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This research aims to provide an in-depth analysis and interpretation of the mechanical behaviour of lime-slag treated pyrite bearing Coode Island Silt (CIS), a widely occurring problematic soft soil in the Melbourne CBD area. This study analyses the time-dependent strength development characteristics, compression behaviour and undrained shear behaviour of CIS treated with different amounts of lime and slag. In the course of this study a new virgin compression model that can reproduce the compression behaviour of a wide range of structured soils including naturally structured soils and artificially cemented soils has been developed. The new model has been employed to interpret the compressibility behaviour of lime-slag treated CIS. An investigation on the time-dependent strength development of the treated soils mass through unconfined compressive strength tests revealed that slag, in combination with a lime content higher than the lime saturation point (the minimum lime content at which the pH of the solution prepared with soil, lime, and water becomes approximately 12.4), can effectively improve the strength and stiffness of soft pyrite bearing CIS. Strength was found to be almost independent of lime content at different curing periods whereas slag was found to have considerable influence on the developed strength at all curing periods. The effect of curing was found to be most prominent at earlier phases of curing. However, its effect on the strength development was found to cease gradually due to gradual decline in the rate of pozzolanic reactions. A comprehensive review of literature revealed that the virgin compression behaviour of structured soils immediately after yield is controlled by the progressive collapse of larger inter-aggregate pores and when these macro pores are collapsed, compression behaviour is controlled by the mineralogy of the soil aggregates. A new virgin compression model based on two easily determinable parameters was developed and the developed model is able to reproduce the compression behaviour of a wide range of structured soils including naturally structured soils and artificially cemented soils. One of the parameters corresponds to the compression caused by the collapse of larger macro-pores while the other parameter corresponds to the compression behaviour at the stress range where the effect of soil mineralogy is the dominant factor. Study on the compressibility behaviour of lime-slag treated CIS revealed that the influences of different experimental variables on the yield strength were very similar to their influences on the UCS values. It was found that compressibility generally increases with an increase in yield strength. The model parameter which characterizes the compression behaviour in the structure controlled zone was found to decrease with an increase in the degree of cementation. On the other hand the parameter that controls the compression behaviour in the de-structured state was found to vary within a much narrow range for all the cases investigated possibly due to comparatively less influence of different experimental variables on the mineralogical alteration of the treated material. Investigation on the undrained shearing behaviour of treated CIS divulged that slag content increases both strength and stiffness of the treated CIS. Consolidation pressure was found to have significant influence on the strength, stiffness and overall stress-strain characteristics. The effect of a change in consolidation pressure on the stress-strain behaviour was found to be more prominent when the treated CIS was sheared from an elevated level of consolidation pressure. It was suggested that at low level of consolidation pressure, the behaviour up to the peak stress is controlled by the response of the cementitious bonds and the post-peak response is characterized by the inhomogeneous deformations of the samples. At elevated consolidation pressure, the brittleness at peak stress state was found to decrease and the stress-strain behaviour was found to be of progressively strain-softening type. The peak strength parameters derived from the result of triaxial tests indicated that peak friction angle does not change significantly with a change in the degree of cementation but the cohesion intercept was found to increase with the degree of cementation. Due to presence of strong discontinuities in the post-peak region, especially for the samples sheared from low level of consolidation pressures, it was not possible to derive a unique set of strength parameters corresponding to de-structured state of the lime-slag treated CIS. The outcomes of this study will equip the geotechnical practitioners with the knowledge required to incorporate cementitious stabilization of pyrite bearing soft soils with lime and slag, which is an abundantly available industrial by-product in Australia.