Strength evolution and carbonation behavior of red mud–fly ash geopolymers under varying pre-curing conditions

Abstract

Harnessing red mud (RM) and fly ash (FA) on a large scale could transform these industrial byproducts into valuable resources, driving greener, low-carbon advancements in the aluminum industry. Mastering mechanical and carbon sequestration behaviors of RM-FA based geopolymer grouting materials will provide theoretical support for achieving this goal. The carbonation characteristics, including depth, area, and CO₂ absorption rate, were evaluated for specimens with different precuring durations via phenolphthalein titration and gravimetric methods. The mechanical features of carbonation-cured materials were then tested. Through wide-ranging microstructural characterization systems (XRD, FTIR, SEM-EDS, TG-DTG, and LF NMR), evolution of phase composition, microstructure, and pore-fracture features during both carbonation and geopolymerization processes was systematically explored under varying precuring conditions. Results demonstrate that extending pre-curing duration significantly mitigates NaHCO3-induced obstruction of CO2 diffusion channels, increasing carbonation depth from 1.01 ± 0.05 mm to 5.07 ± 0.11 mm (RSD＜5.0 %). In specimens with shorter pre-curing periods, elevated free alkali content enhances CO₂ absorption to 2.29 ± 0.07 % (RSD＜5.0 %) (C1P). Notably, C14P specimens achieve maximum CO2 sequestration of 30.9 kg/ton through CaCO3 formation, benefiting from both unimpeded CO2 diffusion (absence of excessive NaHCO3 blockage) and sufficient Ca2 + availability. The 40.58 % strength deterioration in C1P specimens stems from the competition between early-stage geopolymerization and carbonation reactions. Subsequent standard curing promotes the growth of Na2CO3/CaCO2 crystals and further drives geopolymerization, thereby significantly reducing the final strength degradation rate of the specimens. Microstructural analysis confirms that carbonation-induced suppression of N(C)-A-S-H gel/ettringite formation increases total pore volume, constituting the primary reason for strength degradation. These findings systematically elucidate the interaction mechanisms between geopolymerization and carbonation reactions under varying pre-curing conditions, providing a theoretical basis for synergistically optimizing the CO₂ sequestration capacity and mechanical performance of RM-FA based geopolymer materials.