Romans and Concrete

Early architecture began in ancient Rome where the Romans started making concrete more than 2,000 years ago. The concrete they used is around ten times weaker then the concrete we use today. Their main ingredient was volcanic ash, while we use a mix of a lime-based cement, water, sand and gravel.The Romans also used seawater in their mixture.

http://www.smithsonianmag.com/history-archaeology/The-Secrets-of-Ancient-Romes-Buildings.html