Pore structure fractality affects strength and energy dissipation behavior in red mud-based cementitious materials under different activation pathways

The large-scale stockpiling and low reactivity of red mud (RM) constrain its resource utilization. Alkali, thermal, and sulfate activation were investigated by combining uniaxial compression, energy analysis, pore-structure fractal characterization, and microstructural analysis to compare the variations and interrelationships among pore-structure fractality, compressive strength, and dissipated energy of red mud-based cementitious materials under different activation pathways. Compared with the 28-day compressive strength of the control (3.60 MPa), the mixtures activated with 4% sodium silicate, heat-treated RM at 800 °C, and 4% flue gas desulfurization gypsum reached 5.56, 6.08, and 7.33 MPa, respectively. The corresponding pore-structure fractal dimensions increased from 2.4287 for the control to 2.4379, 2.4429, and 2.4757, following the same order as compressive strength and indicating a positive correlation between fractal dimension and compressive strength. In contrast, the final dissipated energy densities of the three activated systems were 0.058, 0.044, and 0.052 MJ/m³, respectively, all higher than the control value of 0.038 MJ/m³, but with a different ranking from fractal dimension and compressive strength. The 4% sodium silicate-activated system exhibited the highest final dissipated energy density and more gradual post-peak softening, whereas the 4% flue gas desulfurization gypsum-activated system showed the highest compressive strength but a steeper post-peak stress drop. These results show that pore-structure fractal dimension increases with compressive strength, whereas dissipated energy varies non-monotonically with fractal dimension and is also influenced by crack propagation and damage evolution during loading.