New technique to determine effective diffusivities for hazardous ions solidified in portland cement
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abstract
Solidification of hazardous waste using portland cement has been and is currently being used in solidification technology, but knowledge of the exact leaching behavior of contaminants is limited. Characterization of the leaching of wastes from the portland cement matrix can be accomplished through a model. Leaching is generally known to be resisted by both chemical and physical mechanisms. The physical mechanism is associated with the structure of the solid matrix and includes such parameters as permeability, tortuosity, and pore dimensions in the matrix. Modeling the leaching of wastes from the solid matrix predicts transport of contaminants through the slab by Fick's law and diffusion. The problem that has arisen is the need to obtain a diffusivity coefficient. Fick's Law is based on the effective diffusivity which depends only on physical factors such as molecular diffusion and pore structure and must be determined in order to have a valid model. The new technique is based on the MacMullin number. The MacMullin number relates the free solution effective diffusivity of an ion to the effective diffusivity of that ion in the pore structure of a solid, and is calculated by a ratio of conductivities. These conductivities are the conductivity of the solid, and the pore water within the solid. The ratio of the conductivities of the pore fluid to the solid yields the MacMullin number. From the MacMullin number, the effective diffusivity through the portland cement matrix of any hazardous ion can be calculated.
