Compaction, direct shear and compression tests have been performed to determine mechanical properties of compacted bentonite-aggregate mixtures with saturated sodium chloride (NaCl) and magnesium chloride (MgCl2) brines for use as backfill materials in salt and potash mine openings. The aggregates include sludge, sand, crushed salt and gravels. Their grain sizes range from 0.425 to 6 mm. The mixing ratios of the bentonite-aggregate mixtures are from 30:70 to 100:0 by weight. The results indicate the decease of the bentonite weight ratio can increase the dry density and decease the optimum brine content, which holds true for both mixing fluids. The cohesions and friction angles increase with increasing aggregate contents and angularity. The compressive strengths and elastic moduli of the mixtures containing finer particles are higher than those with coarser ones. They also increase with the bentonite weight ratio. Higher bentonite contents allow larger dilation of the mixtures under loading, and hence reflecting as higher Poisson’s ratio. The specimens mixed with NaCl brine show dry densities and shear strengths similar to those mixed with MgCl2 brine. The results can be used as initial installation parameters and material selection of backfills emplaced in salt and potash mine openings.