The problem is that modern cement begins to crumble after 30 years, and won't generally last much longer than 50-70 years. It's a big reason why we continually rebuild bridges. The Romans built all sorts of structures that are still standing after 2000 years. Why?
Their concrete didn't fall apart like ours does, because they didn't use Portland cement. They used volcanic rock:
The researchers now know why ancient Roman concrete is so superior. They extracted from the floor of Italy’s Pozzuoili Bay, in the northern tip of the Bay of Naples, a sample of concrete breakwater that dates back to 37 B.C. and analyzed its mineral components at research labs in Europe and the U.S., including at Berkeley Lab’s Advanced Light Source. The analysis, the scientists believe, reveals the lost recipe of Roman concrete, and it also points to how much more stable and less environmentally damaging it is than today’s blend.This isn't just dusty history, this is something that can make a big difference to modern construction. Not often you can get that from an archaeology dig.
That’s why the findings, which were published earlier this month in the Journal of the American Ceramic Society and American Mineralogist, are considered so important for today’s industrial engineers and the future of the world’s cities and ports. “The building industry has been searching for a way to make more durable concretes,” Jackson points out.
The secret to Roman concrete lies in its unique mineral formulation and production technique. As the researchers explain in a press release outlining their findings, “The Romans made concrete by mixing lime and volcanic rock. For underwater structures, lime and volcanic ash were mixed to form mortar, and this mortar and volcanic tuff were packed into wooden forms. The seawater instantly triggered a hot chemical reaction. The lime was hydrated—incorporating water molecules into its structure—and reacted with the ash to cement the whole mixture together.”