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.