A soil containing calcium sulfates was stabilized at 40 degrees C, with ample moisture, by cementitious mixtures producing a range of calcium hydroxide upon hydration. Maximum expansion occurred when the stabilizing agent was lime, whereas a mixture containing portland cement, Class C fly ash, and an amorphous silica stopped the expansion. X-ray diffraction peak width and scanning electron microscopy (SEM) based image analysis showed that maximum expansion correlated with crystallization of colloidal ettringite. Ettringite was seen by SEM within an hour of mixing the soil with lime. Colloidal ettringite was not observed when portland cement, with supplementary cementing materials, was used for stabilization.