Sulfate-alkali balance and fiber-matrix interactions regulate the microstructure, mechanical behavior, and environmental impact of ultra-fine tailings mortar.

This work aims to improve the strength and reduce the brittleness of ultra-fine tailings mortar (FTM) prepared using ultra-fine tailings (UFT) as the fine aggregate. An alkali-sulfate-activated waste-derived binder was developed to prepare FTM. The binder consisted of ground granulated blast-furnace slag (GGBS), red mud (RM), phosphogypsum (PG), and calcium carbide residue (CCR). Recycled tire polymer fibers (RTPF) with rough surfaces were further incorporated into FTM. Isothermal calorimetry, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, thermogravimetry-derivative thermogravimetry (TG-DTG), scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS), mercury intrusion porosimetry (MIP), and compressive strength tests were conducted. These tests examined how PG-CCR composition and RTPF affected the microstructure and macroscopic mechanical behavior of FTM. The results showed that changes in the PG-CCR composition altered the sulfate-alkali balance. This modified the hydration pathway and matrix densification, which in turn regulated the mechanical behavior of FTM. At a PG-CCR composition of 5:15 (wt%), the waste-derived FTM reached a 28 d compressive strength of 6.02 MPa, 326.43% higher than that of ordinary Portland cement (OPC) FTM. With the further addition of 0.4 wt% RTPF, the compressive strength and post-peak energy absorption increased by 15.52% and 46.97%, respectively. These improvements were associated with fiber-matrix mechanical interlocking and crack bridging. A piecewise damage constitutive model was developed for RTPF-FTM. The model describes the stress-strain behavior during initial pore compaction, post-peak softening, and residual load bearing. Compared with OPC FTM, the strength-normalized cost and strength-normalized carbon emissions were reduced by 72.24%-84.03% and 93.43%-95.93%, respectively, while hazardous elements were effectively immobilized. This work provides a basis for the combined utilization of solid wastes and the development of low-carbon FTM.