Early containment of high-alkaline solution simulating low-level radioactive waste in blended cement

Portland cement blended with fly ash and attapulgite clay was mixed with high-alkaline solution simulating low-level radioactive waste at a one-to-one weight ratio. The pastes were adiabatically and isothermally cured at various temperatures and analyzed for phase composition, total alkalinity, pore solution chemistry, and transport properties as measured by impedance spectroscopy. The total alkalinity is characterized by two main drops. The early one corresponds to a rapid removal of phosphorus, aluminum, sodium, and to a lesser extent potassium from the pore solution. The second drop from about 10 h to 3 days is mainly associated with the removal of aluminum, silicon, and sodium. Thereafter, the total alkalinity continues to decrease, but at a lower rate. All pastes display a rapid loss in fluidity that is attributed to an early precipitation of hydrated products. Hemicarbonate appears as early as 1 h after mixing and is probably followed by apatite precipitation. The hemicarbonate is unstable, however, and decomposes at a rate that is inversely related to the curing temperature. At high temperatures, a sodalite-type zeolite appears at about 10 h after mixing. At 30 days the stabilized crystalline composition includes zeolite, apatite and other minor amounts of CaCO₃, quartz, and monosulfate. The impedance behavior correlates with the pore solution chemistry and X-ray diffraction data. The normalized conductivity of the pastes displays an early drop followed by a large decrease from about 12 h to 3 days. At 3 days the permeability of the cement-based waste as calculated by the Katz-Thompson equation is over three orders of magnitude lower than that of Ordinary Portland cement paste. A further decrease in the calculated permeability is not apparent. This particular cement-based system provides rapid stabilization/solidification of the waste material. The transport of waste species is reduced by probable incorporation into apatite, zeolite, and other solid phases.