Increasing global awareness of environmental pollution as well as increasing waste material disposal legislation is providing impetus for material upgrading by stabilisation of in situ soil as an alternative to its export to land-fill and replacement by imported granular fill. The use of ground granulated blast furnace slag (GGBS), an industrial by-product, is well established as a binder in many cement applications where it provides enhanced durability, including high resistance to chloride penetration, resistance to sulphate attack and protection against alkali silica reaction (ASR). This paper reports on efforts to extend the use of GGBS to highway and other foundation layers by determining the beneficial effect on strength of progressively substituting GGBS for lime in lime-stabilised clay soils, particularly in the presence of gypsum. This paper describes the results of laboratory tests on lime-stabilised kaolinite containing different levels of added gypsum and on lime-stabilised gypsum (selenite) bearing Kimmeridge Clay to which, in both cases, the lime has progressively been substituted with GGBS. The tests determine the strength development of compacted cylinders, moist cured in a humid environment at 30°C. The results illustrate that substitution of lime with GGBS in stabilising gypsum-containing clays produces significant improvements in strength development. The process has important practical applications, and the paper reports on-going pilot field trials aimed at realising these applications.