Synergistic Regulation of Strength and pH in Red Mud-Based Backfill Materials by CaCl2: Hydration, Phase Transformation and Microstructural Evolution

The high alkalinity and poor early reactivity of Bayer red mud hinder its large-scale application in mine backfill systems. Red mud-based backfill materials (RBMs) were prepared in this study using untreated red mud and other raw materials with CaCl2 (1–3 wt.%) added to investigate the mechanical properties, aqueous extract pH, and microstructural evolution. The results show that CaCl2 enhances the early age strength of RBMs. Specifically, with 1–3 wt.% CaCl2, the UCS of the samples at 7 d reached 5.63–6.19 MPa, a 24.0–36.3% increase over the reference (4.54 MPa). Moreover, the aqueous extract pH decreased with increasing CaCl2 content, reaching 10.49 at 3 wt.% CaCl2 after 28 d, 0.61 pH units lower than the reference. Additionally, an optimal content of CaCl2 promotes hydration product formation and pore structure refinement. Excessive CaCl2 was associated with pore structure deterioration and reduced mechanical performance at 28 d. Without the cumbersome pretreatment of raw materials, this study achieves synergistic regulation of mechanical properties and aqueous extract pH, providing a theoretical reference and technical support for the resource utilization of red mud.