One of the oldest professions in the world is that of a lime burner, he converted limestone by fire into a product called “quicklime” that would react vigorously with water to make a new product called “hydrated lime” that makes a slurry used to coat rammed earth walls to protect them from the weather.
Rammed earth construction has been used in arid regions since the beginning of time, and is probably one of the oldest construction techniques used in the world. It is still in use in many parts of the world including the Southwestern United States and throughout the Middle East. It was from the early use of hydrated lime that the early use of primitive concrete grew.
Although the technique was well known in the Mid-East it wasn't until the days of the Romans that it was perfected. The development of Roman Concrete no doubt came about by chance. In Italy there are several areas that are volcanic that are covered by volcanic ash. This ash to the naked eye looks like sand, in fact the Romans called it “pit sand.”
The ash in fact has several unique properties, one of them is the ability to combine with hydrated lime to form a cement that is capable of setting underwater. The Romans took advantage of this to build harbor works at the town of Pozzuoli a Roman harbor just north of present day Naples. It was from the volcanic ash found near this town that the Romans made the discovery of their concrete when they mixed hydrated lime with the ash.
It is probable that this discovery was made by accident because the ash found at Pozzuoli looked like sand. At the time the Romans knew when you mixed hydrated lime with sand you produced a plaster that was used to plaster walls. The mixture they made turned hard, and would even harden under water.
They called this pozzolan in reality it contained amorphous silica that reacted with the hydrated lime to make a new kind of cement. It was with this cement that the Romans were able to produce some of their greatest architectural works.
The real secret was in the pozzolan material itself, this was an alumino-silicate rock that had been subjected to the high heat of an exploding volcano. Other pozzolan materials are man made including ceramics, glass, slag, fly ash and silica fume. They can all be mixed with hydrated lime to produce a superior cement like the Romans used.