Rheological properties, mechanical characteristics, and microstructures of gangue-cemented paste backfill: Linking to loess doses

Efficient and sustainable development is the core of green mining. In this paper, natural loess (LO), fly ash (FA), gangue (GA), ordinary Portland cement (OPC), and mixing water were used to prepare gangue-cemented paste backfill (GCPB) for underground filling mining. To investigate the effect of partial replacement of FA with natural LO on GCPB performance, GCPB specimens with varying LO doses were produced. Rheological tests, slump tests, compression tests, X-ray diffraction (XRD), and scanning electron microscopy (SEM) were used to examine the rheological properties, macroscopic strength, and microstructural evolution of GCPB. The results of the experiments reveal that 1) the GCPB slurries were in accordance with the H-B model at various LO doses. The yield stress increased with increasing LO dose because the content of small particles increased. Specifically, the LO doses were positively correlated with the content of fine particles and yield stress. The increase in the amount of small particles resulted in a significant increase in their ability to absorb free water, an increase in particle friction, and an increase in yield stress. Moreover, the slump (219-276 mm) of the GCPB slurry increased and subsequently decreased as the LO dose increased. 2) With increasing curing time and decreasing LO dose, the UCS improved. The UCS with a curing time of 3 d showed a trend of first increasing and then decreasing (due to the obvious influence of pore size) with increasing LO doses. In addition, the UCS decreased from 4.09 MPa (LF-1) to 2.15 MPa (LF-6) when the curing time was 28 d. The strength of the mine filling material was generally 1.5 MPa-2.0 MPa, and all the formulas can meet industrial requirements. 3) XRD and SEM had been used to examine the hydration products of GCPB samples. The hydration products of GCPB with LO doses were mostly composed of calcium silicate hydrate (C-S-H), calcium aluminate hydrate (C-A-H), calcium hydroxide (CH), ettringite (AFt), and gismondite (CaAl2·Si2O8·4H2O). These hydration products with microporous low-density materials filled large pores and cemented them with GA particles, which improved the UCS of the GCPB specimens. Studying the rheological properties, mechanical properties, hydration products, and microstructure of GCPBs is an important prerequisite for green mining.