Study on the effect of soda residue addition on strength of backfill at different curing ages

Soda residue （SR） is the waste residue in the process of producing soda ash by ammonia-alkali process, and its large area accumulation causes serious ecological environmental pollution. In order to discuss the law of influence of soda residue addition on strength characteristics of backfill under different curing ages and reveal the mechanism of influence of soda residue addition on strength characteristics of S-CTB, strength tests of backfill are carried out with soda residue addition and curing age as variables, and physical strength of backfill with soda residue addition of 0%, 5%, 10% and 15% are studied. By means of ultrasonic wave, X-ray diffraction （XRD）, scanning electron microscope （SEM） and energy dispersive spectrometer （EDS）, the pore characteristics, hydration products and micro-morphology of backfill under different conditions are analyzed, and the mechanism of the influence of soda residue addition on the physical characteristics of backfill under different curing ages is revealed. The results show that: the uniaxial compressive strength, elastic modulus and wave velocity of the backfill increase and then decrease with increasing soda residue addition. When the amount of soda residue addition is 5%, the compressive strength of the backfill at different ages is the maximum and the strength threshold of backfill reaches 4.44 MPa at 28 d of curing, which can satisfy the production demand of the mine. The addition of soda residue accelerates the hydration reaction of backfill in the early stage. A 5% soda residue addition increases the amount of ettringite generated in the early stage of backfill, increases the amount of C—S—H gel generated in the later stage, and enhances the density of the backfill. With increasing curing age, the compressive strength, elastic modulus, and wave velocity of the backfill increase significantly. After the addition of soda residue, the internal pores, hydration products and the contact relations between tailings particles of the cemented backfill all change significantly, which is the fundamental mechanism affecting the change of the strength of the backfill after the addition of soda residue. The obtained results provide a theoretical basis for the wide application of soda residue in the field of mining filling to promote the green development of mineral resources.