A couple days ago I prepped a fire pit on the sandy shores of Detroit Lakes, Minnesota. As I shoveled dry ash made from previous fires out of the pit and into a shallow hole in the sand, I imagined myself in pre-Vitruvius days on a Mediterranean coast doing the same thing, or something similar. “Perhaps this is one of the origins of the recipe for concrete?” I thought. I envisioned someone thousands of years ago dumping ash into the sand in a similar way (with lime as well). Maybe it was volcanic ash, or that from a former campfire.
The Romans used this concrete stuff for infinite reasons — from building underwater docks to linear roads to fortifications and infrastructure in general. The architect Vitruvius was the first to scribble down the recipe for concrete. But certainly this knowledge circulated amongst laborers and masons before hand.
One of the concrete secrets is in the texture and quality of the sand. Here is what Vitruvius said in Book 2, Chapter 4: Sand for Concrete Masonry:
In concrete structures one must first inquire into the sand, so that it will be suitable for mixing the mortar and not have any earth mixed in with it. These are the types of excavated sand: black, white, light red, and dark red. Of these the type that crackles when a few grains are rubbed together in the hand will be the best, for earthy sand will not be rough enough. Likewise, if it is thrown onto a white cloth and then shaken off, if it neither dirties the cloth nor leaves behind a residue of earth, it will be suitable.
Thus, keep the soil out of your sand when mixing concrete, this whether you are mixing for a foundation of a tool shed or trying to impose your imperial will on others around you.