Synthesis factors affecting mechanical properties, microstructure, and chemical composition of red mud-fly ash based geopolymers

The influence of synthesis factors, including chemical composition of raw materials and curing conditions, on the microstructure and mechanical properties of geopolymers synthesized from red mud (RM) and class F fly ash (FFA) was investigated. Mechanical properties, microstructure, mineralogy, and chemical composition of the resulting geopolymers were characterized by unconfined compression testing, scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDX), respectively. Geopolymers were successfully synthesized at the ambient condition of 23 °C and 40-50% relative humidity (RH) with the 28-day unconfined compressive strength (UCS) ranging from 11.3 to 21.3 MPa. The formation of amorphous geopolymer gel was confirmed by XRD patterns, SEM images, and EDX spectra. This study suggests that a nominal Na/Al molar ratio in the range of 0.6-0.8 with a fixed nominal Si/Al molar ratio of 2 is a good starting point to synthesize geopolymer from RM and FFA. The ambient condition is also confirmed to be a practically feasible scheme for curing RM-FFA based geopolymers, and exceptional mechanical properties can be obtained at a curing time period of up to 180 days.