How is it that 2000 years later, certain Roman ruins still stand in harbors? Geologists have determined that it pertains to one of the components—aluminum tobermorite, a rare mineral found in volcanic ash. When struck with seawater (another component of Roman concrete, along with lime and rock), a possolanic reaction occurs where the tobermorite crystallizes and spreads—adding further strength. Thus, longterm seawater exposure only reinforces the material, rather than eroding it. This volcanic ash/seawater combination could potentially be used today to produce sustainable concrete for our waterways.