The mention of Roman Concrete is interesting. Was it that we forgot the recipe or didn't know what was special about the recipe in the first place? I can't find

The mention of Roman Concrete is interesting. Was it that we forgot the recipe or didn't know what was special about the recipe in the first place? I can't find the article now, but I remember reading a while back that part of the reason it was lost was that the materials to make it could no longer be found and apparently equivalent materials didn't work the same.