Used by the ancient Greeks and Romans to make concrete, natural pozzolans are now used to replace the amount of clinker used in cement, creating a more sustainable building material.
Pozzolanic materials occur both naturally in the earth’s crust, as well as being produced as by-products of various industrial processes (e.g. fly ash, silica fume, rice husk and some non-ferrous slags).
Natural pozzolans include rocks of volcanic origin (e.g. vitreous rhyolites from the Rocky Mountains in the USA or German and Turkish trasses), as well some sedimentary clays and shales. Some may be used as they are, while others undergo a process of thermal activation (e.g. calcined clays).
The use of natural pozzolans in construction has a long history. They were first used to produce concrete and mortars in combination with lime about 3000 years ago: perhaps the earliest example is the use of Santorinian Earth (a volcanic rock produced by an eruption on the island of Santorini in the eastern Mediterranean) to produce a concrete that was used to line a water storage tank in the Greek city of Camiros on the island of Rhodes in the 8th Century BCE.
More famously, the Romans’ use of volcanic tuffs and pumice in their concrete enabled the construction of such monuments as the Pantheon and the Baths of Caracella. The most significant of these materials came from the town of Pozzuoli (now a part of Naples), from which the word pozzolan is derived.
The use of pozzolans in cement brings several advantages: they increase late compressive strengths (providing adequate curing) and improve resistance to chemically-aggressive environments. They can also inhibit the alkali-silica reaction, which can cause serious damage to concrete structures.
Natural pozzolans are also generally easy to grind, but may require drying when used in higher percentages.
Although beneficial in the development of later strengths, the use of pozzolans reduces the early strength of concrete, making such cements unsuitable for precast applications and potentially increasing construction times. They also tend to increase water demand during concrete production and can lower resistance to carbonation, which raises the risk of corrosion to carbon (black) steel reinforcement.
Ultimately, the addition of pozzolanic materials to cement brings higher durability and resilience to concrete. As a result, the service life of concrete buildings is lengthened, while maintenance requirements may be reduced, improving the economic, social and environmental sustainability of the built environment.