This study was carried out to determine the optimum strength of cemented paste backfill for production of an economic backfill in an underground mine. Tailings samples from a copper mine, Namibia were collected and classified into three particle size range of +75, +150 and +300 µm for preparation of cemented-tailings paste. Tailings compaction was also carried out for a penetrometer (CBR) test to determine the optimum strength of the tailings mix. Cement was added at 2.5, 3.5 and 4% to stabilize the paste and the mix cast in 150 mm cylindrical moulds. Several cast specimens were produced for various tests including compressive strength test, water absorption test and reversed water absorption test. The drying rate was also monitored until the pastes attained a constant mass. Strength properties of the paste for each particle size range were monitored progressively for 30 days and charts produced to determine the optimum strength values and the paste mix that produced them. Results obtained from the study showed that although the 300 µ particle size produced the highest strength for all cement addition, it is found that a reduction of the cement composition to less than 3% for this particle size will produce a support structure of sufficient strength that can prevent failure and subsidence. The tailings could be deslimed and used with about 2.5% cement rather than attempting classification to obtain +300 µ material. The required paste consistency or dilution will be slightly higher than the one used for the mixes in this study to allow for easy pumping. It will require about 15 days for the support to reach its maximum strength which may imply that the mining cycle should be about a little longer than this duration.
Key words: Cemented tailings paste, reverse water absorption, lateral confinement, deslimed tailings, compacted tailings, classified tailings.
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