POZZOLAN

Pozzolan is a finely-divided alumino-silicate material that reacts with calcium hydroxide and alkalies to form compounds possessing cementitious properties.

More than 2000 years ago, Greeks and Romans built structures that survive today that took advantage of the pozzolan-lime reaction. The Romans used a mixture of lime and pozzolan ( a fine volcanic ash), and sea water to produce a hydraulic cement (hardening under water). See this article how Roman concrete differs from today's Portland cement concrete. Recently it has been reported by the University of Calfornia at Berkeley, that  Roman Concrete samples have a very stable calcium-aluminum-silicate-hydrate compound and a very rare hydrothermal mineral called aluminum tobermorite. Romans used pozzolana cement from Pozzuoli, Italy near Mt. Vesuvius to build the Appian Way, the Roman baths, the Coliseum and Pantheon in Rome, and the Pont du Gard aqueduct in south France.  Rome's Pantheon still stands today as the largest unreinforced concrete dome in the world. Vitruvius reported a 2 parts pozzolana to 1 part lime mixture. Animal fat, milk, and blood were used as admixtures (to improve performance.) These structures still exist today!