Chemistry alone, however, will not make good concrete. The ancients mixed their cement, (cement, ash, and rock) with as little water as possible to give a stiff, "no-slump" concrete; and spread it on their cisterns and walls. After this it was pounded in the rock layer. Vitruvius, the noted Roman architect (20 BC) mentioned this process in his history of architecture. He also mentioned that special tamping tools were used to build a cistern wall.
This tamping process was very important. The close packing of the molecular structure by tamping reduced the need of excess water, which is a source of voids and weakness. Also close packing produces more bonding gel than might be normally expected.