Utilisation of alkali-activated blast furnace slag in paste backfill of high-sulphide mill tailings: Effect of binder type and dosage

Abstract

Ordinary Portland cement (OPC) is extensively used as a binder in cemented paste backfill (CPB) of mill tailings. However, it is not particularly suitable for CPB of sulphide-rich tailings mainly due to its low resistance to acid and sulphate attack. In this study, alkali-activated neutral and acidic blast furnace slags (AASs) with aqueous sodium silicate (LSS) and sodium hydroxide (SH) were tested as alternative binders to OPC for CPB of high-sulphide mill tailings. The CPB samples prepared from OPC showed poor mechanical performance (0.39–1.37MPa at 360days) with strength losses (5–40%) occurred in the long term (after 56days) at the binder dosages of 5–7wt.%. However, at the corresponding binder dosages, CPB samples of AAS were found to produce remarkably high strengths (1.54–4.74MPa at 360days) and stability (i.e. no strength losses) over 360days of curing. Strength and stability performance of CPB samples of all binders tested were observed to improve with increasing the binder dosage. Slag characteristics (neutral or acidic) appeared to affect the strength development of CPB samples in short and long terms. Generation of sulphate and acid by the oxidation of pyrite present in the tailings during the hydration and hardening process were monitored to investigate the role of sulphate and acid attack on the short- and long-term mechanical performance of CPB samples. Scanning electron microscope (SEM) and X-ray diffraction (XRD) analysis studies provided an insight into the microstructure and the formation of secondary mineral phases such as gypsum. The findings have revealed that the strength losses observed for CPB samples of OPC after 56days are associated with the oxidation of pyrite i.e. acid and sulphate attack mechanism. Alkali activated slags with their superior short and long term mechanical performance (at ⩾6wt.% binder) can be suitably used in CPB of sulphide-rich tailings.