Utilization of low-alkalinity binders in cemented paste backfill from sulphide-rich mine tailings

Abstract

In this study, the basic properties of waste gypsum (WG), WG and reactive MgO-activated slag (MAS-WG) binders were experimentally explored. The effects of WG content in MAS-WG binders on the unconfined compressive strength (UCS) development of cemented paste backfill (CPB) from sulphide-rich mine tailings were investigated as well. The corresponding stabilization mechanisms were assessed through mineralogical and micro-structure analysis and leaching tests. The results showed that when the content of citric acid exceeded 0.15 wt% of WG content, the initial setting time and 28 d UCS of the WG plaster of 180 mm slump were over 1 h and 15.5 MPa, respectively. The appropriate WG incorporation improved the particle gradation of MAS-WG and MAS-WG-CPB mixtures. The main hydration products of MAS-WG plaster and MAS-WG-CPB from sulphide-rich tailings were crystalline CaSO4·2H2O, ettringite (AFt) and amorphous hydrated products (C-S-H/M-S-H). The proper WG incorporation in the MAS-WG binder also significantly enhanced the early age (14 d and 28 d) UCSs of MAS-WG plaster and MAS-WG-CPB, in which the reason is that appropriate WG incorporation accelerated the hydration process of GGBS in the alkaline solution by forming ettringite (AFt) and enhanced amorphous phase development resulting in the denser microstructure. A lot of WG incorporation made the 28 d UCSs of MAS-WG-CPB decrease due to the loose micro-structure resulted from the decrease of amorphous hydration products. However, the long-stage UCSs of MAS-WG-CPB after 120 d of curing were all stable (i.e., no strength loss) regardless of WG incorporation content, which were attributed to the low-alkalinity solution in MAS-WG-CPB preventing the secondary expansive hydration product formation. The long-stage (180 d) UCS of MAS-WG-CPB had positive correlations with the refined pore structure and the alkalinity of the leaching solution.