Red mud, a residue of Bayer’s process was used synergistically with fly ash to develop geopolymer. An influence of 0–40% red mud addition on the reaction, structure and properties of fly ash geopolymer was studied using isothermal conduction calorimetry (ICC), Fourier transform infrared spectroscopy (FTIR), electron probe microanalysis (EPMA) and mechanical testing. An improvement in intensity of reaction was observed with the red mud addition at all replacement level but improvement in setting time and compressive strength was observed only in the samples containing 5–20% red mud. Structural characterization revealed that rate of reaction was dependent on the NaOH concentration but the development of mechanical properties were related to the compact microstructure which was developed due to the combined effect of NaOH concentration, solubility of silicates and the presence of iron oxides. Based on scientific understanding, paving blocks using 10% and 20% red mud was developed. These blocks were meeting IS 15658: 2006 standard and leaching of toxic metals were within permissible limits.
