Bottom ash-based geopolymer materials: Mechanical and environmental properties

The feasibility of geopolymer synthesis using incinerator bottom ash as the source of Si- and Al- containing phases was tested in the present study. Alkaline activation at moderate temperatures was used to force the rearrangement of the alumino-silicate matrix resulting in the production of a geopolymeric material. Different mixtures were investigated for geopolymer production, which differed in the Si/Al ratio (obtained upon addition of sodium silicate and metakaolin at varying proportions) and the NaOH concentration in the alkaline medium. The obtained geopolymeric materials were characterized in terms of physical and mechanical properties, mineralogical and microstructural characteristics as well as chemical durability. Wide variations in the main characteristics of the obtained products were observed, indicating a significant influence of the process variables on the evolution of the geopolymerization reactions. While the physical and mechanical properties correlated well with the Si/Al ratio, no clear trend was found with the NaOH concentration; however, although the strength development upon geopolymerization was clear, the experimental data suggested that the optimal Si/Al ratio was probably beyond the investigated range. SEM, FT-IR, TG and leaching test data showed that, although the onset of the geopolymer-formation reactions was clear, the degree of matrix restructuring was relatively poor, indicating the need for further investigation to promote the development of the geopolymerization reactions.