Sometime around 200 BC the Romans started using volcanic, pozzolanic ash in their concrete. This was an immediate improvement as a chemical reaction took place between the chemicals in the wall of volcanic ash (silica and small amounts of alumina and iron oxide) and the layer of lime (calcium hydroxide) applied to the wall. Later they found that mixing a little volcanic ash in a fine powder with the moist lime made a thicker coat, but it also produced a durable product that could be submerged in water- something that the plaster product of wet lime and plain sand could not match.