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A B S T R A C T:The results of detailed analyses of the samples discussed in part I are reported. The chemical, morphological and microstructural changes which occurred during moist curing and soaking have been determined using thermal analysis, X-ray powder diffraction analysis, and scanning and transmission electron microscopy combined with EDAX. The analytical results together with the physical observations have shown that the period of volume instability and swelling coincides with the period of gypsum consumption and ettringite formation. However, swelling is not caused by growth of crystalline ettringite but is the result of water absorption. An osmotic mechanism is proposed in which concentration gradients are generated within a surface colloidal layer on the clay particles. Ettringite rods nucleate and grow within this layer and C-A-S-H gel is produced in the areas between the rods. This results in concentration gradients at the clay particle surfaces which provide the driving force for osmosis. The implications of these findings are discussed in relation to limestabilization of soils.In part I of this paper (Abdi & Wild, 1993), the results of unrestrained expansion and swelling pressure for gypsum containing lime-stabilized clays were reported. These results were discussed in relation to the possible swelling mechanisms associated with ettringite formation. The observations indicated an osmotic process promoting water absorption as the most likely mechanism involved in producing expansion and generating swelling pressure.In this paper, the morphological changes which took place in the specimens during expansion are reported in addition to the analysis and characterization of the reaction products. The techniques employed are scanning and transmission electron microscopy (SEM and TEM) with energy dispersive X-ray analysis (EDAX), thermogravimetric analysis (TG) and X-ray powder diffraction (XRD). ANALYTICAL METHODS Thermal analysisThermal analysis was used principally to monitor lime and gypsum consumption during reaction of moist cured, and moist then wet cured kaolinite-lime-gypsum specimens. The instrument used was a Perkin-Elmer thermal analyser (Model TGS-2). The cured samples were analysed in a dry CO2-free nitrogen atmosphere, the gas being dried over silica gel, and then Carbosorb. To ensure consistency and reproducibility of peak temperatures, 7-8 mg of finely powdered sample was used. The heating rate and flow of nitrogen were kept 9 1993 The Mineralogical Society