<p>In-situ ground improvement techniques are often adopted in tunnelling, excavation, hydropower projects, etc. for providing stability and controlling seepage. Such in-situ soil mixing techniques are known to utilise large quantities of cement. Ground granulated blast furnace slag (GGBFS) is often used for partial replacement of cement in large scale ground improvement projects. Although there is plethora of literature available on the strength of cement-GGBFS admixed clays for different mix parameters, such studies are scarce for hydraulic conductivity characteristics. This paper presents a comprehensive laboratory investigation on hydraulic conductivity of cement-GGBFS clay mixes. The effect of confining stresses, cement:GGBFS ratios, binder-contents, water-contents and curing periods are studied on hydraulic conductivity by performing series of accelerated permeability tests using a custom-fabricated triaxial system. The microstructural and mineralogical changes are also studied using scanning electron microscopy and X-ray diffraction analysis, respectively. Results indicate that the values of hydraulic conductivity decrease substantially with the increase in GGBFS substitution up to 50%, but only slightly increase with GGBFS substitution of 75%. The values of 28 days hydraulic conductivity for cement-GGBFS stabilised clays are found to be less than those for the cement-stabilised clays as well as natural clay slurries. Finally, generalized formulations are developed to correlate the unconfined compressive strength, hydraulic conductivity, and water-binder ratios.</p>