The first task after setting up the centering was to mix the mortar and choose the caementa. The best-quality Roman concrete during the imperial period was extraordinarily strong and durable. This is in part due to the addition of pozzolana but also to the high-quality lime that was available. In addition, the choice of caementa played a significant role in the stability of the structures as did the care in placing them within the mortar and ensuring that the mixture was very compact. In the following sections, I examine the individual ingredients of the concrete mixture to show how they interacted with each other and how and why the builders chose the varieties that appear in the extant remains.

MORTAR

The mortar used by the Romans employed pozzolana, a volcanic ash that imparted added strength and hydraulic qualities (the ability to harden under water) that were lacking in the simple lime mortar used by the Greeks. Recent studies show that the resistance to compression of pozzolana-lime mortar is five to eight times stronger than that of lime mortar. A simple lime mortar made of siliceous quartz sand (SiO2), slaked lime (Ca(OH)2), and water (H2O) hardens and gains strength through its contact with carbon dioxide (CO2) in the air as the water evaporates; as a result, the mortar at the center of a mass does not develop the same degree of strength as that in contact with the air.