Bentonite was added to a locally available rock dust to study the behaviour thresholds of the binary mix. Thresholds may be defined as the critical fines content above which the system behaviour changes drastically. By understanding the threshold values of such binary mixtures will help us to have a better control over its behaviour. In this study, the hydraulic conductivity and the swelling behaviour of quarry dust mixed with increasing fractions of bentonite were studied. Index properties of quarry dust were determined by standard sieve analysis, mini-compaction and by cone penetration method. The specific gravity of the quarry dust was about 2.65. The addition of bentonite to quarry dust has found to increase the maximum dry density and reduce the hydraulic conductivity by several orders. The swell potential of the various mix proportions was studied in an oedometer. Samples were of 14 mm thickness and were given a hydration period of 24 h and were compacted at their OMC and MDD. Mixes with low bentonite contents such as a 5% and 10% showed no swelling. The substantial swelling was only observed with 15% bentonite and more. © Springer Nature Singapore Pte Ltd 2019.