This paper aims to identify optimum lime content for lime-slag treated Coode Island Silt (CIS) for sustained improvement of strength. In this study, CIS sampled from the Convention Centre construction site in Melbourne was treated with lime-slag additives in the laboratory to determine optimum lime content. Treatment was conducted with a fixed lime content of 7.5% and varying slag contents of 0-17.5%. The treated samples were cured for 28, 90, 180 and 360 days in a humid chamber. The cured specimens were tested in the laboratory for unconfined compressive strength (UCS), pH and Atterberg limits. Scanning Electron Microscopy (SEM) and X-Ray Diffraction analyses (XRD) were also conducted on specimens cured for one year to study the physico-chemical and microstructural developments responsible for long term strength behaviour. Finally, results of this study were compared with the 5 and 10% lime and varying slag treated CIS by Rex el al. (2008) to establish the optimum percentage of lime required for long-term strength gain. This study found that an optimum lime content of 7.5% was effective in stabilizing and sustaining the strength development of CIS (LL=46, PI=24) based on curing periods up to one year.