Date of Award

Spring 1988

Project Type

Dissertation

Program or Major

Engineering

Degree Name

Doctor of Philosophy

First Advisor

David Gress

Abstract

The use of cement based processes for stabilization-solidification of heavy metal wastes has been extensively studied and characterized. Microstructural characterization of the waste matrix was conducted using X-ray diffraction analysis, scanning electron microscope, energy dispersive spectroscopy, mercury intrusion and helium displacement porosimetry. The physio-chemical characteristics of the solidified waste are affected by the metal type, cement binder, additives, solution to cementitious ratio, and curing regime. All the heavy metals (Cd, Pb, As, Cr) analyzed were detected in the CSH structure; Cr, Cd and As were detected in ettringite, and only Cd was detected in CH. Cd was the only metal detected in the crystalline form as cadmium hydroxide determined by XRD.

All metals increased the total porosity and shifted the pore size distribution to the larger pores within the solidified matrix. Porosity and pore size distribution was not the only parameter affecting the unconfined compressive strength and leaching potential of the solidified waste. Different metal solidified wastes having similar pore characterization exhibited different strength and leaching characteristics.

The seawater leachability of the solidified waste appears to be a function of the metal complex formed and the microstructural changes that occur. Magnesium hydroxide and carbonation of the surface layer affects the apparent surface porosity and permeability and the heavy metal release.

Excessive expansion of the cadmium solidified waste leached in seawater was attributed to a combination of expansive ettringite formation and matrix weakening induced by softening of the CSH phase and gypsum formation. The combined presence of sulfate and chloride solutions is essential for a rapid cadmium waste deterioration as determined by the corrosion in sulfate and chloride solutions.

The sonic method proved very successful as a non-destructive technique for evaluating the freeze-thaw durability of cement solidified heavy metal wastes. These cement solidified wastes have very poor resistance to repeated cycles of freezing and thawing. Destructive forces are created during freezing of water and are a combination of expansive, hydraulic and salt crystalization pressures. Air-entrainment improves freeze-thaw characteristics of the cement solidified waste as expected.

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