Improvement of Fluidity and Long-term Strength of Cemented Paste Backfill with Low Calcium Fly-ash

ABSTRACT Waste rocks and tailings are widely applied materials in underground metal mines to backfill mined-out areas. To test the effect of low calcium fly ash on slurry fluidity and long-term strength of cemented paste backfill, orthogonal experiments of backfill proportions with variable levels of waste, tailing, fly ash, water and their combinations were carried out. Results show that slurry slump is most sensitive to mass concentration, and slurry fluidity is optimal when fly ash-cement ration is 1:1. The longterm strength of cemented paste backfill after curing for 56 days is significantly affected by cement-(tailing + waste) ratio, and slightly evolves regardless of fly ash-cement ratio. Fly ash promotes the generation of active silicon oxide and aluminum oxide, which fills the pores within the backfill. The optimal proportion of cemented paste backfill is 77% mass concentration, 8:2 tailing–waste ratio, 1:30 cement-(tailing + waste) ratio and 1:1 fly ash–cement ratio. The correlative slurry slump and long-term backfill strength meet the requirement of pumping and backfill mining. Additionally, an industrial scale loop pumping test of backfill slurry shows the pipeline transportation resistance is 4 kPa/m, pipeline diameter is optimal at 150 mm, and flow rate is optimal at 90–100 m3/h. The experiment confirms that proper addition of low calcium fly ash is conducive to improve the slurry fluidity and long-term strength of cemented paste backfill.