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 wi th high-alkaline solution simulating low-level radioactive waste at a one-to-one weight ratio. The pastes were adiabatically and isothermall y cured at various temperatures and analyzed for phase composition, to tal alkalinity, pore solution chemistry, and transport properties as m easured by impedance spectroscopy. The total alkalinity is characteriz ed by two main drops. The early one corresponds to a rapid removal of phosphorus, aluminum, sodium, and to a lesser extent potassium from th e pore solution. The second drop from about 10 h to 3 days is mainly a ssociated with the removal of aluminum, silicon, and sodium. Thereafte r, the total alkalinity continues to decrease, but at a lower rate. Al l pastes display a rapid loss in fluidity that is attributed to an ear ly 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 day s 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 diffrac tion 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 day s the permeability of the cement-based waste as calculated by the Katz -Thompson equation is over three orders of magnitude lower than that o f Ordinary Portland cement paste. A further decrease in the calculated permeability is not apparent. This particular cement-based system pro vides rapid stabilization/solidification of the waste material. The tr ansport of waste species is reduced by probable incorporation into apa tite, zeolite, and other solid phases. (C) 1997 Elsevier Science B.V.