Why did ancient Romans make the best concrete? http://on.natgeo.com/2x5JaVc

Well the Romans did not skimp on materials to begin with Erin Nelson. They like the Egyptians and Greeks before them built buildings to last. The idea of temporary structures for government and religion was an alien concept.
Thanks for sharing.
The Romans were masters of the known western world and they knew that aqueducts were required to keep a steady flow of water to the Roman citizens from Rome through the province of Asia.

I saw this addressed on an documentary on one of the more educational channels. But have been unable to find the title of the documentary. I am going off of memory here so the concept is right but the details may be wrong. Concrete has two primary macro components. Aggregate (typically stone) and some type of mortar.

-The Romans used larger pieces of aggregate in their concrete, and no steel rebar. Roman aggregate was typically volcanic rock. The fact that the aggregate was volcanic rock is chemically important in how it binds with the particular ingredients in the Roman recipe for their mortar. Somehow this makes a final product that is lighter weight, more chemically inert once set, and less prone to fracture (plastic and elastic deformation in response to loads). It also gives the final product some macro properties that mimic the micro properties of the volcanically vitrified ingredients. Rebar as an "ingredient" is not chemically inert, and its support within the concrete substantially degrades over time and limits the design life of any cement structure that requires rebar. At the same time rebar is necessary for structural stability but less plastic in response to load stress.

-The use of large pieces of aggregate change how the concrete was applied to forms in Roman concrete. Modern concrete is poured (less labor intensive). Roman concrete was mixed, formed into flexible blocks and then "hand set" in forms more like brick masonry (extremely labor intensive). ***This was a really important concept to the durability and compressive strength of Roman concrete and I know that there are critical details missing from my explanation here.***

-Modern cement uses "Portland cement" as the mortar which sets by carbonatation using free carbon dioxide within the air. Roman concrete used pozzolano mortar which sets by hydration and binds chemically with the volcanic aggregate . The chemical ingredients and temperatures used in preparing those ingredients are key to understanding the difference, but I cannot remember the specifics. The end result is that Roman concrete has some more plastic like characteristics in response to stress. Pozzolano mortar is also much more chemically inert once set. ***This was a really important concept to the durability and compressive strength of Roman concrete and I know that there are critical details missing from my explanation here.***

Thank you for the great share Erin.