Roman concrete is an architectural marvel. It made it possible for the Romans to build structures unlike any built with the techniques of stone masonry. It turns out Roman concrete is also a chemical marvel. The combination of volcanic ash and rock, lime, and seawater gradually becomes stronger over time as the interaction of the volcanic components and the seawater forms new minerals that fill up cracks and reinforce the structure.
An article from the Guardian explains the process:
Over time, seawater that seeped through the concrete dissolved the volcanic crystals and glasses, with aluminous tobermorite and phillipsite crystallising in their place. These minerals […] helped to reinforce the concrete, preventing cracks from growing, with structures becoming stronger over time as the minerals grew.
Given the limitations of Roman science, it’s doubtful that an ancient Roman concrete expert could have explained the chemical processes that happened in concrete, but that doesn’t mean that Romans just stumbled onto this formula by accident. Even with a limited theoretical understanding, smart people can acquire a lot of practical knowledge through experimentation and careful observation.
Thoughts for writers
Something to keep in mind when worldbuilding: practical knowledge doesn’t have to come from theoretical knowledge. In fact, it is often the opposite: theoretical knowledge develops from an attempt to explain what we already know practically to be true. If you want your fictional cultures to be able to make sturdy concrete, or airships, or vaccines, that doesn’t require them to have a modern understanding of chemistry, physics, or biology. Pre-modern peoples discovered lots of useful things by trial and error and paying close attention to the world around them, even if their attempts to explain why those things worked were sometimes wide of the mark, or they never attempted to explain them at all.
History for Writers is a weekly feature which looks at how history can be a fiction writer’s most useful tool. From worldbuilding to dialogue, history helps you write. Check out the introduction to History for Writers here.
Co-Geeking 
