The effect and coupling mechanism of SAP and SRA on the strength and volume stability of full solid waste cemented backfill

This study investigates the action laws and intrinsic mechanisms governing the effects of superabsorbent polymers (SAP) and shrinkage-reducing admixtures (SRA) on the mechanical properties and volume stability of magnesium slag-slag-flue gas desulfurization gypsum-based cemented tailings backfill (MFG-CTB) under both single and composite incorporation conditions. The experimental results demonstrate that the individual or combined addition of SAP and SRA all leads to prolonged setting time, increased internal structural imperfection and reduced early-age strength of MFG-CTB. Notably, the ultrasonic wave velocity of MFG-CTB shows a favorable exponential fitting relationship with its early-age compressive strength. Furthermore, the results of macroscopic and microscopic experiments reveal that the composite incorporation of SRA and SAP at low dosages exerts a prominent synergistic effect on the volume regulation of MFG-CTB. Specifically, the 7-day autogenous expansion rate of MFG-CTB is increased by 195.4%, the drying shrinkage rate at 60 days is reduced by 28.6%, and the cracking age is prolonged by 33.5%. This phenomenon is attributed to the fact that SRA reduces the solubility of cementitious materials at the early stage, and in combination with the internal curing effect of SAP, jointly accelerates the hydration rate of cementitious materials after initial setting. Nevertheless, the MFG-CTB modified by SAP and SRA exhibits an increase in total porosity, median pore diameter, proportion of macropores and number of microcracks, which consequently induces the deterioration of its early-age strength. With the increase in the dosage of either SAP or SRA in the composite system, their improvement effect on the volume stability of MFG-CTB is attenuated, yet still remains superior to that of the control group.