The storage and disposal space of fly ash, bottom ash and mill tailing, by-products of coal fired thermal power plants and beneficiation plants respectively, has imposed a considerable pressure on land availability especially in a populated country like India. Mine backfilling utilizes large volume of such materials for environmentally safe placement in underground voids. Backfilling of underground stopes addresses various issues mainly the backfill material characteristic which enhances the stability of the weak wall rocks and creates competent working floor. The utilization of mine waste as a backfill material has gained momentum to control the environmental hazards and also to raise awareness for mineral resource conservation. Therefore it is imperative to design backfill composite to meet the prescribed operational functions and safety requirements. Design requirements for the structural integrity of a backfill are regulated by physico-mechanical, chemical, mineralogical and morphological characteristics of its ingredients and additives. This study is purported to design a composite backfill using mill tailings from a mine, fly ash, bottom ash from a nearby thermal power plant and cement as binder. All the ingredients of the composite backfill are characterized in laboratory and the most favorable backfill composite in terms of high strength and high slump and optimum binder dosage is determined. The physico-mechanical and rheological characteristics namely specific gravity, particle size distribution, hydraulic conductivity, solid concentration, compressive strength, modulus of elasticity and slump height have been determined. Statistical analyses are also carried out to examine the significant role of backfill ingredients on the mechanical strength parameters and slump of composite backfill material.
Fly ash and bottom ash are the residual by-products of coal burning in coal thermal power plants (Kim & Prezzi, 2008). These are typically treated as waste and disposed together in ash ponds. The mill tailing is also a waste product of a mineral processing plant. The space for storage and disposal of fly ash, bottom ash and mill tailing create a considerable pressure on land availability since these are dumped on surface adjacent to mine as waste material. Backfilling with a mixture of classified mill tailings, fly ash, and bottom ash is a relatively new technique. River sand and mill tailings have been invariably exploited as backfilling material in India (Mishra et al., 2006). Fly ash and bottom ash can be a good alternative to replace the gradually depleting river sand. Mine backfilling is a potential area, which can provide a scope for environmentally safe and large volume disposal of fly ash, bottom ash and mill tailings all mixed together as backfill(Masniyom, 2009). Filling up the voids created by mined-out area addresses three basic issues. These are viz., the utility of mine waste as a backfill material to control surface disposal problems, to employ and facilitate some backfilled stoping methods and also as a material for supporting the weak wall rocks surrounding the excavated stopes. Mine backfilling is gaining importance as a passive mode of support to the weak sidewalls of stopes (Brady & Brown, 2013). In addition, in cut-and-fill stoping methods the backfill provides a working platform and therefore, the fill mass must bear the load of the mining equipment working on the backfill-floor. Thus, it is necessary to prescribe the function and duty of the backfill mass quantitatively.
