Cement.
Actually cement goes back much farther..
The egyptians used cement..
What the romans contributed was the addition of "aggregate", i.e., gravel, etc.. to the cement..
In other words, Romans invented CONCRETE..
Some other interesting things they did with this building material..
They added other minerals, like pumice, to make a water proof concrete.. Then used it to make their shipping docks, canals, and fisheries...
They used Forms, to cast the concrete in a desired shape..
Another additive was wheat gluten.. It acted as an adhesive, allowing the concrete (or cement) to stick to a surface better..
Amazingly, it also increases the life of the concrete/cement as well..
Interestingly, the Chinese used rice paste in exactly the same way.. as an adhesive additive to cement..
Parrallel technologies developing in two seperate societies, thousands of miles apart..