The long-term viability of many construction projects is heavily dependent on the soundness of the underlying subgrade soil which needs to be capable of supporting loads from structures without undergoing excessive deformation or volume changes. However, in certain locations geotechnical engineers face repeated problems in the design of substructures due to the presence of expansive soils. Light structures are especially susceptible to damage in such situations which can result in cracks in walls, beams, columns and on-grade slabs. One extensively used way of overcoming such problems is to improve the soil by stabilizing it with cement and/or lime which prevents this unwanted behavior. Most previous research conducted on expansive soils focused on studying the effects of treatment on the physical properties of the soil without detailed examination of the physicochemical mechanism of the process, although some studies surveyed the effect of the treatment using cement and lime on the mineralogical composition and newly formed pozzolanic compounds. In this research the mineralogical changes due to lime and cement additions to two different expansive soils from Saudi Arabia, in particular, were investigated using X-ray diffraction. The results of this work indicate that addition of lime or cement to the soils tested resulted in the formation of pozzolanic compounds, namely, calcium silicate hydrate and calcium aluminate hydrate together with calcite (CaCO3). The soil improvements are compared for cement and lime. Changes in potassium, aluminum and silicon were also determined and interpreted in terms of the soil mineralogy. It was concluded that pozzolanic compounds play a significant role in reducing the swelling and increasing the strength.
