Stabilization of bentonite by treatment of lime and phosphogypsum

Morocco produces 15 million tons of phosphogypsum residues coming mainly from the phosphate processing industry. Those acid wastes are rich in heavy metals. The release of heavy metals into the environment, and especially in seawater, results in a number of environmental problems. The present study aims at valorizing the phosphogypsum residues for geotechnical applications (e.g, roadbase, subbase, embankment material). In order to stabilize bentonite, we mixed it with different proportions of lime and phosphogypsum . The bentonites was sampled from a TRIBIA’s deposit located 15 km west of the city Nador in Northeast of Morocco,. For that purpose chemical (XRF), mineralogical (XRD), thermal (TAG), geotechnical (Atterberg limits) and mechanical (Proctor and compressive strength) analyzes were performed on the different mixtures of bentonite, phosphogypsum and lime. We observe that lime and phosphogypsum significantly enhance the behavior of bentonite through different reactions. First, a decrease in the plasticity index (PI) and density of the proctor optimum associated with an increase in the optimum water content can be observed. The texture of mixture evolves from a plastic state to a solid, friable, non-tacky state with a partial loss of sensitivity to water. Long-term modifications also occur. Indeed the lime raises the pH of the bentonite and causes an hydrolysis, It forms then crystalline aluminates and hydrated calcium silicates, acting as a binder between the grains thus causing an increase in the compressive strength. In contrast, the phosphogypsum decreases the time of setting