Effects of aggregate size distribution and carbon nanotubes on mechanical properties of cemented gangue backfill samples under true triaxial compression

This study aims to investigate the mechanical properties and damage characteristics of gangue cemented filling materials (CGBM) under true triaxial compression. The true triaxial compression tests, CT scanning, SEM, and EDS tests were conducted on cemented gangue backfill samples (CGBS) with various carbon nanotube concentrations (PCNT) that satisfied fractal theory for particle size distribution (PSD) of aggregates. The mechanical properties, energy dissipations, and failure mechanisms of the CGBS under true triaxial compression were systematically analyzed. The results indicate that appropriate carbon nanotubes (CNTs) effectively enhance the mechanical properties and energy dissipations of CGBS through micropore filling and microcrack bridging, and the optimal effect appears at PCNT = 0.08%. Taking PSD fractal dimension (D) of 2.5 as an example, compared to CGBS without CNT, the peak strength (σp), axial peak strain (ɛ1p), elastic strain energy (Ue), and dissipated energy (Ud) increased by 12.76%, 29.60%, 19.05%, 90.39%, respectively. However, excessive CNTs worse the mechanical properties of CGBS due to CNT agglomeration, manifesting a decrease in σp, ɛ1p, and Δɛv when PCNT increases from 0.08% to 0.12%. Moreover, the addition of CNTs improved the integrity of CGBS after macroscopic failure, and crack extension in CGBS appeared in two modes: detour and pass through the aggregates. The σp and Ud firstly increase and then decrease with increasing D, and porosity shows the opposite trend. The ɛ1p and Δɛv are negatively correlated with D, and CGBS with D = 2.15 has the maximum deformation parameters (ɛ1p = 0.05079, Δɛv = 0.01990) due to the frictional slip effect caused by coarse aggregates. With increasing D, the failure modes of CGBS transition from oblique shear failure to “Y-shaped” shear failure, and then further transform into conjugate shear failure. The study will contribute to a better understanding of the mechanical properties and failure characteristics of CGBS under true triaxial compression.