<p>This paper presents a study on the influence of composite binder of cement and granulated blast furnace slag (GBFS) at different cement/GBFS ratios to the unconfined compressive strength (<i>UCS</i>) and equivalent elastic modulus (<i>E</i>_<i>s</i>) of mixed clay specimens. For this, clay samples were collected from two study sites in the Red River delta, namely, Hai Duong (HD) and Hai Phong (HP), for the laboratory tests. A total of 171 and 540 mixed specimens for HD and HP sites, respectively, were designed and cast in the lab with the use of different binder contents and cement-GBFS ratios. A series of the unconfined compression tests were carried out on the mixed specimens of the HD site at 14, 28, and 56 curing days whereas on the specimens of the HP site at only one curing age of 28 days. The <i>UCS</i> at the HD was found to increase with the increase in curing time and this well agrees with the established fundamental characteristics of cement-stabilised soils. The <i>UCS</i> value of specimens at the HD site reached the maximum at the GBFS ratio of 40% whereas the <i>UCS</i> value at the HP site increased continually from GBFS ratio of 0% to the maximum test ratio of 70%. This behaviour is attributed to the different characteristics of water content and salinity in soil samples from different depositional environments. Statistical analyses resulted in strong correlations between <i>E</i>_s and <i>UCS</i> values and the correlated <i>E</i>_s/<i>UCS</i> ratio depends not only on the binder contents but also strongly on natural characteristics of the native soil. Findings from this study provide useful references for the application of GBFS to partially replace cement for ground improvement work in the delta.</p>