Hydration mechanism of red mud-fly ash based geopolymer

Geopolymer is considered the most promising alternative to Portland cement due to its excellent performance and low cost. In this study, the hydration mechanism of red mud-fly ash based geopolymer (RFG) was analyzed using low-field nuclear magnetic resonance (LNMR), rheological tests, Fourier transform infrared spectrometry (FT-IR), isothermal calorimetry, and scanning electron microscopy coupled with energy dispersive X-ray spectrometry (SEM-EDS). The compressive strength of RFG falls with increasing dosages of red mud, and initially increased and subsequently decreased with the increment of SiO2/Na2O ratio. According to the results of LNMR and rheological tests, the geopolymerical hydration process of RFG is divided into three phases: an induction phase, an acceleration phase, and a stabilization phase. When the SiO2/Na2O ratio of the activator is lower than 2.4, it has an activation effect on the cementitious material, and the optimal value is 2.1. The hydration rate of the cementitious material decreases as the dosage of red mud is increased. The slurry transitions from the Herschel-Barkley model to the Bingham fluid model as the hydration reaction advances. The hydration products is verified with different red mud dosage and SiO2/Na2O ratio by SEM-EDS. The findings of this study enrich our understanding of RFG, promote its engineering applications, and improve the utilization efficiency of red mud.