Around two thousand years ago, several city states and nations flourished around the Mediterranean. However, only one of them rose to become an empire that stretched from England to the Middle East and lasted hundreds of years. What factors led to this greatness? This chapter argues that concrete served as the foundation of the Roman Empire. It enabled the building of harbors, aqueducts, walls, roads and numerous other structures for living, trading and worshiping. These structures enabled the expansion, administration and the flourishing of a vast empire. Concrete played a greater role in the Roman empire than in any other prior empire. Factors that encouraged the innovation of concrete and the stability of the empire were openness to diverse peoples, empowerment of these peoples through a common law and citizenship and competition to outdo standards set by rival nations.

Despite its tremendous contribution to the rise of this empire, historians have paid inadequate attention to this groundbreaking Roman innovation of concrete. Once this powerful innovation declined, the great empire itself faced considerable challenges and began to decline as well. This chapter first details the innovation and then the factors that led to its development and adoption.

The Transformative Innovation of Roman Concrete

What Is Roman Concrete?

The three components of Roman concrete were aggregate, mortar and facing. The aggregate was composed of stones, terra cotta, or tiling and represented the bulk of concrete constructions. In fact, the Latin name for Roman concrete is opus caementicium, after caementa— rough, unhewn quarry stones that reminded the Romans of the fist- sized pieces of rock that were used for aggregate. Aggregate could be composed of natural materials, but builders’ waste was the most common source. Mortar was created by mixing lime, water and a specific type of sand or ash. The main ash utilized in the production of concrete was pozzolana, a volcanic earth that reacted with the water and lime to create an extremely strong and waterproof chemical bond with the aggregate. The chemical bond was a calcium aluminum silicate that is the primary compound of concrete even today. The calcium came from lime, while the silicon and aluminum came from pozzolana. In other parts of the Roman Empire where the volcanic ash was not as readily available, pozzolana was substituted by a similar substance— crushed and burned tile and brick.