Pozzolana

Geochemistry and mineralogy

The major pozzolanically active component of volcanic pumices and ashes is a highly porous glass. The easily alterable, or highly reactive, nature of these ashes and pumices limits their occurrence largely to recently active volcanic areas. Most of the traditionally used natural pozzolans belong to this group, i.e., volcanic pumice from Pozzuoli, Santorin earth and the incoherent parts of German trass.

The chemical composition of pozzolana is variable and reflects the regional type of volcanism. 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 the IUGS. Basic (45–52 wt% SiO₂) and ultrabasic (<45 wt% SiO₂) pyroclastics are less commonly used as pozzolans. Al₂O₃ is present in substantial amounts in most pozzolanas, Fe₂O₃ and MgO are present in minor proportions only, as is typical or more acid rock types. CaO and alkali contents are usually modest but can vary substantially from pozzolana to pozzolana.

The mineralogical composition of unaltered pyroclastic rocks is mainly determined by the presence of phenocrysts and the chemical composition of the parent magma. The major component is volcanic glass typically present in quantities over 50 wt%. Pozzolana containing significantly less volcanic glass, such as a trachyandesite from Volvic (France) with only 25 wt% are less reactive. Apart from the glass content and its morphology associated with the specific surface area, also defects and the degree of strain in the glass appear to affect the pozzolanic activity. Typical associated minerals present as large phenocrysts are members of the plagioclase feldspar solid solution series. In pyroclastic rocks in which alkalis predominate over Ca, K-feldspar such as sanidine or albite Na-feldspar are found. Leucite is present in the K-rich, silica-poor Latium pozzolanas. Quartz is usually present in minor quantities in acidic pozzolanas, while pyroxenes and/or olivine phenocrysts are often found in more basic materials. Xenocrysts or rock fragments incorporated during the violent eruptional and depositional events are also encountered. Zeolite, opal CT and clay 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 the pozzolanic activity, clay formation has adverse effects on the performance of lime-pozzolan blends or blended cements.

Modern use

Pozzolana is abundant in certain locations and is extensively used as an addition to Portland cement in countries such as Italy, Germany, Kenya, Uganda, Turkey, China and Greece. Compared to industrial by-product pozzolans they are characterized by larger ranges in composition and a larger variability in physical properties. The application of pozzolana in Portland 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.

Pozzolanic reaction

The pozzolanic reaction is the chemical reaction that occurs in portland cement containing pozzolans. It is the main reaction involved in the Roman concrete invented in Ancient Rome. At the basis of the pozzolanic reaction stands a simple acid-base reaction between calcium hydroxide (as Portlandite) and silicic acid.