A lot of comments here on Roman concrete are missing the killer application: marine use. Any other concrete we know of will degrade in seawater due to different failure modes acting at once (leaching minerals, pH imbalance, freeze-thaw cycles), and none have the same extent of self-healing.

Roman concrete (pozollanic material, quicklime, seawater) is the only one that resists all failure modes and will sit in the ocean happily for millennia. The main downsides are it's not very strong, it takes a long time to cure, and volcanic ash is hard to come by. The specific of ash you use changes the result, ash is not always that easy to get, and Neapolitan Volcanic ash just happens to be extremely effective at this application.

There are alternatives you can make today. You can make Roman concrete today, but it's still kind of tricky and has the aforementioned downsides. Fly-ash concrete is like volcanic ash concrete but still not as good, and we're gonna run out of fly-ash as coal mines close. High-slag concrete works well but will degrade over time. Alkali-activated concrete is really promising as a Roman concrete alternative but doesn't have long-term test data. Ultra-high-performance concrete is brittle and won't self-heal.

So in truth we still don't know everything about Roman concrete and we still can't make its equivalent without traveling to southern Italy.