Mechanical, mineralogical and chemical characterization of a paste backfill

Mining backfill is now current practice in many mines. Along the years different pratices have evolved in the industry. Hydraulic fills, rockfills and hign density fills have been in used for quite some time now. But more recently, paste backfill, is spreadind in Canadian mines. It has been used by some Quebec mines for the last four years. This type of backfill is prepared from unclassified tailings and put in place as a high density slurry with cement content varying from 3 to 6% by weigth. This technology seems to have significant advantages compared to conventional cemented hydraulic fills. Since the water content of the paste is reduced no dewatering systems is deemed necessary in the backfilled stopes. For a given cement ratio, the achievable strength is better, due in part to a lower water/cement ratio than hydraulic fills. The tailings need not to be classified, so more tailings are returned under ground, minimising the amount of waste to manage on the surface. In the example treated here, the pilot plant trials tests predicted very good strength for this operation with a bonding agents mix of 60% Portland cement and 40% fly ash. Nevertheless, achievable strength at the production stage had to be revised and when the first pillars were mined out exposing the backfill faces the performance was much less satisfactory than expected. Excessive dilution and instability of the backfill were observed. A drilling program was then implemented to collect in situ samples for laboratory testing. The results of triaxial compression tests on samples prepared from the drilling cores showed very little cohesion which contradicted with test results from initial control samples. Mechanical, chemical and microscopic testing of the core samples demonstrate the presence of a chemical alteration in the backfill.