Impact of aging in the presence of reactive gases on cementitious waste forms and barriers

Abstract

The impact of aging in the presence of reactive gases (i.e., carbon dioxide and oxygen) on the physical and chemical microstructure of microconcretes and cast stone cementitious waste forms prepared with different fly ash types was investigated. Geochemical speciation with reactive transport modeling was also performed for the microconcretes to better understand and predict the leaching behavior of cementitious materials after aging. The alkalinity of microconcrete materials due to the fly ash replacement type was found to impact the rate and extent of carbonation and the relative changes in the pore structure within the bulk and interfacial transition zone regions. Migration and deposition of constituents was observed within the microstructure of microconcrete materials in response to changes in constituent solubility due to carbonation. A common mineral and solid solutions reaction set can be used in the geochemical speciation with reactive transport modeling to describe the leaching behavior of non-carbonated and carbonated microconcretes with different fly ash replacement types, with some identified limitations. A multi-ionic diffusion approach and consideration of the minerals controlling solubility at a low liquid-to-solid ratio was required. For cast stone cementitious waste forms, the changes in the mineralogy and the microstructure that occur as a result of carbonation are different from what has been observed for cementitious materials with larger amounts of Portland cement. Oxidation and carbonation were also found to influence the mass transport of constituents due to changes in constituent solubility, material pH, and porosity.