Descriptive microstructure and fracture surface observations of fired volcanic ash

Crystals of the pyroxene group (diopside, augite and enstatite, hedenbergite), series of crystals with the general formula (Mg _x Fe_1–x )₂SiO₄ having various geometry, identified as spinel (and olivine), and plagioclase crystals from anorthite to anorthoclase that grow together in mass having thin parallel groves embedded in a complex matrix together with calcium alumina silicate grains were found to be the descriptive microstructure of fired volcanic ash. Quartz grains were rarely present as confirmed by dilatometry analysis, XRD, SEM and DTA. The presence of dendrites continuously growing to pyroxene crystals indicated the precipitation/crystallization of these crystals from matrix and regions of glass concentration enhance by ions diffusion. Rings of Ti-rich iron micro-crystals observed around spinel (and olivine) suggested the probable nucleating role of these micro-crystals for the precipitation/crystallization phenomenon. The various types of crystals formed, the difference in their geometry and size and their interlocking mechanism result in a contiguous and dense structure with relevant characteristics at relative low temperature (1125–1150 °C) confirming volcanic ash as a promising alternative raw material for vitrified ceramic products. It was concluded that controlled precipitation/crystallization of raw volcanic ash results on microstructure similar to that of glass-ceramic materials. The observation of fracture surface allowed comparison of fracture mechanics of volcanic ash ceramic to that of conventional vitrified ceramics.