Effect of red mud and citric acid on the mechanical properties and water resistance of magnesium oxychloride cement

Despite its excellent mechanical and environmental properties, magnesium oxychloride cement (MOC) suffers from poor water resistance and high production costs, ultimately limiting its application. This study investigates the use of red mud (RM) and citric acid (C 6 H 8 O 7 ) to develop a low-cost, water-resistant RM-based MOC (RM-MOC). The effects of varying RM/C 6 H 8 O 7 contents on the setting time, compressive strength, water resistance, and microstructure of RM-MOC were analyzed. The addition of RM/C 6 H 8 O 7 prolonged the setting time and enhanced water resistance; however, it reduced mechanical strength. Mechanistic analysis revealed that RM reduced 5Mg(OH) 2 ·MgCl 2 ·8H 2 O (Phase 5) formation in RM-MOC, whereas C 6 H 8 O 7 facilitated the generation of M-S-H and Mg-Cl-Si-Al-H gels with homogeneous structures and high polymerization degrees. These gels refined the pore structure and densified RM-MOC, thereby improving water resistance and compensating for the strength loss due to reduced Phase 5 formation. Under the synergistic action of RM and C 6 H 8 O 7 , RM-MOC achieved a compressive strength of 59 MPa and softening coefficients of 0.8 (7 d) and 0.78 (28 d) even at a high RM proportion of 60% (RM/LBMP = 0.6), demonstrating effective waste valorization and simultaneous enhancement of material properties. This study provides a novel strategy for the development of high-performance, low-cost, and environmentally friendly magnesium cement materials.