Strength properties of geopolymers derived from original and desulfurized red mud cured at ambient temperature

This study investigated the feasibility of using red mud derived from the Bayer process in alumina industry to make geopolymer cured at ambient temperature. Two types of red mud were used: the original one and the one after desulfurization of flue gas from coal-burning power plant. The red mud and class C fly ash were mixed at a constant ratio of 50:50 with additional alkaline solution, cured at ambient temperature, and tested for the strength properties at different ages. The geopolymers derived from red mud before and after flue gas desulfurization (FGD) obtained a compressive strength of 15.2 MPa (2.5 M NaOH) and 20.3 MPa (3.5 M NaOH), respectively. The high alkalinity of original red mud was found to contribute to the formation of geopolymer, but additional NaOH solution was needed to achieve a desirable strength. The higher strength of geopolymer made with red mud after FGD was attributed to the effect of Na2SO4, which was the reaction product of NaOH and flue gas. As an activator, Na2SO4 could increase the pH value, accelerate the dissolution of fly ash under alkaline environment. The results from this study indicate a potential synergy among alumina, power plant, and geopolymer industries.