Abstract: To investigate the effect of sulfate and fly ash content on the uniaxial compressive strength of lead-zinc tailings cemented paste backfill，the specimens were soaked in distilled water，solutions with sulfate ions concentrations of 5 000 mg/L and 15 000 mg/L，in which the mass fractions of fly ash were 0%，15% and 30%. The uniaxial compressive strength of specimens and solution pH value were tested in 180 days. The influence of sulfate ions and fly ash on the strength and microstructure of cemented paste backfill were analyzed. The results indicated that the compressive strength of the cemented paste backfill increased first and then decreased with the increase of soaking age. When the concentration of sulfate ions was 5 000 mg / L，sulfate exhibited a significant early strength agent effect. The content of fly ash was 15%，and the compressive strength of the specimen was the highest. The pH value of the soaking solution decreased over time，and in the early stage，the pH value of the sulfate solution soaking was higher than that of distilled water. The pH value of the sulfate solution remained below 9.5 on the 180th day. The scan‐ ning electron microscope results indicated that the size and number of internal pores and cracks in the structure increased under sulfate conditions，while the quantity of cementitious materials reduced. The re‐ search results can provide a theoretical basis for analyzing the safety of sulfur-containing tailings filling.

Key words: sulfate attack, cemented paste backfill, uniaxial compressive strength, fly ash