Evolution of structural characteristics and compressive strength in red mud–metakaolin based geopolymer systems

The structure and mechanical performance of red mud–metakaolin based geopolymers with varying red mud contents (0–40 wt% red mud in red mud–metakaolin raw material mixture) were investigated by X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) Spectroscopy, Scanning Electron Microscopy (SEM) and compressive strength measurements to understand structure–performance relationships in this system. Systematic addition of red mud into geopolymer system resulted in constant loss of intensity and increased broadening of the main features in XRD and FTIR spectra. The compressive strength was at maximum (51.5 MPa) for metakaolin based geopolymer while increasing red mud incorporation in this system resulted in a nearly monotonous decreasing trend in the compressive strength. Iron species originating from red mud were found to be the main factor controlling the evolution of structural characteristics and mechanical performance. The main role of iron species in the system was to prevent dissolution of raw materials in the geopolymer matrix to a great extent. This was supported by diffraction, spectroscopy and microscopy results and was correlated with the decreasing trend in compressive strength with increasing red mud content.