Curing Regime Optimization of Red Mud-Based Geopolymers

Primarily, the current curing regime for red mud-based geopolymer follows cement science, which is less applicable given the distinctions between the strength formation mechanisms of the two systems. While cement curing systems focus on hydration conditions, geopolymer strength formation depends on geopolymerization under highly alkaline conditions, which require elevated temperatures and suitable humidity. This study investigated the effects of curing temperature, curing humidity, and curing duration on the mechanical performance of geopolymers made solely from red mud (RMG). UCS, MIP, SEM, XRD, and FTIR tests were conducted to characterize the strength, microstructure, phase composition, and chemical structure evolutions of RMG under a diverse range of curing regimes. Additionally, a response surface analysis was employed to obtain the optimal curing regime. The results indicate that a high curing temperature facilitated the geopolymerization reactions, increased Si-Al activity, and produced additional gel, which improved the strength development of the RMG solids. The 7-day UCS and 28-day UCS of the thermally cured specimens increased by 27.7% and 14.8%, respectively, as the curing temperature increased from 40 to 80 °C; however, excessively high temperatures resulted in severe shrinkage cracking. Moist curing effectively mitigated the generation of cracks and harmful pores: the 7-day UCS of the specimens cured at 80% relative humidity (RH) was found to be 19.8% higher than those cured at 70% RH. A higher curing humidity (≥80% RH) inhibited the condensation of the RMG paste. The strength of the RMG was enhanced via the extension of the thermal and moist curing duration within 24 h; however, it was sharply reduced when the curing duration increased to 48 hours due to extensive pore generation inside the solid. The optimal curing regime (21.5 h of curing at 73.8 °C and 78.9% RH) was obtained via response surface analysis.