Influence mechanism of limestone powder on red mud-based grouting material

Considering the unstable performance of geopolymeric materials due to the large fluctuation of the raw-material composition and the high alkalinity of the system, this study investigated the effect of limestone powder on red mud–based geopolymeric grouting materials; moreover, the influence mechanism was analyzed via X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM). Also, the study provided some reference to reduce the storage of red mud and realize the collaborative utilization of limestone powder and red mud–based grouting materials. The results show that the mechanical strength of specimens first increases and then decreases with the increase in the limestone powder content. The compressive strength of the specimen with 5% limestone content was the best: the 3-day compressive strength could reach 5.65 MPa, which was 18.94% higher than that of the specimen with 0% limestone powder content. Moreover, the slurry bleeding rate of the 5%-limestone specimen was only 9.85% higher than that of the 0%-limestone specimen, and the porosity of the former on day 28 was 18.35% lower than that of the latter. Therefore, 5% is the best content of limestone powder in red mud–based grouting material. When the mean particle size of limestone powder was 8 μm, the “filling effect” and “nucleation effect” of specimens were significant, and the slurry viscosity rose sharply; the compressive strengths of day-3 and day-28 samples increased by 11.86% and 10% than those of the corresponding bulk-limestone samples, respectively. Thus, the smaller the mean particle size of limestone powder, the more significant the improvement effect of red mud based grouting material. The optimum proportion of red mud–based grouting materials was 47.5% red mud, 47.5% blast furnace slag, and 5% limestone powder. The macro analysis confirms that limestone powder participates in the slurry hydration process, providing nucleation sites for N–A–S–H, C–A–S–H, and C–S–H gel, which can be used for geopolymer gel precipitation and growth and accelerate the slurry hydration.