Pozzolane is a fine, sandy volcanic ash, extracted from the ground, and used as an additive to clinker and gypsum at the grinding level.
Once fully hardened, the Pozzolaneic cement blend may be stronger than Type I Portland cement, due to its lower porosity, which also makes it more resistant to water absorption and spalling.
Many great ancient structures, such as the Colosseum, the Pantheon, the Bath of Caracalla, as well as other structures that are still standing in Italy, Greece, France, Spain and the islands in the Mediterranean Sea, were built with natural Pozzolane-lime mixtures. Many of them have lasted two thousand years. After the invention of Portland cement, natural Pozzolane was used as a concrete strengthening additive to improve characteristics such as durability, compressive strength, chemical resistance. hydration heat, permeability etc..
Natural Pozzolane is formed when silica rich magma meets with a large quantity of under ground water in the volcano conduit.
Under high pressure and high temperature, water in steam form dissolves into the magma mixing with the dissolved carbon dioxide and sulfur gases. When this magma reaches the earth's surface, it blows off the top of the volcano cone.
Because the pressure is suddenly reduced, all the gases inside the magma are released and the magma, blown up like pop-corns, falls to ground then cools into small porous rocks.