Absence of rebar is a huge difference. From what I understand it's the primary reason most modern concrete structures are expected to last only on the order of decades to a century or two. Some modern structures have been built without much if any rebar, for instance the Hoover Dam, and I've heard that might last thousands of years.
But beside that (whch surely is the main reason), there is also a great difference between pozzolanic and portland, nowadays (and since several years) pozzolanic cement (for whatever reasons[1]) came out of use and anything today (and since several years) is portland.
I have worked with both in massive structures (mainly bridge foundations and tunnel lining) and the differences between the two is staggering, in practice portland cement based concrete is very good (compression resistance) already at 3 or 7 days, just fine at 28 days, but tops around the 60-90 days.
Pozzolanic cement based concrete is barely reaching specs at 28 days, but continues to mature (increasing resistance) for years to follow, expecially in massive layers and in humid environments.
If you prefere Portland is a better cememt because it is fast, but - given time - pozzolanic is way "stronger".
In tunnels (which have all the best requisites for concrete to mature correctly) we had at the time (some 30-40 years ago) specs of 250 Kg/cm2 cubic resistance (at 28 days), we used pozzolanic cement for the lower arch and portland for the vault, while both reached specs, after 2-3 years we made some tests and the vault (portland) reached 300, in some cases 350, the lower arch was never below 450, in some cases 500 and even 600.
[1] essentially because it cures faster, allows thinner layers/slabs and for anything where formwork is involved this makes a huge difference
What about non-volcanic pozzolans like fly ash? From what I understand, fly ash is sometimes substituted for some portion of the portland cement that might otherwise have been used, because works similar to volcanic ash to make the concrete stronger.
Fly ash is - generally speaking - an exceptionally good additive, its use is essentially due to the size of its particles, you can consider it as a very good filler with some added value (and it has some very useful side-effects, namely it makes concrete much easier to pump).
Besides the raw resistance, you have to imagine concrete as being a sort of artificial stone, the idea is to fit into a given volume as much material as you can, and you obtain this by mixing together gravel (usually one, two or even sometimes three sizes), crushed sand (rather big in size) and (where available) natural sand (or more finely crushed sand).
Then you add the cement, which is the finer "powder like" material, and water, BUT in many cases the "granulometric curve" remains "empty" in the lower part, and there are practical limits in the amount of cement you can put in the mix, so you need to add something (a filler) that is fine as or finer than cement, and this is often fly ash, which while not being as powerful as cement as a binder has anyway a pozzolanic effect, that helps in reducing the permeability of the set concrete, the matter is briefly explained in the second part of this:
