The "Secret" of roman concrete isn't even really a secret nor really worth it; testing found out that the strength of roman concrete was a by-product of ingredient ratios. i.e. the amount of water and what we would know today as plasticising ingredients.
In short and simplified terms, the higher the water content, the longer the curing time. The longer the curing time, the higher the strength; with a high enough water content and local dampness, a concrete will continue to strengthen for many years, decades, or in the case of roman concrete, centuries.
In reality, the concretes of roman times would've been a lot weaker at their pouring and functional curing times (the amount of time it takes to be "done") than modern concretes are.
The ingredients of concrete are also very simple;
- Cement (Furnace baked limestone+clay, ground to a powder)
- Aggregate (Sand, stones, etc)
- Water