Effects of Carbonation and Calcium Leaching on Microstructure and Transport Properties of Cement Pastes

This thesis presents a comprehensive study of the consequences of exposure of cementitious materials to carbonation and calcium leaching. Due to the extremely slow nature of these degradation processes, NH4NO3 solution was used to accelerate the Ca-leaching degradation kinetics, while pure CO2 at high pressure was applied to speed up the carbonation. The changes in permeability and diffusivity of the degraded materials were measured by novel methods. Microstructural and mineralogical alterations were qualitatively and quantitatively examined by a variety of complementary techniques. In parallel, phenomenological models were also developed to better understand the transient state of degradation and to predict the evolution of the cementitious materials during the degrading processes. Results showed that leaching significantly altered the microstructure of the cement paste to a coarser material resulting in a significant increase in transport properties. In contrast, carbonation led to a refined microstructure and lower permeability and diffusivity. However, both leaching and carbonation processes induced a lower pH with possible loss of beneficial condition for waste package integrity. In combination with accelerated experiments, phenomenological models provide us the possibility to assess the long-term performance of cement-based materials used in disposal systems.