Pozzolans are a broad class of siliceous and aluminous materials which, in themselves, possess little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically with calcium hydroxide (Ca(OH)2) at ordinary temperature to form compounds possessing cementitious properties. The quantification of the capacity of a pozzolan to react with calcium hydroxide and water is given by measuring its pozzolanic activity. Pozzolana are naturally occurring pozzolans of volcanic origin.
History Mixtures of calcined lime and finely ground, active aluminosilicate materials were pioneered and developed as inorganic binders in the Ancient world. Architectural remains of the Minoan civilization on Crete have shown evidence of the combined use of slaked lime and additions of finely ground potsherds for waterproof renderings in baths, cisterns and aqueducts. Evidence of the deliberate use of volcanic materials such as volcanic ashes or tuffs by the ancient Greeks dates back to at least 500–400 BC, as uncovered at the ancient city of Kameiros, Rhodes. In subsequent centuries the practice spread to the mainland and was eventually adopted and further developed by the Romans. The Romans used volcanic pumices and tuffs found in neighbouring territories, the most famous ones found in Pozzuoli (Naples), hence the name pozzolan, and in Segni (Latium). Preference was given to natural pozzolan sources such as German trass, but crushed ceramic waste was frequently used when natural deposits were not locally available. The exceptional lifetime and preservation conditions of some of the most famous Roman buildings such as the Pantheon or the Pont du Gard constructed using pozzolan-lime mortars and concrete testify both to the excellent workmanship achieved by Roman engineers and to the durable properties of the binders they used. Much of the practical skill and knowledge regarding the use of pozzolans was lost at the decline of the Roman empire. The rediscovery of Roman architectural practices, as described by Vitruvius in De architectura, also led to the reintroduction of lime-pozzolan binders. Particularly the strength, durability and hydraulic capability of hardening underwater made them popular construction materials during the 16th–18th century. The invention of other hydraulic lime cements and eventually Portland cement in the 18th and 19th century resulted in a gradual decline of the use of pozzolan-lime binders, which develop strength less rapidly. Over the course of the 20th century the use of pozzolans as additions (the technical term is "supplementary cementitious material", usually abbreviated "SCM") to Portland cement concrete mixtures became common practice. Combinations of economic and technical aspects and, increasingly, environmental concerns have made so-called blended cements, i.e., cements that contain considerable amounts of supplementary cementitious materials (mostly around 20% by weight, but over 80% by weight in Portland blast-furnace slag cement), the most widely produced and used cement type by the beginning of the 21st century.
Pozzolanic materials The general definition of a pozzolan embraces a large number of materials which vary widely in terms of origin, composition and properties. Both natural and artificial (man-made) materials show pozzolanic activity and are used as supplementary cementitious materials. Artificial pozzolans can be produced deliberately, for instance by thermal activation of kaolin-clays to obtain metakaolin, or can be obtained as waste or by-products from high-temperature process such as fly ashes from coal-fired electricity production. The most commonly used pozzolans today are industrial by-products such as fly ash, silica fume from silicon smelting, highly reactive metakaolin, and burned organic matter residues rich in silica such as rice husk ash. Their use has been firmly established and regulated in many countries. However, the supply of high-quality pozzolanic by-products is limited and many local sources are already fully exploited. Alternatives to the established pozzolanic by-products are to be found on the one hand in an expansion of the range of industrial by-products or societal waste considered and on the other hand in an increased usage of naturally occurring pozzolans. Natural pozzolanas are abundant in certain locations and are extensively used as an addition to Portland cement in countries such as Italy, Germany, Greece and China. Volcanic ashes and pumices largely composed of volcanic glass are commonly used, as are deposits in which the volcanic glass has been altered to zeolites by interaction with alkaline waters. Deposits of sedimentary origin are less common, with diatomaceous earths, formed by the accumulation of siliceous diatom microskeletons, a prominent source.
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