Unfortunately sometime during the fall of the Roman empire our ability to make todays most widely used building material was lost in the plunge into the dark ages. The modern recipe that we are all so familiar with wasn't discovered until the 18th century.
When Roman and modern Portland cement are compared it turns out that the old recipe is still better than what we currently use. It is thought that the durability of Roman cement used in construction of places like the coliseum was better because of additives to the mix which help the concrete expand and contract without cracking and breaking down.
I found this out in an online article called "Top 10 Lost technologies" If you would like to read about the other 9 then follow the link, some of these technologies that we have lost we have never regained and not only that some of them are so mysterious that we don't even know what they were actually meant to be!