A review of binder materials used in stabilized backfills
To improve the economic and environmental performance of underground mining operations, it is common practice to return the metallurgical waste material underground as backfill. This type of backfill is called hydraulic backfill and is used to fill the mined-out areas not exposed to further mining. To provide mechanical support for underground mining, a cemented backfill is normally used. An important parameter for determining the suitability of hydraulic and stabilized backfill is its strength. Typically, the unconfined compressive strength required for stabilized fills is between 0.7 and 2 MPa, and a common strength specification is 1 MPa after 28 days.
Since its introduction, cemented backfill has allowed mining companies to increase their ore extraction and improve working and environmental conditions. Cemented backfill continues to be used, however, with the increasing cost of producing and transporting cement to mine sites, backfilling operations have become expensive. Cemented backfill represents 50% to 85% of the mine’s backfill operating cost compared with 5% to 10% for hydraulic backfill.
These economic pressures have led mining companies to carry out research into the partial or total replacement of cement using a variety of alternative materials. The materials investigated are generally site-specific materials, readily available and cost-effective relative to cement, for example, slags, fly ash, calcined gypsum, chemical additives, fibres, etc. This paper provides a historical review of backfilling binder materials and their characteristics. In particular, it reviews the many alternative materials that have been proposed to either partially or fully replace Portland cement as the main binder.
The investigations cited have shown that the cost of some backfill mixes can be reduced while maintaining, and in some cases improving, the physical properties of the stabilized backfills. However, in many cases, cement still remains an integral part of any binder combination. This can be attributed to the alternative binders investigated being either non-binding materials or capable of generating only low-strength backfills, except when used in conjunction with cement.
Slag and fly ash binders require cement in the backfill mix to provide the lime necessary to sustain their hydration reactions, and to produce consistent strength results. Ground-waste glass, sulphides, and clay materials require the addition of cement for best results, however, their main problem is an insufficient quantity of suitable and readily available material in the immediate vicinity of the mining area. The addition of fibres and chemicals are only an attempt to reduce or better use the cement content within the backfill mix.
A potential replacement material for cement is calcined gypsum, which can hydrate into a strong hardened mass when mixed with water. Research has shown that this material can generate high strengths and use considerably less energy to manufacture in comparison with cement. However, significantly more calcium sulphate is required to generate the equivalent strength obtained by cement.
Research into reducing or eliminating cement from the backfill mix will continue, as mining companies attempt to further reduce their costs, and there are still many materials that have not been investigated.