Ultrasonic and Microstructural Evaluation of Sulphide-Rich Tailings Cemented Paste Backfill Properties Containing Alkali-Activated Slag: Effect of Slag Fineness

Cement paste backfill (CPB) is an effective waste management method allowing the storage of fine process tailings into underground mined-out voids. CPB performance generally depends on the properties of the tailings and the type of binder. In recent years, there has been an increasing trend in the use of alkali-activated slag (AAS) to improve the performance properties of CPB. This study focuses on the ultrasonic and microstructural investigation of the effect of slag fineness on the mechanical, geochemical, and durability properties of sulphide-rich tailings CPB made of AAS (AAS-CPB) over 360 days. In this scope, the AAS-CPB samples were prepared at three different slag fineness values (3100–4650–6300 cm2/g). According to the findings, the fineness of the slag significantly improved the early-age and long-term strength (~2.3-fold and ~6.6-fold, respectively) of the CPB samples (CPBs). However, a further increase in the slag fineness was observed to impair the CPB microstructure and strength in the long term. Ultrasonic pulse velocity monitoring displayed a very high relation with the strength evolution of the CPBs and is a very reliable method for the durability assessment of the CPBs. Slag fineness around 4600 cm2/g was found to be sufficient for CPB preparation, and was seen to improve the pore structure evolution of the AAS-CPB. Microstructural studies are in good agreement with the geochemical and durability behaviour of the AAS-CPB at this fineness. Microstructural and ultrasonic findings suggest that, while slag fineness enhances the mechanical and microstructural properties of the AAS-CPB, a further increase in the fineness of the slag has no additional technical advantages.