Effect of Gypsum on Strength Behavior of Lime-Modified Pond Ash as an Underground Stowing Material

Backfilling the mine voids with the suitable material can provide additional ground support by restricting lateral deformation of surrounding coal pillars and roof. This type of support mechanism helps to prevent caving, minimizes surface subsidence, and enhances pillar recovery. River sand is widely used as a stowing/backfilling material, as it is easily available and suitable for its geotechnical properties. River sand is in great demand for many construction projects and is in short supply in many areas. So it is highly desirable to find an alternative stowing material. Some alternative materials used for backfilling are waste rock, mill tailings, quarried rock, sand and gravel. Over the last decade, attention has been made on the use of coal combustion products as an alternate filling material for underground works. Usually, pond ash as a backfill material remains loose and serves only as a temporary working platform rather than offering any lateral strength. Adding low percentages of admixtures to backfill mass permits the development of cohesive strength and self-supporting ability during adjacent pillar mining. This self-supporting nature of the backfill permits higher recovery of pillar ore, which in turn improves the utilization of the mining reserve and the economics of the mining operation. In the present study, laboratory mine model was prepared by using pond ash as a backfilling material with different percentages of lime (2, 6, 8, and 10%) and gypsum (0.5, 1, 1.5, and 2%) as additives. To examine the effect of gypsum on the strength characteristics of stowed pond ash, core samples were collected from the model which were cured for different time periods such as 7, 14, and 28 days. To determine the engineering properties of pond ash composites unconfined compression tests as well as triaxial compression tests were performed. The increase in strength of pond ash composites was observed for lime content up to 6% and gypsum of 1.5%.