There have been a few posts before about the unusual strength of Roman concrete. Here is an excellent short video on the subject. (The secret seems to be the use of volcanic ash):   The amazing strength and durability of many ancient Roman structures, especially those submerged in water, has amazed researchers. Many believe that it is the chemical reaction between seawater with a mixture of volcanic ash and quicklime that creates a highly resistant mineral known as aluminous toberormori