research, but the idea that you can extrapolate the laws of heat transfer to figure out how long some bubble of superheated wet concrete in the middle of the dam will last isn't very reasonable.