ABSTRACT
Natural pozzolans can be subdivided into two categories as materials of volcanic origin (pyroclastic) and materials of sedimentary origin (clastic). The first category includes materials formed by the quenching of molten magma when it is projected to the atmosphere upon explosive volcanic eruptions. The explosive eruption has two consequences: (1) The gases, originally dissolved in the magma are released by the sudden decrease of pressure. This causes a microporous structure in the resulting material. (2) Rapid cooling of the molten magma particles when contacted with the atmosphere results in quenching, which is responsible for the glassy state of the solidified material. Pozzolanic materials of volcanic origin may be
found in loose (incoherent) or compacted (coherent) forms in nature. The latter results from the post-depositional processes such as weathering, compaction, cementation and hardening of the originally loose material. These processes may change the original structure into clayey or zeolitic character. Transformation into clayey structure reduces the pozzolanicity whereas zeolitisation improves it (Massazza, 1988).
