Pozzolana orpozzuolana (/ˌpɒts(w)əˈlɑːnə/POT-s(w)ə-LAH-nə,Italian:[potts(w)oˈlaːna]), also known aspozzolanic ash (Latin:pulvis puteolanus), is a naturalsiliceous or siliceous-aluminous material which reacts withcalcium hydroxide in the presence of water at room temperature (cf.pozzolanic reaction). In this reaction insolublecalcium silicate hydrate and calcium aluminate hydrate compounds are formed possessingcementitious properties. The designation pozzolana is derived from one of the primary deposits ofvolcanic ash used by theRomans inItaly, atPozzuoli. The modern definition of pozzolana encompasses any volcanic material (pumice orvolcanic ash), predominantly composed of finevolcanic glass, that is used as apozzolan. Note the difference with the term pozzolan, which exerts no bearing on the specific origin of the material, as opposed to pozzolana, which can only be used for pozzolans of volcanic origin, primarily composed ofvolcanic glass.
Pozzolanas such asSantorin earth were used in the Eastern Mediterranean since 500–400 BC. Although pioneered by the ancient Greeks, it was the Romans who eventually fully developed the potential of lime-pozzolan pastes as binder phase inRoman concrete used for buildings and underwater construction.Vitruvius speaks of four types of pozzolana: black, white, grey, and red, all of which can be found in the volcanic areas of Italy, such asNaples. Typically it was very thoroughly mixed two-to-one withlime just prior to mixing with water. TheRoman port at Cosa was built of pozzolana-lime concrete that was poured under water, apparently using a long tube to carefully lay it up without allowing sea water to mix with it. The threepiers are still visible today, with the underwater portions in generally excellent condition even after more than 2100 years.
The majorpozzolanically active component of volcanicpumices andashes is a highly porousglass.[1] The easilyalterable, or highly reactive, nature of these ashes and pumices limits their occurrence largely to recently active volcanic areas. Most of the traditionally used naturalpozzolans belong to this group, i.e., volcanic pumice fromPozzuoli,Santorin earth and the incoherent parts of Germantrass.
The chemical composition of pozzolana is variable and reflects the regional type ofvolcanism. SiO2 being the major chemical component, most unaltered pumices and ashes fall in the intermediate (52–66 wt% SiO2) to acid (>66 wt% SiO2) composition range for glassy rock types outlined by theIUGS. Basic (45–52 wt% SiO2) and ultrabasic (<45 wt% SiO2)pyroclastics are less commonly used aspozzolans. Al2O3 is present in substantial amounts in most pozzolanas, Fe2O3 and MgO are present in minor proportions only, as is typical or moreacid rock types. CaO and alkali contents are usually modest but can vary substantially from pozzolana to pozzolana.
Themineralogical composition of unalteredpyroclastic rocks is mainly determined by the presence ofphenocrysts and the chemical composition of the parentmagma. The major component isvolcanic glass typically present in quantities over 50 wt%. Pozzolana containing significantly lessvolcanic glass, such as atrachyandesite fromVolvic (France) with only 25 wt% are lessreactive.[2] Apart from the glass content and its morphology associated with thespecific surface area, also defects and the degree of strain in the glass appear to affect thepozzolanic activity.[3]Typical associated minerals present as largephenocrysts are members of theplagioclasefeldsparsolid solution series. Inpyroclastic rocks in which alkalis predominate over Ca,K-feldspar such assanidine oralbite Na-feldspar[4] are found.Leucite is present in the K-rich, silica-poorLatium pozzolanas.Quartz is usually present in minor quantities in acidic pozzolanas, whilepyroxenes and/orolivinephenocrysts are often found in more basic materials.Xenocrysts or rock fragments incorporated during the violenteruptional and depositional events are also encountered.Zeolite,opal CT andclay minerals are often present in minor quantities as alteration products of the volcanic glass. While zeolitisation or formation of opal CT is in general beneficial for thepozzolanic activity, clay formation has adverse effects on the performance of lime-pozzolan blends or blended cements.
Pozzolana is abundant in certain locations and is extensively used as an addition toPortland cement in countries such as Italy, Germany, Kenya, Uganda, Turkey, China and Greece. Compared to industrial by-productpozzolans they are characterized by larger ranges in composition and a larger variability in physical properties. The application of pozzolana inPortland cement is mainly controlled by the local availability of suitable deposits and the competition with the accessible industrial by-product supplementary cementitious materials. In part due to the exhaustion of the latter sources and the extensive reserves of pozzolana available, partly because of the proven technical advantages of an intelligent use of pozzolana, their use is expected to be strongly expanded in the future.[5]
The pozzolanic reaction is thechemical reaction that occurs inportland cement containingpozzolans. It is the main reaction involved in theRoman concrete invented inAncient Rome. At the basis of the pozzolanic reaction stands a simple acid-base reaction betweencalcium hydroxide (asPortlandite) andsilicic acid.