U. Chinje Melo

The role of iron in the formation of inorganic polymers (geopolymers) from volcanic ash: a 57Fe Mössbauer spectroscopy study

The behavior of the iron present in two volcanic ashes was investigated during geopolymer synthesis using sodium hydroxide as the sole alkali activator. XRD, SEM, and room-temperature 57Fe Mössbauer spectroscopy were used to monitor the behavior of the iron during the synthesis reaction. Geopolymers with very good compressive strengths were formed, especially with the finer ash, in which the iron is present in the crystalline minerals ferroan forsterite and augite. Mössbauer spectroscopy identified the ferrous sites in these minerals, plus a ferric site, probably located in an X-ray amorphous phase. The ferroan forsterite in the original ashes did not react with NaOH, but a substantial proportion of the augite reacted to form new ferric sites with parameters similar to distorted tetrahedral or 5-coordinated environments, suggesting the possible incorporation of ferric iron in the tetrahedral network of the geopolymer product. These results indicate that iron is not necessarily deleterious to geopolymer formation, as has sometimes been suggested.

Sintering behaviour, microstructure and mechanical properties of low quartz content vitrified ceramics using volcanic ash

Abstract A complete investigation on the sintering behaviour, involving ceramic transformation, of volcanic ash is reported. Sintering and softening points, vitrification and fusion of finely ground powders of volcanic ash were obtained by hot stage microscope observation. Then, a suitable thermal cycle, which matches the better microstructure and mechanical properties, has been performed. The low quartz content of the final product, the relative high density together with the particular structural complexity of the matrix consequence of the interlocking of various crystalline phases conferred to fired volcanic ash relevant ceramic characteristics. Nucleation and microcrystallisation of pyroxene together with oxidation and cation enrichment are indicated as the main sintering mechanism of fired volcanic ash. Differently from conventional vitrified ceramics, i.e. quartz and mullite in vitreous matrix, the microstructure of the fired products presents spinel, anorthite, diopside, enstatite, pyroferrite, fayalite and hedenbergite crystals embedded in the high viscous liquid phase resulting in dense and resistant materials.