Static Segregation Of Cemented Coal Gangue Backfill

Cemented coal gangue backfill(CGB)is a mixture of crushed coal gangue,fly ash,cement and water.As an environment friendly filling material,the content of coal gangue and fly ash reach 950 kg/m3 and 380 kg/m3 in CGB respectively.Surface subsidence caused by underground mining activities can be controlled efficiently by backfilling CGB into mine goaf.This would also bring merits to the management of coal gangue and fly ash,and the utilization of underground spaces in mines in the long run.However,due to the high fluidity required by long pumping distance from filling station to mining face,CGB is prone to static segregation during the rest time after being casted into framework.The mechanical performance and durability of backfilling construction will deteriorate because of the static segregation of CGB.Therefore,this paper discussed the effect of mix proportion on the static stability of CGB,and developed ultrasonic pulse velocity method to evaluate static stability both in lab and field.Furthermore,a numerical model was proposed to predict the static segregation of CGB.The main contents and conclusions are as follows:Firstly,research was conducted on the physical and chemical properties of coal gangue,fly ash and cement.It was shown that the apparent density of coarse aggregate is much greater than the apparent density of CGB.Coarse aggregate tends to settle in fresh slurry during the rest time after being casted into formwork.In addition,the performance of naphthalene sulphonate-based superplasticizer was introduced.Secondly,response surface methodology was applied to investigate the effects of key mix proportion parameters,fly ash content,fine gangue ratio and mass concentration,on the static stability of CGB and their interaction on it.Results show that with the increase of fly ash content,the static segregation of CGB become serious.CGB with lager mass concentration tends to be more stable.CGB with fine gangue ratio of 33% performs good fluidity and proper static stability.Mass concentration,fine gangue ratio and fly ash content influence static segregation from high to low.Furthermore,the correlation between static segregation index and rheological parameters was studied and it was found that the static stability of CGB tends to be better with higher plastic viscosity.Thirdly,ultrasonic pulse velocity(UPV)method was developed to evaluate the static stability of CGB.Segregation rate,calculated based on UPV in fresh slurry,can reflect the static stability of CGB with accuracy.Segregation rate of hardened backfilling pillars,calculated based on UPV in backfilling pillars with curing time of 28 d,was proved to be accurate to assess the static stability of CGB.The loss of the compressive strength of the cylinder specimens,drilled from the top section of the hardened backfilling pillars,was almost 31%.Finally,a numerical model based on rheology and fluid dynamics was established to calculate the setting speed of the interface between coarse aggregate and fresh slurry.This model containing six variables,three key composition parameters and three aggregate indexes,was used to predict the static stability of CGB.The aggregate indexes of Xinyang coal mines was tested and applied into the former model to optimize the static stability of CGB.The optimization results show that the performance of CGB with fly ash content of 400 kg/m3,fine gangue ratio of 35 % and mass concentration of 82 % is best,which was proved by segregation column test later.