The main pozzolanic active component of pumice and ash is highly porous volcanic glass.[4] The easily alterable, or highly reactive, nature of this ash and pumice largely limits its presence to recently active volcanic areas. Most of the traditionally used natural pozzolans belong to this group, that is, volcanic pumice from Pozzuoli, soil from Santorini and the material known in German as trass.[5]​.

The chemical composition of pozzolana is variable and reflects the regional type of volcanism. With silica SiO being the major chemical component, most unaltered ash and pumice fall in the intermediate (52–66 wt% SiO) to acidic (>66 wt% SiO) composition range for glassy rock types described by the International Union of Geological Sciences. Basic (45–52 wt% SiO) and ultrabasic (<45 wt% SiO) pyroclasts are less frequently used as pozzolans. AlO is present in substantial quantities in most pozzolans, FeO and MgO are present only in minor proportions, as is usual in more types of acidic rocks. CaO and alkali contents are usually modest, but can vary substantially between different pozzolanic ashes.

The mineralogical composition of unaltered pyroclastic rocks is determined mainly by the presence of phenocrysts and the chemical composition of the original magma. The main component is volcanic glass, typically present in amounts greater than 50% by weight. Pozzolana with a significantly lower content, such as trachyandesite from Volvic (France) with only 25% by weight, has less reactivity.[6] Apart from the glass content and its morphology associated with the specific surface, also defects and the degree of stress in the glass seem to affect the pozzolanic reactivity.[7]​.

Typical associated minerals present as large phenocrysts are members of the feldspar plagioclase series in solid solution. In pyroclastic rocks in which alkalis predominate over calcium, orthoclases such as sanidine or albite with sodium feldspar are found.[8] Leucite is present in Latium pozzolans, rich in potassium and poor in silica. Quartz is generally present in minor quantities in acidic pozzolans, while olivine and/or pyroxene phenocrysts are often found in more basic materials. Xenoliths or rock fragments incorporated during violent volcanic eruptions or the deposition of volcanic materials on the ground are also found.

Zeolites, opals and clay minerals are usually present in minor quantities as alteration products of volcanic glass. While zeolization or CT opal formation is generally beneficial for pozzolanic activity, clay formation has adverse effects on the performance of lime-pozzolana mixtures or in blended cements.