A mixture of lime, clay, sand and water was used in very ancient constructions. The Egyptians were already using it 2,600 years ago.
Around the 1st century, the Romans perfected this "binder". On adding volcanic soil from the region of Pozzuoli, near Naples, they discovered that they could get this mixture to "set" underwater.
We now know that the soil from Pozzuoli - which lends its name to pozzolan - is 60 to 90% clay and 10 to 40% lime, depending on the source. Archaeological vestiges from that time, such as cement drains, have been discovered in several Roman cities.