Free swell potential of lime-treated gypseous soil

Abstract Lime treatment generally enhances the geotechnical properties of the soils. However, some studies have reported an adverse effect, especially in the presence of gypsum. To develop an understanding of the underlying mechanisms, a study of fine-grained soil with different amounts of gypsum was carried out. The influence of curing conditions (curing period and curing temperature) and wetting-drying cycles on the swell potential of fine-grained soil with (0, 5, 15 and 25%) gypsum was investigated to assess the long-term behavior of this type of soil (i.e. gypseous soil) when used in the construction of road and pavement structures. Mineralogical and microstructural studies (porosimetry, X-ray diffraction and scanning electron microscopy) were carried out to evaluate the cementitious materials and the presence of ettringite. The results show that increasing the gypsum content decreased the swell potential of the untreated soil samples. On the other hand, the swell potential values of soil samples started decreasing after the first wetting–drying cycle then reached equilibrium. Lime treatment results show that the suppression of swelling in lime-treated soil samples without gypsum was partly lost after the first cycle, especially for short curing periods and for both temperature values (20 °C and 40 °C), while long-term curing periods (more than 28 days) gave good results with respect to the suppression of swell potential values. Lime treatment of gypsum-containing samples showed an adverse effect on swell potential. The swell potential values increased with curing period and curing temperature, due to the formation of ettringite. Lastly, wetting–drying cycles caused an increase in the swell potential values of gypsum-containing samples, cancelling out the beneficial effect of lime treatment to control swell.