The interfacial chemistry of solidification/stabilization of metals in cement and pozzolanic material systems

The chemistry of cement, its hydration and mechanisms of solidification/stabilization (s/s) of toxic metals by cement-based systems and pozzolanic materials are significantly controlled by surface, near-surface and interfacial phenomena. The adsorption conditions and the selectively strong affinity of hazardous metals towards clay minerals, certain hydrated metal oxides and oxyhydroxides, and cementitous substances also play an important role in the s/s process for the immobilization of contaminants. Recent works from our laboratory involving metal ions and superplasticizers have elucidated the mechanisms of reactions leading to the retardation of cement hydration and subsequent setting and their interactions with silicate-based systems. This article delineates the current status of interfacial chemistry at the solid-liquid boundary and places it in perspective with present and future s/s processes based on Portland cement and pozzolanic materials. The importance of surface charge, the role of interfacial phenomena on adsorption, and the importance of calcium and other types of anions and cations in s/s are also discussed. A surface charge control reaction model that accounts for the importance of calcium and other cations and anions is outlined and used to discuss the chemical nature and microstructure of the interfacial transition zone.