Mechanical Properties and Microstructure of Alkali-Activated Fiber-Reinforced Mortar Incorporating Red Mud and Fly Ash

Red mud (RM) and fly ash (FA) were used as a 30% replacement of cement in a sodium silicate-activated system. Composite mortar specimens with RM/FA ratios of 0:30, 1:5, 1:2, 1:1, and 2:1 were prepared with polypropylene fibers (PPF) for toughness enhancement. The mechanical properties and microstructure of the fiber-reinforced mortar were systematically investigated. The results showed that RM20F10 (RM/FA = 2:1) exhibited the best overall mechanical performance among all tested proportions. At this ratio, the 28-day compressive, flexural, and splitting tensile strengths reached 32.4 MPa, 7.3 MPa, and 4.2 MPa, exceeding the control mortar by 12.5%, 15.9%, and 23.5%, respectively. The RM/FA ratio of 1:1 achieved the highest 7-day flexural-to-compressive strength ratio. At 28 days, autogenous shrinkage increased from 910 με to 1100 με as the RM/FA ratio rose from 0:30 to 2:1, and all RM-containing specimens exhibited higher water absorption than the control mortar. Microstructural analysis by SEM, XRD, and FTIR revealed a denser matrix with reduced porosity, attributed to the synergistic formation of C–S–H, C–A–S–H, and N–A–S–H gels. RM reduced early-age porosity by promoting C–A–S–H gel formation, while FA facilitated late-age densification through delayed activation. PPF effectively bridged microcracks via fiber pull-out, leading to a ductile failure mode.