Solidification–Stabilization of Heavy Metal–Contaminated Clays Using Gypsum: Multiscale Assessment

Abstract

Protecting the environment from hazardous pollutants associated with waste generation and disposal is a major concern that warrants further investigation. Chemical stabilization is commonly used to improve the engineering properties of soils. The current study examines the characteristics of uncontaminated and copper-contaminated clays stabilized by gypsum. Two clay types, including bentonite (with predominantly montmorillonite mineralogy) and kaolin (with predominantly kaolinite mineralogy), were tested, representing high-swelling and low-strength clays, respectively. An extensive laboratory-testing matrix was developed to investigate the effects of gypsum and copper (Cu) concentrations on the engineering properties of the tested clays, including compaction, Atterberg limits, and unconfined compressive strength (UCS). Additionally, X-ray diffraction, field-emission scanning electron microscopy (FESEM), and X-ray fluorescence tests were conducted to understand the microstructural mechanisms controlling the changes in the engineering properties of the stabilized clays. The UCS test results showed that 7 and 9% gypsum content were optimal for uncontaminated bentonite and kaolin, respectively. The microstructural tests revealed that the added gypsum modified the porous network of the stabilized clays. The level of Cu concentration was found to have a considerable influence on the engineering properties, phases of hydration products formed, and microstructural characteristics of the stabilized clays. These changes are attributed to the retardant effect of Cu on hydration and pozzolanic reactions, which in turn alter the phases of hydration products and cementation structure—the bonding of the clays. The findings suggest that gypsum can offer an economic and effective additive for clay stabilization.