The impact of mineral deposition by carbonation and dissolution on tortuosity for leaching of constituents from a cementitious waste form

Multiple processes change the pore structure of cementitious materials during environmental exposure. For cementitious materials, important chemical processes that change pore structure include leaching of calcium phases and reactions forming carbonate precipitates. The resulting change in pore structure can be localized as deposits or depleted thin near surface layers, or distributed amongst substantial depths of the material. In turn, changes in pore structure can result in either decreases in physical retention processes (wash out) or increases in physical retention (pore sealing). The effect of pore structure on physical retention processes is evaluated in this paper as changes in diffusive tortuosity. The geochemical speciation and mass transport solver ORCHESTRA has been implemented to model the leaching of major and minor constituents from a cement-stabilized low-level radioactive waste analogue. Diffusion controlled leaching tests were carried out with deionized water and a simulated groundwater containing magnesium carbonate. Porosity change as a consequence of mineral precipitation and or dissolution has been considered by calculating the net solid volume change of all the minerals present in the system. Constitutive relationships for tortuosity change as a function of porosity change reported in the literature are compared with one another and with experimental results. Sensitivity to initial porosity, initial tortuosity, and external solution composition are investigated.