Incorporation of Cesium by Hydrating Calcium Aluminosilicates

Abstract

The leach behavior of cesium from a series of cement‐based waste forms has been modeled in terms of pore solution concentration and phase formation data. Cement‐based Materials enriched in alumina and silica, and compositionally related glasses, were hydrated in the presence of cesium hydroxide to gauge the degree of partitioning of cesium by the developing hydrates. The compositions of the pore solutions extracted from the cement‐based waste forms were compared both pore solution and leachates, the retention of cesium was found to be directly related to the bulk composition of the waste form; the degree of partitioning of cesium by the hydrated phases increased as more alumina and/or silica were added to the formulation, suggesting that cesium‐containing phases were forming. Additionally, the leachate concentration of cesium for any given composition was also found to be related to the pore solution concentration. This observation led to the development of a leaching model for cement‐based waste forms in which the concentration of cesium in the leachate could be described in terms of pore solution concentration, diffusion, dissolution, and development of cesium host phases. Using this model, effective diffusion coefficients for cesium were calculated to be ∼2 × 10−7 cm2· s−1. Pollucite and cesium‐substituted herschelite were identified as two of the hydration products of the studied glasses. The ability of these zeolites to strongly partition cesium suggests that small quantities of zeolitic phases may also be forming in the cement‐based waste forms, thus explaining their compositionally related leach behavior.