The manufacture and use of concrete was at the heart of most major Roman architectural achievements. From the aqueducts to the Roman baths, this solid but easy to use building material created structures that lasted centuries across the length and breadth of the ancient world. The formula and structure of Roman concrete has been handed down through the years, and is as simple to make today as it was more than a millennium ago.
Create a batch of quicklime by heating pure white limestone in a kiln. Heating the limestone will create a chemical reaction, leeching off some of the carbon and oxygen in the stone and leaving behind the quicklime. The closer to white the limestone is, the greater its purity, as impurities will appear as dark spots in the stone.
Place the quicklime in water. Wait while the quicklime undergoes another chemical reaction and produces a white paste known as hydrated lime.
Mix the hydrated lime with clean sand to create a mortar. For concrete that will be exposed to wet conditions, use pozzolana, which is a volcanic ash found in the volcanic areas of Italy, such as around Mount Vesuvius or Naples. You can emulate pozzolana by pulverizing clay pottery and heating the resulting dust to a temperature just below the melting point. The resulting mortar should be the equivalent of a zero-slump mortar, thick and able to hold its shape when placed onto a surface.
Add small stones to the mixture to strengthen the concrete, mixing the final concrete product with a hoe to completely encompass the stones.
Pour the concrete into place, tamping the concrete down with a wooden tool to make sure the zero-slump mixture completely fills the form you're placing it into.